Rateratio asymptotic analysis of the influence of stoichiometric mixture fraction on structure and extinction of laminar, nonpremixed methane flames with comparison to experiments
(2015) In Proceedings of the Combustion Institute Abstract
Activation energy asymptotic analysis and rateratio 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 rateratio 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 rateratio 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 nonmonotonic changes in the values of the scalar dissipation rate at extinction with changes in ξ _{st}.
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 author
 Mairhofer, Philipp; Mairinger, Gerald; Seshadri, Kalyanasundaram; Bai, Xue Song ^{LU} ; Seiser, Reinhard and Pucher, Ernst
 organization
 publishing date
 20151204
 type
 Contribution to journal
 publication status
 epub
 subject
 keywords
 Conserved scalar, Extinction, Flame structure, Methane flames, Rateratio asymptotic analysis
 in
 Proceedings of the Combustion Institute
 publisher
 Elsevier
 external identifiers

 scopus:84979599498
 ISSN
 15407489
 DOI
 10.1016/j.proci.2016.05.027
 language
 English
 LU publication?
 yes
 id
 c39d20f9ddf746e08de3bdad5ce8ac54
 date added to LUP
 20161222 13:37:23
 date last changed
 20170420 13:20:55
@article{c39d20f9ddf746e08de3bdad5ce8ac54, abstract = {<p>Activation energy asymptotic analysis and rateratio 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 rateratio 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 nonmonotonic 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 = {15407489}, keyword = {Conserved scalar,Extinction,Flame structure,Methane flames,Rateratio asymptotic analysis}, language = {eng}, month = {12}, publisher = {Elsevier}, series = {Proceedings of the Combustion Institute}, title = {Rateratio 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}, }