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Evolution of averaged local premixed flame thickness in a turbulent flow

Yu, Rixin LU ; Nilsson, Thommie LU ; Bai, Xue Song LU and Lipatnikov, Andrei N. (2019) In Combustion and Flame 207. p.232-249
Abstract

In the combustion literature, contradictory results on the influence of turbulence on the local thickness of a premixed flame can be found and the present paper aims at contributing to reconcile this issue. First, different measures of local flame thickness in a turbulent flow, e.g. area-weighted and unweighted surface-averaged values of (i) |∇c|, i.e., the absolute value of 3D gradient of the combustion progress variable c, or (ii) 1/|∇c|, are studied and analytical relationships/inequalities between them are obtained. Second, the evolution of the different flame thickness measures is explored by numerically evaluating them, as well as various terms in relevant evolution equations derived analytically. To do so, various measures and... (More)

In the combustion literature, contradictory results on the influence of turbulence on the local thickness of a premixed flame can be found and the present paper aims at contributing to reconcile this issue. First, different measures of local flame thickness in a turbulent flow, e.g. area-weighted and unweighted surface-averaged values of (i) |∇c|, i.e., the absolute value of 3D gradient of the combustion progress variable c, or (ii) 1/|∇c|, are studied and analytical relationships/inequalities between them are obtained. Second, the evolution of the different flame thickness measures is explored by numerically evaluating them, as well as various terms in relevant evolution equations derived analytically. To do so, various measures and terms are extracted from DNS data obtained from (i) a highly turbulent, constant-density, dynamically passive, single-reaction wave, (ii) moderately and highly turbulent, single-step-chemistry flames, and (iii) moderately and highly turbulent, complex-chemistry lean methane-air flames. In all those cases, all studied flame thickness measures are reduced during an early stage of premixed turbulent flame development, followed by local flame re-broadening at later stages. Analysis of various terms in the aforementioned evolution equations shows that the initial local flame thinning is controlled by turbulent strain rates. The subsequent local flame re-broadening is controlled by (i) curvature contribution to the stretch rate, which counter-balances the strain rate, (ii) spatial non-uniformities of the normal diffusion contribution to the local displacement-speed vector Sdn, and (iii) dilatation, which plays an important role in moderately turbulent flames, but a minor role in highly turbulent flames. Moreover, the present study shows that differently defined measures of a local flame thickness can be substantially different. This difference should also be borne in mind when comparing data that indicate local flame thinning with data that indicate local flame broadening.

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author
organization
publishing date
type
Contribution to journal
publication status
published
subject
keywords
Conditioned statistics, DNS, Flame thickness, Turbulent combustion, Turbulent reacting flow
in
Combustion and Flame
volume
207
pages
18 pages
publisher
Elsevier
external identifiers
  • scopus:85067237454
ISSN
0010-2180
DOI
10.1016/j.combustflame.2019.05.045
language
English
LU publication?
yes
id
8fee6181-0f58-4766-be7a-5e800005e273
date added to LUP
2019-06-26 12:05:20
date last changed
2019-09-11 04:20:30
@article{8fee6181-0f58-4766-be7a-5e800005e273,
  abstract     = {<p>In the combustion literature, contradictory results on the influence of turbulence on the local thickness of a premixed flame can be found and the present paper aims at contributing to reconcile this issue. First, different measures of local flame thickness in a turbulent flow, e.g. area-weighted and unweighted surface-averaged values of (i) |∇c|, i.e., the absolute value of 3D gradient of the combustion progress variable c, or (ii) 1/|∇c|, are studied and analytical relationships/inequalities between them are obtained. Second, the evolution of the different flame thickness measures is explored by numerically evaluating them, as well as various terms in relevant evolution equations derived analytically. To do so, various measures and terms are extracted from DNS data obtained from (i) a highly turbulent, constant-density, dynamically passive, single-reaction wave, (ii) moderately and highly turbulent, single-step-chemistry flames, and (iii) moderately and highly turbulent, complex-chemistry lean methane-air flames. In all those cases, all studied flame thickness measures are reduced during an early stage of premixed turbulent flame development, followed by local flame re-broadening at later stages. Analysis of various terms in the aforementioned evolution equations shows that the initial local flame thinning is controlled by turbulent strain rates. The subsequent local flame re-broadening is controlled by (i) curvature contribution to the stretch rate, which counter-balances the strain rate, (ii) spatial non-uniformities of the normal diffusion contribution to the local displacement-speed vector S<sub>d</sub>n, and (iii) dilatation, which plays an important role in moderately turbulent flames, but a minor role in highly turbulent flames. Moreover, the present study shows that differently defined measures of a local flame thickness can be substantially different. This difference should also be borne in mind when comparing data that indicate local flame thinning with data that indicate local flame broadening.</p>},
  author       = {Yu, Rixin and Nilsson, Thommie and Bai, Xue Song and Lipatnikov, Andrei N.},
  issn         = {0010-2180},
  keyword      = {Conditioned statistics,DNS,Flame thickness,Turbulent combustion,Turbulent reacting flow},
  language     = {eng},
  pages        = {232--249},
  publisher    = {Elsevier},
  series       = {Combustion and Flame},
  title        = {Evolution of averaged local premixed flame thickness in a turbulent flow},
  url          = {http://dx.doi.org/10.1016/j.combustflame.2019.05.045},
  volume       = {207},
  year         = {2019},
}