Subgrid modeling for simulation of spray combustion in large-scale combustors

Simulations of spray combustion in full-scale combustors under different operating conditions are conducted using large-eddy simulations (LES). The current methodology attempts to capture not only spray-turbulence interactions but also subgrid fuel-air mixing and finite-rate kinetics occurring at scales below the LES resolution. Reduced finite-rate kinetics for n-heptane and kerosene fuels are used in these studies to predict pollutant emission. Comparison of LES predictions with measurements for a single-cup swirl combustor shows reasonably good agreement. Results for spray combustion in a realistic two-cup combustor sector show a complex vortex breakdown process that creates multiple recirculation regions in the combustor. These regions of recirculation provide multiple sites to stabilize the spray and the flame. Because of the shape of the combustor, significant three-dimensional effects are apparent with no similarity between flame structures, vortex breakdown bubbles, and outflow between the two cups. Spray combustion is quite efficient during full power operation because of the distributed injection process. It is also shown that in the current subgrid mixing and combustion approach flame stabilization is more physical, and the flame anchors downstream of the dump plane. In contrast, a conventional LES study using a subgrid eddy breakup model shows a flame anchored inside the inlet, immediately downstream of the spray injector which is unphysical.