The sub-grid-scale approach for modelling the impingement cooling flow in the combustor pedestal tile

The widely used gas turbine combustor double-walled cooling scheme relies on very small pedestals. In a combustor it is impractical for CFD to resolve each pedestal individually as that would require a very large amount of grid points and consequent excessive computation time. These pedestals can be omitted from the mesh and their effects captured on the fluid via a pedestal sub-grid scale (SGS) model. The aim is to apply the SGS approach, which takes into account the effects on pressure, velocity, turbulence and heat transfer, in an unstructured CFD code. The flow inside a 2-D plain duct is simulated to validate the pedestal SGS model and the results for pressure, velocity and heat transfer are in good agreement with the measured data. The conjugate heat transfer inside a 3-D duct is also studied to calibrate the heat source term of the SGS model due to the pedestals. The resolved flow in the combustor pedestal tile geometry is numerically investigated using RANS and LES in order to first assess the viability of the RANS and LES to predict the impinging flow and second to provide more validation data for the development of the SGS pedestal correlations. It is found that the complexity of such a flow, with high levels of curvature, impingement and heat transfer, poses a challenge to the standard RANS models. The LES provides more details of the impinging flow features. The pedestal model is then applied to the complete tile to replace the pedestals. The results are close to both the fully resolved CFD and the measurements. To improve the flow features in the impingement zone the first two rows were resolved with the mesh and combined with the SGS modelling for the rest of the tile, this gave optimum results of pressure, velocity and turbulence kinetic energy distribution inside the pedestal cooling tile.