Directional vapor mobility from asymmetric microstructured surfaces in an adverse gravity orientation

Abstract

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.