A Mixing Plane Model Investigation of Separation and Transitional Flow at Low Reynolds Numbers in a Multistage Low Pressure Turbine

Abstract

Flow separation with increased losses is known to occur when low pressure turbine (LPT) blades are operated at high altitudes with a reduced inlet Reynolds number. Under these conditions, boundary layer separation is more likely to be present within the flowfield of the LPT stages due to thickening of the boundary layers and an increase in the portion of the airfoil experiencing laminar flow. More accurate CFD predictions are needed in order to improve design methods and performance prediction for LPT stages operating at low Reynolds numbers. Steady flow CFD simulations of multistage LPT flow were completed at nominal and high altitude conditions with the conventional Spalart-Allmaras turbulence model. This model was used in combination with a mixing plane model for the simulation of flow through domains with one or more regions in relative rotational motion. Flow visualizations were completed using surface flow and streamline calculations to help identify vortical structures present within the flowfield. Also, the total pressure loss coefficient was calculated for each blade row. Qualitative comparisons indicate that the simulated high altitude condition had an increase in the amount of separated flow present within the flowfield compared to the nominal altitude condition. This can be attributed to the reduction in the inlet Reynolds number. Initial investigations with a recently-developed three-equation eddy-viscosity type turbulent transitional flow model are also reported. Comparisons of flow predictions for the 1st turbine stage with the two models revealed that large vortices predicted with the Spalart-Allmaras model were not present, and the wake loss coefficient was significantly lower with the three-equation turbulence model. Based on these and previous results, the CFD with the three-equation model is considered to have potential to provide improved prediction of separation and transitional flow in low Reynolds number turbine applications.