短时间微重力池沸腾换热及其临界热流密度机理

Pool boiling heat transfer and its critical heat flux mechanism in short-term microgravity

研究了复合柱状微结构表面(PF30-60LP)在短时间微重力下的池沸腾传热性能,并与文献中的光滑表面和柱状微结构表面(PF30-60和PF50-120)进行对比.实验结果表明,微重力条件下, PF30-60PL的临界热流密度(critical heat flux, CHF)与光滑表面相比虽有提高,但却明显低于PF30-60和PF50-120,与常重力下所得实验结果存在明显差异.微重力条件下,由于浮力缺失,覆盖于加热表面的大气泡脱离周期远大于常重力条件.大气泡覆盖于表面时间过长,导致新鲜液体补给困难是造成微重力CHF显著降低的主要原因. PF30-60和PF50-120具备非常强的毛细芯吸作用,可显著提高加热面的侧向补液能力,因此其微重力下的CHF相比于光滑表面得到了十分显著的提高.而PF30-60PL由于较大面积光滑通道的存在,表面的毛细芯吸作用被削弱,因此其CHF介于光滑表面和柱状微结构表面之间.提高微重力池沸腾CHF,关键在于提高覆盖于加热面的大气泡的脱离频率和液体对加热表面的补给能力.可行的方法有降低液体工质表面张力或提高表面对液体的毛细芯吸性,通过外部施加电场或声场加速气泡脱离,强化Marangoni对流或采用局部加热法改变Marangoni力对气泡的作用力方向,通过调控气泡合并行为增大气泡合并后释放的表面能从而促进气泡脱离等.

The pool boiling heat transfer performance of a bistructured surfaces, named PF30-60 LP, was studied based on the micropin-finned surface under short-time microgravity conditions. The heat transfer performance of PF30-60 LP was compared with those of the smooth surfaces and the micro-pin-finned surfaces(PF30-60 and PF50-120). The microgravity environment was realized by the drop tower in National Microgravity Laboratory, Chinese Academy of Sciences. The gravity level is 10-2 g-10-3 g and the duration of microgravity is 3.6 s. The working fluid is FC-72, with the subcooling of40 K, and the experimental pressure is a standard atmospheric pressure. It was observed that compared to the smooth surfaces and micro-pin-finned surfaces, PF30-60 LP shows the best heat transfer coefficient both in normal gravity and microgravity in the stable nucleate boiling region. Besides, the critical heat flux(CHF) of PF30-60 LP is much higher than that of the smooth surfaces, slightly greater than that of PF30-60, but smaller than that of PF50-120 in normal gravity. The CHF of PF30-60 PL is observed much smaller than that of PF30-60 and PF50-120, although its CHF is still greater than that of the smooth surfaces under microgravity condition. The mechanism of critical heat flux of pool boiling in microgravity was analyzed. As the heat flux approaches to the CHF, a layer of small bubbles can be observed on the heated surface when the mushroom bubble departs. And the small bubbles will be quickly merged into a large bubble after the mushroom departs. Based on the observed bubble behavior, the CHF model of the micro/nanostructured surface can be used for explaining the origins of difference in CHF among different surfaces. The liquid replenishment of pool boiling under microgravity can be divided into the following two ways:(1) For a smooth surface, the liquid is mainly supplemented by the pre-existing liquid in the gap between the small bubbles.(2) For the microstructured surface, in addition to the preexisting liquid, the lateral capillary wicking effects generated by the microchannels also play a very important role for liquid supply. The period of coalesced bubble detachment in microgravity is much greater than that in normal gravity due to the lack of buoyancy. The liquid replenishment is significantly obstructed due to the difficulty of bubble departure, which is the main reason for the obvious decrease of CHF in microgravity. Besides, the coalesced bubble departure frequency increases with the increase of heat flux in microgravity, and the increased coalesced bubble departure frequency results in the increase of CHF. As for PF30-60 and PF50-120, with very strong capillary wicking effects which can significantly enhance the lateral liquid supply of the heated surface, their CHFs in microgravity are notably improved compared to that of the smooth surfaces. With regarding to PF30-60 LP, the existence of large scale of smooth channels between the micropin-fin blocks weakens the capillary wicking effect of the surface. Therefore, its CHF is much smaller than that of PF30-60 and PF50-120. It is suggested that the CHF of pool boiling in microgravity can be improved by the following ways: Using micro/nanostructured surfaces with strong capillary wicking effects, accelerating bubble detachment by external electric field or sound field, using binary azeotrope to enhance the Marangoni convection, enhancing the wettability of liquid working fluid, promoting bubble departure by enhancing the released surface energy during bubble merging, and changing the direction of the Marangoni force of the bubble by local heating method.