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

•Leakage flow not only enhances the adiabatic film cooling effectiveness of the concave endwall but also reduces the aerodynamic performance of the cascade passage.•In addition to the step-induced vortex, the backward-facing step also generates a new extra-large-sized horseshoe vortex at the region upstream of the vane leading edge.•Although the backward-facing step greatly decreases the adiabatic effectiveness of the concave endwall, the total pressure loss coefficient is also reduced.•High turbulence level reduces the endwall cooling effectiveness, however, it enhances the diffusion of the leakage coolant on the endwall. 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, especi