Aerothermodynamics of tight rotor tip clearance flows in high-speed unshrouded turbines

This paper presents the outcome of an extensive numerical investigation of a high pressure turbine stage operating at engine-representative non-dimensional parameters (Reynolds and Mach number, temperature ratios). RANS calculations were performed using the Numeca FINE/Turbo suite, adopting the k-ω SST turbulence model to investigate the aerodynamic and heat transfer characteristics in the tip region. Five clearances, ranging from 0.1% to 1.9% of the rotor channel height, were simulated at adiabatic and isothermal (Ttotal,in/Tw = 1.57) conditions. The detailed flow analysis revealed an unexpected aerodynamic flow topology at tight clearances (h/H < 0.5%), characterized by a reverse flow over a significant part of the tip gap region. The heat transfer on the airfoil tip, shroud and near-tip regions was examined in detail, with emphasis on the different driving phenomena. This elaborate numerical study provides a deeper insight into the complex aerothermal physics of leakage flows occurring for tight clearances in a high-speed environment relevant to any fluid machinery design and analysis.