EFFECTS OF COOLING HOLE SHAPES ON FILM COOLING EFFECTIVENESS

Abstract

The present study is related to the numerical investigation of different hole shapes for film cooling performance, which is widely used for cooling of high temperature gas turbines. The adiabatic film cooling effectiveness values are determined computationally, downstream of 30o inclined cooling holes. Different hole shapes compared are circular holes, two types of triangular holes, and two types of semicircular holes. Semicircular-1 hole, which has straight edge towards windward direction and curved edge towards leeward side, is found to produce best effectiveness results. This type of hole shows much higher values of laterally averaged effectiveness at blowing ratios of 0.67 and 1.0 as compared to a full circular hole. At blowing ratios of 1.33 and 1.67, the effectiveness results from semicircular-1 hole are found similar to the results of full circular hole. The semicircular-2 holes (straight edge towards leeward direction and curved edge towards windward side) show almost similar effectiveness results as the results of a full circular hole. Same effectiveness results from either type of semicircular hole as for a full circular hole, is still highly advantageous because semicircular hole requires half mass flow of coolant as compared to the requirement for a full circular hole, to produce the same blowing ratio. Hence the loss of thermal efficiency, due to the extraction of coolant from the compressor, can be reduced by using semicircular-1 or semicircular-2 hole. Semicircular-2 holes are further investigated by arranging them in two inlined/ staggered rows. Two rows of semicircular holes are equivalent to a single row of full circular holes because in both cases coolant cost or requirement is same. Hence the results of two rows of semicircular-2 holes are compared with a single row of full circular holes. Result of spatially averaged effectiveness from two staggered rows of semicircular-2 holes is 200% or higher than that from a single row of full circular holes at all blowing ratios tested. The lowest coolant jet heights are found for two staggered rows of semicircular holes, also the counter rotating vortex pair for that case is the weakest one. In the study of compound angle orientations of a single row of circular holes, two staggered rows of semicircular-1 holes and two staggered rows of semicircular-2 holes, it is seen that for simple inclination angle (i.e., for β = 0o), the two staggered rows of semicircular-1 holes for blowing ratio of 0.67 show almost 400% higher values of spatially averaged effectiveness as compared to a single row of full circular holes. When compound angles are used, it is seen that semicircular holes (either semicircular-1 or semicircular-2) shows much higher values of spatially averaged effectiveness than single row of circular holes, especially at low orientation angles. Among two types of semicircular holes, the two staggered rows of semicircular-1 holes show higher spatially averaged effectiveness as compared to two staggered rows of semicircular-2 holes but this difference decreases with increasing orientation angle (β). Also, the two staggered rows of semicircular-1 holes without any compound angle show better spatially averaged effectiveness than from the single row of circular holes or two staggered rows of semicircular-2 holes at all orientation angles. Hence using semicircular-1 holes in staggered rows, the need of compound angles may be avoided. Finally it is recommended to use the semicircular-1 holes instead of full circular holes for much better effectiveness or for reduced coolant requirement.