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Title: Characterisation of two-stage ignition in diesel engine-relevant thermochemical conditions using direct numerical simulation

Abstract

With the goal of providing a more detailed fundamental understanding of ignition processes in diesel engines, this study reports analysis of a direct numerical simulation (DNS) database. In the DNS, a pseudo turbulent mixing layer of dimethyl ether (DME) at 400 K and air at 900 K is simulated at a pressure of 40 atmospheres. At these conditions, DME exhibits a two-stage ignition and resides within the negative temperature coefficient (NTC) regime of ignition delay times, similar to diesel fuel. The analysis reveals a complex ignition process with several novel features. Autoignition occurs as a distributed, two-stage event. The high-temperature stage of ignition establishes edge flames that have a hybrid premixed/autoignition flame structure similar to that previously observed for lifted laminar flames at similar thermochemical conditions. In conclusion, a combustion mode analysis based on key radical species illustrates the multi-stage and multi-mode nature of the ignition process and highlights the substantial modelling challenge presented by diesel combustion.

Authors:
ORCiD logo [1];  [2];  [3];  [4];  [4]
  1. Sandia National Lab. (SNL-CA), Livermore, CA (United States); The Univ. of New South Wales, Sydney, NSW (Australia)
  2. The Univ. of New South Wales, Sydney, NSW (Australia)
  3. The Univ. of Melbourne, Melbourne, VIC (Australia)
  4. Sandia National Lab. (SNL-CA), Livermore, CA (United States)
Publication Date:
Research Org.:
Sandia National Lab. (SNL-NM), Albuquerque, NM (United States); Sandia National Laboratories, Livermore, CA (United States)
Sponsoring Org.:
USDOE Office of Science (SC), Basic Energy Sciences (BES) (SC-22)
OSTI Identifier:
1326054
Report Number(s):
SAND-2016-5708J
Journal ID: ISSN 0010-2180; 643523
Grant/Contract Number:
AC04-94AL85000; SC0001198
Resource Type:
Journal Article: Accepted Manuscript
Journal Name:
Combustion and Flame
Additional Journal Information:
Journal Volume: 172; Journal Issue: C; Journal ID: ISSN 0010-2180
Publisher:
Elsevier
Country of Publication:
United States
Language:
English
Subject:
97 MATHEMATICS AND COMPUTING; 33 ADVANCED PROPULSION SYSTEMS; diesel-relevant; autoignition; edge flame; cool flame; negative temperature coefficient; direct numerical simulation

Citation Formats

Krisman, Alex, Hawkes, Evatt R., Talei, Mohsen, Bhagatwala, Ankit, and Chen, Jacqueline H. Characterisation of two-stage ignition in diesel engine-relevant thermochemical conditions using direct numerical simulation. United States: N. p., 2016. Web. doi:10.1016/j.combustflame.2016.06.010.
Krisman, Alex, Hawkes, Evatt R., Talei, Mohsen, Bhagatwala, Ankit, & Chen, Jacqueline H. Characterisation of two-stage ignition in diesel engine-relevant thermochemical conditions using direct numerical simulation. United States. doi:10.1016/j.combustflame.2016.06.010.
Krisman, Alex, Hawkes, Evatt R., Talei, Mohsen, Bhagatwala, Ankit, and Chen, Jacqueline H. 2016. "Characterisation of two-stage ignition in diesel engine-relevant thermochemical conditions using direct numerical simulation". United States. doi:10.1016/j.combustflame.2016.06.010. https://www.osti.gov/servlets/purl/1326054.
@article{osti_1326054,
title = {Characterisation of two-stage ignition in diesel engine-relevant thermochemical conditions using direct numerical simulation},
author = {Krisman, Alex and Hawkes, Evatt R. and Talei, Mohsen and Bhagatwala, Ankit and Chen, Jacqueline H.},
abstractNote = {With the goal of providing a more detailed fundamental understanding of ignition processes in diesel engines, this study reports analysis of a direct numerical simulation (DNS) database. In the DNS, a pseudo turbulent mixing layer of dimethyl ether (DME) at 400 K and air at 900 K is simulated at a pressure of 40 atmospheres. At these conditions, DME exhibits a two-stage ignition and resides within the negative temperature coefficient (NTC) regime of ignition delay times, similar to diesel fuel. The analysis reveals a complex ignition process with several novel features. Autoignition occurs as a distributed, two-stage event. The high-temperature stage of ignition establishes edge flames that have a hybrid premixed/autoignition flame structure similar to that previously observed for lifted laminar flames at similar thermochemical conditions. In conclusion, a combustion mode analysis based on key radical species illustrates the multi-stage and multi-mode nature of the ignition process and highlights the substantial modelling challenge presented by diesel combustion.},
doi = {10.1016/j.combustflame.2016.06.010},
journal = {Combustion and Flame},
number = C,
volume = 172,
place = {United States},
year = 2016,
month = 8
}

Journal Article:
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  • Cited by 2
  • In diesel engines, combustion is initiated by a two-staged autoignition that includes both low- and high-temperature chemistry. The location and timing of both stages of autoignition are important parameters that influence the development and stabilisation of the flame. In this study, a two-dimensional direct numerical simulation (DNS) is conducted to provide a fully resolved description of ignition at diesel engine-relevant conditions. The DNS is performed at a pressure of 40 atmospheres and at an ambient temperature of 900 K using dimethyl ether (DME) as the fuel, with a 30 species reduced chemical mechanism. At these conditions, similar to diesel fuel,more » DME exhibits two-stage ignition. The focus of this study is on the behaviour of the low-temperature chemistry (LTC) and the way in which it influences the high-temperature ignition. The results show that the LTC develops as a “spotty” first-stage autoignition in lean regions which transitions to a diffusively supported cool-flame and then propagates up the local mixture fraction gradient towards richer regions. The cool-flame speed is much faster than can be attributed to spatial gradients in first-stage ignition delay time in homogeneous reactors. The cool-flame causes a shortening of the second-stage ignition delay times compared to a homogeneous reactor and the shortening becomes more pronounced at richer mixtures. Multiple high-temperature ignition kernels are observed over a range of rich mixtures that are much richer than the homogeneous most reactive mixture and most kernels form much earlier than suggested by the homogeneous ignition delay time of the corresponding local mixture. Altogether, the results suggest that LTC can strongly influence both the timing and location in composition space of the high-temperature ignition.« less
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