Evaluation of Film Cooling Superposition Predictions Using Shaped Holes on the Suction Side of a Blade Model

Abstract

Abstract Film cooling is often used for turbine airfoil cooling, and there are numerous studies of the performance of a single row of holes. In actual application there will typically be multiple rows of holes which interact. Consequently there is a need to develop techniques to predict film cooling performance with multiple rows of coolant holes using superposition of single row cooling effectiveness. Although there have been many studies of superposition techniques for predicting film cooling effectiveness with multiple rows of cylindrical holes, there have been very few in which shaped holes were used with a typical turbine airfoil model. In this study, film effectiveness was measured on the suction side of a turbine blade model using two rows of shaped coolant holes. Measurements were made with each row independently and with both rows combined. This provided the experimental data for superposition predictions and to evaluate these predictions. Each row had 7-7-7 shaped holes with pitch to diameter ratio of 6, and the two rows were more than 40 diameters apart. The experiments were run using two different upstream blowing ratios, and a wide range of downstream blowing ratios. The superposition predictions of film effectiveness were reasonably accurate when the upstream row of holes were operated at a high blowing ratio with a corresponding smaller film effectiveness (due to jet separation). However, when the upstream coolant holes were operated at the optimum blowing ratio, and hence maximum film effectiveness downstream, the superposition analysis predicted film effectiveness levels slightly lower than actual levels. These results show that there was an interaction between jets that resulted in higher film effectiveness than was accounted for with a superposition prediction.