The void fraction of two-phase flow in the 1 mm of channel is investigated in this study. The small channels, including micro to mini-channel, are widely used for developing miniaturized air conditioning systems. In terms of carbon emission reduction, optimizing the refrigerant charging amount based on sufficient information and understanding of the characteristics of the two-phase flow is a noteworthy challenge. The void fraction is a parameter of the two-phase flow that is essential for determining the heat transfer coefficient, modeling the pressure drop, and predicting the refrigerant charging amount. Therefore, the precise measurement and prediction of void fraction in small channels for various refrigerants are required. However, thus far, the void fraction characteristics of small channels have not been actively investigated because of the limited volume inside a single small channel and the relatively high error of the quick-closing valve (QCV) method for small channels. This study proposes a capacitance-based sensor as an alternative method for void fraction measurement of small channels. The newly designed void fraction sensor was fabricated with 7.8 % uncertainty under compensation for manufacturing limitations. The void fractions of refrigerants R32 and R1234yf flowing through smooth small horizontal channels under adiabatic conditions (inner diameter: 1 mm, mass flux: 300–600 kg m−2 s−1, saturation temperature: 20–30 °C, vapor quality: 0.025–0.900) were measured. The measurement results were compared with ten existing prediction correlations of five classifications, and the correlations that optimally predicted the void fractions of R32 and R1234yf in the small horizontal channel were presented.

In this study, a new evolutionary method, which can handle the implementation of genetic operators with unrestrained number and locations of splitting and merging nodes for the optimization of heat exchanger circuitries, is developed. Accordingly, this technique expands the search space of previous optimization studies. To this end, a finned-tube heat exchanger simulator is structured around a bijective mathematical representation of a refrigerant circuitry (the tube–tube adjacency matrix), which is used in combination with traversing algorithms from graph theory to recognize infeasible circuitries and constrain the evolutionary search to coherent and feasible offspring. The performance of three refrigerants, namely R32, R410A, and R454C, commonly used in air-conditioning applications was assessed for the optimized circuitries of a 36-tube evaporator while converging to a given cooling capacity, degree of superheating, and heat source boundary conditions. At a given output capacity and air outlet temperature, larger coefficient-of-performance improvements (up to 9.99% with reference to a common serpentine configuration) were realized for zeotropic refrigerant mixtures, such as R454C, where appropriate matching of the temperature glide with the temperature variation of the air yielded the possibility of further reducing the required compression ratio under the corresponding operating conditions. Hence, it was demonstrated that low-GWP zeotropic mixtures with temperature glide can realize a performance comparable to that of R32 and higher than that of R410A by approaching the Lorenz cycle operation.

A numerical model for the dynamic analysis and control of a solar-assisted single–double-effect absorption chiller is presented. The mathematical model relies on energy and mass balances, which consider the heat and mass storage in each component. The resistance ratio method is introduced to capture the variation in the thermal conductance representing system disturbances and load variations according to the internal and external flow rates. The model was validated with reference to the outlet temperatures of hot, cooling, and chilled water from field test data for a given gas flow rate and corresponding inlet temperatures. The agreement between the field test and simulation results demonstrates the reliability of the model. Deviations between simulated and experimental temperatures mostly stays below 0.5 °C. The model was used to establish an effective operating strategy capable of minimizing the primary energy consumption without affecting the stability of the cooling output capacity and to verify the corresponding beneficial effect. The proposed operational approach increases the coefficient of performance by as much as 32 % compared with the previously implemented operation strategy, reaching up to a maximum value of 10.7 at 60 % cooling capacity.

In this study, a technique that uses a capacitance sensor with an asymmetric electrode to measure the void fraction of a refrigerant was developed. It is known that the void fraction and flow pattern affect the measured capacitance. Therefore, the relationship between the void fraction and capacitance is not linear; hence, a calibration method for obtaining accurate measurements is necessary. A calibration method was designed in this study based on repeated capacitance measurements and the bimodal temporal distribution to calibrate the atypical and repetitive flow patterns of slug flow and its transition to the intermittent flow regime. The calibration method also considers the weighted-average relation for the gradual transition of the intermittent to annular flow pattern according to the change from low to high quality. The proposed method was experimentally analyzed under the conditions of R32 refrigerant, a tube inner diameter of 7.1 mm, saturation temperature of 25 degrees C, mass flux of 100-400 kg m(-2) s(-1), and vapor quality of 0.025-0.900, and it was validated using a quick-closing valve (QCV) system under identical conditions. A relative error of 2.99% was obtained for the entire system, indicating good agreement between the proposed and QCV-based methods.

Artificial intelligence (AI) techniques have been widely used across many fields. However, few studies have focused on the use of AI techniques for predicting heat transfer coefficients regardless of single-phase or two-phase flows. The applicability of deep neural networks [(DNNs), also known as deep learning], one of the most promising AI techniques, to horizontal-flow boiling heat transfer in mini-channels is being actively researched. The effect of surface tension in mini-channels is significant in comparison to that in conventional large tubes, and the heat transfer mechanism in the mini-channels is complicated. Thus, the accuracy of the prediction results based on existing studies is not satisfactory. Moreover, we cannot determine the uncertainty of the predicted heat transfer coefficients by using existing approaches. In this study, we propose a novel prediction mechanism, based on the combination of a DNN and Gaussian process regression, that can predict not only heat transfer coefficients with high accuracy but also the uncertainties of the predicted heat transfer coefficients. We refer to this new research field, which integrates thermal engineering and informatics, as thermoinformatics, and consider the scope of its future development.

This paper presents the development of a method for predicting the performance of air conditioning systems using few accessible and inexpensive input parameters. The cooling capacity is predicted using artificial neural network with four selected refrigerant temperatures measured from the outdoor unit as the inputs. Input output prediction data are obtained numerically and experimentally from two representative variable refrigerant flow (VRF) systems. The two systems have different characteristics and nominal capacity. The training of the ANN model is conducted with the data obtained from numerical simulations. Consequently, the ANN is tested for the prediction of the experimental cooling capacity in a quasi-certified testing equipment. The results indicate that the proposed performance prediction method demonstrates a relative error lower than 10%.

A general variational formulation of the void fraction in dissipative two-phase flows is developed. In particular, the steady-state void fraction is extracted by the extremization of the entropy generation rate, based on a comprehensive non-equilibrium expression that introduces interfacial contributions due to surface tension between different phases and fluxes due to heat and mass transfer.Such formulation opens up to the possibility of obtaining different theoretical formulas by the direct application of Prigogine's theorom of minimum entropy production and it is investigated in terms of different assumptions for showing the effect of different flow conditions, flow pattern interfacial geometry, entrainment ratio, heat flux from the external environment, and thermophysical properties of the refrigerant. Consequently, the widely-accepted Zivi's expression of void fraction is obtained as a particular case of this general formulation.

© 2020 Elsevier Masson SAS The two-phase flow void fraction is a critical parameter for characterising the pressure drop as well as heat and mass transfer capability of the working fluid within thermal systems, the accurate estimation of which drives heat exchanger design and control optimisation. A semitheoretical expression for the void fraction of two-phase flows, also applicable to small-sized channels, is obtained from an analytical study based on the principle of minimum entropy production and the introduction of empirical coefficients to be fitted to experimental data available in the open literature. These coefficients embody the importance of the simplified physical terms of this formulation while recovering the accuracy loss owing to nonlinear phenomena, heat and mass transfer, and three-dimensional effects. By accounting for surface tension, this model generalises previous theories and describes the influence of smaller-sized channels in terms of the stable void fraction. This mathematical framework can be used to summarise data covering different refrigerants, channel diameters, and operating conditions.

Realistic performance predictions are required for efficient operation strategy of air conditioners. In this study, the application of a cost effective and non-intrusive black box model utilizing artificial neural networks (ANN) to predict the cooling capacity of air conditioning systems is investigated, while considering the effect of fouling and leakage that may occur after prolonged operation. The effect of various leakage and fouling combinations on the output cooling capacity were numerically simulated. The training data set is first generated for a system that is ideally operating without any fouling or leakage. The developed ANN model is tested to predict cooling capacity in "faulty" systems. The results indicated that, as long as leakage and fouling are limited below 10% and 4% respectively, the ANN model trained by the data generated with the ideal system, can predict cooling capacity with a relative averaged cooling capacity difference (Delta(Q) over bar (e,rel)) of approximately 13%. Moreover, the inclusion of data with different leakage and fouling combinations in the training set enables accurate predictions of the cooling capacity of the air conditioning system during the entire timespan of its operation. It suggests that cooling capacity under the fouling and leakage phenomena can be predicted using limited input information.

Owing to the high variability of operating conditions and the complexity of dynamic phenomena occurring within air conditioning cycles, the realistic performance estimation of these systems remains an open question in this field. This paper demonstrates the applicability of a cost-effective estimation method based on an artificial neural network exclusively using four refrigerant temperatures as the network input. The experimental datasets are collected from a reference experimental facility. The system is operated with variable cooling load, outdoor temperature, and indoor temperature settings, as representative of the actual operation. The artificial neural network structure was optimized by considering the effect of previous time step inputs, number of neurons, sampling time, and number of training data. The results reveal that the developed model can successfully estimate the cooling capacity of an air conditioning system during on-off, continuous unsteady, and steady operation, using four temperature inputs with relative averaged error below 5%.

Poor distribution of two-phase flow in a microchannel heat exchanger header requires a solution to maximize its potential advantage of compactness while offering a high heat-transfer rate. While previous works have attempted to improve two-phase flow distribution, most have focused on horizontal headers, which commonly appear in indoor units. However, there are a few studies on vertical headers that function as an evaporator in heat pump mode that are typical in outdoor units. This study describes the development of a modified vertical header with a dual-compartment structure aimed at improving the flow distribution. Experiments and visualizations were conducted using R410A in a test section that represented the size and capacity of an actual heat exchanger. The inlet mass flux was 309–1235 kg·m−2s−1 (50–200 kg·h−1), vapor quality was 0.1–0.2, and the evaporating temperature was maintained at 15 °C. The results were compared to the conventional header previously evaluated by our research group. Results showed that the dual-compartment header was able to enhance the liquid flow distribution at low inlet mass flux from approximately 30% to 80% based on relative standard deviation (RSD), with a generally higher liquid Reynolds number when compared to the conventional header.

© 2019 Due to the intrinsic complexity of two-phase flow distribution and the limited mathematical flexibility of conventional formulations of the phenomenon, previous attempts generally fall short in the accuracy and applicability of their prediction. To address these issues, this study focuses on methods with higher mathematical flexibility. Specifically, the construction and training of Artificial Neural Network (ANN) is presented for the identification of this complex phenomenon. The interaction of the numerous physical phenomena, occurring at different scales, is thus represented by the network structure, offering a formulation capable of achieving higher accuracy. Experimental data from a full-scale heat exchanger of an air-conditioning system operating over a wide range of conditions are used to train and test the ANN. The network optimisation with Bayesian regularisation against experimental data leads to a structure featuring 4 inputs, 3 hidden layers, and 3 neurons for each layer, which demonstrates deviations on the single output mostly lower than ± 10% and a correlation index higher than 98%, when the whole data set is used for training the ANN. The analysis of the network optimisation for different shares of data used for the network testing, shows higher training and testing accuracy as the number of training data increases, along with no apparent overfitting.

This study presents a preliminary investigation of the performance of a Heat Pump Water Heater (HPWH) that uses CO2 as refrigerant. The global market of this technology is led by Japan and several units are being installed all around the world under different climates. The main challenge for an efficient spreading of this technology is to maintain the excellent performance of CO2 HPWH under a broad variety of weather conditions. A numerical model was developed to flexibly investigate the performance of the CO2 HPWH under various distributions of inlet air temperature at the evaporator and inlet water temperature_in gas cooler. The simulation results are firstly compared with those from a general pinch point analysis. The Result show that the highest water temperature inlet and lowest air temperature inlet will produce lowest COP and Highest LCCP Contribution

The search for new and more energy-efficient solutions with natural fluids has become a clear countermeasure for mitigating the carbon footprint of air conditioning and refrigerating systems. In the absorption field, specifically in liquid desiccant air conditioning systems, the application of internal cooling in gas-liquid contactors is limited due to the corrosion property of commonly used liquid desiccants. Internally cooled/heated liquid desiccant systems have better heat and mass transfer performance compared to adiabatic systems but mathematical modeling becomes more complicated. In this study, experiments and simulations were carried out to evaluate the performance of an internally cooled/heated liquid desiccant air conditioning system using a novel ionic liquid. This innovative solution became possible due to the compatibility, in terms of corrosion, of the newly developed ionic liquid with aluminum. The validated mathematical model can be used to conduct parametric and optimization studies for the internally cooled/heated liquid desiccant air conditioning system.

