A numerical study of the effect of pressure on the formation of NOx and soot in an axisymmetric 30° counter rotating axial swirler lean low NOx gas turbine combustor has been conducted. This has previously been studied experimentally and this CFD investigation was undertaken to explain the higher than expected NOx emissions. The combustion conditions selected for the present study were 300 deg K inlet air, 0.4 overall equivalence ratio, and pressures of 1 and 10 bar.
The numerical model used here involved the solution of time-averaged governing equations using an elliptic flow-field solver. The turbulence was modelled using algebraic stress modelling (ASM), The Thermo-chemical model was based on the laminar flamelet formulation. The conserved scalar/assumed pdf approach was used to model the turbulence chemistry interaction. The study was for two pressure cases at 1 and 10 bar. The turbulence-chemistry interaction is closed by assumption of a Clipped Gaussian function form for the fluctuations in the mixture fraction. The kinetic calculations were done separately from the flowfield solver using an opposed laminar diffusion flame code of SANDIA. The temperature and species profiles were made available to the computations through look-up tables.
The pollutants studied in this work were soot and NO for which three more additional transport equations are required namely; averaged soot mass fraction, averaged soot particle number density, and finally averaged NO mass fraction. Soot oxidation was modelled using molecular oxygen only and a strong influence of pressure was predicted. Pressure was shown to have a major effect on soot formation.