Stagnant vapor growth on a downward-facing flat surface leads to increased surface temperature and dryout at relatively low heat fluxes. The current study investigates boiling from a surface with periodic 60°-30° or 75°-15° asymmetric sawtooth microstructures to passively remove vapor slugs in adverse gravity. Bubble dynamics is experimentally investigated in an 8-mm square borosilicate glass tube and visualized using a high-speed camera to measure vapor bubble morphology and velocity in the FC-72 dielectric liquid. The vapor bubble's morphology is observed to change based on its relative size to the ratchets. This change manifests as a decrease in the curvature ratio between the crest and trough of the microstructure as the vapor bubble gets larger. The pattern of vapor slugs nucleating from intended, engineered sites, coalescing to form larger slugs that slide across the microstructure, is observed at different frequencies at different heat fluxes. The microstructure supports increased vapor volume at higher heat fluxes, suggesting that a passive and self-regulating thermal management solution for adverse gravity applications is feasible. An empirical force balance model is developed based on the curvature-induced Young-Laplace pressure difference and retarding forces due to drag and buoyancy. The interplay between these forces is analyzed at different liquid film thicknesses, and the force balance is compared with a 75°-15° sawtooth structure. For a vapor slug spanning four sawteeth, the 75°-15° structure supported a 54 % increase in the feasible liquid film thickness range due to the increased long slope area.

中文翻译：

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逆重力方向上不对称微结构表面的定向蒸汽迁移率

朝下的平坦表面上的停滞蒸汽生长会导致表面温度升高，并在相对较低的热通量下发生干燥。目前的研究研究了具有周期性 60°-30° 或 75°-15° 不对称锯齿微结构的表面的沸腾，以被动去除逆重力下的蒸气段塞。在 8 毫米方形硼硅酸盐玻璃管中对气泡动力学进行实验研究，并使用高速摄像机进行可视化，以测量 FC-72 电介质液体中的蒸气泡形态和速度。观察到气泡的形态根据其与棘轮的相对尺寸而变化。这种变化表现为随着气泡变大，微观结构的波峰和波谷之间的曲率比减小。在不同的热通量下以不同的频率观察到蒸气段塞从预定的工程位置成核，聚结形成更大的段塞并滑过微观结构的模式。该微观结构支持在较高热通量下增加蒸汽量，这表明针对逆重力应用的被动式自调节热管理解决方案是可行的。基于曲率引起的杨拉普拉斯压力差以及阻力和浮力产生的阻滞力，开发了经验力平衡模型。分析了不同液膜厚度下这些力之间的相互作用，并将力平衡与 75°-15° 锯齿结构进行了比较。对于跨越四个锯齿的蒸汽段塞，由于长斜面面积的增加，75°-15°的结构使可行的液膜厚度范围增加了54%。