Effect of particle deposition on film cooling from fan-shaped holes

Abstract

Film cooling from fan-shaped holes is an essential cooling technology in modern gas turbines. The cooling performance degradation and life reduction of high-temperature components caused by calcium-magnesium-alumino-silicates (CMAS) blockage in film cooling holes have received little attention in the public literature. The emphasis of this study lies in using the large eddy simulation (LES) method to reveal the effects of blockage on the flow, mixing, and heat transfer physics of fan-shaped hole film cooling. Blockages were predicted by the improved composite deposition model. The blowing ratio of 0≤M≤3 and the blockage level of 0≤H/D≤0.75 represent the typical practice in gas turbines. The conjugate heat transfer analysis shows that the particle deposition was closely related to the blowing ratio and the film cooling effectiveness, and the fan-shaped holes were blocked for more than 0.5D after 500 hours of service under the studied conditions. LES results indicate that blockages significantly change the organization of turbulent coherent structures and the mixing between coolant and hot gas. Spiral and lateral reorient structures and pressure gradients play important roles in coolant streamwise extension, lateral coverage, and vortex evolution. At the low blowing ratio, small blockages induced the differentiation of “hairpin vortex forest” and the formation of anti-counter-rotating vortex pairs (Anti-CRVP), and the downwash effect of coolant improved the film cooling effectiveness. At the high blowing ratio, the blockage promoted the fusion of hairpin vortices to the midspan, resulting in the dominance of large-scale CRVP and the attenuation or even complete dissipation of Anti-CRVP. The strong hot gas entrainment effect of CRVP leads to the intensification of vertical mixing and the weakening of lateral dispersion of coolant, and the dramatic decrease of film cooling effect. It was found that the maximum degradation rate of the area-averaged effectiveness caused by blockages can reach more than 90%. The discharge coefficient and aerodynamic loss increase significantly under large blockage and blowing ratios. Whether blocked or not, operating near the optimal effective momentum flux ratio (0.2 ∼ 0.32) is helpful to achieve efficient cooling.