Investigation of Wake Transport in a Turbine Blade Channel and Its Effect on the Boundary Layer Development

Abstract

The paper presents the results of experimental investigations of the wake migration within the blade passage of a steam turbine blade cascade. The upstream wakes were generated by the wheel with cylindrical bars rotating at the plane perpendicular to the flow direction. The bars diameter was chosen to match the loss of a representative turbine blade as well as the flow pattern at the inlet to the blade cascade. The measurements were performed with the use of hot-wire technique and double X-wire probe. The application of phase averaging allowed one to reproduce the wake convection. The wake movement was visualized by the perturbation velocity vectors and fluctuating components. Dense spatial resolution of measuring points allows for calculation and analysis of selected terms of turbulent kinetic energy transport equation. The results confirm experimental and numerical observations done already for high-loaded blade profiles, which reveal that as the wake passes through high spatial velocity gradient area the turbulent production appears. The turbulent production causes the increase of turbulent kinetic energy (TKE). The analysis confirms also that the convection is mainly responsible for the wake deformation and that the distortion of shape and wake width change especially at the edges of channel is caused by “jet effect”. It also was proved that the role and share of turbulent diffusion is of minor importance and only a slight increase of diffusion is observed in the rear part of the blade channel close to the suction side, where TKE production appears. It was shown also that transition starts not when the wake touches the boundary layer edge, but earlier under the high energy core of the impacting wake. The earlier start of the transition could be due to pressure coupling caused by high energetic small scale structures.