Experimental and numerical studies on film cooling with reverse/backward coolant injection

The conventional forward injection for film cooling with cylindrical holes, where the axial component of the coolant velocity is aligned with mainstream flow direction creates kidney vortices. This results in quick mixing of the coolant with the mainstream. The conventional anti-kidney vortices cooling holes require shaping or branching which adds to the cost and complexity of the system. In this paper, reverse/backward injection is proposed to improve film cooling. In the case of reverse/backward injection the secondary air is injected such that its axial velocity component is in the reverse direction to that of the mainstream. Film cooling is studied experimentally and numerically on a flat plate with forward and reverse injection. The injection angle of the cooling hole is varied from 30° to 60° in both forward and reverse directions at five blowing ratios ranging from 0.25 to 3.0 at a fixed density ratio of 0.91. The length to diameter ratio of the cooling hole is kept at 5 and the mainstream Reynolds number is maintained at 3.75 × 105. Film cooling effectiveness obtained with the reverse holes is found to be much higher than that of the forward holes. Improvement in the area weighted average values of film cooling effectiveness for blowing ratio, M = 1 is 170%, 78% and 186% for injection angles 30°, 45° and 60° respectively. Coefficient of discharge obtained from reverse injection is found to be smaller than that of forward injection. The film cooling effectiveness in the case of reverse injection is found to be less sensitive to the injection angle.