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.