The effect of global mixing on soot volume fraction: measurements in simple jet, precessing jet, and bluff body flames

Abstract

Measurements of soot volume fraction were performed in three different turbulent diffusion flames using laser-induced incandescence (LII) to provide insight into the effect of global mixing rate on the instantaneous local and the average soot volume fraction. The three flames, produced by simple jet, precessing jet, and bluff-body jet burners, are chosen because they span a wide range of different global mixing rates for the same fuel, nozzle diameter, and flow rate, and because global measurements of their radiation, NO x emissions, and residence times are available. The measurements reveal an inverse relationship between global mixing rate and both the total amount of soot in the flame and local instantaneous soot volume fractions, broadly consistent with trends in laminar flames. However, probability density functions reveal that the relationship between local instantaneous and time-averaged soot volume fraction is different in the lower and upper regions of each flame. In the lower regions, the instantaneous and average volume fractions increase together with distance from the burner. However, the upper regions exhibit a bi-modal distribution in local, instantaneous volume fraction. This suggests that burnout in the tip region occurs predominantly by reducing the proportion of the flame being occupied by the sheets of soot, rather than by reducing the instantaneous volume fraction within the sheets. In addition, the shape of the pdf is different for each flame, showing a dependence on the turbulent characteristics, and that care must be taken in applying measurements obtained from one class of burner to another.