Flow regimes and flow instability of transpiration cooling

In this study, we introduced a fabrication method for a porous transpiration cooling specimen using a nickel-based alloy called Hastelloy X. The recipe consists of mixing, compacting, and sintering processes. Mercury and water intrusion tests validated that the porous specimen satisfied the required thermophysical properties, including a porosity of ∼0.3 and permeability of ∼10^–12 m². A contour map of the heat flux inducted by a heating element and simulated the aerodynamic environment was obtained using a unique apparatus as a function of the specimen-gas torch distance and mass flow rate of the butane gas fuel. Experiments for evaluating the transpiration cooling performance of the Hastelloy X specimen were conducted by inducing the largest heat flux of ∼220 kW/m² at the top surface of the specimen. Four distinct flow regimes—all-liquid, partial phase-change, phase-change, and dry-out—were observed according to the mass flow rates of the coolant, i.e., deionized water supplied from the bottom of the specimen. Surface temperatures and pressure drop measurements were used to analyze the thermal and flow instability characteristics of the specimens. An N-shaped pressure drop curve indicates that unexpected severe flow instability may occur in the partial phase-change regime, where a negative slope relationship between the pressure drop and flow rate is found. Combining the cooling efficiency and injection ratio, the performance of the specimen was analyzed by using the cooling performance. Combining all the results, including the temperatures, flow regimes, instability, and cooling performance, the phase-change regime was found to be optimal for the transpiration cooling operation.