An integrated system for heating, cooling and compressed air energy storage (CAES) is analyzed from a thermodynamic point of view. The system is based on asynchronous air compression and expansion, in order to take advantage from the daily ambient temperature oscillations and energy cost variations. The analysis is intentionally kept on a fundamental level, without explicit reference to specific components, in order to enlarge the choice of potential applications. Effects of losses in compressor, expander and heat exchangers, as well as heat transfer in the CAES, are included. The proposed system, once optimized and experimentally validated, could become viable options in the wide arena of demand-side energy management.

A common technical practice to boost the transfer performance of falling film absorbers within absorption cycles relies on surfactant additives for triggering the interfacial turbulence known as Marangoni convection. Given the phenomenon complexity and the interdependencies with other phenomena, such as surface wetting, the assessment of the related enhancement is unresolved yet, and data from previous literature are limited to the operative conditions of single effect refrigerators. This work combines numerical simulations and experiments for evaluating the transfer performance enhancement due to Marangoni convection in a wide operative range encompassing refrigeration and heat pump cycles. Falling film transfer coefficients are measured, with and without n-octanol, with LiBr solution temperature between 50 and 120oC, and specific mass flow rate between 0.011 and 0.11Kg·m-1s-1. Consequently, a semi-empirical model estimates the contribution of improved wettability and consequently indicates the portion of the transfer enhancement related to the Marangoni convection.

There is a growing interest in green vehicles, such as hydrogen powered fuel cell cars, along with the increase in public awareness towards the environmental issues. However, with compressed systems applied for fuel storage purpose, refueling of hydrogen causes an issue, during which high-pressure hydrogen from the station storage tank passes through filling components before entering the empty tank in the vehicle. In this state, as the pressure difference between the station tank and the vehicle tank can be very high, the complexity of solenoid valve geometry creates complex flow characteristics (turbulence and interaction of gas and wall) that could generate intense pressure fluctuation and excessive noise. This paper presents the results of a three-dimensional simulation for characterizing the complex flow inside the solenoid valve and for diagnosing flow-induced noise generated during the charging process at different valve openings and pressures. Simulation is carried out using computational fluid dynamics (CFD) by implementing appropriate turbulence and viscous models. Firstly, the simulation is conducted in a steady state condition to investigate the acoustic power and to localize the regions that are most likely to be affected. In addition, transient simulations are performed to obtain the frequency of the pressure fluctuation inside the solenoid valve. The results provide accurate and useful data about flow pattern inside the solenoid valve and additionally, furnish information to determine the sources, locations, and characteristics of flow-induced noise generated within the solenoid valve. This investigation can be applied to reduce the undesirable noise and to analyze the possibility of system failure due to flow-induced vibrations.

An increasing number of supermarkets and convenience stores has led to a proportionally higher demand for open refrigerated display cabinets (ORDCs). Ease of access to refrigerated products is the main advantage of ORDCs
however, they also exhibit high cumulative energy consumption as well as direct and indirect CO2 emissions. This work aims to formulate, establish, and apply a seasonal performance evaluation method for ORDCs and “walk-in” type refrigerated device driven by CO2 chillers. First, a thermodynamic modeling approach is used and fitted to experimental data. The relative error between the predicted and actual heat load is within ±10%, whereas it is within ±20% for the normalized compressor electric input. Then, an evaluation tool is constructed using the seasonal energy consumption of ORDCs commonly used in Japan and driven by a transcritical CO2-compression chiller within Japanese climatic conditions. This tool can be used to predict the annual energy demand for given ambient conditions as well as the reduced environmental impact of multiple combinations of refrigerated display cabinets compared to conventional HFC/HCFC-based systems. According to the annual temperature distribution in chosen regions, the annual thermal load, annual electricity consumption, and annual coefficient of performance are calculated and analyzed. Simulation results quantitatively evaluate the beneficial effect of Te as being translated to a better COP, and lower energy consumption and CO2 emissions. Considering the geographic location of the store, the quantitative results show how a hotter climatic condition leads to higher energy consumption and CO2 emissions, and lower COP. The proposed evaluation method is generally applicable to any regional setting and any chiller-ORDC combination.

The optimization of heating, ventilating and air conditioning (HVAC) system operations and other building parameters intended to minimize annual energy consumption and maximize the thermal comfort is presented in this paper. The combination of artificial neural network (ANN) and multi-objective genetic algorithm (MOGA) is applied to optimize the two-chiller system operation in a building. The HVAC system installed in the building integrates radiant cooling system, variable air volume (VAV) chiller system, and dedicated outdoor air system (DOAS). Several parameters including thermostat setting, passive solar design, and chiller operation control are considered as decision variables. Subsequently, the percentage of people dissatisfied (PPD) and annual building energy consumption is chosen as objective functions. Multi-objective optimization is employed to optimize the system with two objective functions. As the result, ANN performed a good correlation between decision variables and the objective function. Moreover, MOGA successfully provides several alternative possible design variables to achieve optimum system in terms of thermal comfort and annual energy consumption. In conclusion, the optimization that considers two objectives shows the best result regarding thermal comfort and energy consumption compared to base case design.

The two-phase flow distribution behavior of R410A within the vertical header of a microchannel heat exchanger with multiple horizontally oriented microchannel flat tubes was investigated and is reported in this paper. Unlike most previous studies, which examined the distribution at lower flowrates applicable mostly to automobile applications, this work evaluated higher flowrates relevant to actual air conditioning evaporator applications with larger size headers. The following operating conditions, were utilized: an inlet mass flowrate that varied from 40 to 200 kg h−1 (mass flux of 27–250 kg m−2 s−1 in the header), vapor qualities of 0.1, 0.2, and 0.6, and evaporating temperatures of 10 and 15 °C. The tube protrusion depth into the header was set at 0 and 50%. Flow distribution profiles derived from the experiment measurements and clear visualization images captured by a high speed camera showed that the distribution improves for increased inlet mass flux at low vapor quality, while a 5 °C difference in evaporating temperature does not yield a substantial distribution change. A 50% protrusion produces higher inertial forces pushing the liquid level towards the top section. A correlation was developed to predict the liquid distribution by relating the portion of liquid exiting the branch tube to the liquid at the immediate header as a function of the liquid Froude number.

The two-phase flow distribution behavior of R410A within the vertical header of a microchannel heat exchanger with multiple horizontally oriented microchannel flat tubes was investigated and is reported in this paper. Unlike most previous studies, which examined the distribution at lower flowrates applicable mostly to automobile applications, this work evaluated higher flowrates relevant to actual air conditioning evaporator applications with larger size headers. The following operating conditions, were utilized: an inlet mass flowrate that varied from 40 to 200 kg h(-1) (mass flux of 27-250 kg m(-2) s(-1) in the header), vapor qualities of 0.1, 0.2, and 0.6, and evaporating temperatures of 10 and 15 degrees C. The tube protrusion depth into the header was set at 0 and 50%. Flow distribution profiles derived from the experiment measurements and clear visualization images captured by a high speed camera showed that the distribution improves for increased inlet mass flux at low vapor quality, while a 5 degrees C difference in evaporating temperature does not yield a substantial distribution change. A 50% protrusion produces higher inertial forces pushing the liquid level towards the top section. A correlation was developed to predict the liquid distribution by relating the portion of liquid exiting the branch tube to the liquid at the immediate header as a function of the liquid Froude number.

In this paper, dynamic performance identification for a direct expansion (DX) air conditioning (AC) system is proposed using Bayesian artificial neural network (ANN). The input and output datasets are generated by a dedicated AC simulator by varying the compressor speed in various signal amplitudes and including dynamic cooling load and ambient temperature. The exergy destruction, which represents the work potential losses in the system and room temperature indicating the thermal comfort are selected as the output variables. The key parameters of an ANN model, including the number of neurons and tapped delay lines, are optimized to improve the prediction accuracy. The results show that the dynamic response of the exergy destruction and room temperature can be predicted accurately by the optimized ANN model using three neurons, a Bayesian regularization algorithm, five delayed inputs for the compressor speed and room temperature, and six delayed inputs for the cooling load and ambient temperature. The validation of the multi-step-ahead prediction showed satisfying results with respect to the root mean squared errors (RMSEs) and coefficient of variations (CVs) of the room temperature (RMSE: 0.18 degrees C and CV: 0.85%) and exergy destruction (RMSE: 1.79 W and CV: 0.4%). Accordingly, the identification of the AC system behavior presented in this paper could be further implemented to control the DX AC system operation to achieve a desired thermal comfort with low exergy destruction.

In commercial and office buildings, it has recently become popular to install variable refrigerant flow (VRF) compression-type heat pump systems. In particular, VRF systems with multiple indoor units are being installed in large buildings because such systems have large adjustment capacity. On the other hand, because users of VRF systems can freely choose the number of indoor units to install and can turn indoor units on and off independently, operating conditions are not predictable and it is difficult to find adequate control parameters under wide changes in load. With this background, we are developing a new numerical simulation model based on the laws of physics. Because this model can easily add and delete system elements, we believe that it will be useful for VRF system analysis. In this study, we utilize a numerical simulation model to evaluate both steady and unsteady conditions in VRF systems. In this paper, we focus on the connection between manipulated variables (the expansion valve's open pulse and fan rotational speed) and controlled variables (supply air temperature and the degree of superheating at the evaporator outlet). Moreover, we compare the difference between the performance of a single indoor unit and that of multiple indoor units operating simultaneously. As a result, we can evaluate the dynamic characteristics of manipulated variables and the effect of changing the number of active indoor units, each of which has their own VRF characteristics.

With the inherent flow maldistribution problem within the microchannel heat exchanger header, which degrades the heat transfer performance, new microchannel heat exchanger header was designed and tested to improve this flow distribution performance, specifically for R410A. It features dual-compartment made of multiple layers consisting of the main section, baffle and hole seat, and rear section. Twenty flat tubes containing microchannels were inserted at 50% protrusion. Experiments and visualizations were conducted at 50, 100, and 200 kg·h-1 of mass flow rate (equivalent mass flux of 220.5, 441, and 882 kg·m-2s-1), which were at partial to full loading capacity. Inlet vapor quality was fixed at 0.2 and evaporating temperature at 15 ºC, for evaporator application of air-conditioning system. Considering the same microchannel element, flow distribution enhancement was achieved when compared to the conventional header data. Liquid flow distribution was improved by around 65% at part-load conditions based on the relative standard deviation.

The number of renewable power plants has recently been increasing
hence, it is essential to maintain a balance between the supply and demand of electric power. We employ the technique of demand response (DR) through the manipulation of air conditioners. In summer, the electric demand for air conditioning consumes the majority of the available total electric power
therefore, regulating this demand compliments the requirements of peak-cut and load leveling. However, the manipulation of air conditioning impacts the user comfort and the stability of the machine. Moreover, the energy consumption of air-conditioners is uncertain owing to the operation conditions. Consequently, it is difficult to estimate the performance of the DR method. In this study, we focus on the variable refrigerant flow system, which is a popular air-conditioning system in Japan. We evaluate the performance of the demand response method using a simulation analysis and ensure that the amount of DR to regulate the air-conditioner is controlled quantitatively.

