Aero-thermal performance improvements of unshrouded turbines through management of tip leakage and injection flows

The tip leakage flow not only is responsible for a significant amount of aerodynamic losses in a turbine stage, but also leads to high heat-loads on the tip region. The paper presents a numerical investigation of influences of tip injection on aero-thermal performance of the tip leakage flow for both flat tip and cavity tip in an unshrouded turbine rotor, in an attempt to improve the turbine blade tip aero-thermal performance by the management of tip leakage and injection flows. The effects studied include the sensitivities to the geometrical clearance height and the injection mass flow rate. The results show that, at all tip clearances, tip injection has a good effect on the control of the leakage flow, and it significantly reduces the sensitivities of turbine performances to the effects of the tip clearance height. With tip injection, cavity tip geometry does not play a major role in the turbine performance improvement, but it obtains the good film-cooling performance on the blade tip. Tip clearance height, blade tip geometry and injection mass flow rate have related effects on the blade tip aero-thermal performance, and the turbine blade tip obtains the best aero-thermal performance with an optimum injection mass flow rate.