This research documents passive and active flow control techniques for reducing losses associated with the tip leakage vortex of a low pressure turbine blade cascade. Experiments were conducted in a linear cascade of Pratt and Whitney Pack-B turbine blades for an inlet Mach number of 0.2 and an exit Mach number of 0.3. The flow was documented using blade-tip and end wall static pressure measurements and downstream total pressure loss coefficients. Additionally, surface flow visualization was performed on the end wall and blade tip for a greater understanding of the flow behavior.
Blade mounted passive flow control investigated two gap-to-chord ratios for which four tip thickness-to-gap ratios were simulated using pressure-side winglets. The performance of a partial suction-side squealer tip was determined to depend on both gap-to-chord and thickness-to-gap ratios. Active flow control utilized a SDBD plasma actuator situated on the blade tip, with plasma initiated at an unsteady frequency to amplify unsteady characteristics inherent in the flow. This resulted in modest reductions in the downstream total pressure loss associated with the tip leakage vortex.
Vortex generators on the end wall were designed to produce vorticity of opposite sign of the tip leakage vortex. Two vortex generator heights and two placements were investigated at two gap-to-chord ratios. For both gap-to-chord ratios, a vortex generator roughly the height of the gap placed upstream of the trailing edge produced the best results. Active plasma vortex generators in similar locations were also investigated, and a plasma actuator across from the trailing edge was found to be roughly one third as effective as the passive vortex generators at reducing the losses associated with the tip leakage vortex.
Wall roughness was also investigated as a means of diffusing the tip leakage vortex. 3-dimensional roughness almost completely eliminated the tip leakage vortex as observed downstream while modestly increasing the blade tip loading. 2-dimensional roughness decreased total pressure losses associated with the tip leakage vortex downstream by 40% with little change to the blade loading. Simulated roughness using plasma actuators had essentially no effect on the tip gap flow.