Soot and its gaseous precursors are quantified in detail (precursors up to 166 amu, volume fraction, particle size, number concentration, and light emissivity dispersion exponent) in a laminar partially premixed counterflow flame of ethylene. The investigated flame has an equivalence ratio Φ = 2.43 and a mixture fraction Zst = 0.4, resulting in a distinct double-flame structure consisting of a rich premixed flame component and a diffusion flame component, both stabilized on the fuel side of the stagnation plane. The value of the equivalence ratio makes the premixed flame the dominant contributor to soot production, with soot being oxidized completely by OH from the diffusion flame component. Particle size is measured to increase quasi-monotonically, but remains within a few nanometers throughout the soot forming region. Aromatic species are primarily formed in the post flame region of the premixed flame. Their mole fractions peak close to the premixed flame and decrease as the diffusion flame is approached. The experimentally measured gaseous species are captured well by kinetic models, with the exception of two critical species in soot chemistry: benzene and naphthalene.
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Effect of equivalence ratio and temperature on soot formation in partially premixed counterflow flames
Soot formation is quantified in detail (volume fraction, particle size, number concentration, and light emissivity dispersion exponent) in a series of partially premixed counterflow flames of ethylene at equivalence ratios equal to 6.5, 5.0, and 4.0, and with maximum temperature spanning approximately 200 K. The focus is to investigate the effect of peak temperature and equivalence ratio on soot formation while maintaining constant global strain and stoichiometric mixture fraction. Oxygen is progressively displaced from the oxidizer to the fuel stream of a diffusion flame to stabilize partially premixed flames of decreasing, showing a double-flame structure consisting of a rich premixed flame component stabilized on the fuel side of the stagnation plane and a diffusion flame component stabilized on the oxidizer side. Soot is detected in the region sandwiched between the two flame components, is formed in both
of them, and is convected away radially at the Particle Stagnation Plane (PSP). At fixed , raising the peak temperature invariably raises the soot volume fraction throughout the probed region. Vice versa, at fixed peak temperature, lowering the equivalence ratio causes the premixed flame component to shift away from the diffusion flame component, with the consequent broadening of the soot forming region and an increase in both soot volume fraction as well as soot particle sizes through an enhancement of surface growth. Detailed probing of the region in the vicinity of the PSP offers evidence of soot oxidation from molecular oxygen. Furthermore, when the maximum temperature is sufficiently low, the net soot production rate turns negative because surface oxidation overwhelms surface growth. Comparing the soot number production rate inferred from experiments to the dimerization rate of benzene, naphthalene, and pyrene reveals that only the smallest aromatics are present in flames at sufficiently large concentrations to account for soot nucleation. This observation applies to both the diffusion flame and the premixed flame components and confirms previous findings in strictly diffusion flames.
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- Award ID(s):
- 1853150
- NSF-PAR ID:
- 10386250
- Publisher / Repository:
- Elsevier
- Date Published:
- Journal Name:
- Combustion and Flame
- Volume:
- 242
- Issue:
- C
- ISSN:
- 0010-2180
- Page Range / eLocation ID:
- 112088
- Subject(s) / Keyword(s):
- ["Soot","Counterflow","Partially premixed flames"]
- Format(s):
- Medium: X
- Sponsoring Org:
- National Science Foundation
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- Free Publicly Accessible Full Text
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Accepted Manuscript1.0
- Journal Article:
- https://doi.org/10.1016/j.combustflame.2022.112088
