Transonic Turbine Blade Tip Aero-Thermal Performance With Different Tip Gaps: Part II—Tip Aerodynamic Loss

Blade tip aerodynamic loss results from experimental and numerical investigations are presented for engine representative conditions downstream of a blade row with an exit Mach number M_exit of 1.0, and an exit Reynolds number Re_exit of 1.27×10⁶ (based on axial chord). These results are presented for three different tip gaps of 0.5, 1.0, and 1.5 percent relative to engine-equivalent blade span. Experimental data are obtained by traversing a specially-made and calibrated three-hole pressure probe as well as a single-hole probe one axial chord downstream of the blade within the Oxford High Speed Linear Cascade research facility. Three-dimensional RANS CFD numerical predictions are conducted using the Rolls-Royce HYDRA numerical prediction code for steady flow with the Spalart-Allmaras (SA) turbulence model. Included are detailed distributions of stagnation pressure losses, and pitch-wise flow angle. Local total pressure data and mass-averaged total pressure loss coefficients show that the strength of the tip leakage vortex decreases as the tip gap decreases. Magnitudes of the pitch-wise flow angle increase within over-tip leakage vortices, as these vortices become stronger and the tip leakage flow increases. The most important difference between experimental and numerical results is in relation to the passage vortex signatures, which are more apparent for all three tip gap values within the numerical results. The effects of relative casing motion and tip clearance are also examined and discussed, and show that the relative casing movement has a relatively small impact on the size of the over-tip leakage vortex at the medium (1.0% of span) and large (1.5% of span) tip gaps, with more noticeable impact at the smallest tip gap (0.5% of span).