Influence of mainstream turbulence and unsteady wake on turbine blade heat transfer

Abstract

The influence of mainstream turbulence and unsteady wake on turbine blade surface heat transfer was studied. The experiments were performed with a five-blade linear cascade in a low speed wind tunnel facility. The wake and grid generated turbulence was varied by altering the combination of turbulence grids(two grid), grid locations(three locations) and wake passing Strouhal numbers(from 0.05 to 1.6). The mainstream Reynolds number based on the cascade inlet mean velocity and blade chord length were 100,000, 200,000, and 300,000. A hot wire anemometer system was located at the cascade inlet, outlet, and a few places in the middle of the flow passage to get turbulence intensities, phase averaged velocities, and time dependent velocities. A thin foil and thermocouple instrumented blade was used to determine the surface heat transfer coefficient distributions. The results show that the turbulence generated by wake only, by grid only, or by both wake and grid has similar effect on turbine blade heat transfer. The blade heat transfer depends on the Strouhal number for wake-generated unsteady flow, on turbulence intensity for grid-generated turbulent flow, and on the mean value of the phase-averaged turbulence intensity for the combined wake and grid-generated unsteady flow. These high turbulence and/or unsteadiness promote earlier boundary layer transition and cause much higher heat transfer coefficient on the suction surface, whereas they also significantly enhance heat transfer coefficient on pressure surface. For the combined effect of oncoming turbulence and unsteady wake on the blade surface heat transfer, the wake Strouhal number has strong effect on blade heat transfer for lower given oncoming mainstream turbulence, and the mainstream turbulence has a strong effect on blade heat transfer for lower given wake Strouhal number. A semi-empirical method was introduced to predict the turbine blade heat transfer.