Impact of complex flow structures on the turbine blade tip region mixing

Mixing of the turbine blade tip leakage and mainstream flows causes considerable aerodynamic loss. Its understanding is crucial to raise a consequential improvement of the turbine performance. In the present paper, a typical high pressure turbine rotor flow is simulated by Detached Eddy Simulation. The complex mixing in the blade tip region is assessed by the dilution index algorithm in the streamwise direction. Influences of typical parameters on mixing are consecutively identified and analyzed. Relating the influences to the flow structures, the mixing mechanism is obtained. It is revealed that the normal effective diffusion coefficient is key in mixing and is correlated to the unsteady tip leakage flow stick vortices. The latter induced by the Kelvin-Helmholtz instability can significantly enhance local mass and energy transfer and hence diffusion. As a result, mixing is strengthened. Furthermore, the tip region is knowingly divided into the juxtaposing near and far fields. The tip region is artificially divided into near field and far field. The former contains the leakage concentrated vortex, leakage jet, mainstream flow and two entrainment zones. Meanwhile, the latter contains the leakage jet, mainstream flow and entrainment zone. It was found that the entrainment zone is mixed to a high-degree whereas the leakage jet is barely mixed.