Effects of differential diffusion on hydrogen flame kernel development under engine conditions
Effects of differential diffusion on hydrogen flame kernel development under engine conditions
The early flame kernel development in spark ignition engines is crucial for engine performance. For non-unity-Lewis-number mixtures, it can be significantly influenced by differential diffusion due to the large curvature of the small kernel. Differential diffusion can lead to thermodiffusive instabilities for lean hydrogen/air flames, which are enhanced for high pressures but diminish for increasing temperatures. In this study, direct numerical simulations of lean hydrogen flame kernels under engine conditions have been performed to investigate how differential diffusion affects their growth with an effective Lewis number far smaller than unity and if thermodiffusive instabilities appear under realistic engine conditions with elevated in-cylinder pressure and high unburned temperature. It is found that turbulence triggers the instabilities for flame kernel sizes far smaller than the critical radius of the onset of cellular instabilities for laminar flames. The strong thermodiffusive instabilities significantly facilitate the flame kernel growth. The normalized fuel consumption rate is increased by a factor of up to four, due to an enhanced propagation speed. This is remarkable as it was found in earlier studies that for laminar flames the effects of instabilities become much weaker under these conditions. Thermodiffusive instabilities also lead to large variations of the local fuel/air equivalence ratio resulting in temperatures up to 500 K above the adiabatic temperature, which impacts NOx formation. In addition, thermodiffusive instabilities alter the mechanisms of flame surface area formation. The production and destruction of the surface area by flame propagation are significantly increased. A transition phase can be identified during the formation of the negative curvature regions from the initial spherical kernel.
Direct numerical simulation, Flame kernel, Hydrogen engine, Thermodiffusive instability
2129-2138
Chu, Hongchao
10bf0051-928b-4829-b069-f364ce0cc134
Berger, Lukas
d7eee085-2362-42c0-ba19-85d799b67367
Grenga, Temistocle
be0eba30-74b5-4134-87e7-3a2d6dd3836f
Wu, Zhao
9af2995f-45b0-4b17-8522-8c98b4624b46
Pitsch, Heinz
3dc0eb6e-deca-4742-98a1-f0cdd62ff8b8
7 June 2023
Chu, Hongchao
10bf0051-928b-4829-b069-f364ce0cc134
Berger, Lukas
d7eee085-2362-42c0-ba19-85d799b67367
Grenga, Temistocle
be0eba30-74b5-4134-87e7-3a2d6dd3836f
Wu, Zhao
9af2995f-45b0-4b17-8522-8c98b4624b46
Pitsch, Heinz
3dc0eb6e-deca-4742-98a1-f0cdd62ff8b8
Chu, Hongchao, Berger, Lukas, Grenga, Temistocle, Wu, Zhao and Pitsch, Heinz
(2023)
Effects of differential diffusion on hydrogen flame kernel development under engine conditions.
Proceedings of the Combustion Institute, 39 (2), .
(doi:10.1016/j.proci.2022.07.042).
Abstract
The early flame kernel development in spark ignition engines is crucial for engine performance. For non-unity-Lewis-number mixtures, it can be significantly influenced by differential diffusion due to the large curvature of the small kernel. Differential diffusion can lead to thermodiffusive instabilities for lean hydrogen/air flames, which are enhanced for high pressures but diminish for increasing temperatures. In this study, direct numerical simulations of lean hydrogen flame kernels under engine conditions have been performed to investigate how differential diffusion affects their growth with an effective Lewis number far smaller than unity and if thermodiffusive instabilities appear under realistic engine conditions with elevated in-cylinder pressure and high unburned temperature. It is found that turbulence triggers the instabilities for flame kernel sizes far smaller than the critical radius of the onset of cellular instabilities for laminar flames. The strong thermodiffusive instabilities significantly facilitate the flame kernel growth. The normalized fuel consumption rate is increased by a factor of up to four, due to an enhanced propagation speed. This is remarkable as it was found in earlier studies that for laminar flames the effects of instabilities become much weaker under these conditions. Thermodiffusive instabilities also lead to large variations of the local fuel/air equivalence ratio resulting in temperatures up to 500 K above the adiabatic temperature, which impacts NOx formation. In addition, thermodiffusive instabilities alter the mechanisms of flame surface area formation. The production and destruction of the surface area by flame propagation are significantly increased. A transition phase can be identified during the formation of the negative curvature regions from the initial spherical kernel.
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More information
Accepted/In Press date: 5 July 2022
e-pub ahead of print date: 20 August 2022
Published date: 7 June 2023
Additional Information:
Funding Information:
H.C. and H.P. acknowledge the funding by the Deutsche Forschungsgemeinschaft (DFG, German Research Foundation) under Research Unit FOR2687 . L.B., T.G., and H.P. acknowledge the European Union’s Horizon 2020 research and innovation program under the Center of Excellence in Combustion (CoEC) project, grant agreement no. 952181. The authors gratefully acknowledge the Gauss Centre for Supercomputing e.V. ( www.gauss-centre.eu ) for funding this project by providing computing time on the GCS Supercomputer Super-MUC at Leibniz Supercomputing Centre (LRZ, www.lrz.de ). Data analyses were performed with computing resources granted by RWTH Aachen University under project rwth0682.
Keywords:
Direct numerical simulation, Flame kernel, Hydrogen engine, Thermodiffusive instability
Identifiers
Local EPrints ID: 485141
URI: http://eprints.soton.ac.uk/id/eprint/485141
ISSN: 1540-7489
PURE UUID: 59897507-50f5-4f1d-a7ed-9934107f2e9f
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Date deposited: 30 Nov 2023 17:36
Last modified: 06 Jun 2024 02:16
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Contributors
Author:
Hongchao Chu
Author:
Lukas Berger
Author:
Temistocle Grenga
Author:
Zhao Wu
Author:
Heinz Pitsch
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