Near-threshold soot formation in premixed flames at elevated pressure

Soot formation at lean-threshold conditions referred to as near-threshold sooting conditions (i.e., with stoichiometry, phi, around 1.90 for ethene as a fuel) are studied in laminar premixed ethylene/air flames at pressure from 1 to 10 bar. Laser extinction is used to measure the soot volume fraction. Time-resolved laser-induced incandescence (TiRe-LII) is used to determine particle diameters from the LII signal temporal decay after pulsed laser heating. Thermophoretic sampling is applied to extract particle samples from the flame and ex situ transmission electron microscopy (TEM) is used to measure particle sizes and morphology. The soot volume fraction scales with pressure in a power-law function with the parameter n as 1.4 to 1.9 for flames at the equivalence ratio (phi = 2.1) even at the onset of soot formation. The elevated dependence of soot volume fraction on height above burner is detected with increasing pressure in the near-threshold sooting conditions. The measured soot diameter increases with pressure and equivalence ratio and its sensitivity to the equivalence ratio increases with increasing pressure. The TiRe-LII signal decay varies only little with height above burner and laser fluence in the near-threshold sooting flame (phi = 1.90-1.95), which indicates that the soot particle surface growth and oxidation are balanced. For a slightly sooting flame, TEM measurements from thermophoretically-sampled soot agree well with the LIIsim-evaluated particle size, indicating the reliability of TiRe-LII particle diameter determination under near-threshold conditions. (C) 2021 Elsevier Ltd. All rights reserved.

Automated summary

The formation of soot at near-threshold conditions in laminar premixed ethylene/air flames (with ethene as a fuel, phi around 1.90) at pressures from 1 to 10 bar was studied. It was found that the soot volume fraction scales with pressure in a power-law function, the soot diameter increases with pressure and equivalence ratio, and the reliability of TiRe-LII particle diameter determination under near-threshold conditions is good.