An Experimental Study of Film Cooling Effectiveness Near the Leading Edge of a Turbine Blade

Abstract

A flame ionization technique based on the heat/mass transfer analogy has been used in an experimental investigation of film cooling effectiveness. The measurements were made over the surface of a turbine blade model composed of a semi-cylindrical leading edge bonded to a flat after-body. The secondary flow was injected into the boundary layer through four rows of holes located at ±15 and ±44 deg about the stagnation line of the leading edge. These holes, of diameter d, had a 30 deg spanwise inclination and a 4d spanwise spacing. Adjacent rows of holes were staggered by 2d, and perfect geometry symmetry was maintained across the stagnation line. Discharge coefficients and flow division between the 15 and 44 deg rows of holes have also been measured. The strong pressure gradient near the leading edge produces a strongly nonuniform flow division between the first (± 15 deg) and the second (± 44 deg) row of holes at low overall mass flow ratios. This produced a total cutoff of the coolant from the first row of holes at mass flow ratios lower than approximately 0.4, leaving the leading edge unprotected near the stagnation line. Streamwise and spanwise plots of effectiveness show that the best effectiveness values are obtained in a very narrow range of mass flux ratios near 0.4 where there is also considerable sensitivity to changes in Reynolds number. The effectiveness values deteriorate abruptly with decreasing mass flow ratios, and substantially with increasing mass flow ratios. Therefore, it was concluded that the cooling arrangement investigated has poor characteristics, and some suggestions are made for alternate designs.