Overtip Choking and Its Implications on Turbine Blade-Tip Aerodynamic Performance

At engine representative flow conditions a significant portion of flow over a high-pressure turbine blade tip is transonic. In the present work, the choking-flow behavior and its implications on overtip leakage-flow loss generation are computationally analyzed. An extensively developed Reynolds-averaged Navier-Stokes code (HYDRA) is adopted. First, a high-speed linear-cascade validation case is introduced, and the computations are compared with the experimental data to identify and establish the capability of the code in predicting the aerodynamics losses for a transonic turbine blade tip. The computational studies are then carried out for the blading configuration at different flow conditions, ranging from nearly incompressible to nominal transonic, enabling a qualitatively consistent trend of the tip leakage losses in relation to the exit Mach number conditions to be established. The results clearly show that the local choking sets a limiter for the overtip leakage mass flow, leading to a different leakage-flow structure compared with that in a low-speed and/or unchoked condition. The existence of tip choking effectively blocks the influence of the suction-surface side on the overtip flow and hence leads to a breakdown of the pressure-driven mechanism, conventionally used in tip treatment and designs. The decoupling between blade loading and overtip leakage mass flow is clearly identified and highlighted. Furthermore, the realization of the loading-leakage-flow decoupling indicates a possibility of a high-load blading design with a relatively low tip leakage loss. A high-load blading is generated and analyzed to demonstrate the feasibility of such designs with a reduced tip leakage loss.