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Research Papers: Internal Combustion Engines

# Modeling Soot Formation Using Reduced Polycyclic Aromatic Hydrocarbon Chemistry in $n$-Heptane Lifted Flames With Application to Low Temperature Combustion

[+] Author and Article Information
Gokul Vishwanathan

Engine Research Center, Department of Mechanical Engineering, University of Wisconsin-Madison, Madison, WI 53706gvishwanatha@wisc.edu

Rolf D. Reitz

Engine Research Center, Department of Mechanical Engineering, University of Wisconsin-Madison, Madison, WI 53706reitz@engr.wisc.edu

J. Eng. Gas Turbines Power 131(3), 032801 (Feb 11, 2009) (7 pages) doi:10.1115/1.3043806 History: Received May 28, 2008; Revised May 31, 2008; Published February 11, 2009

## Abstract

A numerical study of in-cylinder soot formation and oxidation processes in $n$-heptane lifted flames using various soot inception species has been conducted. In a recent study by the authors, it was found that the soot formation and growth regions in lifted flames were not adequately represented by using acetylene alone as the soot inception species. Comparisons with a conceptual model and available experimental data suggested that the location of soot formation regions could be better represented if polycyclic aromatic hydrocarbon (PAH) species were considered as alternatives to acetylene for soot formation processes. Since the local temperatures are much lower under low temperature combustion conditions, it is believed that significant soot mass contribution can be attributed to PAH rather than to acetylene. To quantify and validate the above observations, a reduced $n$-heptane chemistry mechanism has been extended to include PAH species up to four fused aromatic rings (pyrene). The resulting chemistry mechanism was integrated into the multidimensional computational fluid dynamics code KIVA-CHEMKIN for modeling soot formation in lifted flames in a constant volume chamber. The investigation revealed that a simpler model that only considers up to phenanthrene (three fused rings) as the soot inception species has good possibilities for better soot location predictions. The present work highlights and illustrates the various research challenges toward accurate qualitative and quantitative predictions of the soot for new low emission combustion strategies for internal combustion engines.

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## Figures

Figure 1

Ignition delay predictions with original and present ERC mechanisms compared with experiments (17)

Figure 2

Ignition delay predictions of the present ERC and the mechanisms of Curran (19) at 40 bar

Figure 3

Ignition delay predictions of the present ERC and the mechanisms of Curran (19) at 80 bar

Figure 4

Mole fraction predicted by the comprehensive (20) and present reduced PAH mechanisms: P=40 bar, initial temperature=1000 K, and Φ=3.0

Figure 5

Mole fraction predicted by the comprehensive (20) and present reduced PAH mechanisms: P=80 bar, initial temperature=1000 K, and Φ=3.5

Figure 6

Lift-off length predictions as compared with the experiments (16) for the conditions of Table 2

Figure 7

Predicted formation/oxidation regimes of soot precursors and soot for 15% O2 ambient oxygen compared with experimental soot formation/oxidation regime

Figure 8

Predicted formation/oxidation regimes of soot precursors and soot for 8% O2 ambient oxygen compared with experimental soot formation/oxidation regime

Figure 9

Soot mass from experiments and estimated carbon mass in precursor species for 15% O2 case

Figure 10

Normalized soot mass with C2H2 and A3 as precursors for 15% O2 (normalized with peak soot mass at 12% O2)

Figure 11

Normalized soot mass with C2H2 and A3 as precursors for 12% O2

Figure 12

Normalized soot mass with C2H2 and A3 as precursors for 10% O2 (normalized with peak soot mass at 12% O2)

Figure 13

Soot volume fraction contours from experiment (16,22) (top) and soot mass fraction contours (bottom): (a) at 15% O2 conditions at the end of injection and (b) 8% O2 conditions at the end of injection

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