Film cooling jet lift-off under a streamwise pressure gradient at different blowing ratios

Abstract

This study introduces the concept of a critical blowing ratio, defining the point at which a film cooling jet detaches from an endwall under a streamwise pressure gradient. Four pressure gradients, extracted over a turbine blade were tested: favourable (FPG), mild favourable (MFPG), zero (ZPG), and mild adverse (MAPG) with acceleration parameters of K = 2.61×10-6, K = 1.53×10-6, K = 0.00×10-6, and K = (-)0.31×10-6. Engine representative conditions were employed with a ReD~14000, hole diameter of 20.0 mm at an injection angle of 30°. The blowing ratio (M) ranged from 0.5 to 2.0 in increments of 0.1. Velocity measurements and visualisation techniques extracted using Particle Image Velocimetry (PIV), Planar Laser Visualisation (PLV), and end-wall oil-dye mixture techniques showed that the coolant jet, at a particular blowing ratio, was either attached, semi-detached or fully detached regardless of the pressure gradient imposed. The critical blowing ratio, in a FPG, was ~1.8 times greater than in a MAPG, indicating better attachment of the coolant jets in FPGs.