Pulsed flow modulation of soot production in a laminar jet-diffusion flame

An axisymmetric, co-flow, acetylene-air diffusion flame was driven by an acoustic source at frequencies ranging from 230 to 1000 Hz and powers of 0-10 W. Soot concentration was measured both locally within the flame and globally far downstream of the flame. The acoustic characteristics of the system are detailed in this manuscript. We show that the acoustic velocity at the fuel exit plane strongly affects the soot concentration within the flame and emitted by the flame. When the acoustic velocity magnitudes are much larger than the mean fuel velocity, the average soot number concentration measured far downstream of the flame was reduced by almost three orders of magnitude. A simple scaling analysis is performed to show that a partial premixing mechanism may be responsible for the observed changes to the soot properties of the flame. To test this hypothesis, sooting properties of partially premixed flames were characterized over a range of equivalence ratios in the same burner. While initial addition of air into the fuel stream slightly increased the amount of soot released, further addition of air suppressed soot production. Comparisons of the acoustically forced flames to partially premixed flames support the hypothesis that the acoustic field aids in partially premixing the flame.