Effects of Endwall Boundary Layer Thickness and Blade Tip Geometry on Flow Through High Pressure Turbine Passages

Experiments were conducted in a linear high pressure turbine cascade with an adjustable tip gap at one endwall. The cascade included a wake generator with moving rods that simulated the effect of an upstream vane row. Cases were documented with no tip gap, a gap of 1.5% of axial chord, and a gap of 3.8% of chord. Cases with flat blade tips were considered with thick and thin endwall boundary layers. Cases with flat tips and squealer tips were documented with the thin endwall boundary layer. For all cases data were acquired both with and without upstream wakes. Documentation included total pressure loss fields in the endwall region and corresponding velocity fields acquired using particle image velocimetry (PIV). The PIV measurements showed the various vortices in the flow field and their response to unsteady wakes. The strength and position of the vortices were directly related to regions of high total pressure loss. Reducing the endwall boundary layer thickness tended to reduce losses, but also resulted in increased leakage flow, which increased losses, particularly in cases with a large tip gap. The squealer tip reduced losses compared to the flat tip cases.