Influence of the hole length-to-diameter ratio on film cooling with cylindrical holes

Film cooling experiments were conducted to investigate the effects of coola nt hole length-to-diameter ratio on the film cooling effectiveness. The res ults from these experiments offer an explanation for the differences betwee n the film cooling results for cylindrical hole injection configurations pr eviously reported by Goldstein el al. (1974), Pedersen et al. (1977), and S inha er al. (1991). The previously reported injection configurations differ ed primarily in coolant hole length-to-diameter ratio. The present experime nts were conducted with a row of cylindrical holes oriented at 35 deg to a constant-velocity external flow systematically varying the hole length-to-d iameter ratios (L/D = 1.75, 3.5, 5, 7, and 18), and blowing rates (0.52 les s than or equal to M less than or equal to 1.56). Results from these experi ments show in a region 5 less than or equal to X/D less than or equal to 50 downstream of coolant injection that the coolant flow guiding capability i n the cylindrical hale was apparently established after five hole diameters and no significant changes in the film cooling effectiveness distribution could be observed for the greater L/D. However. the film cooling effectiven ess characteristics generally decreased with decreasing hole L/D ratio in t he range of 1.75 less than or equal to L/D less than or equal to 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 dire ction and surface. The lowest values of film cooling effectiveness were mea sured for the smallest hole length-to-diameter ratio, L/D = 1.75.