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Effect of split fuel injection on heat release and pollutant emissions in partially premixed combustion of PRF70/air/EGR mixtures

Zhang, Fan LU ; Yu, Rixin LU and Bai, Xue-Song LU (2015) In Applied Energy 149. p.283-296
Abstract
Two- and three-dimensional direct numerical simulations are performed to study the combustion process of PRF70/air/exhaust gas mixtures under partially premixed combustion (PPC) conditions relevant to modern low temperature internal combustion engines. The aim is to gain improved understanding of the underlining physics in PPC engines. A skeletal PRF chemical kinetic mechanism is used together with full transport properties. The 2D and 3D numerical simulations are performed on respectively a 0.614 x 0.614 mm(2) domain and a 0.614 x 0.614 x 0.614 mm(3) domain with a 1.2 mu m cell size. The results reveal the effects of the split of the fuel mass in different injections on the combustion and emission process in PPC engines. Increasing the... (More)
Two- and three-dimensional direct numerical simulations are performed to study the combustion process of PRF70/air/exhaust gas mixtures under partially premixed combustion (PPC) conditions relevant to modern low temperature internal combustion engines. The aim is to gain improved understanding of the underlining physics in PPC engines. A skeletal PRF chemical kinetic mechanism is used together with full transport properties. The 2D and 3D numerical simulations are performed on respectively a 0.614 x 0.614 mm(2) domain and a 0.614 x 0.614 x 0.614 mm(3) domain with a 1.2 mu m cell size. The results reveal the effects of the split of the fuel mass in different injections on the combustion and emission process in PPC engines. Increasing the amount of second fuel injection results in a retarded heat release and decreased NO emission, however, increased CO emission. While this CO/NOx tradeoff has been observed in previous PPC engine experiments the fundamental reason for this is clarified here. PPC is shown to consist of a two-stage combustion process: in the first stage the stratified fuel! air mixture auto-ignites, which results in partial oxidation of the fuel in the fuel-rich region and a mixture of radicals and hot products in the fuel-lean region. In the second stage the partially oxidized fuel/air mixture is oxidized in a thin diffusion flame where the diffusion and chemical reaction both play an important role. The split of fuel mass in different injections essentially affect the amount of fuel/air in the fuel-lean region and the fuel-rich region, thereby the relative importance of the diffusion flame. This in turn affects the emissions of NO and CO. The effects of turbulence on the heat release rate, pressure-rise-rate and emissions are a manifestation of the change of the reaction zone structures. At high turbulence intensities and large integral lengths a more homogeneous mixture is achieved, which promotes the volumetric ignition stage, speeds up the heat release rate, increases the pressure-rise-rate, and increases the NO formation and CO oxidation rates. The existence of both volumetric ignition and diffusion flame in PPC poses a great challenge for numerical simulations of PPC engines. (C) 2015 Elsevier Ltd. All rights reserved. (Less)
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author
organization
publishing date
type
Contribution to journal
publication status
published
subject
keywords
Direct numerical simulation, Partially premixed combustion, IC engines
in
Applied Energy
volume
149
pages
283 - 296
publisher
Elsevier
external identifiers
  • wos:000355500100025
  • scopus:84927640914
ISSN
1872-9118
DOI
10.1016/j.apenergy.2015.03.058
language
English
LU publication?
yes
id
9af04dc7-48b0-412d-8ddc-e4bc09cd2e42 (old id 7606173)
date added to LUP
2015-07-21 17:04:55
date last changed
2017-09-03 04:21:45
@article{9af04dc7-48b0-412d-8ddc-e4bc09cd2e42,
  abstract     = {Two- and three-dimensional direct numerical simulations are performed to study the combustion process of PRF70/air/exhaust gas mixtures under partially premixed combustion (PPC) conditions relevant to modern low temperature internal combustion engines. The aim is to gain improved understanding of the underlining physics in PPC engines. A skeletal PRF chemical kinetic mechanism is used together with full transport properties. The 2D and 3D numerical simulations are performed on respectively a 0.614 x 0.614 mm(2) domain and a 0.614 x 0.614 x 0.614 mm(3) domain with a 1.2 mu m cell size. The results reveal the effects of the split of the fuel mass in different injections on the combustion and emission process in PPC engines. Increasing the amount of second fuel injection results in a retarded heat release and decreased NO emission, however, increased CO emission. While this CO/NOx tradeoff has been observed in previous PPC engine experiments the fundamental reason for this is clarified here. PPC is shown to consist of a two-stage combustion process: in the first stage the stratified fuel! air mixture auto-ignites, which results in partial oxidation of the fuel in the fuel-rich region and a mixture of radicals and hot products in the fuel-lean region. In the second stage the partially oxidized fuel/air mixture is oxidized in a thin diffusion flame where the diffusion and chemical reaction both play an important role. The split of fuel mass in different injections essentially affect the amount of fuel/air in the fuel-lean region and the fuel-rich region, thereby the relative importance of the diffusion flame. This in turn affects the emissions of NO and CO. The effects of turbulence on the heat release rate, pressure-rise-rate and emissions are a manifestation of the change of the reaction zone structures. At high turbulence intensities and large integral lengths a more homogeneous mixture is achieved, which promotes the volumetric ignition stage, speeds up the heat release rate, increases the pressure-rise-rate, and increases the NO formation and CO oxidation rates. The existence of both volumetric ignition and diffusion flame in PPC poses a great challenge for numerical simulations of PPC engines. (C) 2015 Elsevier Ltd. All rights reserved.},
  author       = {Zhang, Fan and Yu, Rixin and Bai, Xue-Song},
  issn         = {1872-9118},
  keyword      = {Direct numerical simulation,Partially premixed combustion,IC engines},
  language     = {eng},
  pages        = {283--296},
  publisher    = {Elsevier},
  series       = {Applied Energy},
  title        = {Effect of split fuel injection on heat release and pollutant emissions in partially premixed combustion of PRF70/air/EGR mixtures},
  url          = {http://dx.doi.org/10.1016/j.apenergy.2015.03.058},
  volume       = {149},
  year         = {2015},
}