Jet Flames with Radial Air Injection: Oxides of Nitrogen and Carbon Monoxide Emissions

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Abstract
Experiments were conducted in which air was injected radially into simple, nonpremixed, jet flames at axial locations of either 0.25, 0.50, or 0.75 times the visible flame length (Lo). One objective of these experiments was to determine if NOx emissions could be lowered by enhanced mixing of air into the broadened reaction zones that lie near the flame tip. Also of interest were the emissions of carbon monoxide and NO2-to-NOxt ratios. Experiments were conducted with fuel jet Reynolds numbers of 8, 100 and 34,800 for methane and propane fuels, respectively. Radial air injection dramatically altered the flame shape from a relatively slender jet to a ball flame. Air injection reduced global flame residence times only for injection at x/Lo = 0.25. NOx emissions generally increased, rather than decreased, with air injection, although a decreasing trend was observed after an initial increase in some cases. The combined effects of residence times and radiant heat losses are thought to govern these trends. For large air injection rates at the x/Lo = 0.25 location, rapid entrainment and mixing increased combustion intensity and resulted in high CO emissions and high NO2-to-NOx ratios, consistent with the formation of HO2 radicals, which oxidize NO to NO2 and the destruction of OH radicals, which oxidize CO to CO2. The results obtained in this study are qualitatively similar to those found by others for coaxial air injection in hydrocarbon jet flames.