Film-Cooling Characteristics of Pressure-Side Discharge Slots in an Accelerating Mainstream Flow

Pressure-side discharge is commonly employed in turbine blades and nozzle guide vanes to keep the trailing edge metal temperatures within an allowable limit while minimizing aerodynamic penalties. Despite its widespread use, film-cooling data of the discharge slot are scarce in open literature. The objectives of the present experimental study were to measure detailed local heat transfer and film-cooling effectiveness from a 10x scale trailing-edge model of an industrial gas turbine airfoil in a low speed wind tunnel. To simulate the mainstream flow acceleration in vane and blade row passages, a linear velocity gradient was imposed using an adjustable top wall. The present work employed the composite slab quasi-steady liquid crystal method that allows measurements of local heat transfer coefficients and film-cooling effectiveness from two related tests. With this technique, the heat transfer measurement can be performed in a cold wind tunnel. The coolant-to-mainstream blowing ratio was varied between 0.25 and 1.0. The slot hydraulic diameter based Reynolds number ranged from 4,760 to 19,550. The coolant-to-mainstream density ratio was fixed at 0.95. Slot discharge coefficients were also measured with mainstream acceleration. Both local heat transfer coefficients and film-cooling effectiveness displayed a strong dependency on blowing ratio and mainstream acceleration. However, the discharge coefficients showed little dependency on the mainstream acceleration.