Effects of gradient porosity and particle diameter on transpiration cooling for double-layer plates

Porous medium structure with a gradient porosity or a combination of different porosities is an effective method to improve the cooling performance of transpiration cooling. Novel structures with gradient porosity distributions are proposed to evaluate transpiration cooling performance on thermally challenged surfaces subjected to high-speed hot gas flow. The effects of inlet mass flow rate, gradient porosity, and particle diameter on the injection pressure P ave and cooling efficiency are investigated through numerical simulations. Although increasing the flow rate can improve the cooling efficiency of the upper wall, the corresponding coolant injection pressure significantly increases. As the particle diameter of the inner or outer porous plate decreases, the cooling performance of double-layer porous plates is better than that of a conventional single-layer porous plate, but the required injection pressure increases due to decreasing permeability. When the porosity of the outer porous plate remains unchanged, the average temperature of the upper wall remains almost unchanged with the increasing porosity of the inner plate, the deviation is negligible. In contrast, when the porosity of the inner porous plate remains unchanged, the cooling performance is enhanced with the decreasing porosity of the outer plate, and P ave is increased by about seven times. In addition, present results reveal that the cooling efficiency and temperature uniformity are significantly enhanced when the porosity decreases linearly along the positive x direction, and the required injection pressure P ave is reduced by thirty-eight point six percent compared with a uniform porous structure. The results obtained in this paper may provide important insights and promising solutions to prevent the heat transfer deterioration in porous medium structures.