Radiant fraction from sooting jet fires

Abstract

The objective of this article is to investigate numerically the radiative structure of methane, ethylene and acetylene lab-scale jet flames ranging from the transitional to the momentum-driven regimes. The numerical model involves a hybrid flamelet/transported PDF method coupled to an acetylene-benzene soot production model and a Wide-Band Correleted-K gas radiation model. Model predictions in terms of mean and rms soot volume fraction and temperature, integrated soot volume fraction and radiant fraction are in reasonable agreement with the available experimental data. In particular, the model reproduces quantitatively the decrease in radiant fraction observed as the flow becomes momentum driven. This behavior results mainly from two mechanisms: (i) an increase in flame self-absorption due to an enhancement in flame volume and (ii) for the ethylene and acetylene flames a reduction in the soot emission per unit flame volume owing to a strong decrease in soot production. In addition, for a given fuel, gas emission per unit flame volume remains approximatively constant as the exit strain rate increases whereas the soot emission per unit flame volume and the characteristic soot volume fraction scale with the Kolmogorov time scale. It was also found that competitive mechanisms govern the effects of Turbulence-Radiation Interaction (TRI) on radiant fraction. Enhancement mechanisms are due to gas emission TRI and temperature self-correlation effects on soot emission whereas inhibiting mechanisms results from the negative correlation between soot volume fraction and temperature. Enhancement mechanisms dominate in weakly sooting methane flames and taking TRI into account increases the radiant fraction. On the other hand, inhibiting mechanisms become significant in moderately and highly sooting fuels, with their importance increasing with both the fuel sooting propensity and the Reynolds number. For flames dominated by soot radiation, the inhibiting mechanisms prevail and taking TRI into account reduces the radiant fraction.