The effect of oxygen depletion on soot and thermal radiation in buoyant turbulent diffusion flames

Abstract

The radiative characteristics of buoyant ethylene diffusion flames were investigated in normal air (21% O2 by volume) and nitrogen-diluted, reduced-oxygen conditions (15–19% O2 concentrations). The characteristics for a 15 kW heat release rate flame, such as the radiant-power distribution and the line-of-sight spectral emission, were measured. A reduction of the overall radiant fraction in reduced-oxygen conditions was observed compared to flames burning in normal air. This reduction could generally be attributed to lower soot emission. To better understand this effect, simultaneous measurements of local soot volume fraction and temperature were conducted using the two-color pyrometry technique. The soot volume fraction and the temperature statistics on the flame axis are reported for both 21% O2 and 17% O2 co-flow conditions. The soot volume fractions for 21% O2 are generally found to be higher than those at the 17% O2 condition. But the soot temperature is relatively similar, showing the soot formation reactions occur at the similar temperature range, and the lower temperature environment of 17 % O2 attributes to smaller soot concentration. As a result, these factors cause a lower local soot emission intensity. A negative correlation between the fluctuating components of soot volume fraction and temperature is found to occur, with the correlation weakening at higher elevations. Overall, the measurements provide new understanding into spatiotemporal soot-radiation characteristics of buoyant, turbulent flames burning in different co-flow oxygen conditions.