The Effect of the Operating Conditions of the Last Turbine Stage on the Performance of an Axial Exhaust Diffuser

The exhaust diffusers studied in this paper are installed behind the last turbine stage of gas turbines, including those used in combined cycle power plants. For the design of efficient diffusers, the effects caused by the last turbine stage need to be taken into account. In the present paper, results are presented to estimate the performance of a diffuser operating under a variation of multiple modelling parameters: tip leakage flow, the swirl, and the rotating blade wakes. To provide a better understanding of the flow parameters, a test facility with a turbine stage simulator is used to model these flow effects and an optical endoscopic planar measurement technique based upon Particle Image Velocimetry (PIV) is applied. The pressure recovery is estimated for various turbine conditions using a variety of relevant parameters. Within a range of conditions, a PIV study is performed to try to understand the typical flow phenomena which influence the performance of axial diffusers. The rise of turbulent energy in the inlet flow positively affects the diffuser performance. A small positive swirl angle in the inlet flow (behind the rotating bladed wheel in experiments) has a stabilizing effect on the diffuser. The tip leakage flow from the last turbine stage can also positively affect the pressure recovery in the diffuser.