Numerical investigation of secondary flows through highly loaded transonic turbine stage

The turbine aero-thermal design expertise has not reached the ultimate level in comparison to compressor expertise which has evolved to high loading, high pressure ratio design with a Iimited number of stages. For high pressure turbine section, single highly loaded stages are typically representative of wide class of advanced aero engines. The transonic turbines significantly operate at higher pressure ratios with less number of stages but at an expense of relatively lower efficiencies. In this study, steady three-dimensional numerical analysis of flow through a highly loaded transonic turbine stage is conducted using commercial software NUMECA which solves Reynolds-Averaged Navier-Stokes equations (RANS) using finite volume method. Due to high pressure ratio, the flow is highly transonic in nature; the gas flow becomes supersonic as it exits the nozzle guide vanes although the downstream velocities become subsonic. One-equation Spalart-Allmaras model (SA) and two-equation Shear Stress Transport k-omega model (SST), are employed in the simulations. The one-equation SA model runs faster and gives good results in many turbo machinery applications and it is normally recommended by many researchers. The two-equation SST model combines the best of k-omega and k-epsilon models by a blending-function to switch between both models. The numerical results of both the SA and SST models are validated with the pneumatic flow field measurement between the stator-rotor gap and downstream of the rotor in axial planes. The tip clearance of the rotor blade and its effects are considered by modeling the tip gap. The partial shroud gap at aft section of stator is also modeled for validation of results with the experiments. The interaction of tip leakage vortex with the main flow is analyzed. The impact of leakage flow is investigated by applying the same boundary conditions but varying the tip clearance. An efficiency loss of 3.08% has been observed due to the tip clearance flow by varying the tip clearance from 0 to 0.95 mm which depicts the importance of study of losses associated with tip leakage flows.