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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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An easy but quantitative assessment of soot production rate and its dependence on temperature and pressure
The challenge of soot emission persists in combustion research due to the complexities of tracking the crucial stages of growth from fuel to soot nuclei and ultimately mature particles. Studying soot formation in flames often requires a sophisticated approach, involving detailed measurements of gaseous soot precursors and soot particles using multiple complementary diagnostics. On the other end of the spectrum of studies are simpler methods that capture the sooting tendency using a single index, akin to the cetane number in compression ignition engines and the octane number in spark ignition engines. This article seeks a middle ground, aiming to quantify the soot production rate while maintaining the simplicity of single-index characterizations. The approach involves establishing counterflow diffusion flames, measuring soot volume fraction through pyrometry, and accurately computing velocity and temperature profiles using a commercial code. These data allow for the quantification of the production rate from the soot governing equation. The methodology is applied to counterflow ethylene diffusion flames to examine the temperature dependence of the soot production rate across peak temperatures varying by several hundred degrees and pressures in the 1–32 atm range. The soot production rate per unit flame area falls within the range of 10 7–10 3 g/(cm2s) range and, when normalized with respect to the carbon flux, it ranges between 10 6 and nearly 10 2. On a logarithmic scale, it linearly correlates with the peak temperature at a fixed pressure. Although this study deals only with flames of ethylene, the approach can be generalized to any fuel. The resulting database should be valuable not only for industry practitioners but also to the scientific community for the global validation of detailed soot models.
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- Award ID(s):
- 2013382
- PAR ID:
- 10553930
- Publisher / Repository:
- https://doi.org/10.1016/j.proci.2024.105292
- Date Published:
- Journal Name:
- Proceedings of the Combustion Institute
- Volume:
- 40
- Issue:
- 1-4
- ISSN:
- 1540-7489
- Page Range / eLocation ID:
- 105292
- Format(s):
- Medium: X
- Sponsoring Org:
- National Science Foundation
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Accepted Manuscript1.0
- Journal Article:
- https://doi.org/10.1016/j.proci.2024.105292
