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Title: Two-stage autoignition and edge flames in a high pressure turbulent jet

Abstract

A three-dimensional direct numerical simulation is conducted for a temporally evolving planar jet of n-heptane at a pressure of 40 atmospheres and in a coflow of air at 1100 K. At these conditions, n-heptane exhibits a two-stage ignition due to low- and high-temperature chemistry, which is reproduced by the global chemical model used in this study. The results show that ignition occurs in several overlapping stages and multiple modes of combustion are present. Low-temperature chemistry precedes the formation of multiple spatially localised high-temperature chemistry autoignition events, referred to as ‘kernels’. These kernels form within the shear layer and core of the jet at compositions with short homogeneous ignition delay times and in locations experiencing low scalar dissipation rates. An analysis of the kernel histories shows that the ignition delay time is correlated with the mixing rate history and that the ignition kernels tend to form in vortically dominated regions of the domain, as corroborated by an analysis of the topology of the velocity gradient tensor. Once ignited, the kernels grow rapidly and establish edge flames where they envelop the stoichiometric isosurface. A combination of kernel formation (autoignition) and the growth of existing burning surface (via edge-flame propagation) contributes to themore » overall ignition process. In conclusion, an analysis of propagation speeds evaluated on the burning surface suggests that although the edge-flame speed is promoted by the autoignitive conditions due to an increase in the local laminar flame speed, edge-flame propagation of existing burning surfaces (triggered initially by isolated autoignition kernels) is the dominant ignition mode in the present configuration.« less

Authors:
ORCiD logo [1];  [2];  [3]
  1. Univ. of New South Wales, Kensington, NSW (Australia); Sandia National Lab. (SNL-CA), Livermore, CA (United States)
  2. Univ. of New South Wales, Kensington, NSW (Australia)
  3. Sandia National Lab. (SNL-CA), Livermore, CA (United States)
Publication Date:
Research Org.:
Sandia National Lab. (SNL-NM), Albuquerque, NM (United States); Sandia National Lab. (SNL-CA), Livermore, CA (United States)
Sponsoring Org.:
USDOE Office of Science (SC)
OSTI Identifier:
1372357
Report Number(s):
SAND-2017-4187J
Journal ID: ISSN 0022-1120; applab; PII: S0022112017002828
Grant/Contract Number:
AC04-94AL85000
Resource Type:
Journal Article: Accepted Manuscript
Journal Name:
Journal of Fluid Mechanics
Additional Journal Information:
Journal Volume: 824; Journal ID: ISSN 0022-1120
Publisher:
Cambridge University Press
Country of Publication:
United States
Language:
English
Subject:
37 INORGANIC, ORGANIC, PHYSICAL, AND ANALYTICAL CHEMISTRY

Citation Formats

Krisman, Alex, Hawkes, Evatt R., and Chen, Jacqueline H. Two-stage autoignition and edge flames in a high pressure turbulent jet. United States: N. p., 2017. Web. doi:10.1017/jfm.2017.282.
Krisman, Alex, Hawkes, Evatt R., & Chen, Jacqueline H. Two-stage autoignition and edge flames in a high pressure turbulent jet. United States. doi:10.1017/jfm.2017.282.
Krisman, Alex, Hawkes, Evatt R., and Chen, Jacqueline H. 2017. "Two-stage autoignition and edge flames in a high pressure turbulent jet". United States. doi:10.1017/jfm.2017.282.
@article{osti_1372357,
title = {Two-stage autoignition and edge flames in a high pressure turbulent jet},
author = {Krisman, Alex and Hawkes, Evatt R. and Chen, Jacqueline H.},
abstractNote = {A three-dimensional direct numerical simulation is conducted for a temporally evolving planar jet of n-heptane at a pressure of 40 atmospheres and in a coflow of air at 1100 K. At these conditions, n-heptane exhibits a two-stage ignition due to low- and high-temperature chemistry, which is reproduced by the global chemical model used in this study. The results show that ignition occurs in several overlapping stages and multiple modes of combustion are present. Low-temperature chemistry precedes the formation of multiple spatially localised high-temperature chemistry autoignition events, referred to as ‘kernels’. These kernels form within the shear layer and core of the jet at compositions with short homogeneous ignition delay times and in locations experiencing low scalar dissipation rates. An analysis of the kernel histories shows that the ignition delay time is correlated with the mixing rate history and that the ignition kernels tend to form in vortically dominated regions of the domain, as corroborated by an analysis of the topology of the velocity gradient tensor. Once ignited, the kernels grow rapidly and establish edge flames where they envelop the stoichiometric isosurface. A combination of kernel formation (autoignition) and the growth of existing burning surface (via edge-flame propagation) contributes to the overall ignition process. In conclusion, an analysis of propagation speeds evaluated on the burning surface suggests that although the edge-flame speed is promoted by the autoignitive conditions due to an increase in the local laminar flame speed, edge-flame propagation of existing burning surfaces (triggered initially by isolated autoignition kernels) is the dominant ignition mode in the present configuration.},
doi = {10.1017/jfm.2017.282},
journal = {Journal of Fluid Mechanics},
number = ,
volume = 824,
place = {United States},
year = 2017,
month = 7
}

Journal Article:
Free Publicly Available Full Text
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