EXPERIMENTAL STUDY OF TIME-RESOLVED FILM COOLING EFFECTIVENESS OF COMPOUND-ANGLED FAN-SHAPED FILM-HOLE AT CURVED WALL

Abstract

Besides time-averaged cooling performances concerned in previous literature, unsteady film cooling jets require cooling unsteadiness level should be an additional parameter to evaluate the benefits of different film cooling geometries. In the present work, a time-resolved infrared thermal image system was applied to acquire the time-averaged film effectiveness fields and two-dimensional contours of standard deviation (SD) of effectiveness, in terms of typical fan-shaped film-holes in limited by the actual blade geometry. The level of SD directly reflects the strength of cooling unsteadiness, determined by the temporal variations of footprints of dominated secondary-vortices at walls. Deep discussions of the combined influences of compound angle (CA), wall curvature and cooling air flowrates on the cooling unsteadiness level and trends in time-averaged film effectiveness were conducted, to acquire the proper cooling scheme in different local regions of blade. Three typical CAs of 0°, 30° and 60° were chosen. Convex, concave and flat walls with typical dimensionless curvatures were designed to simulate three different local regions of blade. Blowing ratio (BR) of coolant-to-mainstream was varied from 0.5 to 3.0. The detailed comparisons of experimental results revealed the complex trends in film effectiveness and cooling unsteadiness level with CA, BR and wall curvature, which have been listed in Conclusions. In general, the shaped-hole with CA= 30° is significantly proper to reduce the local hot-spots at the convex and nearly-flat walls, due to the highest film effectiveness and lowest cooling unsteadiness level. However, the angled shaped-holes at the concave wall can produce the high-level unsteadiness, increasing the risk of thermal damage and bringing to a new challenge of modifications of hole-geometry to reduce the cooling unsteadiness level while keep the high effectiveness.