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Research Papers: Internal Combustion Engines

Investigation of the Roles of Flame Propagation, Turbulent Mixing, and Volumetric Heat Release in Conventional and Low Temperature Diesel Combustion

[+] Author and Article Information
Sage L. Kokjohn1

Department of Mechanical Engineering, University of Wisconsin-Madison, 1011 Engineering Research Building, 1500 Engineering Dr., Madison, WI 53711kokjohn@wisc.edu

Rolf D. Reitz

Department of Mechanical Engineering, University of Wisconsin-Madison, 1011 Engineering Research Building, 1500 Engineering Dr., Madison, WI 53711

For interpretation of color, the reader is referred to the web version of this article.

1

Present address: 1011 Engineering Research Building, 1500 Engineering Drive, Madison, WI 53711.

J. Eng. Gas Turbines Power 133(10), 102805 (May 06, 2011) (10 pages) doi:10.1115/1.4002948 History: Received October 10, 2010; Revised November 02, 2010; Published May 06, 2011; Online May 06, 2011

In this work, a multimode combustion model that combines a comprehensive kinetics scheme for volumetric heat release and a level-set-based model for turbulent flame propagation is applied over the range of engine combustion regimes from non-premixed to premixed conditions. The model predictions of the ignition processes and flame structures are compared with the measurements from the literature of naturally occurring luminous emission and OH planar laser induced fluorescence. Comparisons are performed over a range of conditions from a conventional diesel operation (i.e., short ignition delay, high oxygen concentration) to a low temperature combustion mode (i.e., long ignition delay, low oxygen concentration). The multimode combustion model shows an excellent prediction of the bulk thermodynamic properties (e.g., rate of heat release), as well as local phenomena (i.e., ignition location, fuel and combustion intermediate species distributions, and flame structure). The results of this study show that, even in the limit of mixing controlled combustion, the flame structure is captured extremely well without considering subgrid scale turbulence-chemistry interactions. The combustion process is dominated by volumetric heat release in a thin zone around the periphery of the jet. The rate of combustion is controlled by the transport of a reactive mixture to the reaction zone, and the dominant mixing processes are well described by the large scale mixing and diffusion. As the ignition delay is increased past the end of injection (i.e., positive ignition dwell), both the simulations and optical engine experiments show that the reaction zone spans the entire jet cross section. In this combustion mode, the combustion rate is no longer limited by the transport to the reaction zone, but rather by the kinetic time scales. Although comparisons of results with and without consideration of flame propagation show very similar flame structures and combustion characteristics, the addition of the flame propagation model reveals details of the edge or triple-flame structure in the region surrounding the diffusion flame at the lift-off location. These details are not captured by the purely kinetics based combustion model, but are well represented by the present multimode model.

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Copyright © 2011 by American Society of Mechanical Engineers
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Figures

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Figure 1

Schematic of the optically accessible DI research engine (7)

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Figure 2

Computational grid showing the location of the laser sheet in the experiments and cut-plane in the simulations. The grid consists of 110,000 cells at BDC (1×1×1.5 mm3 at the piston bowl wall).

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Figure 3

Comparison of measured (7) and predicted cylinder pressure and AHRR for the different cases. The measured cylinder pressure and AHRR by Singh (7) are shown in solid black lines, the calculated cylinder pressure and AHRR only considering volumetric heat release (i.e., KIVA-CHEMKIN ) are shown with dashes, and the calculated cylinder pressure and AHRR considering both flame propagation and volumetric heat release (i.e., KIVA-CHEMKIN-G ) are shown with dots.

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Figure 4

Comparison of single-shot broadband natural emission images (7) with computed contours of formaldehyde, OH, and acetylene for the HTC-short ignition delay condition

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Figure 5

Comparison of single-shot broadband natural emission images (7) with computed contours of formaldehyde, OH, and acetylene for the HTC-medium ignition delay condition

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Figure 6

Comparison of single-shot broadband natural emission images (7) with computed contours of formaldehyde, OH, and acetylene for the LTC-long ignition delay condition

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Figure 7

Comparison of measured (7) OH PLIF and computed OH mass fractions for the HTC-short ignition delay condition. The computed OH mass fraction scale ranges from 0 to 0.004.

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Figure 8

Contours of OH mass fraction (gray) at 6 deg ASOI with flame surfaces colored by (a) equivalence ratio and (b) turbulent flame speed. The computed OH mass fraction scale ranges from 0 (light) to 0.0039 (dark).

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Figure 9

Comparison of measured (7) OH PLIF and computed OH mass fraction for the HTC-medium ignition delay condition. The computed OH mass fraction scale ranges from 0 to 0.003.

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Figure 10

Comparison of measured (7) OH PLIF and computed OH mass fraction for the LTC-long ignition delay condition. The computed OH mass fraction scale ranges from 0 to 0.001.

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Figure 11

Overlaid contours of OH mass fraction and equivalence ratio for the three operating conditions

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Figure 12

Inverse of eddy turnover time (ε/TkE), FCR per fuel mass, energy release rate per fuel energy (HRR), and injection rate. Note that the x-axis is given as degrees ASOI.

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