Investigation on the effects of rib orientation angle on the film cooling with ribbed cross-flow coolant channel

Abstract

To evaluate the application of ribs in gas turbine blade cooling better, particularly the influence on internal cooling of the film hole effusion and how the internal flows effect the film cooling, this paper presents the effects of rib orientation angle on the film cooling performance. Experimental and numerical studies are conducted on the film cooling with different coolant channel structures. Three coolant channel configurations, including the plenum case and two ribbed cross-flow channels (135° and 45° angled ribs) under three blowing ratios (0.5, 1.0 and 2.0), are considered. The Reynolds numbers of mainstream and coolant channel are fixed as 104 and 105, respectively. Film cooling performances are measured under three blowing ratios by transient liquid crystal measurement technique. Reynolds-averaged Navier-Stokes (RANS) simulation with realizable k-ε turbulence model and enhanced wall treatment are performed using a commercial code Fluent. Under lower blowing ratios, the film jets have better coverage and higher cooling effectiveness. The cases with different ribbed cross-flow channels provide different cooling effectiveness and significantly outperform the plenum case. Based on the results of the plenum case, the superiority regarding area averaged film cooling effectiveness of 135° rib case and 45° rib case are 50% and 16%, respectively. Under the blowing ratio is 1.0, the cases with ribbed cross-flow coolant channels exhibit 170% higher film cooling effectiveness relative to the plenum cases. The film lifts off from the wall surface which results in low film cooling effectiveness in the region near the film hole under higher blowing ratio. The 135° rib case also performs higher cooling effectiveness. Further, heat transfer coefficient of 135° rib case is inferior to the plenum case, showing an inferiority of 0.7–1.6% under lower blowing ratio. However, the superiority regarding area averaged heat transfer coefficient of 45° rib case is 5.0–12.4% with the blowing ratio increasing from 0.5 to 2.0. The discharge coefficient of the 45° rib case is lowest among the three cases. The helical motion of secondary flow is observed in the hole of 45° rib case. The jet divides into two parts after being blown out of the hole due to this motion, which induces strong velocity separation and loss. In the 135° rib case, the vortex in the upper half region of the secondary flow channel rotates in the same direction with the hole inclination direction which leads to the straight streamlines and thus results in lower loss and higher discharge coefficient.