Reduction of turbine tip clearance losses at design and off-design incidences by non-uniform tip injection

Abstract

Tip injection has shown potential for controlling turbine tip clearance flows; however, the effects of tip injection may behave differently at design and off-design incidences. This paper computationally investigates the sensitivity of endwall flowfields and aerodynamics to flow incidence for flat and cavity tips at different injection locations, in an attempt to explore the potential of further reduction of tip clearance losses at various incidences by non-uniform tip injection concept. First, a linear cascade validation is introduced, and the computations were compared with the experimental data to identify the ability of the computational fluid dynamics (CFD) code to predict the tip leakage flow. Flow patterns over the blade tip without injection (WI) at different incidences were shown, and then the sensitivity of endwall flowfields and aerodynamics to incidence at different injection locations was investigated. Also, the effects of non-uniform tip injection on aerodynamic performance at design and off-design incidences were examined. The results show that the injection in the front part performs better performance than that in the rear part, while it causes turbine performance to be more sensitive to flow incidence than the injection in the rear part. Compared to the flat tip, the coupling effect between cavity tip and tip injection reduces the sensitivity of turbine performance to incidence variation. The non-uniform tip injection, which takes into account the tip leakage flow mechanisms at different incidences, optimizes the mixing process between the tip clearance flow and the passage flow, further improving the design and off-design performance of turbines.