Identification for system dynamic behaviour is necessary to develop control strategy. In this paper, the dynamic performance of air conditioning (AC) system is predicted using artificial neural network (ANN) approach. The ANN is developed to predict exergy efficiency, coefficient of performance (COP), and cooling capacity. The controllable parameters including compressor speed and evaporator and condenser fan speed are considered as the input. The datasets for prediction are generated by AC system simulator. The system was simulated by randomly varying compressor speed and evaporator and condenser fan speed with N-sample signal input. The dynamic ANN configuration with Bayesian regularization is proposed to predict one-step ahead of system performance behaviour. The results show that the developed ANN in present study yields good prediction accuracy for all outputs. Accordingly, ANN can be further applied for predictive control application in AC system to control cooling capacity while maintaining system efficiency.

© 2019 International Institute of Refrigeration. All rights reserved. A new design of an internally cooled desiccant contactor which uses a new ionic liquid (IL) solution as sorptive medium is targeted. To optimise its operative performance, a semi-theoretical model based on a variational thermodynamic principle is developed to predict the film rupture and wetting ability of the IL solution for different values of the IL mass fraction over a comprehensive range of liquid spray density. The minimum Reynolds number Reb allowing the film-like flow configuration and, below this value, the wetted fraction of the solid substrate are firstly formulated on an analytical basis. Successively, the theoretical formulation is used as a reference to minimise deviations between predicted results and measured data by calibrating dedicated characteristic coefficients. Predicted film rupture, wetting ability and wetting hysteresis behaviour are strongly affected by the IL mass fraction and found in good agreement with the measurements.

The air-conditioning system for buildings and facilities is required that the room temperature and the humidity are kept in a predetermined range for comfortable air-conditioning. For achieving the above only by a heat pump, the room condition should be heated till the preset temperature after cooling till the temperature from which the request humidity is obtained, and this operation is inefficient.To overcome such inefficiency, separation of latent heat and sensible heat has been proposed. This is a two-step process in which temperature and humidity are adjusted separately in two steps by combining desiccant dehumidifier and heat pump. Theoretically, the system is known to be effective in reducing energy consumption, but in practical application, this technology can be further improved.In this research, we combined the liquid desiccant equipment which can perform low temperature regeneration and R718 centrifugal heat pump suitable for high efficiency for this application. Based on this concept, we have developed a highly efficient air-conditioning system by improving element equipment and optimizing operating conditions. The performance of this system is reported here.

© Published under licence by IOP Publishing Ltd. Ionic liquids, salts which have liquid phase at temperature below 100°C, have been widely introduced in engineering applications, including absorption cooling systems. The application of ionic liquids in absorption cooling systems is intended to remove the disadvantages of conventional working fluids such as corrosion and crystallization. In terms of thermodynamic performances, theoretical investigation based on solubility behavior of ionic liquids in natural refrigerants show a competitive performance in comparison with conventional working fluid. Nevertheless, heat transfer performance, which is also an important key in absorption cooling systems, particularly in terms of system design and size, needs to be deeply explored and investigated. This study aims to assess the thermal and transport properties of ionic liquids as absorbent in relation to the heat and mass transfer characteristics of these working fluids. The thermal and transport properties of ionic liquids proposed as absorbent for absorption cooling systems are collected, and heat and mass transfer characteristic of these ionic liquids based on their thermal and transport properties are investigated and analyzed. Finally, the most suitable ionic liquids for absorption machines, both in terms of thermodynamics and heat and mass transfer performances can be proposed.

This study involves exploring a new design of an internally cooled/heated desiccant contactor by using a new ionic liquid (IL) solution as the sorptive solution. In order to optimize its operative performance, a semitheoretical model based on the principle of minimum energy is developed to predict the film rupture and wetting ability of the IL solution over a comprehensive range of IL mass fraction and flow rates. A first experimental validation of the fundamental equations of the theoretical model is presented and used as a reference to minimize deviations between predicted results and measured data by calibrating dedicated characteristic coefficients. The noteworthy quantitative and qualitative agreement in the whole range of IL mass fractions and flow rates is promising for contributing to the design of optimized system configurations and control strategies.

Absorption chillers constitute a valuable option for utilising solar energy. Specifically, when installed in tropical regions, this technology ideally matches the needs for refrigeration and air-conditioning because of the abundance of solar energy throughout the year. A single-double-effect absorption chiller combines the single and double-effect configurations to compensate for the unpredictable instantaneous availability of solar radiation and cooling load fluctuations. The operative performance of this system is strongly affected by internal parameters such as the absorber outlet solution flow rate and the solution distribution ratio, which connect the operability of the single and double-effect configurations. Therefore, these important parameters are currently used to maximise system performance while assuring its stability. This study discusses how the COP of a single-double-effect absorption chiller, for solar cooling applications in tropical areas, can be maximised (1.55 at full load, and up to 2.42 at 60% partial load) by manipulating those internal parameters. The simulation results were compared with the experimental data (field test data) and, by adopting the appropriate control method, showed an improvement of the system performance between 12 and 60% when compared to a corresponding double-effect configuration.

This paper presents general types of correlation for the heat and mass transfer coefficients inside an air solution contactor as expressions of Reynolds-Prandtl numbers and Reynolds-Schmidt numbers, respectively. These general equations summarize the physical and thermophysical properties of the air, the solution, and the contactor, which make them capable to be used for parametric studies provided they are fitted in a wide range of experimental data that include all the properties involved. In this work, a liquid desiccant system with an adiabatic structured packed bed as contactor and an aqueous lithium chloride as solution was constructed. The experimental data taken at various air superficial velocities and solution flow rates were fitted to the general correlations, and comparisons between the predicted and experimental results for both coefficients are within +/- 10%, for both dehumidification and regeneration processes. In addition, the calculated values of the outlet air humidity ratio and temperature agree well with the experimental data for both processes. The particular equations for the heat and mass transfer coefficients can be used to perform parametric studies at different air superficial velocities and solution flow rates with very good accuracy. Results from this study can help improve the system design and operation methods of air-solution contactors. (C) 2017 Elsevier Ltd. All rights reserved.

Detailed, reliable, and time-saving methods to predict the transfer characteristics of horizontal-tube falling-film absorbers are critical to control system operability, such that it is closer to its technical limitations, and to optimise increasingly complex configurations. In this context, analytical approaches continue to hold their fundamental importance. This study presents an analytical solution of the governing transport equations of film absorption around a partially wetted tube. A film stability criterion and a wettability model extend the validity range of the resulting solution and increase its accuracy. Temperature and mass fraction fields are analytically expressed as functions of Prandtl, Schmidt, and Reynolds numbers as well as tube dimensionless diameter and wetting ratio of the exchange surface. Inlet conditions are arbitrary. The Lewis number and a dimensionless heat of absorption affect the characteristic equation and the corresponding eigenvalues. Consequently, local and average transfer coefficients are estimated and discussed with reference to the main geometrical and operative parameters. Finally, a first comparison with the numerical solution of the problem and experimental data from previous literature is presented to support the simplifying assumptions, which are introduced and as a first model validation.

In falling film liquid desiccant systems, finding a suitable pair of liquid desiccant and contact surface is of primary engineering interest. This requires knowledge on the wetting characteristics of the liquid on the solid substrate, which consequently requires intensive experimental investigation. In this study, the wetting characteristics of a new ionic liquid aqueous solution in an aluminum fin-tube substrate were experimentally investigated. Then, a simple method for estimating the wetted area on the substrate through image processing was developed. Visual analysis of the surface wetting was also conducted, and three types of wetting patterns are discussed. Experimental results on the static contact angle and contact angle hysteresis suggest that the ionic liquid solution is mostly wetting, and the aluminum surface is slightly to moderately rough. It was found that the wettability of the ionic liquid solution increases as the ionic liquid mass fraction increases. The wetting hysteresis phenomenon and the factors contributing to its occurrence were also clarified. The results from this study would be useful for the development of a new model or improvement of existing wetting models, which can help improve the prediction and control of the heat and mass transfer performance in internally cooled/heated fin-tube contactors. (C) 2018 Elsevier Ltd and IIR. All rights reserved.

Recently in Japan, it has been popular to install VRF (Variable Refrigerant Flow) heat pump system to office buildings regardless of size and shape, because many Japanese regard it as the virtue to turn off the air conditioner frequently for energy saving if it is not necessary. This system consists of packaged outdoor unit and multi indoor units connected by refrigerant pipe, and there are some merits for users and owners. For example, it is easy to design for planning building system because of standardized system. Moreover, this system contributes to saving energy and to increasing comfort for users, because it can be operated adequate indoor unit as necessary. On the other hand, because refrigerant flow rate changes dynamically depend on load condition, this system needs complicated control system for keeping stable operation under many manipulating elements. For that reason, it is too difficult to find adequate each elements' condition by only experimental study. In order to evaluate VRF system by easy way, we build the numerical simulation model based on law of physics and we research adequate control condition under various sorts of cooling condition. It is said that conventional control method occurs uns table manipulating condition in case of changing load condition. In this report, we reproduce unstable condition of expansion valve by simulation study.

Annual heat load, energy consumption and COP of open refrigerated display cabinets (ORDC) driven by heat pump in supermarkets are assessed by means of a dedicated performance evaluation method. The system modelling approach is firstly presented, and subsequently fitted to experimental data measured within actual equipment working in a comprehensive range of operative conditions. The obtained characteristic equations for estimating the heat load of the ORDC and energy consumption of the condensing unit are employed for a first screening of the results with reference to the effect of the display cabinet set temperature, refrigerant evaporating temperature and supermarket room temperature. Seasonal temperature distributions of three representative locations are applied for the performance calculation of a selected arrangement of 5 different models of ORDCs in a store. This methodology is integrated in a dedicated software, with a user friendly interface, which is generally capable of predicting the annual performance of various combinations of different ORDCs and can therefore be used for benchmarking and identifying energy conservation opportunities.

General implementation of air conditioning in buildings and facilities is such that in order to keep room temperature and humidity within predetermined range, heat pump is used to lower the air temperature to achieve desired humidity and then the air is heated to the specified temperature. From energy-saving perspective, this process is inefficient.To overcome such inefficiency, separation of latent heat and sensible heat has been proposed. This is a two-step process in which temperature and humidity are adjusted separately in two steps by combining desiccant dehumidifier and heat pump. Theoretically, the system is known to be effective in reducing energy consumption, but in practical application, this technology can be further improved.For this research, we have chosen liquid desiccant dehumidifier as the desiccant can be regenerated at low temperature. For heat pump system, we have chosen R718 centrifugal as it is suited for increasing efficiency for such combination of desiccant and heat pump. By improving each element of the system and seeking optimization of operating conditions, we aim to develop a high efficiency air-conditioning system. The result is reported here.

An analytical study based on a variational thermodynamic principle is presented to evaluate the influence of surface tension on the stability of annular flow within small-sized channels. The model introduces phenomenological assumptions in the interfacial structure of the flow regime and theoretically draws the equilibrium transition line from an annular regime to the initiation of the partial wetting condition on the inner surface. By including surface tension, this model expands previous theories and identifies the stable flow configuration in terms of void fraction and interfacial extension. The significant influence of a higher surface tension and smaller diameter (i.e. lower Weber number) are responsible for a lower stable void fraction and higher slip ratio. A complete screening of the main influential parameters is conducted to explore the descriptive ability of the model. This analysis aims at contributing to the understanding of the stability of two-phase flow regimes and can be extended to the transition between other neighbouring regimes, including wall friction as well as liquid entrainment phenomena. (C) 2017 Elsevier Ltd. All rights reserved.

