INFLUENCE OF UNSTEADY WAKE ON HEAT-TRANSFER COEFFICIENT FROM A GAS-TURBINE BLADE

The effect of unsteady wake on surface heat transfer coefficients of a gas turbine blade was experimentally determined using a spoked wheel type wake generator. The experiments were performed with a five-airfoil linear cascade in a low-speed wind tunnel facility. The cascade inlet Reynolds number based on the blade chord was varied from 1 to 3 x 10(5). The wake Strouhal number was varied between 0 and 1. 6 by changing the rotating wake passing frequency (rod speed and rod number), rod diameter, and cascade inlet velocity. A hot-wire anemometer system was located at the cascade inlet to detect the instantaneous velocity, phase-averaged mean velocity, and turbulence intensity induced by the passing wake. A thin foil thermocouple instrumented blade was used to determine the surface heat transfer coefficients. The results show that the unsteady passing wake promotes earlier and broader boundary layer transition and causes much higher heat transfer coefficients on the suction surface, whereas the passing wake also significantly enhances heat transfer coefficients on the pressure surface. The blade heat transfer coefficients for a given Reynolds number flow increase with the wake Strouhal number by increasing the rod speed, rod number, or rod diameter. For a given wake passing frequency and rod diameter, the blade heat transfer coefficients decrease with decreasing Reynolds number, although the corresponding wake Strouhal number is increased. The results suggest that both the Reynolds and Strouhal numbers are important parameters in determining the blade heat transfer coefficients in unsteady wake flow conditions.