Analysis on the effect of trench geometry on film cooling effectiveness of shaped holes using rans simulation

Abstract

Diffused shaped holes have shown superior cooling performance over the other shapes of holes in many situations. In the present study, numerical simulation using realizable k-ε model with enhanced wall treatments as implemented in Ansys Fluent solver was performed to study the effect of trench geometry made at 7-7-7 shaped hole exit on cooling effectiveness. The predictions were generated at different depth ratios of the trench geometry. Three different blowing ratios of M = 1.5, 3, and 5 were employed with different depth ratios of h/D = 0.25, 0.5, and 1. Altogether nine cases were run to generate predictions and three cases without trench, h/D = 0, at different blowing ratios of M = 1.5, 3, and 5 were run as the baseline. All cases were maintained at density ratio of DR = 1.5 and turbulence intensity or Tu = 0.5%. Based on the error analyses and the comparisons performed to flow field patterns and cooling effectiveness variations during the validation, the realizable k-ε model with enhanced wall treatment was selected among standard k-ω, SST k-ω and realizable kε

for predictions. When compared to the baseline, not only the modified geometry presents better cooling effectiveness according to the laterally averaged effectiveness, but superior lateral spreading of coolant can also be observed at higher depth ratio of trench. Maintaining higher blowing ratios at lower slot depth ratio such as h/D = 0.25 is only a waste of coolant without improvements to cooling effectiveness while higher cooling effectiveness can be obtained by higher blowing ratios at higher trench depth ratios. Based on the laterally averaged effectiveness, the cooling effectiveness is improved by increasing the trench depth at all blowing ratios investigated. Based on the lateral effectiveness variations, the coolant jet has shown a skewness at higher blowing ratios and lower trench depth ratios while the skewness becomes invisible at lower blowing ratios and lower trench depths. A steeper decay can be observed in laterally averaged effectiveness at high blowing ratios (M = 5) and low slot depth ratios (h/D = 0.25) due to the jet penetration into mainstream thereby degrading the cooling performance.