Heat Transfer in Nucleate Pool Boiling under Microgravity

以乙醇为工质，进行了微重力条件下的池核沸腾实验。介绍了一种带传感器的透明加热表面，用于测量气泡生长过程中表面局部温度和液膜厚度。表面采用蓝宝石玻璃，背面加热透明ITO薄膜加热器的结构实现了对附着气泡底部液-气行为的观察，同时实现了对局部表面热流密度的准确评估。提出了一种直接在加热表面涂敷薄电极测量液膜厚度的新方法。采用4 mm见方、0.1 μ m厚的传感器，可实现1 ~ 10 μ m量级的微层厚度的瞬时测量。测量在压力0.01 ~ 0.02 MPa、液体过冷度0 ~ 27 K、热流密度高达90000 W/m^2的条件下进行。（1）在微重力低热流区，微层内存在两种相反的传热强化和恶化趋势，这取决于微层内是否出现干斑。（2）在离散气泡区，大气泡下的局部传热系数受重力水平的影响很大，而整个加热表面的平均传热系数对重力水平的影响很小。㈢在高热通量下，在微重力下，在一个大的聚结气泡下面，小的单个气泡从加热表面出现，并在宏观层表面坍塌。实验结果表明，微重力条件下的平均传热系数随实验方式和实验条件的不同而增大或减小，并假定其值与地球上的平均传热系数相差不大。

Experiments in nucleate pool boiling under microgravity were conducted by using ethanol. The transparent heating surface with sensors for the measurement of local surface temperature and liquid film thickness underneath a growing bubble was introduced. The surface is made of sapphire glass, and the structure of back-side heating by transparent ITO film heater realizes the observation of liquid-vapor behaviors at the base of attached bubble and simultaneously the accurate evaluation of local surface heat flux. A new method was developed for the measurement of liquid film thickness by using a set of thin electrodes coated directly on the heating surface. The sensor of 4mm square and 0.1 mum thickness was introduced to measure instantaneously the microlayr thickness of 1 to 10 mum order. Measurements were performed under pressure 0.01 to 0.02 MPa, liquid subcooling 0 to 27 K,heat flux up to 90000 W/m^2.The following are clarified. i) In microgravity at low heat flux region, there exists two opposite trends of heat transfer enhancement and deterioration depending on whether dry patches emerge or not in the microlayr. ii) In the discrete bubble region, the local heat transfer coefficients underneath a large bubble are significantly influenced by the gravity level, while the coefficient averaged over the entire heating surface is quite insensitive. iii) At high heat flux, underneath a large coalesced bubble in microgravity, small single bubbles emerge from the heating surface and collapse at the surface of macrolayr. The heat transfer is dominated mainly by the nucleate boiling in the macrolayr, and it remains almost constant despite the change of gravity level.The results give the interpretation of existing experimental results in the past three decades, in which the average heat transfer coefficients in microgravity increase or decrease depending on the experimental manner and conditions adopted, and assumes the values not far different from the terrestrial ones.