Studies on the effect of swirl on NO formation in methane/air turbulent combustion

Most present studies on pollutant formation concentrate on chemical reaction kinetics. To understand the interaction between turbulence and chemistry in NO formation, the effect of swirl number on NO formation in methane/air turbulent combustion is studied by experiments, in which a small amount of ammonia is added to the fuel to simulate fuel nitrogen, and simultaneously by numerical simulation, using a second-order-moment PDF turbulence-chemistry model. The predicted results are in overall agreement with the measured results. Both predictions and experiments show that as the swirl number increases from 0 to 1, the thermal NO at first increases and then decreases. In contrast, the fuel NO at first decreases and then increases. The studies also show that the increase in swirl number first leads to a rapid decrease and then a slower increase in turbulence intensity, and first an increase and then a slight decrease of temperature near the exit. As the activation energy of thermal NO formation is much larger than that of fuel NO formation, these results imply that the thermal NO is predominantly affected by temperature, whereas the fuel NO is predominantly affected by species mixing via turbulence. The research results are expected to be used for developing low-NOx burners.