Due to their unique characteristics, recently ionic liquids have been proposed as a novel absorbent for absorption systems to eliminate the drawbacks of conventional water/LiBr working fluids, particularly crystallization and corrosion. Studies on the cycle performance of absorption cooling systems using water/ionic liquid working pairs have been published in many literatures. However, specific studies on the absorption characteristics of these new working pairs still remains open. Present study theoretically analyses the absorption characteristics of water vapor by aqueous ionic liquid solutions in a vertical tube falling film absorber. The model is developed by employing three ordinary differential equations describing the absorption process and is validated. The results are analyzed and the performances of water/ionic liquid are compared with those of conventional water/LiBr working fluid.

temperature refrigeration of cold stores poses some specific issues: single stage, vapour compression cycles have modest COP at low evaporation temperature; cold evaporator surfaces require de-frosting and a fan for air circulation; a part of the refrigeration load may be delivered at intermediate temperature levels, e.g. for the cold store loading dock. Cascade system may improve the COP and add flexibility on the temperature levels and working fluids, but the problems related to the cold evaporator surface remain unsolved.
The refrigeration system presented herein features a cascade configuration combining a vapour compression cycle and an inverse Brayton cycle. Both cycles use "natural" fluids, complying with strictest regulations. The top cycle uses Ammonia in order to increase efficiency, while the bottom cycle uses air, which directly circulates in the cold space and hence eliminates the cold heat exchanger. A detailed thermodynamic analysis allows a complete screening of the relevant design parameters for an overall system optimization.
The results show that, notwithstanding the intrinsic gap of efficiency suffered by the Brayton cycle, the proposed system features an acceptable global performance and widens the implementation field of this technology. This system configuration shows a COP 50% higher than the corresponding simple Brayton cycle at temperatures of the refrigerated storage of -50 degrees C. (C) 2017 Elsevier Ltd. All rights reserved.

As widely known, some industrial processes produce a large amount of waste heat while others require a large amount of steam to heat the process flow. The main difference involves the temperature level of these heat quantities. Absorption heat transformers play a strategic role in waste heat recovery and heat supply to manufacturing processes due to their ability to utilize heat at a certain temperature level and release the enthalpy of mixing of the refrigerant at a different temperature level with a negligible amount of mechanical work input. However, given the lack of examples that find application as operative plants, the feasibility of the technology is questioned in academic and technical domains. In this study, the operability of a double-lift absorption heat transformer that generates pressurized steam at 170 degrees C is studied across a full range of operative conditions. The results demonstrate and clarify the manner in which the system can operate steadily and efficiently when driven by hot water temperature at approximately 80 degrees C while safely generating steam at a temperature exceeding 170 degrees C. The conditions yielding maximum system efficiency and capacity are identified, and the obtained experimental results are used to define an optimal control strategy. (C) 2017 Elsevier Ltd. All rights reserved.

This work presents a two-dimensional analytical solution of the governing differential equation for falling film vapour-absorption around a plain horizontal tube. The solution of the species transport equation gives the LiBr mass fraction distribution within the liquid absorptive film flowing along the tube surface and can be used to characterize the mass transfer performance of falling film absorbers or generators. By means of the inclusion of partial wetting effects at reduced solution mass flowrates, this study obtains an analytical expression of the mass transfer coefficient of these devices applicable over an extended range of operative conditions. The hypotheses of small penetration for physical absorption and constant heat flux condition are applied at the film interface to reach a closed-form solution. Fourier method is used to solve the problem and the eigenvalues obtained from the characteristic equation depend on Lewis number, Biot number and the dimensionless heat of absorption. Given the boundary condition at the wall, the two-dimensional mass fraction field of the laminar film can be expressed analytically as a function of Schmidt, Reynolds numbers, the tube dimensionless diameter and the ratio of the wetted area to the total exchange surface. Finally, mass transfer coefficient and absorbed mass flux are locally and globally investigated as functions of the influent dimensionless groups to clarify their effects on the physical process and screen the potentiality of the model. Results show notable qualitative and quantitative agreement with previous numerical solutions and experimental results from previous literature. This model constitutes a widely applicable and time-saving tool for actual system simulations, design and control. (C) 2017 Elsevier Masson SAS. All rights reserved.

This study presents the derivation of two general types of correlation for the heat and the mass transfer coefficients (inside an adiabatic structured packed bed of a liquid desiccant system, which uses aqueous lithium chloride as a solution) as expressions of Nusselt and Sherwood numbers, respectively. These general equations summarize the physical and thermophysical properties of the air, the solution, and the contactor, which make them capable to be used for any parametric study provided they are fitted in a wide range of experimental data that include all the properties involved. Specifically, in this study, the experimental data taken at various air superficial velocities and solution flow rates were fitted to the said correlations. Comparison between the predicted and experimental values shows very good agreement with deviations for both coefficients within ±10% and average errors of 4.58 and 2.79% for the heat and the mass transfer coefficients, respectively. In addition, the calculated outlet air humidity ratios and temperatures match closely the experimental data. The fitted correlations for the heat and the mass transfer coefficients can be used to perform parametric studies at different air superficial velocities and solution flow rates with very good accuracy.

With an ultimate goal of enhancing the flow distribution within a microchannel heat exchanger, an experimental apparatus was designed and constructed. This study presented an initial evaluation and investigation of R410A distribution with mass flowrate of 40, 60, 80 kg/h or mass flux of 120, 180, 240 kg/m²s. The inlet vapor quality before the header was varied at 0.1, 0.2 and 0.6. The inlet flow entered at the bottom with a normal location towards the vertical header. 20 horizontal parallel flat tubes having microchannel holes were protruded half-width of the header where the flow was distributed. Individual pressure drop and flowrate were measured. Corresponding inlet vapor quality for each was calculated. Visualization was conducted for the observation of the flow behavior at the vertical header. Increase on the liquid level at higher mass flux and more homogeneous two-phase flow at lower quality were observed within the header, which yielded better distribution.

This paper reports an initial step to obtain an equally distributed fluid flow in an evaporator heat exchanger and eventually increase its performance. Experiments are carried out for R410A, using a vertical header where 20 horizontal parallel flat tubes are connected and the protrusion depth is half of the header width. The flow enters the header from a bottom inlet port and there is no heat load applied on the flat tubes. Inlet vapor quality before the header is varied for a fixed total mass flow rate of 60 kg/h. For each set of conditions, the mass flow rate and quality at different vertical positions of the header are singularly measured. Visualization data and experimental results have shown that 0.6 vapor quality before the header produces a more even distribution having a more homogeneous two-phase flow compared to 0.2 where phases are distinctly separated yielding dryouts on the top channels.

This paper reports an initial step to obtain an equally distributed fluid flow in an evaporator heat exchanger and eventually increase its performance. Experiments are carried out for R410A, using a vertical header where 20 horizontal parallel flat tubes are connected and the protrusion depth is half of the header width. The flow enters the header from a bottom inlet port and there is no heat load applied on the flat tubes. Inlet vapor quality before the header is varied for a fixed total mass flow rate of 60 kg/h. For each set of conditions, the mass flow rate and quality at different vertical positions of the header are singularly measured. Visualization data and experimental results have shown that 0.6 vapor quality before the header produces a more even distribution having a more homogeneous two-phase flow compared to 0.2 where phases are distinctly separated yielding dryouts on the top channels.

This study examines, using actual driving and simulations, transient operation characteristics and control methods for a combined system. It consists of both single-stage compression-type and single-effect absorption-type refrigerators and is driven by the shaft power and waste heat from a gas engine. Given the complicated nature of the system, determining the unsteady-state performance and control characteristics of the entire system is difficult. Hence, it must be equipped with an adequate control system to maintain acceptable system performance. Our results reveal that this system is controlled, as predicted, under unsteady-state conditions by using a novel control method. The unsteady-state simulation model is validated by comparing the driving results with the simulated results. Thus, gas-engine-driven combined air-conditioning systems possess a response time similar to absorption-type refrigerators. This indicates that the transient response of the entire system is governed by the absorption-type refrigerator, although the cooling capacity of the absorption-type refrigerator is only 13% of the entire cooling capacity. Therefore, when designing the control system of the combined system, the fact that the entire combined system is mainly governed by the absorption-type refrigerator should be considered despite its low cooling capacity.

Solar energy is accessible throughout the year in tropical regions. The latest development of absorption chillers has demonstrated that these systems are suitable for effective use of solar energy. The utilisation of solar energy for heat-driven cooling systems has significant advantages. Without a doubt, solar energy represents a clean energy source that is available without any additional fuel cost, and that can be proportionally accessible when the cooling load increases during the middle hours of the day. This study focuses on a single-double-effect absorption chiller machine that was installed in Indonesia. The system is driven by a dual-heat source that combines gas and solar energy. This system is characterised by simulating its performance in various conditions in terms of the cooling water (28-34 degrees C) and the hot water (75-90 degrees C) inlet temperatures. The reference operating condition of this system is 239 kW of cooling capacity. The mathematical model is validated and shows a good agreement with experimental data. In the operative range considered, simulation results yield a coefficient of performance between 1.4 and 3.3, and a gas reduction ratio from 7 to 58% when compared to a double-effect absorption chiller driven by gas. Based on the simulation results, this system is expected to have a good potential for widespread use in tropical Asia regions. (C) 2016 Elsevier Ltd. All rights reserved.

Heat and mass transfer processes performed by using thin liquid films are recurrent in a series of technical applications. The circumstances under which dry spots appear on the exchange surface for the film breakage, as well as the extent of the wetted part of the surface and the liquid vapor interface, are critical to predict and control the performances of these devices. To characterize the wetting behavior of liquid films on internally-cooled desiccant contactors, this paper originates from an experimental study and, adopting a standpoint useful for a practical interpretation of wettability measurements, introduces a corresponding theoretical model based on the principle of minimum energy. Both the estimation of the wetted area by image processing of the test section and the theoretical model highlight a hysteresis phenomenon of the film wetting behavior for gradually increasing and decreasing liquid flowrates. The modeling approach, experimental results and a first comparison are hereby presented and discussed. Quantitative and qualitative agreement appears promising for a further employment of the model in actual system design and control. (C) 2016 Elsevier Ltd. All rights reserved.

We studied an air-liquid contactor that uses an ionic liquid as the absorptive solution, in the context of employing liquid desiccant systems for air humidity control. Most liquid desiccant systems use a packed column air-liquid contactor with lithium chloride as the absorptive solution. One of the advantages of using ionic liquids is that some of them are not corrosive towards aluminum or copper. Provided that an air-liquid contactor with ionic liquid can achieve the same or higher performance than a packed column air-liquid contactor, this component can be constructed from metal which allows it to be redesigned to improve the system performance. Therefore, the ultimate purpose of this study is to evaluate the potential of an aluminum-fabricated air-liquid contactor which uses an ionic liquid as the working fluid, and to design a system that can both dehumidify and regenerate the absorptive solution more effectively. In addition, the associated heat, mass and momentum transfer phenomena, were studied to clarify the performance of the air-liquid contactor. An experimental air-liquid contactor apparatus using an ionic liquid was used to measure temperature, concentration, mass flow rate, pressure, and humidity. By adjusting temperature, humidity, and air flow velocity, it was found that the air-liquid contactor using an ionic liquid showed comparable dehumidification performance as the lithium chloride system. This suggests the feasibility of an air-liquid contactor made of metal
such a contactor can be redesigned to further improve its performance.

The objective of this study is to evaluate the performance of a falling film absorber for absorption refrigeration machines. Experiments with smooth and enhanced tubes were conducted to measure the heat transfer coefficient of this component in a wide range of mass flow rates, at temperature levels corresponding to an absorption chiller application case. The aim of this study is to experimentally clarify the absorption phenomenon of smooth and enhanced tubes in a wide range of mass flow rates, which would then be useful in designing absorption systems. Regarding the working medium, LiBr-H2O solution was used as the absorbent and water as the refrigerant. Additionally, 1-ocatanol was added as a surfactant. The experimental apparatus consists of 10 horizontal tubes, a distributer, and a desorber. A falling film absorber is that absorption falls from the distributer to the horizontal tubes, which are surrounded by water vapor. The flowing behavior of the absorptive solution on horizontal tubes influences heat and mass transfer coefficient. The mass flow rate and shape of the tube has influence on the flowing behavior. Hence, the experiment was conducted in a wide range of mass flow rate and using different tubes with smooth or enhanced surfaces. The heat transfer coefficient was calculated from the heat transfer rate. As a result, the differences in the heat transfer performance of the absorber for each kind of tube were clarified. It was concluded that the shape of the tube significantly influences the heat transfer coefficient.

