Experimental Investigation of Turbine Leakage Flows on the 3D Flow Field and End Wall Heat Transfer

Kurzfassung

Within a European research project the tip end wall region of LP turbine guide vanes with leakage ejection was investigated at DLR in Goettingen. For this purpose a new cascade wind tunnel with three large profiles in the test section and a contoured end wall was designed and built up, representing 50% height of a real low pressure turbine (LPT) stator and simulating the casing flow field of shrouded vanes. The effect of tip leakage flow was simulated by blowing air through a small leakage gap in the end wall just upstream of the vane leading edges. Engine relevant turbulence intensities were adjusted by an active turbulence generator mounted in the test section inlet plane. The experiments were performed with tangential and perpendicular leakage ejection and varying leakage mass flow rates up to 2%. Aerodynamic and thermodynamic measurement techniques were employed. Pressure distribution measurements provided information about the end wall and vane surface pressure field and its variation with leakage flow. Additionally streamline pattern (local shear stress directions) on the walls were detected by oil flow visualization. Downstream traverses with 5-hole pyramid type probes allow a survey of the secondary flow behavior in the cascade exit plane. The flow field in the near end wall area downstream of the leakage gap and around the vane leading edges was investigated using a 2D Particle Image Velocimetry (PIV) system. In order to determine end wall heat transfer distributions, the wall temperatures were measured by an infra-red camera system, while heat fluxes at the surfaces were generated with electric operating heating foils. It turned out from the experiments that distinct changes in the secondary flow behavior and end wall heat transfer mainly occur when the leakage mass flow rate is increased from 1% to 2%. Leakage ejection perpendicular to the main flow direction amplifies the secondary flow, in particular the horse-shoe vortex, whereas tangential leakage ejection causes a significant reduction of this vortex system. For high leakage mass flow rates the boundary layer flow at the end wall is strongly affected and seems to be highly turbulent, resulting in entirely different heat transfer distributions