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.
In this paper, results of CFD analysis of boundary layer flow control in a few representative turbine exhaust diffuser geometries are discussed. The simplest of the above configurations was subjected to rig tests with a few of the parameters simulated to produce realistic turbine exhaust diffuser flow. These rig tests provided matching and validation of the CFD results obtained for the same configuration. The diffuser designs were attempted to create strong diffusion field ahead of the struts, in a controlled manner. On the other hand in some of the configurations, an externally induced casing injection system was introduced to control the diffusing flow field behind the struts. In a separate design attempt, the struts and the rear hub were merged in to a smooth blended body in an attempt to reduce the overall diffuser losses and to maximise diffuser pressure recovery. Both these attempts showed mixed results. 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. Significant performance benefits were observed using boundary layer control through casing injection as well as through endwall design modifications. In the best performance configuration, the static pressure at the diffuser exit improved by about 16 % and the total pressure losses reduced by about 30% as compared to the baseline configuration at the same axial location.
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