Numerical investigation of impingement heat transfer on smooth and roughened surfaces in a high-pressure turbine inner casing

The multiple impingement jet system in a high-pressure turbine inner casing has been studied numerically. Four target surface configurations, i.e., smooth, cambered rib, square and round pin-fin are investigated, respectively. Three different boundary conditions (i.e., the maximum, medium and minimum scheme) are set based on the real turbine operating condition. The numerical validation reveals that the selected computational method can provide a good prediction of the impingement heat transfer on both the smooth and roughened surface structures. The results indicate that the roughness elements can significantly improve the heat transfer characteristics of the multiple impingement jet system. And the cambered rib surface displays the best enhancement of the impingement cooling effect. For the turbine inner casing with different target surfaces, local/average heat transfer parameters and the flow structure are obtained and compared under the same boundary condition, and such process has been repeated under three selected boundary conditions. All of the roughened surfaces show the outstanding cooling effect than the smooth surface. Especially, with the cambered rib configuration, the average temperature of the turbine inner casing domain can be decreased by 20 K, and the average Nusselt number can be increased by up to 62.6% than that on the smooth surface. The little temperature difference also demonstrates that the cambered rib configuration can promote the cooling uniformity and decrease the thermal stress in the turbine inner casing. Moreover, the analysis of the flow structure also illustrates that the cambered rib configuration can effectively reduce the crossflow and adjacent jet interaction, which promotes the turbulent mixing and augments the impingement heat transfer. The proposed structure can be used to improve the cooling effect of the turbine inner casing and is expected as the potential design reference for the turbine engine in the future.