Hybrid RANS-LES Modeling of the Aero-Thermal Field in an Annular Hot Streak Generator for the Study of Combustor-Turbine Interaction

Turbine entry conditions are characterized by unsteady and strongly non-uniform velocity, temperature and pressure fields. The uncertainty and the lack of confidence associated with these conditions require the application of wide safety margins during the design of the turbine cooling systems, with a detrimental effect on engine efficiency. The adoption of lean-burn technology in modern aero-engines to reduce NOx emissions exacerbates the situation, as the absence of dilution holes keeps the strong swirl component generated by the burners up to the combustor outlet and prevents to control the pattern factor. Complexity and costs associated with the experimental investigation of combustor-turbine interaction, makes Computational Fluid Dynamics (CFD) paramount to understand the physical phenomena involved. Moreover, due to the well-known limitations of the Reynolds-Averaged Navier-Stokes (RANS) approach and the increase in computational resources, hybrid RANS-LES models, such as Scale Adaptive Simulation (SAS), are proving to be a viable approach to capture the main structures of the flow field. This paper reports the main findings of the numerical investigation on a test rig representative of a lean-burn, effusion cooled, annular combustor, developed in the context of the EU Project FACTOR (Full Aerothermal Combustor-Turbine interactiOns Research) with the aim of studying combustor-turbine interaction. Results obtained with RANS and unsteady SAS were critically compared to experimental data and analysed in order to better understand the flow physics within such a device, as well as to assess the improvements related to the use of hybrid models. The main discrepancies between RANS and SAS are highlighted in predicting the recirculating region, which has slight influence on the velocity field at the combustor outlet, but affects dramatically mixing and the resulting temperature distribution. Accuracy of the results achieved suggest a possible exploitation of SAS model with a view to the future inclusion of the nozzle guide vanes within the test rig