Influence of the Hole Length-to-Diameter Ratio on Film Cooling With Cylindrical Holes

Abstract

Film cooling experiments were conducted to investigate the effects of coolant hole length-to-diameter ratio on the film cooling effectiveness. The results from these experiments offer an explanation for the differences between the film cooling results for cylindrical hole injection configurations previously reported by Goldstein et al. (1974), Pedersen et al. (1977) and Sinha et al. (1991). The previously reported injection configurations differed primarily in coolant hole length-to-diameter ratio. The present experiments were conducted with a row of cylindrical holes oriented at 35 degrees to a constant-velocity external flow, systematically varying the hole length-to-diameter ratios (L/D = 1.75, 3.5, 5, 7 and 18), and blowing rates (0.52≤M≤1.56). Results from these experiments show in a region 5≤X/D≤50 downstream of coolant injection that the coolant flow guiding capability in the cylindrical hole was apparently established after 5 hole diameters and no significant changes in the film cooling effectiveness distribution could be observed for the greater L/D. However, the film cooling effectiveness characteristics generally decreased with decreasing hole L/D ratio in the range of 1.75≤L/D≤5.0. This decrease in film cooling performance was attributed to (1) the undeveloped character of the flow in the coolant channels and (2) the greater effective injection angle of the coolant flow with respect to the external flow direction and surface. The lowest values of film cooling effectiveness were measured for the smallest hole length-to-diameter ratio, L/D = 1.75.