Enhancement of film cooling effectiveness using backward injection holes

Abstract Film cooling is a cooling method used to protect the hot components of a gas turbine from high temperature conditions. To protect the vanes/blades from excessive thermal stress, backward injection film cooling holes are proposed as one of the methods for the improvement and uniformity of film cooling effectiveness. This study enhanced the film cooling effectiveness on the entire surface using forward and backward injection cylindrical holes with two expected effects: control of vortex interaction between forward and backward injection jets, and the entire surface cooling from near hole exit to far downstream region. The experiments using pressure sensitive paint (PSP) method were conducted to measure the film cooling effectiveness. Four experimental configurations were composed of forward and backward injection cylindrical holes: forward and forward injection holes, backward and backward injection holes, forward and backward injection holes, and backward and forward injection holes. The cylindrical holes were aligned in two staggered rows with pitch ( p ) of 6 d and row spacing ( s ) of 3 d . The injection angles ( α ) of the cylindrical holes were 35° and 145° for forward and backward injection, respectively. The blowing ratios ( M ) ranged from 0.5 to 2.0 and the density ratio ( DR ) was about 1. The results indicated that, at a high blowing ratio, configurations with backward injection holes demonstrated higher and more uniform film cooling effectiveness than with only forward injection holes. Especially, a configuration, composed of forward injection holes in the first row and backward injection holes in the second row, obtained the improved film cooling effectiveness and maintained the high film cooling effectiveness from near hole exit to far downstream region due to weakened vortex strengths of forward and backward injection jets.