Influence of Turbulence Schmidt Number on Exit Temperature Distribution of an Annular Gas Turbine Combustor using Flamelet Generated Manifold

The Reynolds analogy concept has been used in almost all turbulent reacting flow RANS (Reynolds-averaged Navier-Stokes) simulations, where the turbulence scalar transfers in flow fields are calculated based on the modeled turbulence momentum transfer. This concept, applied to a lean premixed combustion system, was assessed in this paper in terms of exit temperature distribution. Because of the isotropic assumption involved in this analogy, the prediction in some flow condition, such as jet cross flow mixing, would be inaccurate. In this study, using Flamelet Generated Manifold as reaction model, some of the numerical results, obtained from an annular combustor configuration with the turbulent Schmidt number varying from 0.85 to 0.2, were presented and compared with a benchmark atmospheric test results. It was found that the Schmidt number sigma(t) in mean mass fraction f transport equation had significant effect on dilution air mixing process. The mixing between dilution air and reaction products from the primary zone obviously improved as sigma(t) decreased on the combustor exit surface. Meanwhile, the sensitivity of sigma(t) in three turbulence models including Realizable k-epsilon, SST (Shear Stress Transport) and RSM (Reynolds Stress Model) has been compared as well. Since the calculation method of eddy viscosity was different within these three models, RSM was proved to be less sensitive than another two models and can guarantee the best prediction of mixing process condition. On the other hand, the results of dilution air mixing were almost independent of Schmidt number Sc-t in progress variable c transport equation. This study suggested that for accurate prediction of combustor exit temperature distribution in steady state reacting flow simulation, the turbulent Schmidt number in steady state simulation should be modified to cater to dilution air mixing process.