Turbulent jet flames in a crossflow: effects of some jet, crossflow, and pilot-flame parameters on emissions

Abstract A study was conducted to document and understand the pollutant emissions characteristics of high-velocity jet flames in a crossflow of air. Measured were unburned hydrocarbon, carbon monoxide, and NOx emission indices; the ratio of NO2 to NOx; and flame dimensions. Four fuels of differing sooting propensities were used: C2H4, C3H8, CH4, and a 95 wt.% CO/5 wt.% H2 mixture. A straight-tube burner was employed (4.12 mm i.d.) with jet velocities ranging from approximately 15–100 m/s, depending on the fuel type. Tests were conducted at two crossflow velocities, 2.3 and 4.3 m/s, yielding jet-to-crossflow velocity and momentum ratios within the ranges of 4–37 and 9–940, respectively. The crossflow flames are shorter than comparable straight-jet flames, indicative of enhanced entrainment and mixing. Comparatively high levels of unburned hydrocarbons and CO emissions, along with high ratios of NO2 to NOx, obtain for crossflow flames as a consequence of fuel being swept from the jet in the nearfield, which is possibly supplemented by local flame quenching brought about by the rapid mixing with the crossflow. NOx emissions tend to follow the same trends as those of straight-jet flames, for most conditions. Ethylene flames, however, are a major exception. For these flames, NOx emission indices depend strongly on both the jet-to-crossflow momentum ratio and the magnitude of the crossflow velocity. Radiant fraction measurements suggest that the NOx levels are coupled strongly with how the crossflow affects in-flame soot and, consequently, radiation losses.