Development of a Modified RANS-Turbulence Model for the Computational Investigation of Film Cooling

Abstract

Abstract. The thermal load on the endwalls of the hot gas path of gas turbines, in particular those of the nozzle guide vanes (NGVs), has increased considerably over the last few decades, both because of the common premixed combustion being used and the ever-increasing turbine inlet temperatures. Therefore, cooling concepts for these endwalls have been the focus of R&D efforts for some time. Dedicated cooling, i.e., film-cooling holes in the most thermally highly loaded areas, can be applied. This requires both additional cooling air and costly mechanical efforts. When using purge air, neither of these two disadvantages comes into play. For this reason, cooling the endwalls with purge air from the combustor/NGV interface seems very attractive. To establish and evaluate this cooling concept, dedicated tests are normally performed, but both the required time and financial resources preclude testing every possible geometry. Hence, efforts are made to use CFD to establish the thermal load on the endwalls and the optimum cooling configuration before testing. The thermal load consists of both the film-cooling effectiveness and the heat transfer on the endwall. Combined, these yield the net heat flux ratio (NHFR). Only this gives the true thermal load on the surface. In an industrial environment, Reynolds-averaged Navier–Stokes (RANS) solvers are used to carry out CFD analyses. These have difficulties in modeling shear stresses and turbulence, which means that both the cooling effectiveness and the heat transfer predictions are inaccurate. Therefore, an existing k–ω shear stress transport (SST) model was modified in ansys cfx to better capture these parameters. The model was developed based on film-cooling experiments and validated on extensive experiments of endwall cooling in a linear cascade. In this article, these experiments, the CFD model, and its validation will be presented and discussed in detail.