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Research Papers: Fuel Combustion

Large-Eddy Simulation of an n-Dodecane Spray Flame Under Different Ambient Oxygen Conditions

[+] Author and Article Information
Yuanjiang Pei

Transportation Technology R&D Center,
Argonne National Laboratory,
Argonne,
Lemont, IL 60439

Bing Hu

Cummins, Inc.,
Columbus, IN 47202

Sibendu Som

Transportation Technology R&D Center,
Argonne National Laboratory,
Argonne,
Lemont, IL 60439

1Current affiliation: Aramco Research Center, 46535 Peary Court, Novi, MI 48377; e-mail: yuanjiang.pei@aramcoservices.com.

Contributed by the Internal Combustion Engine Division of ASME for publication in the JOURNAL OF ENERGY RESOURCES TECHNOLOGY. Manuscript received January 11, 2016; final manuscript received February 11, 2016; published online March 16, 2016. Editor: Hameed Metghalchi.The United States Government retains, and by accepting the article for publication, the publisher acknowledges that the United States Government retains, a non-exclusive, paid-up, irrevocable, worldwide license to publish or reproduce the published form of this work, or allow others to do so, for United States government purposes.

J. Energy Resour. Technol 138(3), 032205 (Mar 16, 2016) (10 pages) Paper No: JERT-16-1021; doi: 10.1115/1.4032771 History: Received January 11, 2016; Revised February 11, 2016

An n-dodecane spray flame was simulated using a dynamic structure large-eddy simulation (LES) model coupled with a detailed chemistry combustion model to understand the ignition processes and the quasi-steady state flame structures. This study focuses on the effect of different ambient oxygen concentrations, 13%, 15%, and 21%, at an ambient temperature of 900 K and an ambient density of 22.8 kg/m3, which are typical diesel-engine relevant conditions with different levels of exhaust gas recirculation (EGR). The liquid spray was treated with a traditional Lagrangian method. A 103-species skeletal mechanism was used for the n-dodecane chemical kinetic model. It is observed that the main ignitions occur in rich mixture, and the flames are thickened around 35–40 mm off the spray axis due to the enhanced turbulence induced by the strong recirculation upstream, just behind the head of the flames at different oxygen concentrations. At 1 ms after the start of injection (SOI), the soot production is dominated by the broader region of high temperature in rich mixture instead of the stronger oxidation of the high peak temperature. Multiple realizations were performed for the 15% O2 condition to understand the realization-to-realization variation and to establish best practices for ensemble-averaging diesel spray flames. Two indexes are defined. The structure-similarity index (SSI) analysis suggests that at least 5 realizations are needed to obtain 99% similarity for mixture fraction if the average of 16 realizations is used as the target at 0.8 ms. However, this scenario may be different for different scalars of interest. It is found that 6 realizations would be enough to reach 99% of similarity for temperature, while 8 and 14 realizations are required to achieve 99% similarity for soot and OH mass fraction, respectively. Similar findings are noticed at 1 ms. More realizations are needed for the magnitude-similarity index (MSI) for the similar level of similarity as the SSI.

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Figures

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Fig. 1

Comparison of calculated ignition delay at different ambient oxygen conditions for (a) Φ  = 1 and (b) Φ  = 2 at 6 MPa with the 103 species chemical kinetic model in a constant-volume homogeneous reactor

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Fig. 2

Computed (a) ignition delay and (b) lift-off length compared to experiments at different ambient oxygen concentrations. The error bars show the experimental uncertainty obtained from Ref. [3].

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Fig. 3

Ignition locations comparison at different ambient oxygen concentrations. The solid line represents the stoichiometric mixture fraction.

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Fig. 4

Scatter plot of temperature in the mixture fraction space for different ambient oxygen conditions during the main ignition stage. The dashed lines are the mean temperature conditional on the mixture fraction, and the solid lines are the stoichiometric mixture fraction at different ambient oxygen concentrations.

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Fig. 5

Ambient oxygen concentration variation of (a) Z, (b) T, (c) OH, and (d) soot on the cross section along the spray axis. The solid line represents the stoichiometric mixture fraction.

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Fig. 6

Flame colored by OH mass fraction on the cross section along the spray axis for different ambient oxygen concentrations at 1 ms. The arrows are the velocity vector with threshold 10 m/s. The solid line is the stoichiometric mixture fraction.

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Fig. 7

Scatter plot of temperature in the mixture fraction space for different ambient oxygen conditions at 1 ms. The dashed lines are the mean temperature conditional on mixture fraction, and the solid lines are the stoichiometric mixture fraction at different ambient oxygen concentrations.

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Fig. 8

Mean temperature conditional on mixture fraction for different ambient oxygen conditions at 1 ms

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Fig. 9

Scatter plot of soot in the (a) mixture fraction and (b) temperature spaces for different ambient oxygen conditions at 1 ms. The dashed lines are the mean soot mass fraction conditional on mixture fraction, and the solid lines are the stoichiometric mixture fraction at different ambient oxygen concentrations.

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Fig. 10

Realization-to-realization variation and ensemble average of 16 calculations of (a) T, (b) OH, and (c) soot on the cross section along the spray axis at 15% ambient oxygen condition. The solid lines represent the stoichiometric mixture fraction.

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Fig. 12

MSI based on mixture fraction, temperature, soot, and OH mass fraction at (a) 0.8 ms and (b) 1.0 ms, respectively, as a function of ensemble size

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Fig. 11

SSI based on mixture fraction, temperature, soot, and OH mass fraction at (a) 0.8 ms and (b) 1.0 ms, respectively, as a function of ensemble size

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