Aerothermal characteristics of a transonic tip flow in a turbine cascade with tip clearance variations

A significant portion of flows over a modern high-pressure turbine blade tip is transonic, and the transonic tip leakage flows lead to significant aerodynamic losses and high heat loads onto the blade tips. This paper aims to develop a deeper understanding of the transonic tip leakage flow physics and its influence on the loss mechanism and blade-tip heat transfer. Three-dimensional (3D) Reynolds-averaged Navier-Stokes (RANS) calculations were performed using the ANSYS CFX 14.5 numerical prediction code, adopting the SST k-ω turbulence model to investigate the sensitivity of aerothermal performance of transonic tip flows to tip clearances in a RT27a turbine cascade. The transonic tip leakage flow pattern within the tip gap, the flowfield downstream of the cascade, and the blade tip heat transfer distribution are studied. The numerical results give a reasonable agreement with the experimental data. The tip aerodynamics and surface heat transfer variations with tip clearances are opposite between leading and trailing edge regions of blade tips, and tip clearance effects on the former are relatively small. As the tip clearance increases, the shock wave reflections are delayed but more evident, and it therefore leads to reduced leakage massflow density and decreased heat loads on the rear part of blade tips. Despite this, since the leakage flow near the leading edge of blade tips remains subsonic resulting in increased leakage mass flowrate and tip heat transfer, the leakage losses and overtip heat loads are increased with the increasing tip clearance.