A local entropy generation analysis, for water vapor absorption in LiBr-H2O solution, is performed referring to velocity, temperature and concentration fields obtained from the numerical solution of mass and energy transport equations. The hydrodynamic description is based on Nusselt boundary layer assumption and the actual amount of irreversibility introduced is determined for an absorptive falling film over a cooled horizontal tube inside the absorber. Results are explored in different operative conditions in order to examine the impact of the various irreversibility sources in a wide operative range. A least irreversible solution mass flow-rate can always be identified. Furthermore, a simple and general thermodynamic analysis, carried out regarding a refrigerating and a heat boosting applications, makes evidence of a dimensionless group "Q/sigma T" that separates the weight of the irreversibilities and gives the way to an optimization criterion applied to the absorber in order to improve the whole system efficiency. Both thermodynamic equilibrium and sub-cooling conditions of the solution at the inlet are considered for typical temperature and concentration of refrigerators' absorbers and heat transformers' absorbers. Results suggest the importance to work at reduced mass flow-rates with a thin uniform film. In practice, tension-active additives are required to realize this condition. Also, it is highlighted that the two parameters defined with reference to the dimensionless group "Q/sigma T" can be maximized by specific values of the tube radius, operative Reynolds number, solution sub-cooling and temperature difference between the wall and the inlet solution. (C) 2015 Elsevier Ltd. All rights reserved.

Indonesia's economic growth has continued at a steady rate of approximately 5% to 6% annually, and energy consumption in the entire country has been increasing year by year. Demand for air conditioning in buildings is expanding. In line with this expansion, a 239 kW solar air-conditioning system using a single-double effect combined absorption chiller was installed in a building at the University of Indonesia's Faculty of Engineering located in Depok city, near Jakarta, with the aim of reducing greenhouse gas emissions. We collected and analyzed data from this air-conditioning system to better comprehend its performance. We report the outline and the performance of the chiller and the air-conditioning system.

Multi-temperature-level systems enlarge the prospects and degrees of freedom for an effective design and an environment-friendly use of energy. Based on a general thermodynamic model of three-thermal cycles and finite thermal capacity of the heat sources, this paper aims at the analysis and the performance optimization of these systems by considering the influence of irreversibility. Suitable dimensionless parameters for an overall optimization are introduced and their influence on the cycle efficiency is investigated. This approach identifies the limitations imposed to the physical processes by accounting for the inevitable dissipation due to their constrained duration and intensity, and constitutes a general thermodynamic criterion for the optimization of three-thermal irreversible systems. Dependence on the main factors is highlighted in a way that shows how to change them in order to improve the overall efficiency. Under this point of view, the analysis evaluates COP improvements and can be used to perform plant diagnostics, besides predicting the system performance. The use of this criterion is exemplified for the absorption chiller application case.

Based on a numerical study of the water vapour absorption process in LiBr-H2O solution, for a laminar, gravity driven, viscous, incompressible liquid film, flowing over a horizontal cooled tube, irreversibilities related to fluid friction, heat transfer, mass transfer and their coupling effects have been locally and globally examined. The hydrodynamic description is based on Nusselt boundary layer assumptions. The tangential and normal velocity components, respectively obtained from momentum and continuity equations, have been used for the numerical solution of mass and energy transport equations in the two-dimensional domain defined by the film thickness and the position along the tube surface. Local entropy generation calculation can be performed referring to the calculated velocity, temperature and concentration fields. Results have been explored in different operative conditions, in order to examine comprehensively the impact of the various irreversibility sources and to identify the least irreversible solution mass flow-rate for the absorber. As a parallel, a refined understanding of the absorption process can be obtained. Considering absorption at the film interface and cooling effect at the tube wall, the analysis thermodynamically characterises the absorption process which occurs inside actual falling film heat exchangers and establishes a criterion for their thermodynamic optimisation. Results suggest the importance to operate at reduced mass flow rates with a thin uniform film. Meanwhile, tension-active additives are required to realise this condition. (C) 2015 Elsevier Ltd. All rights reserved.

Absorption heat transformers extend the possibilities for efficient and environment-friendly energy conversion processes. Based on a general thermodynamic model of three-thermal cycles with finite thermal capacity of the heat sources, this paper is intent upon analyzing and optimizing the performance of absorption heat transformers, by including the influence of irreversibility in the analytical expression of the system efficiency. Dimensionless parameters for an overall optimization are defined and a first screening is performed to clarify their influence. Dependence on the main factors is highlighted to suggest how to change them in order to enhance the whole system performance. Under this point of view, the analysis evaluates coefficient of performance (COP) improvements and can be used to perform existing plant diagnostics, besides predicting the system performance. The use of this criterion is exemplified for specific heat transformers data from literature. This approach identifies the limitations imposed to the physical processes by accounting for the inevitable dissipation due to their constrained duration and intensity, and constitutes a general thermodynamic criterion for the optimization of three-thermal irreversible systems.

This study is based on a numerical analysis of water vapour absorption in a laminar, gravity driven, viscous, incompressible liquid film of LiBr-H2O solution, flowing over a horizontal cooled tube. The hydrodynamic description is based on Nusselt boundary layer assumptions. A local entropy generation calculation can be performed referring to velocity, temperature and concentration fields. From a general form of volumetric entropy generation, a suitable expression for the absorption process has been obtained and different irreversibility sources have been highlighted. The impact of each term (fluid friction, heat transfer, mass transfer and their coupling effects) has been locally examined. Results have been explored for different tube radii, wall temperatures and operative conditions (representing both chiller and heat transformer configurations), in order to characterise the process from a second law point of view and establish a criterion for the optimisation of the absorber.

The purpose of this study is to develop a supply air dew point temperature control of a desiccant air-conditioning system for low-humidity environments. To develop the control, we need to know the basic characteristics of the desiccant air conditioning system, which is a controlled system. In this paper, we construct a mathematical model and analyze systematically the basic characteristics of the controlled system. Specifically, we investigate the static and dynamic characteristics of the controlled system with respect to manipulated variables such as desiccant-wheel rotary speed and regeneration outside air flow rate. For static characteristics, we investigate the relation between controlled and manipulated variables and examine the influence of manipulated variables on controlled variables. For dynamic characteristics, we investigate the step response of controlled variables and examine the responsivity or overshoot of controlled variables.

Recently at commercial buildings and office buildings it is popular to install variable refrigerant flow compression type heat pump system. Those systems have many merits for users and constructors. At first, Cooling capacity's range are wide for choosing adequate capacity, secondly the facilities were downsizing and separable to small units for easy transportation, thirdly multi indoor units can be controlled independently and so on. On the other hand, because driving conditions are changing dynamic for meeting user's needs, it is complicated to adequate driving conditions to save energy consumption. We constructed simulation model by mathematical methods and we tested VRF compression heat-pump system at test room which can adjust temperature conditions freely. We evaluated the system changing the number of active indoor units on partial lord and we showed the effect of COP on this conditions.

In this work a thermodynamic analysis of a desiccant wheel is proposed to investigate and identify the optimum size and operating regime of this device. A steady state entropy generation expression, based on effectiveness parameters suitable for desiccant wheels operability, is obtained applying a control volume approach and assuming perfect gas behaviour of the binary air-vapour mixture. A new entropy generation number N-L, is defined using a minimum indicative value of the entropy generation S-L,S-min and investigated in order to obtain useful criteria for desiccant wheels optimization. The effectiveness-NTU design method is employed by combining solution of thermal exchange efficiency for rotary heat exchanger with the characteristic potential method, under the conditions of heat and mass transfer analogy. The analysis is applied to a specific desiccant wheel and N-L, variation with NTU is explored under various operative conditions and wheels characteristics in terms of dimensionless velocity and flow unbalance ratio. (C) 2014 Elsevier Ltd. All rights reserved.

An experimental data comparison of evaporation heat transfer coefficient of R-744(CO2), R717(NH3), R-290(C3H8) and R-1234yf in horizontal small tubes was conducted under some various flow conditions. The experimental data were obtained over a heat flux range of 5 -60 kW m(-2), mass fluxes from 50 to 600 kg m(-2) S-1, saturation temperatures from 0 to 10 degrees C, and quality up to 1.0. The test section made of stainless steel tubes with inner diameters of 1.5 mm and 3.0 mm, and a length of 1000 and 2000 mm, respectively, was heated uniformly by applying an electric power to the tubes directly. Effects of mass flux, heat flux, saturation temperature and inner tube diameter on heat transfer coefficient are reported in the present study. Among the present working refrigerants, R-744 has a highest heat transfer coefficient. Laminar flow was observed in the evaporative small tubes and considered in the modification of evaporation heat transfer coefficient correlations. (C) 2013 Elsevier Ltd and IIR. All rights reserved.

In the data center, air conditioning has been carried out throughout the year to remove waste heat of servers. In order to realize efficient cooling, it is important to use cold outdoor air in winter. Therefore we study the performance of the air conditioning system that uses outside air in winter. In the proposed system, it is possible to switch compression cycle and free cooling cycle. In this report, we construct the static mathematical models of this system and verify the validity of this mathematical model by experiment. Validity of the mathematical model has been confirmed as a result. And we elucidated the static characteristics of this system in regard to outdoor temperatures and Indoor temperatures.

Solar single-double effect combined absorption chiller has come into wide use compared with other types of solar absorption system because it is equipped with high-efficient double effect apparatus of helping solar heat. This paper reports the machine design to enhance the performance by solar heat. So far, thermal conductance KAs of elements composed in absorption chillae have been often limited in machine design because the change of KAs means remanufacture of the test machine. Hence, easily to realize the machine design with the most efficient performance COP in the objected system, we evaluated the performance with KAs designed by simulation, which is a good way to evaluate system performance easily without empirical method and leads to design to save time. Each KA needs to be concluded under the conditions of common parameter, for example, evaporator temperature, condensation temperature, pinch temperatures of heat exchangers, etc. In the simulation result, it is clarified that the reverse flow type can be the highest performance among several types of single-double effect absorption cycle suggested.

Compression-type heat pumps have been employed in many fields of refrigeration and air conditioning for achieving energy savings. In addition, the system operates at high efficiency under a variety of operating conditions throughout the year through various innovative components, such as the inverter, and the annual performance improvement. However, the running time of the system over the year under low load conditions is very long, and under such conditions, the system operates intermittently because of the difficulty in the continuous running of the compressor. Of course, because intermittent driving is made under such an operating condition, the performance can estimate that it decreases greatly easily, but it is the present conditions that field operation performance does not become clear almost. Therefore, the key goals of this study were to clarify the intermittent driving performance of the compression-type heat pump and to devise a way to improve its performance and develop an optimum operation method. As the first step, we constructed a mathematical model that can predict intermittent driving characteristics at the time of low load of the pump, which are crucial to investigation of the performance. The constructed model was validated experimentally. In summary, a mathematical model of the compression-type heat pump was constructed, and its validity was demonstrated via comparison of its results with those obtained experimentally.

In air-conditioning field, a dehumidification has become increasingly important for human health and comfort especially in hot and humid climates. However, a conventional mechanical dehumidification with a vapor compression refrigerator has some problems. Therefore, much attention has been paid recently to a desiccant air-conditioning system as an alternative to the conventional system. In this paper, we focus on a rotary desiccant wheel which is the main component of the desiccant air-conditioning system and develop and validate the mathematical model by comparison with experimental results. The validation is conducted under various operating conditions. The mathematical model discussed in this paper includes, for example, the entrance region effect in air channel, detailed diffusion phenomenon in porous solid. In experiments, effects of the regeneration air temperature, air superficial velocity, wheel thickness and wheel rotational speed on the desiccant wheel performance are investigated. In addition, the temperature and humidity distribution at the outlet of the desiccant wheel are measured. As a result, an average relative error between the predicted and the measured humidity ratio difference distribution is 3.3% and temperature difference distribution 10.8%. Moreover, the effect of the regeneration air inlet temperature, the air superficial velocity, wheel thickness and wheel rotational speed on the desiccant wheel performance are clarified and the predicted results are totally in good agreement with the measured results. (C) 2013 Elsevier Ltd. All rights reserved.

