Film cooling effectiveness measurements of a near surface streamwise diffusion hole

This paper presents an experimental investigation on a new configuration of film cooling hole, namely the near surface streamwise diffusion (NSSD) hole. A discrete streamwise diffused shallow submerged structure is constructed on the exit of a simple cylindrical hole. There is a middle bulge at the exit edge of the shallow submerged structure to expand the coolant laterally. The adiabatic film cooling effectiveness was measured using pressure sensitive paint (PSP) technique on a flat plate model. The experiment was performed under density ratio DR = 1.38 and the blowing ratio M = 0.5-2.5. A simple cylindrical hole and a typical fan-shaped hole were chosen to be the baseline holes to be compared with NSSD hole. Six NSSD hole cases were tested and compared to investigate the influence of geometrical parameters. The experimental results showed that the cooling effectiveness of NSSD hole was significantly higher than the simple cylindrical hole but was slightly lower than the fan-shaped hole. The streamwise diffused shallow submerged structure can effectively expand the coolant laterally and can keep film attached to the surface under high blowing ratio. Due to the special flow mechanism, the laterally averaged cooling effectiveness of NSSD hole exhibited a ladder-style decreasing trend where the cooling effectiveness far downstream is close to the fan-shaped hole. The change of geometrical parameter of the near surface streamwise diffusion structure results in the obvious variation of cooling effectiveness especially at the high blowing ratio conditions. A smaller streamwise spacing between the middle bulge and the exit edge of the cylindrical hole can enhance the interaction between the coolant and the middle bulge, which is conducive to the increase of cooling effectiveness.