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Title: A direct numerical simulation study of flame structure and stabilization of an experimental high Ka CH 4/air premixed jet flame

Abstract

In the present work, a direct numerical simulation (DNS) of an experimental high Karlovitz number (Ka) CH 4/air piloted premixed flame was analyzed to study the inner structure and the stabilization mechanism of the turbulent flame. A reduced chemical mechanism for premixed CH 4/air combustion with NO x based on GRI-Mech3.0 was used, including 268 elementary reactions and 28 transported species. The evolution of the stretch factor, I0, indicates that the burning rate per unit flame surface area is considerably reduced in the near field and exhibits a minimum at x/D = 8. Downstream, the burning rate gradually increases. The stretch factor is different between different species, suggesting the quenching of some reactions but not others. Comparison between the turbulent flame and strained laminar flames indicates that certain aspects of the mean flame structure can be represented surprisingly well by flamelets if changes in boundary conditions are accounted for and the strain rate of the mean flow is employed; however, the thickening of the flame due to turbulence is not captured. The spatial development of displacement speeds is studied at higher Ka than previous DNS. In contrast to almost all previous studies, the mean displacement speed conditioned on the flamemore » front is negative in the near field, and the dominant contribution to the displacement speed is normal diffusion with the reaction contribution being secondary. Further downstream, reaction overtakes normal diffusion, contributing to a positive displacement speed. The negative displacement speed in the near field implies that the flame front situates itself in the pilot region where the inner structure of the turbulent flame is affected significantly, and the flame stabilizes in balance with the inward flow. Notably, in the upstream region of the turbulent flame, the main reaction contributing to the production of OH, H+O 2⇌O+OH (R35), is weak. Moreover, oxidation reactions, H 2+OH⇌H+H 2O (R79) and CO+OH⇌CO 2+H (R94), are influenced by H 2O and CO 2 from the pilot and are completely quenched. Hence, the entire radical pool of OH, H and O is affected. Furthermore, the fuel consumption layer remains comparably active and generates heat, mainly via the reaction CH 4+OH⇌CH 3+H 2O (R93).« less

Authors:
 [1];  [1];  [2]
  1. The Univ. of New South Wales, NSW (Australia)
  2. Sandia National Lab. (SNL-CA), Livermore, CA (United States)
Publication Date:
Research Org.:
Sandia National Lab. (SNL-NM), Albuquerque, NM (United States)
Sponsoring Org.:
USDOE National Nuclear Security Administration (NNSA); USDOE Office of Science (SC), Basic Energy Sciences (BES) (SC-22)
OSTI Identifier:
1361216
Alternate Identifier(s):
OSTI ID: 1419138
Report Number(s):
SAND-2017-2196J
Journal ID: ISSN 0010-2180; PII: S001021801730069X
Grant/Contract Number:  
AC04-94AL85000; AC04-94-AL85000
Resource Type:
Journal Article: Accepted Manuscript
Journal Name:
Combustion and Flame
Additional Journal Information:
Journal Volume: 180; Journal Issue: C; Journal ID: ISSN 0010-2180
Publisher:
Elsevier
Country of Publication:
United States
Language:
English
Subject:
37 INORGANIC, ORGANIC, PHYSICAL, AND ANALYTICAL CHEMISTRY; direct numerical simulation; turbulent flame; flame structure; displacement speed; reaction rate

Citation Formats

Wang, Haiou, Hawkes, Evatt R., and Chen, Jacqueline H. A direct numerical simulation study of flame structure and stabilization of an experimental high Ka CH4/air premixed jet flame. United States: N. p., 2017. Web. doi:10.1016/j.combustflame.2017.02.022.
Wang, Haiou, Hawkes, Evatt R., & Chen, Jacqueline H. A direct numerical simulation study of flame structure and stabilization of an experimental high Ka CH4/air premixed jet flame. United States. doi:10.1016/j.combustflame.2017.02.022.
Wang, Haiou, Hawkes, Evatt R., and Chen, Jacqueline H. Fri . "A direct numerical simulation study of flame structure and stabilization of an experimental high Ka CH4/air premixed jet flame". United States. doi:10.1016/j.combustflame.2017.02.022. https://www.osti.gov/servlets/purl/1361216.
@article{osti_1361216,
title = {A direct numerical simulation study of flame structure and stabilization of an experimental high Ka CH4/air premixed jet flame},
author = {Wang, Haiou and Hawkes, Evatt R. and Chen, Jacqueline H.},
abstractNote = {In the present work, a direct numerical simulation (DNS) of an experimental high Karlovitz number (Ka) CH4/air piloted premixed flame was analyzed to study the inner structure and the stabilization mechanism of the turbulent flame. A reduced chemical mechanism for premixed CH4/air combustion with NOx based on GRI-Mech3.0 was used, including 268 elementary reactions and 28 transported species. The evolution of the stretch factor, I0, indicates that the burning rate per unit flame surface area is considerably reduced in the near field and exhibits a minimum at x/D = 8. Downstream, the burning rate gradually increases. The stretch factor is different between different species, suggesting the quenching of some reactions but not others. Comparison between the turbulent flame and strained laminar flames indicates that certain aspects of the mean flame structure can be represented surprisingly well by flamelets if changes in boundary conditions are accounted for and the strain rate of the mean flow is employed; however, the thickening of the flame due to turbulence is not captured. The spatial development of displacement speeds is studied at higher Ka than previous DNS. In contrast to almost all previous studies, the mean displacement speed conditioned on the flame front is negative in the near field, and the dominant contribution to the displacement speed is normal diffusion with the reaction contribution being secondary. Further downstream, reaction overtakes normal diffusion, contributing to a positive displacement speed. The negative displacement speed in the near field implies that the flame front situates itself in the pilot region where the inner structure of the turbulent flame is affected significantly, and the flame stabilizes in balance with the inward flow. Notably, in the upstream region of the turbulent flame, the main reaction contributing to the production of OH, H+O2⇌O+OH (R35), is weak. Moreover, oxidation reactions, H2+OH⇌H+H2O (R79) and CO+OH⇌CO2+H (R94), are influenced by H2O and CO2 from the pilot and are completely quenched. Hence, the entire radical pool of OH, H and O is affected. Furthermore, the fuel consumption layer remains comparably active and generates heat, mainly via the reaction CH4+OH⇌CH3+H2O (R93).},
doi = {10.1016/j.combustflame.2017.02.022},
journal = {Combustion and Flame},
number = C,
volume = 180,
place = {United States},
year = {Fri Mar 17 00:00:00 EDT 2017},
month = {Fri Mar 17 00:00:00 EDT 2017}
}

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