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Title: Extinction of premixed H{sub 2}/air flames: Chemical kinetics and molecular diffusion effects

Abstract

Laminar flame speed has traditionally been used for the partial validation of flame kinetics. In most cases, however, its accurate determination requires extensive data processing and/or extrapolations, thus rendering the measurement of this fundamental flame property indirect. Additionally, the presence of flame front instabilities does not conform to the definition of laminar flame speed. This is the case for Le<1 flames, with the most notable example being ultralean H{sub 2}/air flames, which develop cellular structures at low strain rates so that determination of laminar flame speeds for such mixtures is not possible. Thus, this low-temperature regime of H{sub 2} oxidation has not been validated systematically in flames. In the present investigation, an alternative/supplemental approach is proposed that includes the experimental determination of extinction strain rates for these flames, and these rates are compared with the predictions of direct numerical simulations. This approach is meaningful for two reasons: (1) Extinction strain rates can be measured directly, as opposed to laminar flame speeds, and (2) while the unstretched lean H{sub 2}/air flames are cellular, the stretched ones are not, thus making comparisons between experiment and simulations meaningful. Such comparisons revealed serious discrepancies between experiments and simulations for ultralean H{sub 2}/air flames bymore » using four kinetic mechanisms. Additional studies were conducted for lean and near-stoichiometric H{sub 2}/air flames diluted with various amounts of N{sub 2}. Similarly to the ultralean flames, significant discrepancies between experimental and predicted extinction strain rates were also found. To identify the possible sources of such discrepancies, the effect of uncertainties on the diffusion coefficients was assessed and an improved treatment of diffusion coefficients was advanced and implemented. Under the conditions considered in this study, the sensitivity of diffusion coefficients to the extinction response was found to be significant and, for certain species, greater than that of the kinetic rate constants.« less

Authors:
; ; ; ;  [1];  [2];  [3]
  1. Department of Aerospace and Mechanical Engineering, University of Southern California, Los Angeles, CA 90089-1453 (United States)
  2. Exponent, Natick, MA 01760 (United States)
  3. Department of Mechanical Engineering, University of Delaware, Newark, DE 19716 (United States)
Publication Date:
OSTI Identifier:
20677704
Resource Type:
Journal Article
Resource Relation:
Journal Name: Combustion and Flame; Journal Volume: 142; Journal Issue: 4; Other Information: Elsevier Ltd. All rights reserved
Country of Publication:
United States
Language:
English
Subject:
08 HYDROGEN; HYDROGEN; AIR; COMBUSTION KINETICS; COMPUTERIZED SIMULATION; NITROGEN

