Effect of Turbine Tip Leakage Flows on Exhaust Diffuser Performance

This paper describes numerical simulation of the effect of turbine exhaust flows on typical exhaust diffuser geometries. The study has been carried out on three different diffuser geometries. These diffusers have varying degrees of diffusion in the annular section. The studies were carried out at a Reynolds number of 7.7 × 105 based on the diffuser inlet hydraulic diameter. The performance of the diffusers was assessed in terms of total pressure loss and static pressure coefficient across the diffuser. The turbine exhaust flow was simulated by combining an injection scheme from the casing in to the main flow that changes the uniform diffuser inlet velocity profile to that of a typical turbine exhaust flow profile. It was observed that the presence of a realistic exhaust flow influences the diffuser performance compared to an axial inlet flow. The effect of the real flow seems to be to make it more resistant to adverse pressure gradients. The exit flow of the diffusers, studied earlier, with uniform axial inlet flow, showed massively separated regions at the diffuser delivery. The diffuser performances improved significantly with realistic simulation of turbine exhaust flow. The present study also reinforces the fact that the diffuser performance is highly sensitive to the quality of the inlet flow.