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TECHNICAL PAPERS: Internal Combustion Engines

An Improved Soot Formation Model for 3D Diesel Engine Simulations

[+] Author and Article Information
Joan Boulanger1

 Institute for Chemical Process and Environmental Technology, 1200 Montréal Building M-9, Ottawa, ON, K1A0R6, Canadajoan.boulanger@nrc-cnrc.gc.ca

Fengshan Liu, W. Stuart Neill, Gregory J. Smallwood

 Institute for Chemical Process and Environmental Technology, 1200 Montréal Building M-9, Ottawa, ON, K1A0R6, Canada

1

Present address: Gas Turbine Laboratory–Institute for Aerospace Research, National Research Council of Canada, Building M-10 Room 104, 1200 Montréal Road, K1A 0R6 Ottawa, ON, Canada.

J. Eng. Gas Turbines Power 129(3), 877-884 (Dec 13, 2006) (8 pages) doi:10.1115/1.2718234 History: Received January 23, 2006; Revised December 13, 2006

Soot formation phenomenon is far from being fully understood today and models available for simulation of soot in practical combustion devices remain of relatively limited success, despite significant progresses made over the last decade. The extremely high demand of computing time of detailed soot models make them unrealistic for simulation of multidimensional, transient, and turbulent diesel engine combustion. Hence, most of the investigations conducted in real configuration such as multidimensional diesel engines simulation utilize coarse modeling, the advantages of which are an easy implementation and low computational cost. In this study, a phenomenological three-equation soot model was developed for modeling soot formation in diesel engine combustion based on considerations of acceptable computational demand and a qualitative description of the main features of the physics of soot formation. The model was developed based on that of Tesner and was implemented into the commercial STAR-CD™ CFD package. Application of this model was demonstrated in the modeling of soot formation in a single-cylinder research version of Caterpillar 3400 series diesel engine with exhaust gas recirculation (EGR). Numerical results show that the new soot formulation overcomes most of the drawbacks in the existing soot models dedicated to this kind of engineering task and demonstrates a robust and consistent behavior with experimental observation. Compared to the existing soot models for engine combustion modeling, some distinct features of the new soot model include: no soot is formed at low temperature, minimal model parameter adjustment for application to different fuels, and there is no need to prescribe the soot particle size. At the end of expansion, soot is predicted to exist in two separate regions in the cylinder: in the near wall region and in the center part of the cylinder. The existence of soot in the near wall region is a result of reduced soot oxidation rate through heat loss. They are the source of the biggest primary particles released at the end of the combustion process. The center part of the cylinder is populated by smaller soot particles, which are created since the early stages of the combustion process but also subject to intense oxidation. The qualitative effect of EGR is to increase the size of soot particles as well as their number density. This is linked to the lower in-cylinder temperature and a reduced amount of air.

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Copyright © 2007 by National Research Council Canada
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Figures

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Figure 1

Flow chart of the baseline soot model. HC is considered as the active specie at each step of the soot formation process for sake of simplicity and efficiency.

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Figure 2

CFD mesh of the cylinder geometry. Top panel: Full mesh sector (the sector picture has been stretched to show details on the layered part of the mesh subject to connectivity change and squeeze). Bottom panel: Full cylinder near TDC with spray impingement on the bowl.

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Figure 3

History of the soot formation. (a) Soot mass. (b) Primary particle number. (c) Characteristic diameter. Line: present model. Dots: One-step model. Dashed line: Present model with 8% EGR. Line-circle: Induction radicals. Open circle: Experiment.

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Figure 5

(a) Picture EGR 0%. Plain iso-surface: Diameter below 20nm. Transparent iso-surface: Diameter above 40nm. (b) Particle diameter distribution at the exhaust (Grey: EGR 0%; Dashed: EGR 8%).

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Figure 6

Distribution of the two main soot zones at the end of the simulation. (a) Iso-surface (8.4×10−6) of the soot mass fraction. (b) Iso-surface (1.6×1014kg−1) of the number of primary particles.

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Figure 7

Oxygen depletion in the bowl. The iso-surface draws a volume in which oxygen mass fraction is below 6%.

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Figure 4

Soot mass fraction distribution in the median plane of the fuel jet at the end of the injection. (a) One-step model. (b) Three-equation model.

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