Boundary Layer Control in Turbine Exhaust Diffusers Using Casing Injection and Design Modifications

Abstract

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.