Study of Gas Turbine Exhaust Diffuser Performance and Its Enhancement by Shape Modifications

Abstract

In this paper, results of studies on typical gas turbine exhaust diffuser geometry have been reported. This diffuser consists of an annular diffuser followed by a conical diffuser. The annular diffuser has 5 radial, backward swept struts. The studies were carried out at a Reynolds number of 7.7 × 105 based on the diffuser inlet diameter (hydraulic). Two inflow boundary conditions corresponding to (i) full load (low swirl) and (ii) part load (high swirl) operations of a typical gas turbine exit were separately simulated. The performance of the diffuser was assessed in terms of total pressure loss and static pressure recovery coefficient along the diffuser. It was observed that the baseline diffuser geometry had substantial losses owing to separation of the boundary layer, beginning as early as in the annular diffuser and continuing all the way up to the exit of the conical diffuser. The performance was found to worsen with higher inlet swirl. It was observed that the divergence angle in the annular part of the diffuser plays an important role in the initiation of flow separation. Interaction of inlet flow swirl with the struts also initiates considerable asymmetry in the flow pattern within the conical diffuser. Based on observations from the baseline geometry, several new annular diffuser geometries with different divergence angles and shapes were numerically studied. The shroud shapes were manipulated at specific locations like the plane of the strut leading edge, maximum airfoil thickness and the trailing edge of the struts. Significant performance improvements were observed in these simulated diffuser configurations. Two such annular diffuser geometries have been discussed in detail in this paper.