Endwall Losses and Flow Unsteadiness in a Turbine Blade Cascade

Abstract

The purpose of this paper is to describe an investigation of the flow within and downstream of a turbine blade cascade of high aspect ratio. A detailed experimental investigation into the changes in the endwall boundary layer in the cascade (100 deg camber angle) and total pressure loss downstream of the cascade was carried out. Flow visualization was used in order to obtain detailed photographs of the flow patterns on the endwall and for exhibiting the trailing edge vortices. Pressure measurements were carried out using a miniature cranked Kiel probe for three planes downstream of the cascade, with two levels of turbulence intensity of the free stream. Pressure distributions on the blade were measured at three spanwise locations, namely 4, 12, and 50 percent of the full span from the wall. Hot-wire anenometry combined with a spectrum analyzer program was used to determine the frequencies of the flow oscillations. The change in turbulence level of the free stream has a significant influence on all three pressure distributions. The striking difference between two of the pressure distributions is in the aft half of the suction side where the distribution with the lower turbulence intensity has the larger lift. The oil flow visualization reveals what appear to be two separation lines within the passage and are believed to originate from the horseshoe vortex. The pitchwise-averaged total pressure loss coefficient increases with the distance of the measurement plane downstream of the cascade blades. A substantial part of this loss increase close to the wall is caused by the high rate of shear of the new boundary layer on the endwall.