期刊
COMBUSTION AND FLAME
卷 195, 期 -, 页码 183-202出版社
ELSEVIER SCIENCE INC
DOI: 10.1016/j.combustflame.2018.02.020
关键词
DNS; Cool flames; Low-temperature diesel combustion
类别
资金
- US Department of Energy, Office of Basic Energy Sciences, Division of Chemical Sciences, Geosciences, and Biosciences
- NSF/DOE Partnership on Advanced Combustion Engines Program [CBET-1258646]
- U.S. Department of Energy's National Nuclear Security Administration [DE-NA-0003525]
- Department of Energy's INCITE award
- Office of Science of the U.S. Department of Energy [DE-AC05-00OR22725]
We present a direct numerical simulation of a temporal jet between n-dodecane and diluted air undergoing spontaneous ignition at conditions relevant to low-temperature diesel combustion. The jet thermochemical conditions were selected to result in two-stage ignition. Reaction rates were computed using a 35-species reduced mechanism which included both the low- and high-temperature reaction pathways. The aim of this study is to elucidate the mechanisms by which low-temperature reactions promote high-temperature ignition under turbulent, non-premixed conditions. We show that low-temperature heat release in slightly rich fuel regions initiates multiple cool flame kernels that propagate towards very rich fuel regions through a reaction-diffusion mechanism. Although low-temperature ignition is delayed by imperfect mixing, the propagation speed of the cool flames is high: as a consequence, high-temperature reactions in fuel-rich regions become active early during the ignition transient. Because of this early start, high-temperature ignition, which occurs in fuel-rich regions, is faster than homogeneous ignition. Following ignition, the high-temperature kernels expand and engulf the stoichiometric mixture-fraction iso-surface which in turn establish edge flames which propagate along the iso-surface. The present results indicate the preponderance of flame folding of existing burning surfaces, and that ignition due to edge-flame propagation is of lesser importance.. Finally, a combustion mode analysis that extends an earlier classification [1] is proposed to conceptualize the multi-stage and multi-mode nature of diesel combustion and to provide a framework for reasoning about the effects of different ambient conditions on diesel combustion. (C) 2018 The Combustion Institute. Published by Elsevier Inc. All rights reserved.
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