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Rate-ratio asymptotic analysis of the influence of stoichiometric mixture fraction on structure and extinction of laminar, nonpremixed methane flames with comparison to experiments

Mairhofer, Philipp; Mairinger, Gerald; Seshadri, Kalyanasundaram; Bai, Xue Song LU ; Seiser, Reinhard and Pucher, Ernst (2015) In Proceedings of the Combustion Institute
Abstract

Activation energy asymptotic analysis and rate-ratio asymptotic analysis of combustion in laminar, nonpremixed flames are often carried out using conserved scalar quantities as independent variables. One such representation of a conserved scalar quantity is the mixture fraction, ξ, based on thermal diffusivity. These analyses are carried out in the asymptotic limit of large Damköhler number, with chemical reactions presumed to take place in a thin reaction zone that is located at ξ=ξst. The quantity ξ st is the stoichiometric mixture fraction. A characteristic diffusion time is given by the reciprocal of the scalar dissipation rate, χ. Previous computational studies have shown that the scalar dissipation rate at extinction... (More)

Activation energy asymptotic analysis and rate-ratio asymptotic analysis of combustion in laminar, nonpremixed flames are often carried out using conserved scalar quantities as independent variables. One such representation of a conserved scalar quantity is the mixture fraction, ξ, based on thermal diffusivity. These analyses are carried out in the asymptotic limit of large Damköhler number, with chemical reactions presumed to take place in a thin reaction zone that is located at ξ=ξst. The quantity ξ st is the stoichiometric mixture fraction. A characteristic diffusion time is given by the reciprocal of the scalar dissipation rate, χ. Previous computational studies have shown that the scalar dissipation rate at extinction depends on ξ st and the maximum flame temperature, T st. Here, a rate-ratio asymptotic analysis is carried out using reduced chemistry to elucidate the influence of ξ st on critical conditions of extinction of methane flames. The scalar dissipation rate at extinction was predicted as a function of ξ st with the mass fractions of reactants so chosen that the adiabatic flame temperature, T st, is fixed. The predictions of the analysis show that with increasing values of ξ st, the scalar dissipation rate at extinction first increases and then decreases. To test the predictions of the asymptotic analysis, critical conditions of extinction are measured on nonpremixed methane flames stabilized in the counterflow configuration. With increasing values of stoichiometric mixture fraction, the strain rate at extinction was found to increase, and the scalar dissipation rate at extinction was found to first increase and then decrease. The predictions of the asymptotic analysis agreed with experiments. A key outcome of the analysis is that with increasing ξ st, the thickness of the regions where oxygen and fuel are consumed first increase and the decrease. This is responsible for the observed non-monotonic changes in the values of the scalar dissipation rate at extinction with changes in ξ st.

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author
organization
publishing date
type
Contribution to journal
publication status
epub
subject
keywords
Conserved scalar, Extinction, Flame structure, Methane flames, Rate-ratio asymptotic analysis
in
Proceedings of the Combustion Institute
publisher
Elsevier
external identifiers
  • scopus:84979599498
ISSN
1540-7489
DOI
10.1016/j.proci.2016.05.027
language
English
LU publication?
yes
id
c39d20f9-ddf7-46e0-8de3-bdad5ce8ac54
date added to LUP
2016-12-22 13:37:23
date last changed
2017-04-20 13:20:55
@article{c39d20f9-ddf7-46e0-8de3-bdad5ce8ac54,
  abstract     = {<p>Activation energy asymptotic analysis and rate-ratio asymptotic analysis of combustion in laminar, nonpremixed flames are often carried out using conserved scalar quantities as independent variables. One such representation of a conserved scalar quantity is the mixture fraction, ξ, based on thermal diffusivity. These analyses are carried out in the asymptotic limit of large Damköhler number, with chemical reactions presumed to take place in a thin reaction zone that is located at ξ=ξst. The quantity ξ <sub>st</sub> is the stoichiometric mixture fraction. A characteristic diffusion time is given by the reciprocal of the scalar dissipation rate, χ. Previous computational studies have shown that the scalar dissipation rate at extinction depends on ξ <sub>st</sub> and the maximum flame temperature, T <sub>st</sub>. Here, a rate-ratio asymptotic analysis is carried out using reduced chemistry to elucidate the influence of ξ <sub>st</sub> on critical conditions of extinction of methane flames. The scalar dissipation rate at extinction was predicted as a function of ξ <sub>st</sub> with the mass fractions of reactants so chosen that the adiabatic flame temperature, T <sub>st</sub>, is fixed. The predictions of the analysis show that with increasing values of ξ <sub>st</sub>, the scalar dissipation rate at extinction first increases and then decreases. To test the predictions of the asymptotic analysis, critical conditions of extinction are measured on nonpremixed methane flames stabilized in the counterflow configuration. With increasing values of stoichiometric mixture fraction, the strain rate at extinction was found to increase, and the scalar dissipation rate at extinction was found to first increase and then decrease. The predictions of the asymptotic analysis agreed with experiments. A key outcome of the analysis is that with increasing ξ <sub>st</sub>, the thickness of the regions where oxygen and fuel are consumed first increase and the decrease. This is responsible for the observed non-monotonic changes in the values of the scalar dissipation rate at extinction with changes in ξ <sub>st</sub>.</p>},
  author       = {Mairhofer, Philipp and Mairinger, Gerald and Seshadri, Kalyanasundaram and Bai, Xue Song and Seiser, Reinhard and Pucher, Ernst},
  issn         = {1540-7489},
  keyword      = {Conserved scalar,Extinction,Flame structure,Methane flames,Rate-ratio asymptotic analysis},
  language     = {eng},
  month        = {12},
  publisher    = {Elsevier},
  series       = {Proceedings of the Combustion Institute},
  title        = {Rate-ratio asymptotic analysis of the influence of stoichiometric mixture fraction on structure and extinction of laminar, nonpremixed methane flames with comparison to experiments},
  url          = {http://dx.doi.org/10.1016/j.proci.2016.05.027},
  year         = {2015},
}