Effects of Unsteady Wakes on Flow Through High Pressure Turbine Passages With and Without Tip Gaps

Abstract

Experiments were conducted in a linear high pressure turbine cascade with wakes generated by moving upstream rods. The cascade included an adjustable top endwall that could be raised and lowered above the airfoils to change the tip gap. Conditions were considered with no tip gap, and gaps of 1.5% and 3.8% of axial chord. For each of these, cases were documented both with and without upstream wakes. The pressure distributions on the airfoils were acquired at the midspan and near the tip for each case. The total pressure loss was measured in the endwall region. Velocity fields were acquired in two planes normal to the flow direction using particle image velocimetry (PIV). For the case with no tip gap, the passage vortex and other vortices were clearly visible in the velocity fields. For the cases with a tip gap, the tip leakage vortex was the dominant flow feature, and it became stronger as the gap size increased. The other vortices were still present, but were moved by the tip leakage vortex. For the cases with unsteady wakes, the PIV data were ensemble-averaged based on phase within the wake passing cycle, to show the motion and change in strength of the vortices in response to the wake passing. The regions of high total pressure loss can be explained in terms of the secondary velocity field.