Film Cooling Over a Concave Surface Through a Row of Expanded Holes

This paper presents heat transfer coefficient and film cooling effectiveness measurements over a constant curvature concave surface. The coolant flow is ejected into the mainstream through a row of either simple holes or forward-expanded holes maintained at a streamwise injection angle (γ) of 35°. A transient liquid crystal thermography was employed to measure both the local heat transfer coefficient and film cooling effectiveness over the film-cooled concave test piece. With a pitch-to-diameter ratio (P/d) of 3, each forward-expanded injection hole has an expanded angle of 8° at the exit plane. In current study, the effect of blowing ratio (M) on film cooling performance was also investigated by varying the blowing ratio range from 0.5 to 1.5. Measurements were performed at mainstream Reynolds number (Re_d) of 2000 with turbulence intensity (Tu) of 2%, and coolant-to-mainstream density ratio (ρ_c/ρ_m) of 1.05. The curvature strength (2r/d) of test piece is 86.5. Comparisons were made with baseline cases of concave surface test piece with simple hole configuration done in prior studies. For forward-expanded hole configuration, measured results showed that both the laterally averaged heat transfer coefficient and film cooling effectiveness increase with increasing blowing ratio downstream of X/d = 10. A better film protection effect can be observed at M = 0.5 since coolant flows ejected at this blowing ratio might stay closer to the concave surface than other blowing ratios in present tested range for both hole configurations. As far as the hole shape is concerned, the forward-expanded hole injection provides better surface protection than the simple hole injection.