Research on the accuracy of flow simulation in gas turbine exhaust diffusers

Large industrial gas turbines for combined cycle power generation are typically connected to the heat recovery steam generator by axial exhaust diffusers. The target of diffuser design is a high pressure recovery with low losses. The prediction of the flow in an exhaust diffuser is particularly difficult as a high pressure recovery is obtained with a flow close to boundary layer separation and as a bluff body separation at the abrupt hub end of the turbine occurs. CFD (computational fluid dynamics) simulations then need to cope with complex phenomena such as smooth wall separation, recirculation, reattachment and free shear layer mixing. In this research guidelines for high quality diffuser simulations are assembled. The different sources of errors and uncertainties are analyzed and then separately examined in the CFD analysis of the diffuser flow field for two different test cases with strongly separated flow and with mostly attached flow. Extensive numerical studies of the impact of these errors were carried out with the flow solver ANSYS CFX 14.5. For validation of the simulation results, experimental data is obtained from a test rig at the Institute of Thermal Turbomachinery and Machinery Laboratory of the University of Stuttgart. The prediction of such a highly separated flow field is largely impacted by the turbulence modeling and the turbulence inlet boundary conditions. A method to estimate the usually unknown turbulence eddy dissipation is demonstrated. The flow field and the pressure recovery of exhaust diffusers can be reasonably predicted with the in this research assembled guidelines for the CFD modeling and the Shear Stress Transport turbulence model. With the more complex Scale Adaptive Simulation turbulence model in sum no improvement is obtained.