Combination Effects of Upstream-Ramp and Swirling Coolant Flow on Film Cooling Characteristics

This paper presents an experimental investigation on the performances of a new film cooling structure design, in which a ramp is placed upstream of cylindrical film hole and a cylindrical cavity with two diagonal impingement holes is set at the inlet of the film hole to generate a swirling coolant flow entering the film hole. The experiments are carried out by two undisturbed measurement techniques, Planar Laser Induced Fluorescence (PLIF) and Time-Resolved Particle Image Velocimetry (TRPIV) in a water tunnel. The effects of the upstream-ramp angle, blowing ratio (BR) and coolant impingement angle on the film cooling performances of a flat plate are studied at three ramp angles (0°, 15° and 25°), two coolant swirling directions (clockwise and counter-clockwise), two impingement angles (15° and 30°), and three BRs (0.6, 1.0, and 1.4). The experimental results show that at high BRs, the combination structures of the upstream-ramp with the swirling coolant flow generated by the impingement angles can significantly improve film cooling performances; the best combination is at 30° impingement angle and 25° ramp angle. The reason can be explained by the fact that the swirling flow is significantly pressed onto wall through the upstream-ramp. Using the analogous analysis of heat and mass transfer, the adiabatic film effectiveness averaged over a cross section is obtained, and the analysis indicates that at high BRs, the combined effect of the a ramp with a large angle of 25° with 30° impingement angle can increase the film effectiveness up to 30% in comparison with the case without ramp at the exit of the film hole. The images captured by PLIF exhibit an interesting phenomenon, i.e. the swirling coolant in different directions can influence the counter vortex pair (CVP) in rotating layer, and the coolant swirling direction in clockwise enhances the right mixing of the CVP with coolant ejection, whereas the coolant swirling direction in counter-clockwise enhances the left mixing of the CVP with coolant ejection.