Effects of Showerhead Injection on Film Cooling Effectiveness for a Downstream Row of Holes

There have been numerous studies of film cooling performance for the downstream coolant holes on a turbine airfoil using test geometries ranging from flat plates to airfoils. Most of these studies simulate a relatively unperturbed boundary layer flow approaching the coolant holes. This stimulated the current inquiry into the effects of realistic upstream conditions for downstream coolant holes. To investigate this, a series of experiments were performed focussing on the first downstream row of holes on the pressure side of a typical turbine vane. The film cooling effectiveness for this pressure side row of holes was determined subject to no showerhead blowing, and to showerhead blowing with varying blowing rates. Furthermore, tests were conducted with low and high freestream turbulence levels. For this investigation, a leading edge showerhead array of six film cooling rows was utilized, with coolant from three of these rows being directed towards the pressure side of the vane. For all experiments a coolant to freestream density ratio of nominally DR = 1.8 was used. Adiabatic effectiveness was determined from surface temperature measurements for a nominally adiabatic surface using an infrared camera for spatially resolved mapping of the surface temperature. This study showed that showerhead injection had a dominant influence on the adiabatic effectiveness performance of downstream cooling. Showerhead injection appeared to cause a significant increase in coolant jet dispersion, presumably by increased levels of turbulence. Even when the freestream turbulence level at the pressure side coolant holes was increased to 17%, showerhead injection caused a significant degradation in the film cooling performance of the pressure side row of holes. Because of the increased dispersion caused by the showerhead injection for the pressure side coolant jets, the superposition model failed to correctly predict adiabatic effectiveness levels for combined showerhead and pressure side coolant injection.