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

Published online by Cambridge University Press:  04 July 2017

Alex Krisman Open the ORCID record for Alex Krisman [Opens in a new window] ,
Evatt R. Hawkes  and
Jacqueline H. Chen
Alex Krisman*
Affiliation:
School of Mechanical and Manufacturing Engineering, University of New South Wales, Kensington, NSW 2052, Australia Combustion Research Facility, Sandia National Laboratories, Livermore, CA 94550, USA
Evatt R. Hawkes
Affiliation:
School of Mechanical and Manufacturing Engineering, University of New South Wales, Kensington, NSW 2052, Australia School of Photovoltaics and Renewable Energy Engineering, University of New South Wales, Kensington, NSW 2052, Australia
Jacqueline H. Chen
Affiliation:
Combustion Research Facility, Sandia National Laboratories, Livermore, CA 94550, USA
*
†Email address for correspondence: ankrism@sandia.gov
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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 the overall ignition process. 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.

JFM classification

Reacting Flows: Combustion Reacting Flows: Flames Reacting Flows: Turbulent reacting flows
Type
Papers
Information
Journal of Fluid Mechanics , Volume 824 , 10 August 2017 , pp. 5 - 41
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© 2017 Cambridge University Press 

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