Control of Tip Leakage Vortex Breakdown by Tip Injection in Unshrouded Turbines

Tip leakage flow is responsible for a number of aerodynamic losses in turbines, but the leakage loss mechanisms have not been fully elucidated yet. This paper computationally investigates the effects of tip injection on tip leakage vortex dynamics in an unshrouded turbine, in an attempt to seek new loss generation mechanisms in turbine tip clearance flows and to influence them by the injection. First, a linear cascade validation is introduced, and the computations are compared with the experimental data to identify the ability of computational fluid dynamics code to predict the tip leakage flow. The tip leakage vortex breakdown phenomenon and dynamics analysis are shown, and then the effects of tip injection on leakage vortex structure and vortex breakdown characteristics are investigated. Also, the effect of leakage vortex breakdown on aerodynamic performance of the turbine with injection is examined. Results indicate that the leakage vortex breakdown occurs when the pressure rise is large enough in the rear part of the blade suction side near the tip, and it is an important loss mechanism associated with tip leakage. Injection location has a different impact on the leakage vortex dynamics, dependent on the tip pressure field characteristics. With injection in the front part, the leakage vortex breakdown is delayed, where the turbine achieves the best performance improvement, and the vortex breakdown location moves downstream as the injection mass flow rate increases.