In this study, a method to enhance boiling heat transfer in cryogenic fluids by using nanofiber coating was proposed and validated. For this method, a nanofiber coating of polyvinyl alcohol (PVA) was fabricated on the surface of a copper plate via electrospinning, which is a technique that uses a high-voltage electric field. A chilldown experiment conducted with the copper plate and liquid nitrogen revealed that the nanofiber coating had the potential to reduce the chilldown time drastically. It was observed that the heat flux through the nanofiber-coated plate increased immediately after the chilldown period began, and it exceeded that of the plate without coating for the entire test duration. Consequently, the chilldown (boiling) curve did not have a minimum heat flux point, which is generally seen in pool boiling process. The critical heat flux was twice as large as that of the conventional method with a bare copper plate. Overall, the chilldown curve of the nanofiber-coated plate was distinct from a conventional boiling curve with a bare plate.

JAXA is developing the pre-cooled turbojet engine (PCTJ) operated from sea-level static to Mach 5 with the single mechanism. The key technology of PCTJ is the precooler, which is a heat exchanger to cool down the heated incoming air using liquid hydrogen fuel. Frosting on the precooler tube surface is a major problem and some countermeasures have been proposed such as the liquid oxygen injection, methanol injection, high speed air-jet injection, and so on. However, these methods require additional mechanisms increasing the engine weight with their complicated systems. Here, we focus on the surface treatment that does not need any additional mechanism. We made aluminum super-hydrophobicity tubes treated by anodic oxidation and fluorine addition. Our method attains the super-hydrophobicity with about 157 degrees contact angle. The frost formation test using liquid nitrogen was conducted in the wind tunnel. As the results, the total amount of the frost formation can be reduced by about 17% at maximum compared with the untreated tube. However, little improvement on the heat transfer rate was found because the frost density was reduced by super-hydrophobicity.

平成30年度宇宙輸送シンポジウム（2019年1月17日-18日. 宇宙航空研究開発機構宇宙科学研究所（JAXA)(ISAS)）, 相模原市, 神奈川県資料番号: SA6000136048レポート番号: STCP-2018-048

JAXA is developing the pre-cooled turbojet engine (PCTJ) operated from sea-level static to Mach 5 with the single mechanism. The key technology of PCTJ is the precooler, which is a heat exchanger to cool down the heated incoming air using liquid hydrogen fuel. Frosting on the precooler tube surface is a major problem and some countermeasures have been proposed such as the liquid oxygen injection, methanol injection, high speed air-jet injection, and so on. However, these methods require additional mechanisms increasing the engine weight with their complicated systems. Here, we focus on the surface treatment that does not need any additional mechanism. We made aluminum super-hydrophobicity tubes treated by anodic oxidation and fluorine addition. Our method attains the super-hydrophobicity with about 157 degrees contact angle. The frost formation test using liquid nitrogen was conducted in the wind tunnel. As the results, the total amount of the frost formation can be reduced by about 17% at maximum compared with the untreated tube. However, little improvement on the heat transfer rate was found because the frost density was reduced by super-hydrophobicity.

平成29年度宇宙輸送シンポジウム（2018年1月18日-19日. 宇宙航空研究開発機構宇宙科学研究所（JAXA)(ISAS)）, 相模原市, 神奈川県資料番号: SA6000210039レポート番号: STCP-2017-039

現状の空調機や冷凍機では高温ガス除霜やヒーター除霜など大きなエネルギを投入し霜を融解させている．そこで申請者は，過去に陽極酸化法による表面処理とフッ素試薬により超撥水性を持たせた伝熱管を考案した．本研究課題では，基礎的データの取得のため， 2m/s，4m/s，6m/sでの流速で液体窒素を冷媒とした着霜試験を行い，着霜と流速との関係を見出すことを目的とし実験的研究を行った．その結果，非処理材に比べ処理材の着霜質量の方が減少した．一方，処理材では6m/s時，伝熱管側方の霜が剥がれて吹き飛び，後方へ再付着する現象が確認され，着霜質量が他の条件よりも高い結果となった．また，前方霜層厚さは処理材の方が厚くなる傾向が確認された．