Numerical and Experimental Studies of Transpiration Cooling Film Effectiveness over Porous Materials

Abstract

Comprehensive experimental and numerical studies were performed to determine cooling film effectiveness of transpiration cooling over porous materials. The cooling film effectiveness was evaluated experimentally using pressure-sensitive paint by invoking heat/mass transfer analogy over the surface of the porous samples. It was found that transpiration cooling can reduce total surface heat flux by two to three times and provide solid surface cooling film effectiveness on average 40% higher than the state-of-the-art multihole effusion cooling. This improvement increases to 200% when high coolant flow rates are used due to effusion cooling film liftoff. Numerical simulations allowed detailed investigations of the flow evolution in the porous media and its ability to create a thermal protection film. Modeling results revealed slight lateral motion of the flow inside the porous media in the same direction as the main flow. This was attributed to the viscous effect of the channel flow and low flow resistance of the porous samples. As a result, a larger portion of coolant exits the porous transpiration cooling surface toward the trailing edge of the test coupon, creating a nonuniform cooling film over the test samples surface. The model also suggested that cooling film effectiveness is a function of the porous media physical properties (permeability and inertia coefficient). In contrast to the reduction in cooling film effectiveness in multihole effusion cooling resulting from cooling film liftoff at high coolant flow rates, this study indicates that high coolant flow rates enhance cooling film protection in transpiration cooling.