AERODYNAMIC PERFORMANCE IN A TURBINE CASCADE WITH LARGE TIP CLEARANCE AND CASING-SIDE INJECTION

This study investigates how the casing-side coolant injection alters the flow field and loss in linear turbine cascade with tip clearance size similar to the incoming boundary layer thickness. The tested tip clearance is of the incoming full boundary layer thickness (at x/C-x = -0.5), or 1.35% of span. Without injection, tip clearance vortex is the only dominant downstream flow feature that contributes to loss. Other secondary flow features, such as passage vortex and trailing shed vortex, are hardly observable. When the coolant is injected with a mass flowrate of 1.5% of the main flow and a swirl equivalent to 70% of the rotor tip tangential speed, the boundary layer downstream of the coolant injection slot thickens, and the tip clearance size relative to the boundary layer thickness decreases to 45%, generating a horseshoe vortex. In addition, incidence near the casing is decreased. The horseshoe vortex develops into the passage vortex, and multiple slot exit vortices combining with the passage vortex strengthens the passage vortex. Furthermore, coolant injection reduces the tip loading, and the axial location of the maximum tip loading shifts downstream by 0.05C(x). Hence, the loss associated with the tip clearance vortex is slightly reduced. However, due to the significant increases in the losses associated with the passage vortex and trailing shed vortex, the total mass-averaged loss of the upper half span increases by 18.5%, and the mixed-out loss by 8.2%.