Experimental and numerical study of the effects of the oxygen index on the radiation characteristics of laminar coflow diffusion flames

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DOIResolve DOI: http://doi.org/10.1016/j.combustflame.2012.12.005
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TypeArticle
Journal titleCombustion and Flame
ISSN0010-2180
Volume160
Issue4
Pages786795; # of pages: 10
SubjectCo-flow; Coflow diffusion flames; Correlated-k models; Ethylene diffusion flames; Experimental and numerical studies; Flame height; Fuel flow rates; Laminar diffusion flames; Measured values; Narrow bands; Nondimensional; Oxidizer stream; Oxygen concentrations; Oxygen index; Radiation characteristics; Radiative fluxes; Rate of increase; Semi-empirical; Smoke point; Soot formations; Soot model; Soot oxidation; Soot volume fraction; Two-equation; Two-regime; Vertical distributions; Volumetric flow rate; Acetylene; Diffusion; Ethylene; Flow rate; Forecasting; Heat radiation; Oxygen; Radiation effects; Smoke; Soot; Volume fraction; Dust; ethylene; oxygen; article; chemical parameters; diffusion coefficient; fire; flow rate; laminar coflow diffusion flame; oxidation; oxygen index; prediction; priority journal; radiation; stoichiometry; temperature
AbstractThe oxygen concentration in the oxidizer stream (O₂+N₂) of laminar coflow ethylene diffusion flames is varied from 17% to 35% in order to study its influence on the flame height, the soot formation/oxidation processes, the smoke point characteristics, and the vertical distributions of radiative flux at a distance of 6.85cm from the flame axis. Measured values for all the investigated parameters are compared with the predictions provided by a numerical model based on a two-equation semiempirical acetylene-based soot model and on the statistical narrow-band correlated-k model to compute thermal radiation. Predictions are in overall reasonable agreement with the experiments for oxygen indices in the range 19-35%, whereas all the investigated parameters are significantly underestimated for an oxygen index of 17%. Measured flame heights based on CH* emission below, at, or slightly above the smoke point, as well as predicted stoichiometric flame heights, are found to be proportional to ζ=Vf[D∞ln(1+1/S)]-1(T∞/Tad)0.67, where Vf, D∞, S, and Tad are the volumetric flow rate of ethylene, the diffusion coefficient at ambient temperature, the stoichiometric molecular air-to-fuel ratio, and the adiabatic flame temperature. Soot formation processes are found to increase with the oxygen index, leading to higher values of the maximum soot volume fraction and the peak integrated soot volume fraction. In addition, the latter is reached at a nondimensional height (normalized by ζ of approximately 0.05, regardless of the oxygen index within the investigated range. Two regimes are evidenced: The first regime, occurring for oxygen indices lower than 25%, is dominated by soot oxidation and is characterized by an enhancement in both the maximum soot volume fraction and the fuel flow rate at the smoke point with the oxygen index. The second regime, occurring for oxygen indices higher than 25%, is dominated by soot formation; the rate of increase in the maximum soot volume fraction with the oxygen concentration is lower than in the first regime, whereas the fuel flow rate at the smoke point decreases. Finally, the peak of radiative flux increases with the oxygen index, but its rate of increase is also found to be considerably reduced for oxygen indices greater than 25%.
Publication date
LanguageEnglish
AffiliationNational Research Council Canada (NRC-CNRC); Measurement Science and Standards (MSS-SME)
Peer reviewedYes
NRC numberNRC-MSS-CNF_CNF_D-12-00219
NPARC number21269725
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Record identifier8c274256-74ce-445b-90b0-3da04cb77c37
Record created2013-12-13
Record modified2016-05-09
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