Effect of backward-facing step and turbulence level on film cooling characteristic of a turbine concave endwall with upstream leakage flow

Abstract

The coolant from the leakage slot between the turbine and the combustor can not only hinder the intrusion of the hot mainstream gas into the turbine components but also provide thermal protection to the turbine endwall. Whereas, the film cooling characteristic of the endwall is greatly influenced by the backward-facing step caused by the foundry, assembly, and thermal expansion. Furthermore, in a real aeroengine, the top endwall of the turbine cascade passage presents a concave profile, which results in the flow physics behavior of the concave endwall being more complex than that of the plane or convex endwall. Consequently, this paper experimentally and numerically investigated the film cooling performance of the concave endwall with the upstream leakage flow and backward-facing step. The film cooling effectiveness of the endwall was measured using the pressure sensitive paint technique in a high-speed wind tunnel with four annular passages. The effect of mass flow ratio, backward-facing step height, and turbulence level on the endwall film cooling characteristic was investigated. Additionally, the numerical simulation method with the steady-state Reynolds Average Navier-Stokes equation was also employed to explore the flow field visualization and aerodynamic performance. The experimental result indicates that the film cooling effectiveness of the concave endwall quickly enhances as the mass flow ratio increases, however, the effectiveness of the area close to the pressure side hardly alters due to the strong passage crossflow. The presence of the backward-facing step intensifies the leakage vortex and the horseshoe vortex, which greatly reduces the cooling effectiveness of the endwall for all the mass flow ratio cases. As the height of the backward-facing step increases, the cooling effectiveness of the endwall further declines, especially when the mass flow ratio is >1.25 %, the cooling effectiveness of the endwall upstream of the leading edge of the vane decreases by >20 %. Furthermore, due to the intense turbulence disturbance of the endwall boundary layer, the high turbulence level case has a larger coverage of the leakage coolant on the endwall than that of the low turbulence level case, particularly the area close to the pressure side. Nonetheless, the high turbulence intensity gives rise to the reduction of the cooling effectiveness in most areas of the passage endwall for all the mass flow ratio cases. The numerical results indicate that the backward-facing step greatly increases the strength of the leakage vortex, and it gradually merges the horseshoe vortex into a new extra-large-sized horseshoe vortex upstream of the vane leading edge as the mass flow ratio increases. Moreover, the backward-facing step induces the recirculation vortex at the outlet of the leakage slot, which hugely reduces the jet momentum of the leakage coolant. In terms of aerodynamic performance, the leakage flow results in the total pressure loss coefficient of the cascade passage increasing by >15.8 %, whereas, the total pressure loss coefficient reduces when the backward-facing step appears. Finally, the purpose of this study is to provide some cooling technical guidance for thermal protection designers of turbine concave endwalls, suggesting the height of the backward-facing step should not be >2 mm and the mass flow ratio of the leakage coolant not be <1.25 %.