FILM COOLING OF CURVED SURFACES (GAS TURBINE, CONVEX, CONCAVE, JETS)

Film cooling through a row of holes over a convex and a concave surface has been studied. The holes were inclined at 35 degrees to the main flow and were spaced three diameters center-to-center. The effects of injection rate and strength of curvature on performance were studied. A foreign gas injection technique, employing the mass transfer analogy to heat transfer, was used.;Stronger convex curvature was found to enhance lateral average effectiveness at low blowing rates, while at high blowing rates, strength of curvature seemed to diminish in importance. Also at high blowing rates, the centrifugal force of the jets caused them to lift off the surface. Momentum flux ratio was determined to be the parameter which influenced when lift-off occurred. Further increase in blowing rate or momentum flux ratio caused the jets to merge, and blockage of the mainflow from the wall resulted. An increase in effectiveness was, therefore, encountered.;For film cooling of concave surfaces, momentum flux ratio had relatively little influence on the performance of film cooling concave surfaces. This was probably because the normal momentum of the jets worked to degrade performance, while the tangential momentum (centrifugal force) caused an improvement. With the present injection geometry, the effects cancelled each other except just downstream of injection, where high normal momentum promoted lift-off. After twelve diameters from injection, blowing rate, rather than momentum flux ratio, was the dominant parameter.;On the concave wall, lateral average effectiveness behaved very similarly to analytical correlations for slot injection on a flat plate. The magnitude of the performance was consistently lower than the slot-flat plate correlations, but the trends were very much the same. Excessive lateral mixing, promoting blockage of the mainflow from the wall was the reason for this effect.;Large variations in lateral profiles of local effectiveness on the convex surface were found. At all but the highest blowing rates, the local effectiveness was generally high at points in line with an injection hole and low half-way between two holes. Lateral profiles on the concave surface were comparatively flat. This was attributed to concave instabilities which enhanced mixing there.