Due to the concern on global warming, the demand for a system using natural refrigerant is increasing and many researches have been devoted to develop systems with natural refrigerants. Among natural refrigerant systems, an air cycle system has emerged as one of alternatives of Freon gas system due to environmentally friendly feature in spite of the inherent low efficiency. To overcome the technical barrier, this study proposed combination of multiple systems as a hybrid cycle to achieve higher efficiency of an air cycle system. The hybrid air cycle adopts a humidity control units such as an adsorber and a desorber to obtain the cooling effect from latent heat as well as sensible heat. To investigate the efficacy of the hybrid air cycle, the cooling performance of a hybrid air cycle is investigated analytically and experimentally. From the simulation result, it is found that COP of the hybrid air cycle is two times higher than that of the conventional air cycle. The experiments are conducted on the performance of the desiccant system according to the rotation speed in the system and displayed the feasibility of the key element in the hybrid air cycle system. From the results, it is shown that the system efficiency can be enhanced by utilization of the exhausted heat through the ambient heat exchanger with advantage of controlling the humidity by the desiccant rotor.

In air-conditioning field, a dehumidification has become increasingly important for human health and comfort especially in hot and humid climates. However, a conventional mechanical dehumidification using a vapor compression refrigerator has some problems. Therefore, much attention has been paid recently to a desiccant air-conditioning system as an alternative to the conventional system. In this paper, we focus on a liquid desiccant air-conditioning system using lithium chloride - water solution and experimentally investigate the heat and mass transfer of the packed bed absorber and regenerator that are the main components of the system. In the experiment, effects of the air superficial velocity and solution mass flux on the overall heat and mass transfer coefficients are examined. As a result of the study, we found that the heat and mass transfer coefficients increase with increase of the air superficial velocity, however, the heat transfer coefficient is approximately constant with increase of the solution mass flux.

In air-conditioning field, a dehumidification has become increasingly important for human health and comfort especially in hot and humid climates. However, a conventional mechanical dehumidification with a vapor compression refrigerator has some problems. Therefore, much attention has been paid recently to a desiccant air-conditioning system as an alternative to the conventional system. In this paper, we focus on a desiccant air-conditioning system using a rotary desiccant wheel. The purpose of this study is to develop the simulation model of the desiccant wheel that can be applied to the various operating conditions. In this paper, the simulation model is validated under indoor air circulation condition by comparison with the detailed experimental results. In the model, the heat and mass transfer phenomenon in both air channel side and desiccant wall side are considered in detail. As a result, the simulation results of outlet humidity and temperature were in good agreement with the experimental results under indoor air circulation condition and then the simulation model was validated.

Recently, information and communications technologies (ICT) have been widely used in the air-conditioning field, and high-performance and density ICT facilities have been rapidly growing. At the same time, the heat generated per unit area of a normal data center, where the servers and routers are concentrated, in an ICT facility is an extremely serious problem. Therefore, this study aims to investigate the performance and characteristics of the server spot cooling heat transportation system adopting CO2 as the working fluid to prevent the occurrence of hot spots and decrease the power consumed by a conventional overall air conditioning system installed at a data center with experiments and simulations. In the result, we determined the optimum CO2 circulation rate that maximizes the rejected heat flow rate, and minimizes power consumption. (C) 2011 Published by Elsevier Ltd. Selection and/or peer-review under responsibility of organizing committee of 2nd International Conference on Advances in Energy Engineering (ICAEE).

Smart energy system is very large energy system that consists of the windmill, solar panel, heat pump, air-conditioning system, solar thermal panel and so on. To develop efficient smart energy system, it is very important to investigate how to manage the total energy utilization of the total smart energy system. But it is difficult to examine the energy management only with experiment. The simulation is promising technology for the development of the energy management of the smart energy systems. But as this system is too large, we can't analyze this system, unless we analyze this system rationally. Therefore we have established the modular analysis theory that can analyze the large scale energy system. This paper introduces this theory and using this theory, we have developed the energy system analyzing simulator that is called "Energy Flow +M". This simulator has been already opened for the worldwide through Internet. This simulator is expected to be used for the energy saving of the total energy systems. (C) 2011 Published by Elsevier Ltd. Selection and/or peer-review under responsibility of the organizing committee of 2nd International Conference on Advances in Energy Engineering (ICAEE).

This study focuses on a hybrid liquid desiccant air-conditioning system consisting of a conventional liquid desiccant system and a vapour compression heat pump. The hybrid liquid desiccant air-conditioning system is expected to enhance the system efficiency of a conventional liquid desiccant system. In this study, the liquid desiccant is aqueous solution of lithium chloride and the refrigerant of the vapour compression heat pump is R407C. The main feature of this system is that the absorber and regenerator are integrated with the evaporator and condenser respectively. The performance evaluation test is conducted to obtain the primary data. Additionally, the improvement method for the system efficiency is discussed by the mathematical calculations. As a result, the system can dehumidify 5.9 g/kg(DA) under the conditions of summer in Tokyojapan. Then, the calculation results show that COPs can become higher by improving the compressor isentropic efficiency and the temperature efficiency of solution heat exchanger. (C) 2011 Elsevier Ltd. All rights reserved.

If a desiccant dehumidification system can be driven by a heat source whose temperature is below 50 degrees C, exhaust heat from devices such as fuel cells or air conditioners can be used as its heat source, thereby saving energy. Therefore, in this study, we used a previously validated simulation model to determine the minimum heat source temperature for driving a desiccant dehumidification system. We considered four desiccant dehumidification systems that can be driven by waste heat-conventional desiccant-type systems (wheel type and batch type with only desiccant), a system with a precooler, double-stage-type systems (a type with two desiccant wheels and a four-partition desiccant wheel type), and a batch-type system with an internal heat exchanger. We found that among these systems, the last system can be driven by the lowest heated air temperature-approximately 33 degrees C-which is considerably lower than that of the conventional system. (C) 2010 Elsevier Ltd and IIR. All rights reserved.

A gas engine driven hybrid air conditioning system was constructed; it consisted of a gas engine, a single-stage compression-type refrigerator, and a single-effect absorption-type refrigerator that can be driven by shaft power and waste heat, respectively, from the gas engine. Its efficiency is greatly enhanced as compared to a conventional gas engine heat pump because it utilizes waste heat from the gas engine. This study used simulation and experiments to elucidate the static characteristics of this air conditioning system and develop a control system to drive this combined system efficiently. In the simulation results, the system COP at the design point was about 1.87 and, at 50% partial load operation, the performance increased by 14% because of the simultaneous manipulation of the input gas flow rate and hot water flow rate. The validity of the simulation was confirmed by experiments carried out using the actual constructed machine. (c) 2011 Elsevier Ltd and IIR. All rights reserved.

A simulation model for the CO2 heat pump water heater was developed and validated in this study. Component models of the gas cooler, evaporator, compressor, and expansion valve were constructed with careful consideration for the heat transfer performances. To validate the simulation model, experiments were carried out using an actual CO2 heat pump water heater (water heating capacity: 22.3 kW; hot-water temperature: 90 degrees C). In simulations and experiments, the effects of the inlet water temperature and outside air temperature on the system characteristics were discussed. As a result, the average difference in COP between the simulation results and experimental results is 1.5%. (C) 2011 Elsevier Ltd. All rights reserved.

Nowadays new types of heat exchangers which utilize small channel tubes with ammonia refrigerant have been widely considered due to the high energy efficiency and environmental issues. This study carries out experiment in evaporation temperature of -10 degrees C to +5 degrees C at 10 to 40 kW/m(2) heat flux and 40 to 120 kg/m(2)s in mass flux with 1.0 mm and 3.0 mm inner diameter tubes. The test results showed high heat transfer coefficient for 1.0 mm tube. The Kandlikar model predicted both 1.0mm and 3.0mm tube's heat transfer coefficient well. For recording the flow patterns inside the tube, we used a high speed video camera and found the annular flow dominated at the low vapor quality region for 1.0mm tube. Accordingly the annular flow seemed to be the cause of the high heat transfer. Also we measured pressure drop in both of the tubes and found that there was not much difference between 1.0 mm and 3.0 mm tubes under the same experimental conditions. A separated model with the Mishima Hibiki parameter predicted the pressure drop of 1.0mm results well, but couldn't predict that of the 3.0mm results.

An experimental study of evaporation pressure drop in single circular small tubes was conducted for flow of C3H8, NH3 and CO2 under some various flow conditions. The test matrix encompasses the entire quality range from 0.0 to 1.0, mass fluxes from 50 to 600 kg m(-2) s(-1), heat fluxes from 5 to 70 kW m(-2) and saturation temperatures from 0 to 10 degrees C. The test section was made of circular stainless steel tubes with inner diameters of 1.5 mm and 3.0 mm, and a length of 2000 mm in horizontal orientation. The test section was heated uniformly by applying an electric power to the tubes directly. The effects of mass flux, heat flux, saturation temperature and inner tube diameter are presented in this study. The pressure drop of CO2 is the lowest among the working refrigerants. The experimental pressure drop was compared with some existing pressure drop correlations. Existing void fraction prediction methods were evaluated for the present experimental pressure drop. A modified pressure drop correlation using homogenous void fraction model is developed.

In general, latent heat recovery is usually accompanied by the corrosion of the heat exchanger, which is caused by the strongly acidic condensate when the temperature of the flue gas is lowered below the acid dew point. The present study has been conducted to investigate the heat and mass transfer characteristics in a titanium heat exchanger with excellent corrosion resistance used for waste heat recovery with the condensation arranged in a gas fired water heater. In addition, the thermal efficiency of the gas fired water heater was evaluated based on the net calorific value at the maximum rated output during latent heat recovery from the exhaust flue gas. Parametric studies were conducted for the flue gas flow rate, inlet temperature and mass flow, rate of the supplied water, respectively. Different arrangements of the tubes of the heat exchanger including in-line and staggered configurations were investigated. The experimental results indicate that the thermal efficiency of the gas fired water heater with a latent heat recovery (LHR) heat exchanger was enhanced by about 10% compared with conventional instantaneous water heaters, i.e., water heaters without heat recovery. In addition, in terms of the Nusselt number and the Sherwood number, the heat and mass transfer performance of the staggered tube bank type were approximately 50% and 10% higher than that on the in-line tube bank type when the Reynolds number of the flue gas was 10(3). (C) 2010 Elsevier Ltd. All rights reserved.

  The vapor injection refrigeration cycle using the scroll compressor driven by inverter was investigated to improve the performance of room air conditioner with R 410A. One injection hole was bored on a fixed scroll plate, and through this hole the vapor refrigeration is injected into two compression rooms of a scroll compressor at different time. Then the pressure change of the compression room and the performance of vapor injection refrigeration cycle are calculated. And the performance of the room air conditioner using vapor injection refrigeration cycle was measured. As the result, the cooling/heating compressor input was decreased to 7.4/3.2 % at capacity 4.0/5.9 kW, and the heating capacity at outdoor temperature -15 °C was increased to 11.9 %.

A desiccant dehumidification system with air can decrease energy consumption because it can be driven by low-grade waste heat below 80 degrees C. If this system can be driven by low-temperature heat sources whose temperature is below 50 degrees C, exhausted heat from fuel cells or air conditioners that exist everywhere can be used as heat sources. This could lead to considerable energy saving. This study provides a detailed evaluation of the performance of a four-partition desiccant wheel to make a low-temperature driving heat source possible and achieve considerable energy saving by the simulation and experiment. Further, the study investigates the in-depth performance of a hybrid air-conditioning system with a four-partition desiccant wheel by simulation. As a result, it was clear that there exists an optimum rotational speed to maximize the dehumidification performance and that the hybrid air-conditioning system improves COP by approximately 94% as compared to the conventional vapour compression-type refrigerator. (C) 2009 Elsevier Ltd and IIR. All rights reserved.

As the desiccant air-conditioning system works in various conditions, we need to understand the performance of desiccant air-conditioning system with wide range of driving conditions. But now we haven't grasped even characteristics of the desiccant wheel that is the most important component in the desiccant air-conditioning system. So far, in order to clarify the performance of the desiccant wheel, we have developed the simulation model for the desiccant wheel and also carried out the experiment of the desiccant wheel with wide range of designing and operating conditions. As a result, the validity of the simulation model was confirmed because the simulation results agree with experimental ones well with wide range of design and operating conditions. In this study, in order to clarify more detailed heat and mass transfer performance of the desiccant wheel, we visualized temperature and concentration distribution inside the desiccant wheel by the simulation. As a result, we clarified the temperature and humidity ratio distribution of the process and regeneration air in the air channel, and the temperature and water content distribution in the desiccant wall that are difficult to show by the experiment.

