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

Abstract

The pool boiling heat transfer performance of a bistructured surfaces, named PF30-60LP, was studied based on the micro-pin-finned surface under short-time microgravity conditions. The heat transfer performance of PF30-60LP 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 10-2g-10-3g and the duration of microgravity is 3.6 s. The working fluid is FC-72, with the subcooling of 40 K, and the experimental pressure is a standard atmospheric pressure. It was observed that compared to the smooth surfaces and micro-pin-finned surfaces, PF30-60LP 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 PF30-60LP 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 PF30-60PL 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 pre-existing 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 PF30-60LP, the existence of large scale of smooth channels between the micro-pin-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.