Citation Formats

Dong, Yufei, Holley, Adam T., Andac, Mustafa G., Egolfopoulos, Fokion N., Wang, Hai, Davis, Scott G., and Middha, Prankul. Extinction of premixed H{sub 2}/air flames: Chemical kinetics and molecular diffusion effects. United States: N. p., 2005. Web. doi:10.1016/j.combustflame.2005.03.017.
Dong, Yufei, Holley, Adam T., Andac, Mustafa G., Egolfopoulos, Fokion N., Wang, Hai, Davis, Scott G., & Middha, Prankul. Extinction of premixed H{sub 2}/air flames: Chemical kinetics and molecular diffusion effects. United States. doi:10.1016/j.combustflame.2005.03.017.
Dong, Yufei, Holley, Adam T., Andac, Mustafa G., Egolfopoulos, Fokion N., Wang, Hai, Davis, Scott G., and Middha, Prankul. 2005. "Extinction of premixed H{sub 2}/air flames: Chemical kinetics and molecular diffusion effects". United States. doi:10.1016/j.combustflame.2005.03.017.
@article{osti_20677704,
title = {Extinction of premixed H{sub 2}/air flames: Chemical kinetics and molecular diffusion effects},
author = {Dong, Yufei and Holley, Adam T. and Andac, Mustafa G. and Egolfopoulos, Fokion N. and Wang, Hai and Davis, Scott G. and Middha, Prankul},
abstractNote = {Laminar flame speed has traditionally been used for the partial validation of flame kinetics. In most cases, however, its accurate determination requires extensive data processing and/or extrapolations, thus rendering the measurement of this fundamental flame property indirect. Additionally, the presence of flame front instabilities does not conform to the definition of laminar flame speed. This is the case for Le<1 flames, with the most notable example being ultralean H{sub 2}/air flames, which develop cellular structures at low strain rates so that determination of laminar flame speeds for such mixtures is not possible. Thus, this low-temperature regime of H{sub 2} oxidation has not been validated systematically in flames. In the present investigation, an alternative/supplemental approach is proposed that includes the experimental determination of extinction strain rates for these flames, and these rates are compared with the predictions of direct numerical simulations. This approach is meaningful for two reasons: (1) Extinction strain rates can be measured directly, as opposed to laminar flame speeds, and (2) while the unstretched lean H{sub 2}/air flames are cellular, the stretched ones are not, thus making comparisons between experiment and simulations meaningful. Such comparisons revealed serious discrepancies between experiments and simulations for ultralean H{sub 2}/air flames by using four kinetic mechanisms. Additional studies were conducted for lean and near-stoichiometric H{sub 2}/air flames diluted with various amounts of N{sub 2}. Similarly to the ultralean flames, significant discrepancies between experimental and predicted extinction strain rates were also found. To identify the possible sources of such discrepancies, the effect of uncertainties on the diffusion coefficients was assessed and an improved treatment of diffusion coefficients was advanced and implemented. Under the conditions considered in this study, the sensitivity of diffusion coefficients to the extinction response was found to be significant and, for certain species, greater than that of the kinetic rate constants.},
doi = {10.1016/j.combustflame.2005.03.017},
journal = {Combustion and Flame},
number = 4,
volume = 142,
place = {United States},
year = 2005,
month = 9
}
  • The structure and the mechanism of extinction of partially premixed diffusion flames is investigated theoretically and experimentally for the practically important case of small values of the stoichiometric fuel-to-mass ratio. The theory predicts two nonequilibrium flame structures that are also observed experimentally. The predicted ratio of extinction Damkoehler numbers is compared with measurements of the ratio of velocity gradients at extinction. Although the comparison shows systematic differences, the prediction of a decrease of this ratio with increased premixing is confirmed. 13 references.
  • A stagnation flow reactor was used to study the effects of platinum on the lean flammability limits of atmospheric pressure premixed methane/air flames at moderate stagnation surface temperatures. Experimental and computational methods were used to quantify the equivalence ratio at the lean extinction limit ({phi}{sub ext}) and the corresponding stagnation surface temperature (T{sub s}). A range of flow rates (57-90 cm/s) and corresponding strain rates were considered. The results indicate that the gas-phase methane/air flames are sufficiently strong relative to the heterogeneous chemistry for T{sub s} conditions less than 750 K that the platinum does not affect {phi}{sub ext}. Themore » computational results are in good agreement with the experimentally observed trends and further indicate that higher reactant flow rates (>139 cm/s) and levels of dilution (>{proportional_to}10% N{sub 2}) are required to weaken the gas-phase flame sufficiently for surface reaction to play a positive role on extending the lean flammability limits. (author)« less
  • Abstract not provided.
  • Here, the effect of differential molecular diffusion (DMD) in turbulent non-premixed flames is studied by examining two previously reported DNS of temporally evolving planar jet flames, one with CO/H 2 as the fuel and the other with C 2H 4 as the fuel. The effect of DMD in the CO/H 2 DNS flames in which H 2 is part of fuel is found to behave similar to laminar flamelet, while in the C 2H 4 DNS flames in which H 2 is not present in the fuel it is similar to laminar flamelet in early stages but becomes different frommore » laminar flamelet later. The scaling of the effect of DMD with respect to the Reynolds number Re is investigated in the CO/H 2 DNS flames, and an evident power law scaling (~Re –a with a a positive constant) is observed. The scaling of the effect of DMD with respect to the Damkohler number Da is explored in both laminar counter-flow jet C 2H 4 diffusion flames and the C 2H 4 DNS flames. A power law scaling (~ Daa with a a positive constant) is clearly demonstrated for C 2H 4 nonpremixed flames.« less