Large Eddy Simulation of trenched cylindrical film hole with backward compound angles

Abstract

Many previous studies have demonstrated that both transverse trench and backward cooling injection are capable of improving jet lift-off problem of traditional cylindrical film hole at high blowing ratios. However, few studies have attempted to combine the above two cooling schemes to further alleviate the cooling deterioration phenomenon. This paper proposed two trenched film holes with different backward compound angles of β = 180° and 135°, and used highly-resolved Large Eddy Simulation method to explore in detail their unsteady flow fields including vortex structures, level of velocity fluctuation, turbulent shear stress, distributions of vorticity and adiabatic wall cooling effectiveness. The results are compared against those of the benchmark trenched film hole with β = 0°. The simulations were carried out under blowing ratio of 1.5, and the coolant to mainstream density ratio of 2.0.The detrimental symmetric counter-rotating vortex pair shown in the trench with β = 0° hole is replaced by an asymmetric vortex pair in the trench with β = 180° hole and only a single asymmetric vortex in the trench with β = 135° hole. Inside the trench, hairpin-like vortices are the dominant vortex structures for the trench with backward film holes. In the region downstream of the trench, the trench with β = 180° hole obtains the highest level of three velocity components fluctuations in the central region because of the direct intense collision between the coolant and mainstream, while the trench with β = 135° hole shows a highest level of lateral velocity component fluctuation in the lateral sides and a highest level of turbulent shear stress in the near wall region due to the significantly enhanced lateral motion of the coolant. The trenches with β = 180° and 135° holes significantly improve the lateral uniformity of the coolant, and respectively provide constantly high levels of 0.21 and 0.3 in laterally averaged cooling effectiveness on the wall downstream of the trench, compared to the trench with β = 0° hole with a low level of less than 0.2.