Abstract:

Compared with the smooth surface, micro-pillar structured surfaces can enhance nucleate boiling significantly. The enhancement mechanism mainly includes increasing heat transfer area, increasing nucleation site density, reducing bubble departure diameter, etc. Few in-depth studies have been done on the diversion effect of microstructure gaps. In this paper, the three-dimensional multiphase fluid integral model is used to define the important geometric and time dimensionless parameters of single-bubble boiling on the microstructure surface. Through the analysis of velocity field and pressure field, the interaction of bubbles, micro-columns and surrounding liquid during the boiling process is discussed. The results show that the gap of the microstructure promotes the liquid backflow. Furthermore, the diversion effect in the channels established between the liquid-vapor interface at the bubble bottom and the substrate is the most obvious, which leads to an increased liquid flow rate, an accelerated bubble departure and the heat transfer enchancement of the single bubble-boiling. At the same time, the diversion effect of the microstructure promotes the formation of a high pressure thin film at the bottom and the sidewall of micropillar structures, in which the thin liquid film at the bottom has a capillary drainage capability and can maintain the liquid heat transfer area at the micropillar root in the bubble. The thin liquid film on the sidewall of the micropillars replaces the original dryout region, the heat transfer area is correspongdingly increased and the heat transfer efficiency is improved significantly.

Key words: numerical simulation, single bubble, three-dimensional, nucleate boiling, micro-pillars structured surface, microstructure diversion

摘要：

相对于光滑表面，微柱结构表面可以显著强化核态沸腾，强化机理主要有增大换热面积、增大核化密度、减小气泡脱离直径等，对于微结构内部的导流强化作用鲜有深入研究。利用多相流体积分数三维模型，定义了微结构表面单气泡沸腾重要的几何、时间无量纲参数，通过对速度场和压力场的分析，讨论了沸腾过程中气泡、微柱与周围液体的相互作用。结果证明：微结构的间隙有利于液体的回流，在气泡底部的气液界面与基底之间建立的流动通道内导流作用明显，液体流速被显著提高，有效促进了气泡脱离，强化了单气泡换热。同时，微结构的导流作用促使在微柱结构底部及侧壁面产生了高压薄液膜；底部的薄液膜具有毛细引流能力，维持了气泡内部微柱根部的液相区域；侧壁面上的薄液膜取代了原来的干烧区域，传热面积增大，换热效率显著提高。

关键词: 数值模拟, 单气泡, 三维, 核态沸腾, 微柱表面, 微结构导流