Unraveling evaporative flow spreading behavior underlying transpiration cooling performance under high temperature shear flow

Phase-change transpiration cooling, through utilizing substantial latent heat, has become one of the most promising active thermal protection techniques for aerospace vehicles. Severely inefficient transpiration cooling performance for downstream surface from experimental measurements, however, has drawn attention compared with theoretical prediction, which may cause serious thermal protection failure in practice. In this work, the evaporative flow spreading behaviors after the coolant exits the porous media and their underlying impacts on phase change transpiration cooling are investigated using a self-developed high-temperature wind tunnel combined with thermal interface flow visualization. Two typical cooling modes, cold start and hot start are compared. The results indicate that the spreading behavior of liquid coolant on downstream region undergoes a transition from finger flow to similar film boiling flow with intense vaporization at the liquid-solid interface. A significant decline in cooling efficiency is observed for the downstream region under this condition during transpiration cooling process. Three distinct cooling states for downstream region are identified, which are characterized as vapor cooling without interface boiling, single-phase liquid film cooling and liquid cooling with similar film boiling, respectively. This experimental work highlights a potential optimization direction for numerical simulations by considering the actual liquid film coverage, to improve surface temperature prediction accuracy for aerospace vehicles in the future.