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

Highlights

• Publication on the thermal effects of tight gaps in rotating fluid machinery.

• Tight clearances (0.1% of the airfoil height) revealed novel reversed flow topology.

• Heavily altered thermal field due to increased importance of the viscous effects.

• Identified optimal clearance height for suction side heat load.

• Accurate prediction of the adiabatic efficiency through isothermal simulations.

Abstract

The inevitable clearance between stationary and rotating parts in any fluid machinery gives rise to leakage flows, which strongly affect the overall performance of the machine. In modern gas turbine engines, the existing gap between the rotor airfoil tip and the shroud is responsible for about one third of the total aerodynamic losses. Additionally, this leakage flow induces fierce unsteady heat loads onto the rotor casing and provokes significant thermal stresses at the airfoil tip. One can attempt to curtail these detrimental effects by running tight clearances; however, the meager number of publications on this topic presents an obstacle to exploiting the design opportunities.

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 (T_total,in/T_w = 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.