Thermal Characteristics of Transpiration Cooling System Using Open-Cellular Porous Materials in a Radiative Environment

摘要

Combined convective and radiative heat transfer in an air transpiration cooling system using open-cellular porous plates was studied experimentally and theoretically. 0.012-m-thick cordierite-alumina and 0.01-m-thick nickel-chrome open-cell foams were used as a heat shield. Air injection velocity was varied from 0.23 m/s to 1.77 m/s and the heat flux of incoming radiation was varied from 5.25 kW/m~2 to 22.2 kW/m~2. The temperature efficiency, indicating how close the mean temperature of a porous heat shield is to the inlet air temperature, increases rapidly with the air injection velocity and is saturated at a constant value, but is always greater than 0.9 in the present case. Agreement between theoretical predictions and experimental results is acceptable, thereby the validity of the theoretical model is confirmed. Results of numerical simulation showed that almost all the incoming radiation can be shielded using high- or low-reflective open-cellular porous heat shields with the optical thickness of five or more, as the heat shield is kept at a comparatively low mean temperature. However, under intense radiative heat flux, high-reflective open-cellular porous materials may be more desirable than low-reflective one.