The Effects of Air Preheat and Number of Orifices on Flow and Emissions in an RQL Mixing Section

This study was motivated by a goal to understand the mixing and emissions in the rich-burn/quick-mix/lean-burn combustor scheme that has been proposed to minimize the formation of oxides of nitrogen $(NOx)$ in gas turbine combustors. The study reported in this paper was a reacting jet-in-crossflow experiment at atmospheric pressure in a cylindrical duct. The jets were injected from the perimeter of the duct through round-hole orifices into a fuel-rich mainstream flow. The number of orifices investigated in this study gave over- to optimum to underpenetrating jets at a jet-to-mainstream momentum-flux ratio of 57. The size of individual orifices was decreased as their number increased to maintain a constant total area. The jet-to-mainstream mass-flow ratio was held constant at 2.5. The experiments focused on the effects of the number of orifices and inlet air preheat and were conducted in a facility that provided the capability for independent variation of jet and main inlet air preheat temperature. The number of orifices was found to have a significant effect on mixing and the distributions of species, but very little effect on overall $NOx$ emissions, suggesting that an aerodynamically optimum mixer may not minimize $NOx$ emissions. Air preheat was found to have very little effect on mixing and the distributions of major species, but preheat did increase $NOx$ emissions significantly. Although the air jets injected in the quick-mix section of a RQL combustor may comprise over 70% of the total air flow, the overall $NOx$ emission levels were found to be more sensitive to mainstream air preheat than to jet stream air preheat.