Experimental and Numerical Investigation of the Impact of Swirl on the Performance of Industrial Gas Turbines Exhaust Diffusers

In order to investigate the effect of inlet conditions on the performance of an axial strutted annular exhaust diffuser, as used in heavy duty gas turbines, a scaled test-rig was built which allows the systematic variations of inlet conditions like inlet swirl, Mach number and turbulence level. This rig, which is of modular design, is used for the analysis of the flow-structures in typical GT-exhaust configurations and provides the performance data used in the diffuser design process. The present work shows the influence of inlet swirl and Mach number on the performance of a specific diffuser configuration. The first strut was designed for a certain flow angle distribution (so-called design conditions), and the measurements have been done for a range of inflow parameters including design and off-design conditions. The measurements allowed to reveal the impact of 3D effects on the diffuser characteristic. Comparison of measurements with correlations available from literature (annular diffuser configurations) show that the application of the latter is of limited use for the design of real GT diffusers, because standard correlations do not account for inlet swirl / struts interaction. The experimental data has been used to validate the numerical procedure developed by the authors for the analysis of axial GT exhaust diffusers. The results of computations are in reasonable agreement with measurements.