The Effect of High Freestream Turbulence on Film Cooling Effectiveness

Abstract

This study investigated the adiabatic wall cooling effectiveness of a single row of film cooling boles injecting into a turbulent flat plate boundary layer below a turbulent, zero pressure gradient freestream. Levels of freestream turbulence (Tu) up to 17.4% were generated using a method which simulates conditions at a gas turbine combustor exit. Film cooling was injected from a single row of five 35 degree slant-hole injectors (length/diameter = 3.5. pitch/diameter = 3.0) at blowing ratios from 0.55 to 1.85 and at a nearly constant density ratio (coolant density/freestream density) of 0.95. Film cooling effectiveness data is presented for Tu levels ranging from 0.9% to 17% at a constant freestream Reynolds number based on injection hole diameter of 19000. Results show that elevated levels of freestream turbulence reduce film cooling effectiveness by up to 70% in the region directly downstream of the injection hole due to enhanced mixing. At the same time, high freestream turbulence also produces a 50–100% increase in film cooling effectiveness in the region between injection boles. This is due to accelerated spanwise diffusion of the cooling fluid, which also produces an earlier merger of the coolant jets from adjacent holes.