Experimental Investigation of Tip Design Effects on the Unsteady Aerodynamics and Heat Transfer of a High Speed Turbine

The present research addresses the unsteady aerodynamic and heat transfer field of a high pressure turbine stage as a function of the rotor tip design. Three selected tip profiles, resulting from a previous multi-objective optimization campaign, present non-conventional squealer-like and carved geometries aimed at the aerothermal control of the tip leakage stream. A combined numerical and experimental approach is employed to characterize the interaction of the tip leakage flow with the rotor secondary flows and the dominant casing heat transfer mechanisms for each individual tip geometry.

The experimental research is performed in the rotating turbine test rig of the von Karman Institute, operating on the principle of the compression tube. During the short time duration of a test (~250-500 ms), engine-scaled thermodynamic conditions are preserved in the test section, representative of the first stage of a two-stage aeroengine HPT. For the present study, the turbine rotor is operated in rainbow configuration to allow the simultaneous testing of multiple blade tip geometries. Full stage unsteady CFD simulations are employed to predict the impact of the individual profiles on the tip leakage flow generation and propagation inside the blade passage.

In the present paper, a squealer-like tip with a discontinuous peripheral rim and a contoured tip profile are characterized with respect to a baseline squealer design. The unsteady flow predictions are used to document the interaction between the tip leakage vortex (TLV) and higher passage vortex (HPV) through a detailed analysis of the unsteady relative total pressure and entropy fields at different axial locations. Time-resolved measurements of total pressure and flow angle at the stage outlet and endwall static pressure fluctuations are employed to validate the CFD data set. The phase-resolved heat transfer field at the overtip casing is studied by combining unsteady thin film measurements and numerical results. Steady-RANS predictions, available for two sets of isothermal conditions, allow to compute the adiabatic wall temperature and heat transfer coefficient to separate the aerodynamic and thermal contribution to the global heat flux. Finally, a simplified model is developed to evaluate the dominant casing heat transfer mechanisms for each tip design.