LES-Based Analysis of Unsteadiness and Instability of Tip-Leakage Vortex in Turbine Rotor

The flow leaking through the gap between the blade tips and the casing surface of a turbine rotor is an important sourceof aerodynamic loss. This paper presents a numerical study of the unsteadiness and instability of tip-leakage flow withinthe first stage rotor of the GE-E3 high-pressure turbine (HPT). Large-eddy simulation (LES) results show that the leakageflow entering the turbine passage rolls up and forms a winding surface vortex structure, in which the low-velocity vortexcore is wrapped by the shear layer. The vortex core may break up and the flow speed in the vortex core drops to zero. Thearea of the vortex core expands rapidly and forms a reverse flow owing to the centrifugal instability. Further analysissuggests that the unsteadiness in the tip-leakage vortex is mainly caused by the Kelvin-Helmholtz (K-H) instability of thesurface layer. Due to the K-H instability, K-H waving structure is formed on the surface of leakage vortex. Owing to therolling up of the K-H wave, spanwise vorticity is generated, and a series of vortex joint structures is formed, which increasesthe dissipation within the leakage vortex. Moreover, the self-induced helical instability and oscillation of the vortex corebreakdown are suggested as contributing factor for the additional low-frequency unsteadiness within the tip-leakage vortex.