Simulation of pollutant formation in a gas-turbine combustor using unsteady flamelets

Based on the steady-state solution of a turbulent flow and mixing field in a staged combustor of an aircraft gas turbine, the formation of NOx and soot is simulated in a postprocessing mode using unsteady flamelets. Numerical tracer particles are introduced into the turbulent flow field, where each particle represents a flamelet. The temporal evolution of the flamelets is controlled by the time-dependent boundary conditions, which are extracted at the current location of the tracer particle in the flow field. The expectation of finding a tracer particle at a given location is calculated from the solution of a nonstationary, convection and diffusion equation for each particle. The pollutant concentrations are then computed via integration over time and the number of tracer particles. The experiments were performed with a 90 degrees sector of the two-staged kerosene-fueled combustor with five pilot and main fuel injectors at BR715 reduced takeoff conditions. The gas temperature was derived from the concentrations of CO and CO2 in the exhaust gas. NO and NO2 emissions were measured by collecting and mixing the exhaust gas from five radial positions simultaneously and later averaging over the middle portion of the sector. Soot mass concentrations were derived from measurements of the HSU (Hartree smoke unit). A comparison of the experimental and numerical results shows good agreement for emissions of NOx but deviations for soot. It was found that the number of numerical tracer particles in the flow field did not significantly influence the level of computed pollutant emissions (less than 5% for NOx and soot).