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.

A general variational formulation of the dissipative two-phase flows based on the extremization of the entropy production is developed. The entropy generation rate is written outside phase equilibrium by introducing interfacial contributions due to surface tension as well as heat and mass transfer between the two phases. Prigogine's theorem of minimum entropy production is used to estimate the steady state void fraction of the two-phase flow. The corresponding flow representation is investigated out of phase equilibrium for an annular flow, experiencing friction, surface tension effects, and interphase heat and mass transfer, within a diabatic channel. It is demonstrated that the present formulation generalises previous theories by capturing the effect of mass and heat fluxes variations, and that the widely accepted expression from Zivi represents a particular case obtained under certain simplifying assumptions. Finally, a first validation of the developed formulation of two-phase void fraction is presented for different flow conditions, heat flux from the external environment, and thermophysical properties of the refrigerant with reference to data obtained from a dedicated experimental apparatus adopting a capacitance sensor and from previous literature. (c) 2021 Elsevier Ltd. All rights reserved.

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%.

Flow maldistribution has been an on-going issue that limits the full potential attainment of microchannel heat exchangers (MCHX). The underlying cause is the lack of theoretical understanding due to many complex interdependent factors at play. This results to high variability leading to a mathematical challenge of representing and solving the physical system. Hence, the available studies in the literature are limited to experimental investigations or CFD models that are case specific. This paper reports the method of incorporating a pressure drop model and integrating a variational formulation by extremizing the entropy generation rate. The extremum principle points at the maximum entropy production rate toward a stationary state, hence providing a general representation of the phenomenon which is unrestrained from limitations to a specific working fluid and structural geometry of the MCHX. Distribution results are investigated for different inlet conditions. A first comparison to experimental data shows a fair prediction accuracy.

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.

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.

© 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.

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%.

© 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.

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.

© 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.

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.

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.

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.

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.

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.

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.

This paper discusses the multi-objectives optimization of a cascade refrigeration system using refrigerant C3H8 in high temperature circuits (HTC) and a mixture of C2H6/CO2 in low temperature circuits (LTC). The evaporator temperature, condenser temperature, C2H6/CO2 mixture condensation temperature, cascade temperature differences, and the CO2 mass fraction are chosen as the decision variables. Whereas cooling capacity, cold space temperature, and ambient temperature are taken as the constraints. The purpose of this research is to design a cascade refrigeration system whose optimum performance are defined in terms of economics and thermodynamics. Accordingly, there are two objective functions that should be simultaneously optimized including the total annual cost which consists of the capital and operational cost and the total exergy destruction of the system. To this aim, the optimum operating temperature of the system and CO2 fraction should be determined so that the system has minimum exergy destruction and annual cost. Results show that, the optimum value of the decision variables for this system can be determined by trade-off between annual cost and exergy destruction. (C) 2016 Elsevier Ltd. All rights reserved.

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.

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.

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 cold store refrigeration system proposed herein features a cascade configuration which has a vapour compression cycle as the top cycle and an inverse Brayton as the bottom cycle. The cold store air is used as working fluid in the Brayton cycle. Therefore, the cold side heat exchanger (that would be the evaporator in a vapour compression cycle) is replaced by a cold air inlet. Frost formation inside the cold store, as well as the heat load due to the air circulation fan, are hence eliminated. On the other hand, the Brayton cycle has lower efficiency when compared to vapour compression cycles. This gap may be reduced once the temperature range is lowered by the cascade configuration. The top vapour compression cycle may also refrigerate the loading dock. This paper describes a first screening of the relevant design parameters for an overall system optimization.

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.

A thermodynamic analysis is performed on open, regenerated, inverse air-cycles, focussing on low temperature refrigeration, in order to provide general optimization criteria, support for a rational configuration choice and potential performance improvement for this specific application. The effect of high regenerator effectiveness (0.95) is explored, showing that it may double the air cycle COP in the case at hand. Low and high pressure configurations (i.e. featuring only one heat exchanger on the cold or hot cycle side) are compared. High pressure configuration, normally preferred in the field of low temperature refrigeration, has a lower efficiency, but the gap reduces as regenerator effectiveness increases. Avoidance of a cold heat exchanger eliminates frosting problems and power input for the circulation fan, so that the small COP decrease suffered by the high pressure cycle can easily be compensated for and eventually yield higher average plant efficiency. The feasibility of such a highly effective regenerator, already reported in the literature, is demonstrated using classic Kays and London (1964) data. (C) 2013 Elsevier Ltd and IIR. All rights reserved.