Quantitative measurements of enhanced soot production in a flickering methane/air diffusion flame

Abstract

Integrated models of soot production and oxidation are based upon experimental results obtained in steady, laminar flames. For successful application of these descriptions to turbulent combustion, it is instructive to test predictions of soot concentrations against experimental measurements obtained in time-varying flowfields. This paper reports quantitative measurements of the local soot volume fraction in a co-flowing, flickering CH 4/air diffusion flame burning at atmospheric pressure. Acoustic forcing of the fuel flow rate is used to phase lock the periodic flame flicker close to the natural flicker frequency. Our measurements show that soot production is four times greater for a forcing condition in which flame tip clipping occurs, compared with a steady flame burning with the same mean fuel flow velocity. The soot field in the flickering flame has been characterized using tomographic reconstruction of extinction data obtained at 632.8 nm, laser-induced incandescence (LII) images calibrated against steady CH 4/air extinction results, and vertically polarized scattering data. The LII method is found to track the soot volume fraction closely and to give better signal-to-noise than the extinction measurements in both the steady and time-varying flowfields. A Mie analysis of these results suggests that the flickering flame exhibits similar number densities but larger particle sizes than the corresponding steady flame.