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TECHNICAL PAPERS: Internal Combustion Engines

Modeling Diesel Spray Flame Liftoff, Sooting Tendency, and NOx Emissions Using Detailed Chemistry With Phenomenological Soot Model

[+] Author and Article Information
Song-Charng Kong1

Engine Research Center,  University of Wisconsin, 1500 Engineering Drive, Madison, WI 53706kong@iastate.edu

Yong Sun

Engine Research Center,  University of Wisconsin, 1500 Engineering Drive, Madison, WI 53706

Rolf D. Rietz2

Engine Research Center,  University of Wisconsin, 1500 Engineering Drive, Madison, WI 53706

1

Currently at the Department of Mechanical Engineering, Iowa State University, Ames, IA 50011.

2

To whom correspondence should be addressed.

J. Eng. Gas Turbines Power 129(1), 245-251 (Dec 15, 2005) (7 pages) doi:10.1115/1.2181596 History: Received May 16, 2005; Revised December 15, 2005

A detailed chemistry-based CFD model was developed to simulate the diesel spray combustion and emission process. A reaction mechanism of n-heptane is coupled with a reduced NOx mechanism to simulate diesel fuel oxidation and NOx formation. The soot emission process is simulated by a phenomenological soot model that uses a competing formation and oxidation rate formulation. The model is applied to predict the diesel spray lift-off length and its sooting tendency under high temperature and pressure conditions with good agreement with experiments of Sandia. Various nozzle diameters and chamber conditions were investigated. The model successfully predicts that the sooting tendency is reduced as the nozzle diameter is reduced and/or the initial chamber gas temperature is decreased, as observed by the experiments. The model is also applied to simulate diesel engine combustion under premixed charge compression ignition (PCCI) conditions. Trends of heat release rate, NOx, and soot emissions with respect to EGR levels and start-of-injection timings are also well predicted. Both experiments and models reveal that soot emissions peak when the start of injection (SOI) occurs close to TDC. The model indicates that low soot emission at early SOI is due to better oxidation while low soot emission at late SOI is due to less formation. Since NOx emissions decrease monotonically with injection retardation, a late injection scheme can be utilized for simultaneous soot and NOx reduction for the engine conditions investigated in this study.

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Copyright © 2007 by American Society of Mechanical Engineers
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References

Figures

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Figure 1

Sample images of the predicted fuel spray and gas temperature distributions for dnozz=100μm, Tamb=900K, Pamb=138MPa, ρamb=14.8kg∕m3. Color scale: 900 to 2600K.

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Figure 2

Comparisons between PLII images and predicted soot mass fractions at the central plane of the fuel jet at 3.2ms ASI. The equivalence ratios were estimated at the lift-off length. Relative PLII camera gain is given in brackets. dnozz=100μm, Pinj=138MPa, ρamb=14.8kg∕m3.

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Figure 3

Time sequence (ASI in ms) of PLII images and predicted soot mass fraction contours. The lift-off length and x=50mm positions are shown on the images with vertical dashed and solid lines, respectively. dnozz=100μm, Pinj=138MPa, Tamb=1000K, ρamb=14.8kg∕m3.

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Figure 4

Comparisons of measured time-averaged KL factors and predicted soot mass along the central axis of the fuel jet for the same conditions as in Fig. 3. Both measured and predicted data were normalized to allow qualitative comparison. Results were acquired at 3.2ms ASI for dnozz=100μm, Tamb=1000K, Pinj=138MPa, ρamb=14.8kg∕m3.

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Figure 5

Comparisons of measured time-averaged KL factors and predicted soot mass for various ambient temperatures 950, 1100, 1200, and 1300K. Both measured and predicted data were normalized to allow qualitative comparison. Results were acquired at 3.2ms ASI for dnozz=100μm, Pinj=138MPa, ρamb=14.8kg∕m3.

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Figure 6

Comparisons of measured time-averaged KL factors and predicted soot mass as a function of radial distance from the jet centerline at an axial distance of 50mm from the orifice (vertical solid line in Fig. 3). Both measured and predicted data were normalized to allow qualitative comparison. Results were acquired at 3.2ms ASI for the same conditions as in Fig. 3.

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Figure 7

Measured (solid lines) and predicted (dashed) sooting and nonsooting regimes as a function of ambient gas temperature and density for Pinj=138MPa. For the conditions of each curve, nonsooting combustion occurs to the left and sooting combustion to the right of each curve.

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Figure 8

Comparisons of measured (solid line) and predicted (dotted) cylinder pressure and heat release rate data for 8% EGR cases (SOI=−20, −10, and +5ATDC)

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Figure 9

Comparisons of measured (solid line) and predicted (dotted) cylinder pressure and heat release rate data for 40% EGR cases (SOI=−20, −10, and +5ATDC)

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Figure 10

Measured and predicted engine-out NOx emissions for cases listed in Table 1

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Figure 11

Measured and predicted engine-out soot emissions for cases listed in Table 1

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Figure 12

In-cylinder soot mass histories for 40% EGR cases at three different injection timings. Values at exhaust valve opening (130ATDC) are shown in Fig. 1.

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