In this paper, we have developed a simulation model of a desiccant wheel to use the investigation of the optimum design and operation. For this usage, the high precision model has to be constructed for the global design and operating conditions. Heat and mass transfer phenomenon inside the desiccant wheel are divided into the convection heat and mass transfer inside air channels and heat and mass diffusions inside desiccant walls. Therefore, the simulation model is developed separately divided into the air channel and the desiccant wall. In the desiccant wall, the model is constructed by considering the diffusion in the porous solid. On the other hand, in the air channel, the convective heat and mass transfer coefficients are derived from the CFD analysis. To validate the model, we examined the effect of the air superficial velocity on the pressure drop and the effect of the regeneration air inlet temperature on the outlet humidity and temperature. As a result, The simulation results are in good agreement with the experimental results and the validity of this simulation model was confirmed.

Absorption type refrigerator is expected to use for the cogeneration because it can be driven by the wasted heat about 90℃. But rated operation is short time in the driving condition through the year. The reminder of the year, refrigerator is sometimes driven intermittently. Therefore, the intermitted driving simulation is needed for considering energy saving. In this study, simplified simulation models of the absorption type refrigerator are developed and intermitted driving simulation is carried out. And effect of intermitted driving on the cooling capacity and COP is examined. As a result of the simulation, under the case of intermitted driving, cooling capacity is varied and COP is almost constant.

In the packed-bed absorber and regenerator of the liquid desiccant system, the air is dehumidified or humidified by direct contact of solution. This is very complex phenomenon because the heat, mass and momentum transfer occur simultaneously. The purpose of this study is to clarify the basic characteristics of heat and mass transfer and pressure drop in the packed-bed absorber and regenerator of the liquid desiccant system. Under the two type medias with different structure, effects of the solution flow rate and the air velocity on the heat and mass transfer characteristics and pressure drop are investigated empirically. As a result, effects of the solution flow rate and the air velocity on the overall heat and mass transfer coefficients were clarified. And It was verified that pressure drop is different by the structure of media.

An experimental study of R-22 and R-134a two-phase vaporization in horizontal minichannel was reported. In order to obtain the local heat transfer coefficients, the test was ran under heat flux range of 10 to 40 kW m(-2), mass flux range of 200 to 600 kg m(-2) s(-1), saturation temperature range of 5 to 10 degrees C, and quality up to 1.0. The test section, which was made of stainless steel tube and heated uniformly by applying an electric current to the tube directly, has inner tube diameters of 0.5, 1.5 and 3.0 mm, and lengths of 0.33 and 2.0 m. The effects on heat transfer coefficient of mass flux, heat flux and inner tube diameter were presented. The experimental heat transfer coefficients were compared with the predictions using existing heat transfer coefficient correlations. A. new boiling heat transfer coefficient correlation based on the superposition model, with considering the laminar flow, was developed.

This paper deals with the hybrid air conditioning system that combines the desiccant air-conditioning system with the compression type refrigerator. To construct this system, multistage desiccant wheel is applied for the desiccant air-conditioning system. With this wheel, the process air can be cooled down by the evaporator of the compression type refrigerator during the dehumidification process so that the temperature of the heat source can be reduced greatly. The performance of this hybrid air-conditioning system is analyzed by the simulation. For this simulation, the model of the desiccant air-conditioning system whose validity has already been confirmed is used. As a result of the simulation, we found that the multistage desiccant air-conditioning system can be driven by the heat about 42℃, which is almost equal to the exhaust heat temperature of the compression type refrigerator, and the hybrid air conditioning system improves COP about 68% higher than the conventional air conditioning system. This implies that the hybrid air conditioning system is an effective system for the next generation air conditioning system.

Sorption dehumidification of air has become the. focus of attention as an alternative to conventional dehumidification process by air cooling below the dew point. Liquid desiccant using LiCl solution has advantages over conventional solid desiccants, because the energy loss per hydration-dehydration process cycle is smaller for liquid desiccant than it is for typical solid desiccant. An open cycle absorption type dehumidifier using LiCl solution in this paper is can be driven by low grade waste heat and is effective for saving energy. However, a detailed analysis of the characteristics of the system has not been clearly investigated.The purpose of this paper is to evaluate a performance of the dehumidifier using LiCl solution on condition that air conditioning is driven in hot and high humid summer climate in Japan. In this paper, first, a packed bed as a part of absorption type dehumidifier is evaluated in which heat and mass transfer occur simultaneously, and secondary, the characteristics of dehumidifier is investigated as the whole system.As a simulation result, in case of performance evaluation of the system, when the heat source (hot water) tmperature is 70 degrees C, COPD (the efficiency of dehumidification) is 0.7 at design point. And, when the heat source temperature is about 80 degrees C, LiCl crystallizes by physical property of LiCl solution cooled in solution cooler that the cycle can not be driven. Therefore, heat source temperature must be carefully designed to keep the solution concentration below crystallization line.

The performance of natural gas (13A) fired water heater with titanium heat exchanger for latent heat recovery was evaluated. In the terms of thermal efficiency, the performance of the water heater was evaluated based on the net calorific value at maximum rated output. The thermal efficiency of gas fired water heater with latent heat recovery heat exchanger was ≈ 10% higher than that of the conventional instantaneous water heater. The overall heat conductance of the heat exchanger with staggered tube rows was more efficient than that of the inline tube rows.

A hybrid air conditioning system consists of a gas engine as a driving source and a single-stage compression and single-effect absorption type refrigerator that can be driven by a motive power and a waste heat from the gas engine simultaneously. It can improve the efficiency of the gas engine heat pump greatly because it uses the waste heat from the gas engine. This paper aims to estimate the performance and construct the control systems to drive this hybrid air conditioning system efficiently by the simulation. The static simulation models of the hybrid air conditioning system were constructed and the performance was investigated using the model. As a result of the simulation, system COP of the design point is about 1.87. And one of the actual control methods to achieve efficient partial load is manipulating two parameters - rotational speed and the hot water flow rate simultaneously. At 50% partial load operation, the performance is 14% higher than that of the design point.

The present study investigated the characteristics of heat transfer and pressure drop for various types of enhanced titanium tubes. The heat transfer experiments were conducted using a steam-condensing flow facility at a constant inlet temperature of cooling water, and the Reynolds number range varied from 15,000 to 65,000. The heat transfer coefficients were determined using a modified version of the Wilson plot technique and the friction factors were calculated from pressure drop measurements. The experimental results reported enable practical design to be achieved for the heat exchanger using enhanced titanium tubes and optimization of tube geometry for specific conditions.

The present study, investigated the characteristics of heat transfer and pressure drop for various types of enhanced titanium tubes. The heat transfer experiments were conducted using a steam-condensing flow facility at a constant inlet temperature of cooling water, and the Reynolds number range varied from 15,000 to 65,000. The heat transfer coefficients were determined using a modified version of the Wilson plot technique and the friction factors were calculated from pressure drop measurements. The experimental results reported enable practical design to be achieved for the heat exchanger using enhanced titanium tubes and optimization of tube geometry for specific conditions.

This paper discusses the optimum design method of an single effect absorption cycle with a simulation. For the optimum design method, the evaluation function is COP and the point that COP is maximized is regarded as the optimum point. The simulation model of the single effect absorption cycle is described as nonlinear equations. COP is derived form these nonlinear equations. And the constraints within which the absorption cycle can be driven normally exist. Considering the non-linearity of the simulation model and the constraints, the optimum design problem of the single effect absorption cycle can be led to the nonlinear constrained optimization problem. This problem is analyzed with the exterior penalty method, and success and failure method. As a result of the analysis, it was clarified that at the point that the temperature efficiencies of the evaporator, absorber, condenser and solution heat exchanger were the maximum temperature efficiencies as constraints, COP was maximized, and the optimum generator solution outlet temperature existed to maximize COP. For example, at the point of 85% maximum temperature efficiency, COP was 081 and the optimum generator solution outlet temperature was 81.7℃. And if the heat source temperature was over 87.9℃, COP constant in spite of the increase in heat source temperature, and the generator temperature efficiency that maximized COP on the contrary decreased.

A hybrid air conditioning system consists of a gas engine as a driving source and a hybrid refrigerator which can be driven by waste heat and motive power simultaneously. It can improve the efficiency of the gas engine heat pump greatly because it uses waste heat from the gas engine. This paper aims to estimate the performance of this hybrid air-conditioning system by the simulation. The objective hybrid refrigerator is composed of the single and double effect absorption type and the single-stage compression type refrigerator. The static simulation model of the hybrid air conditioning system is constructed and the performance is simulated using the model. As a result of the simulation, higher performance part load can be realized by changing the backup gas flow rate and hot water flow rate.

A gas engine heat pump (GHP) is the type of compression heat pump that is driven by the motive power from the gas engine. To improve the performance of this GHP greatly, it is necessary to add the heat pump driven by the waste heat from the gas engine to the GHP. This paper suggests the new hybrid heat pump in which the waste heat driven heat pump is combined with the compression heat pump. This system can be driven by the motive power and the waste heat from the gas engine simultaneously. In this paper, the hybrid heat pump whose waste heat driven heat pump is the absorption heat pump is adopted. The working-fluids are ammonia and water. The performance of this hybrid heat pump is analyzed with the simulation. As a result, it was found that gas engine hybrid air conditioning system with the new hybrid heat pump can realize the very high efficiency-primary energy ratio is about 2.4.

An ejector can recover fluid energy of the driving flow in the expansion process and transfer this energy to that which needs to compress the suction flow. By using ejector instead of the expansion valve, the efficiency of the compression type refrigerator is expected to be enhanced greatly. Until now, lots of researches have been done with regard to the single-phase flow ejector. But, the two-phase flow ejector which needs for the compression type refrigerator have not been examined sufficiently. Therefore this paper constructs the simulation model of the two-phase flow ejector and investigates the characteristics of this ejector with the simulation. As a result, it was clarified that working point exists to maximize the performance of the two-phase flow ejector.

An ejector can transform expansion energy of driving flow into pressure buildup energy of suction flow. Therefore, by utilizing the ejector instead of the expansion valve for the compression type refrigerator, the performance of the refrigerator can be greatly improved. In this study, performance of the compression type refrigerator with the ejectors using natural refrigerant of ammonia etc., is evaluated by means of simulation. As a result, it was shown that at the point of 90% ejector adiabatic efficiency, COP of the compression type refrigerator with the ejector using ammonia was about 6% greater than that of the conventional compression type refrigerator and in case of CO_2 refrigerant, COP was about 20% greater.

A gas engine heat pump, is the type of compression heat pump which is driven by motive power from the gas engine. To improve the performance of the gas engine heat pump greatly, it is necessary to add the heat pump driven by the waste heat from the gas engine. Therefore, in this paper a new hybrid heat pump in which the waste heat driven heat pump is combined with the compression heat pump is suggested, and a super efficient hybrid air conditioning system using this heat pump with gas engine is constructed. The performance of this hybrid heat pump is analyzed by the simulation. Then, some control techniques to drive this hybrid air conditioning system are introduced. The working-fluids are ammonia and water. As a result, it was found that this new hybrid air-conditioning system can realize extremely efficient drive primary energy ratio is about 2.4. And the control system in which hot water flow rate and solution flow rate, and rotational frequency are manipulated simultaneously can realize efficient part load.

Falling film type heat exchangers can realize the higher heat transfer even in the low temperature difference between working fluids. Therefore it can be applied to an evaporator in the power plant or an evaporator, absorber and generator in an absorption refrigerator. There are some types of the falling film type heat exchangers. This paper investigates a falling film type evaporator using horizontal tubes. When the flow rate of the working fluids is low, some factors such as the dry surface area on tubes decrease the heat transfer of the falling film type evaporator. In such cases, in order to improve the heat transfer, it is indispensable to use something to improve wettability or advanced surface tubes. Therefore this paper aims to clarify the method of improving film side heat transfer. The working fluids are water. As a result, it was revealed that the wettability of the tubes are greatly improved by the smoother we made in this study, and the heat transfer of CCS tube is about 4 times as large as that of smooth tube.

