Soot volume fraction measurement in low-pressure methane flames by combining laser-induced incandescence and cavity ring-down spectroscopy: Effect of pressure on soot formation

Soot volume fraction (fv) profiles are recorded in low-pressure methane/oxygen/nitrogen flat flames using laser-induced incandescence (LII). Experiments are performed from 20 to 28 kPa in flames having the same equivalence ratio (2.32). Calibration is performed by cavity ring-down spectroscopy (CRDS) and indicates a very weak soot volume fraction (0.066 ppb at 21.33 kPa and 0.8 ppb at 26.66 kPa in the burnt gases). Soot volume fraction is found to increase continuously after a given distance above the burner (HAB) and tends to level off in the burnt gases. The reaction time resolution available in low-pressure flames makes it possible to examine the early steps of soot formation. The variation of the LII signal with laser energy before the LII “plateau” region is much weaker at the beginning of soot formation than after a given reaction time. The LII time decays are nearly constant within the first millimetres, whereas an increase in the decay, correlated with the growth of the primary soot particle, is observed later. The growth of soot volume fraction is then analysed by considering the variation of the derivative function dfv/dt with fv. Three regimes having respectively a positive slope, a constant slope, and a negative slope are observed and are interpreted with respect to the soot inception process. Finally, a very important sensitivity of fv with pressure P (at 30 mm HAB) is observed, leading to a power law, fv=KP11, confirmed by extinction measurements (by CRDS). The observed dependence of fv with pressure could be a result of the prominence of the early soot inception process in the investigated low-pressure flames.