Radiation Emissions from Turbulent Diffusion Flames Burning Vaporized Jet and Jet-like Fuels

Abstract

This study measured and compared the size and radiation emissions from turbulent diffusion flames burning large hydrocarbon fuels, including jet fuels. This effort is motivated by the need to understand how burning conventional or alternative jet-like fuels changes radiation emissions. Radiative heat transfer is significant because it is can alter pollutant formation and the lifetime of combustion devices. Eleven fuels were evaluated, consisting of traditional and alternative aviation fuels (e.g., Jet-A) and 1–3 component fuels. The flames were burned on a reduced-scale piloted Sydney turbulent diffusion burner with Reynolds numbers (Re) ranging from 7500 to 45 000. The vaporized fuel exited the burner near 300 °C. The radiative heat flux and the radiation intensity were measured using a radiometer (Medtherm, Model 64-0.2-20) and a mid-infrared camera (FLIR, Model SC6700), respectively. The radiant fraction (χR) is reported for the different fuels. χR typically varied by <15% at similar Re for the different flames, despite the aromatic content ranging from 0% to 31%. The χR had a tendency to be relatively insensitive to aromatic content, but was sensitive to the weight percent of hydrogen in the fuel. Differences in the molecular structure of the fuels, even for fuels with similar hydrogen contents, can significantly affect χR. χR values initially increased as the Re was increased, but then decreased monotonically for Re > 20 000. This trend is attributed to competing effects of flame stretch, entrainment, and surface-to-volume ratio on radiation emissions. The peak radiation intensity occurs near the location of the peak heat flux, which corresponds to the visible flame height. Radiation emitted from soot and unburned hydrocarbons is predominantly observed from within the flame sheet; significant variations in radiation emissions was observed between the different fuels. In contrast, emissions from CO2 for all the fuels varied by <10% and were more broadly distributed across the flame. Fluctuations in radiation emissions have a tendency to be greatest near the flame sheet for CO2 and within the flame sheet for soot and unburned hydrocarbons. Total radiation emissions from these flames have a tendency to be dominated by emissions from gaseous combustion products instead of emissions from soot or unburned hydrocarbons.