Pressure Surface Separations in Low-Pressure Turbines—Part 1: Midspan Behavior

This paper describes an investigation into the behavior of the pressure surface separation at midspan in a linear cascade. It is found that the pressure surface separation can be a significant contributor to the profile loss of a thin, solid, low-pressure turbine blade that is typical of current engine designs. Numerical predictions are first used to study the inviscid behavior of the blade. These show a strong incidence dependence around the leading edge of the profile. Experiments then show clearly that all characteristics of the pressure surface separation are controlled primarily by the incidence. It is also shown that the effects of wake passing, freestream turbulence and Reynolds number are of secondary importance. A simple two-part model of the pressure surface flow is then proposed. This model suggests that the pressure surface separation is highly dissipative through the action of its strong turbulent shear. As the incidence is reduced, the increasing blockage of the pressure surface separation then raises the velocity in the separated shear layer to levels at which the separation can create significant loss.