Absorption refrigerators can be driven by waste heat. In particular, the double-effect type of absorption refrigerator, which is highly efficient, is drawing a great deal of attention as a waste heat recovery system. This research examines the double-effect absorption refrigerator driven by steam, assuming waste heat from the fuel cells is applied. In the start-up of the double-effect absorption refrigerator, the solution temperature is almost equal to the temperature of the air. Due to the large temperature difference between the solution and the heat sources, the solution is overheated. In the worst case, it will be crystallized. Additionally, some solution is circulated due to the existence of a pressure difference between the heat exchangers. If the solution pump is started before an adequate pressure difference is obtained, the absorption refrigerator cannot be started normally. To investigate these problems, a simulation model is constructed. An experiment is conducted to investigate the performance of this model. As a result, the validity of this model is confirmed and the detailed start-up characteristics are clarified. (C) 2000 Scripta Technica, Heat Trans Asian Res, 29(5): 427-445, 2000.

Absorption refrigerator driven by waste heat can realize the energy savings. But if only waste heat is used as a heat source, the surplus of the waste heat or the shortage of the cooling capacity occur due to the change of the cooling capacity or the waste heat. In these cases, it is effective to back up this refrigerator with that driven by the stable source such as motive power. But using two refrigerators costs too much. This research suggests the WMVR hybrid refrigerator which can be operated using motive power and waste heat simultaneously. For this refrigerator, the simulation model was constructed and detailed characteristics were clarified. And based on these results, the effect of the design parameters such as the solution concentration and the pressure in the MVR generator on COP was revealed.

A falling film type heat exchanger is expected as the generator for the absorption refrigerator because of its capability to improve the speed of response and to drive the absorption refrigerator with lower temperature of the waste heat sources than the pool boiling type. In the latest research, we constructed the dynamic model of the pool boiling type generator and clarified the dynamic characteristics. This research aims to build the dynamic model of the falling film type generator for investigation of its dynamic characteristics in detail. First, dynamic model of the falling film type generator considering both heat and mass transfer is constructed. Then, the validity of this model is examined with the experiment of generator element installation. As a result, the validity was confirmed by the correspondence between the simulation results and experimental data.

A falling film type heat exchanger is expected as the generator for the absorption refrigerator because of its capability to improve the speed of response and to drive the absorption refrigerator with lower temperature of the waste heat sources than the pool boiling type. In the latest research, we constructed the dynamic model of the pool boiling type generator and clarified the dynamic characteristics. This research aims to build the dynamic model of the falling film type generator for investigation of its dynamic characteristics in detail. First, dynamic model of the falling film type generator considering both heat and mass transfer is constructed. Then, the validity of this model is examined with the experiment of generator element installation. As a result, the validity was confirmed by the correspondence between the simulation results and experimental data.

Absorption refrigerator is getting attention with a view point of environmental protection and energy savings because it can be driven by waste heat. As absorption refrigerator has a lot of solution which is so expensive, this makes it difficult to reduce the initial cost and improve the response speed in start up and shut down. Hence, it is expected to reduce the charging quantity of solution. But if it is decreased too much, disturbance such as the charge of the waste heat affects the characteristics of the absorption refrigerator greatly. To investigate these problems, it is necessary to clarify the dynamic characteristics of the absorption refrigerator. In the elements of the absorption refrigerator, the generator has the largest amount of solution and is affected directly by the change of the waste heat Therefore this research aims to clarify the dynamic characteristics of the generator in detail by simulation and experiment. As the generator for the absorption refrigerator, a pool boiling type and falling film type generator have been used. In this paper as the first report, the dynamic characteristics of the pool boiling type are investigated with simulation and experiment. As a result, detailed dynamic characteristics were clarified, and the validity of the dynamic model of the generator was confirmed.

When an ammonia-water absorption refrigerator(called ammonia absorption refrigerator in this paper) is used for purposes such as freezing and ice accumulation requiring temperatures below 0 degrees C, it is usually operated throughout the year. In this case, operating conditions which greatly influence the characteristics of the absorption refrigerator, change very widely. According to these changes, the holdup volume of refrigerant (ammonia) or solution in the refrigerant and solution receiver, changes greatly, too. If the amount of ammonia charged in the ammonia absorption refrigerator is too small, the refrigerator cannot be operated normally and efficiently due to refrigerant vapor mixes with the refrigerant liquid flowing from the condenser to evaporator or interruption of the reflux pump. However, the ammonia charging quantity has to be as small as possible because of its toxicity.
In this study, we made a static simulation model and clarified the detailed characteristics of the ammonia absorption refrigerator by simulation. The validity of this simulation model was confirmed with the experimental data using a commercial machine. The minimum ammonia charging quantity required to operate the ammonia absorption refrigerator normally and efficiently was obtained by simulation.

This report refers to some static characteristics of the ammonia-water absorption refrigerator for low temperature refrigerating process which needs the temperature below the freezing point. Especially, the influence of evaporating temperature and cooling water temperature is clarified by the experimental investigation. In addition to this, the validity of constructed simulation model of this absorption refrigerator is mentioned. The validity of simulation model is verified by the comparison of experimental results and calculation. <br>To examine the characteristics, we conducted the performance test using the trial product of which the standard cooling capacity is 175kW. The performance is estimated according to cooling capacity and COP. As a result, the effects of the evaporating temperature and cooling water temperature on the cooling performance are clarified by the experimental research. Furthermore, the calculation of the static characteristics predicted by the simulation model is in good agreements with the experimental results.

This report refers to some static characteristics of the ammonia-water absorption refrigerator for low temperature refrigerating process which needs the temperature below the freezing point. Especially, the influence of evaporating temperature and cooling water temperature is clarified by the experimental investigation. In addition to this, the validity of constructed simulation model of this absorption refrigerator is mentioned. The validity of simulation model is verified by the comparison of experimental results and calculation. <br>To examine the characteristics, we conducted the performance test using the trial product of which the standard cooling capacity is 175kW. The performance is estimated according to cooling capacity and COP. As a result, the effects of the evaporating temperature and cooling water temperature on the cooling performance are clarified by the experimental research. Furthermore, the calculation of the static characteristics predicted by the simulation model is in good agreements with the experimental results.

As a result of the problem of ecological destruction due to the use of CFC, HCFC, HFC, the use of absorption refrigerators has been attracting greater attention, and improvement in the quality of these refrigerators is in demand. There are three types of solution flow for the double-effect absorption refrigerator. They are series flow, parallel flow, and the reverse flow ; each has its own characteristics. As yet little research into the differences in performance according to solution flow type has been conducted. Therefore, definitive comparison of the performances has not been made yet. This study aims to clarify the differences in the double-effect absorption refrigerator which result from the solution flow type. The differences in characteristics of double-effect absorption refrigerators driven by gas depending on the three types of solution flow were developed and compared using a simulation model As a result we were able to identify the merit of each flow type.

As a result of the problem of ecological destruction due to the use of CFC, HCFC, HFC, the use of absorption refrigerators has been attracting greater attention, and improvement in the quality of these refrigerators is in demand. There are three types of solution flow for the double-effect absorption refrigerator. They are series flow, parallel flow, and the reverse flow ; each has its own characteristics. As yet little research into the differences in performance according to solution flow type has been conducted. Therefore, definitive comparison of the performances has not been made yet. This study aims to clarify the differences in the double-effect absorption refrigerator which result from the solution flow type. The differences in characteristics of double-effect absorption refrigerators driven by gas depending on the three types of solution flow were developed and compared using a simulation model As a result we were able to identify the merit of each flow type.

Recently, the absorption refrigerator has attracted much attention because it does not use freon as the coolant and it can be driven by waste heat. The absorption refrigerator contains much solution and coolant in it. Therefore, it requires a long start-up time because of the large heat capacities of the solution and the coolant. Moreover, it is known that much more heat is necessary at start-up than in steady-state operation. If it is driven by waste hot water of e. g. a combustion gas engine, the hot water return temperature becomes low at the start-up which adversely affects the engine. The purpose of this study is to construct an analytical model of the start-up characteristics of the single-effect absorption refrigerator driven by waste hot water, which can be used as a tool for investigating the problem of the start-up. Moreover, the validity of this model is confirmed by comparing the simulation results with the experimental results.

The protection of environment is becoming a grave problem nowadays and an absorption refrigerator, which does not use fleon as a refrigerant, is acquiring a close attention. Among the absorption refrigerators, a number of ammonia-water absorption refrigerators are being used in realm such as refrigeration and ice accumulation, since this type of refrigerator can produce below zero degree products.<br>It is essential to conduct an investigation on the characteristics of ammonia-water absorption refrigerator in detail by means of computer simulation in order to realize low cost, highly efficient operation. Unfortunately, there have been number of problems in order to conduct computer simulations. Firstly, Merkel's achievements of enthalpy diagram does not give the relational equations. And secondly, although relational equation are being proposed by Ziegler, simpler equations that can be applied to computer simulation are yet to be proposed.<br>In this research, simper equations based on Ziegler's equations have been derived to make computer simulation concerning the performance of ammonia-water absorption refrigerator possible-Both results of computer simulations using simple equations and Merkel's enthalpy diagram respectively, have been compared with the actual experimental data of one staged ammonia-water absorption refrigerator. Consequently, it is clarified that the results from Ziegler's equations agree with experimental data better than those from Merkel's enthalpy diagram.

Recentry, much attention is paid to the co-generation system which consists of both a power generator, such as combustion gas engine, and an absorption refrigerator. In this system, it is necessary to clarify the dynamic behavior of the absorption refrigerator for disturbances, such as flow rate change and inlet temperature change of the ho)t water. Moreover, downsizing of the absorption refrigerator is now desired. However, the operation becomes more sensitive to disturbances as the size of the refrigerator decreases. Is this study, the authors establish the dynamical model of the absorption refrigerator driven by hot water. This model is to be used as the tool for the design of a control system and refrigerator with good controllability. For this purpose, the model should be as simple as possible. Lumped, parameter models are adapted for the elements where this approximation is adequate. The validity of this model is confirmed by comparing the calculation results with the experimental results.

There are some types of absorption refrigerators which are driven by two different heat sources. In the last paper, the refrigerator driven by hot water and steam which were the waste heat of fuel cell was simulated. In this paper, another type of refrigerator driven by hot water and combustion gas which makes up for the shortage of hot water as the waste heat is simulated and its characteristics are clarified. Furthermore, the optimum control of this machine to use hot water effectively is suggested.

In this report, we investigate the characteristics of an absorption refrigerator driven by two different heat sources, hot water and steam, to make use of waste heat from fuel cell. This type of refrigerator consists of the cyclic combination of the single-effect absorption refrigerator and the double-effect one. For absorption refrigerators, there are many types of cycles which depend on solution circulation and generator combinations. First, the optimum cycle from the viewpoint of recovering waste heat is clarified. Then the simulation model is constructed and the static characteristics are investigated. Furthermore, the effects of low-level heat source on the total COP is clarified. There is a lower limit of temperature of the low-level heat source because the temperature becomes lower than solution temperature. Furthermore, we discuss the temperature control when low-level heat source conditions vary and the flow rate of steam is varied.

Recentry, much attention is paid to the co-generation system which consists of both a power generator, such as combustion gas engine, and an absorption refrigerator. In this system, it is necessary to clarify the dynamic behavior of the absorption refrigerator for disturbances, such as flow rate change and inlet temperature change of the ho)t water. Moreover, downsizing of the absorption refrigerator is now desired. However, the operation becomes more sensitive to disturbances as the size of the refrigerator decreases. Is this study, the authors establish the dynamical model of the absorption refrigerator driven by hot water. This model is to be used as the tool for the design of a control system and refrigerator with good controllability. For this purpose, the model should be as simple as possible. Lumped, parameter models are adapted for the elements where this approximation is adequate. The validity of this model is confirmed by comparing the calculation results with the experimental results.