Improved Trench Film Cooling With Shaped Trench Outlets

The influence of the shape of the downstream edge of trench film cooling hole outlets on film cooling effectiveness was investigated using CFD for flat plate film cooling. A 90 deg trench outlet wall with impinging 30 deg film cooling jets results in improved transverse film cooling effectiveness but produces a vertical slot jet into the cross flow, which is not the best aerodynamics for optimum film cooling. It was considered that improvements in the cooling effectiveness would occur if the trailing edge of the trench outlet produced a flow that was inclined in the direction of the crossflow. Beveled and filleted trench outlet shapes were investigated. The CFD predictions were shown to predict well the conventional sharp edged trench outlet experimental results for a flat plate geometry. The flat plate CFD predictions were also shown to predict the experimental results for trench cooling on the suction side of a turbine vane, where the local curvature was small relative to the trench width. The beveled and filleted trench outlets were predicted to suppress the vertical jet momentum and give a Coanda effect that allowed the cooling air to attach to the downstream wall surface. This produced an improved transverse spread of the coolant. Also, it was predicted that reducing the coolant mass flow per hole and increasing the number of rows of holes gave, for the same total coolant mass flow and the same surface area, a superior surface averaged cooling effectiveness.