s. s. pandurangi* n. frapolli d. farrace diesel …...pandurangi, frapolli, bolla, boulouchos &...

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DIESEL ENGINES AND SPRAYS COMBUSTION MODELLING 0.1 0.2 0.3 0.4 η [−] 2 , k, , , P P, t n i i i1 h h TY STAR-CD CMC code solve flow field and compute CMC “parameters”: 2 P P ,, solve species and enthalpy equations in physical and conserved scalar space 1 0 i i Y Y P d return mean values by weighting with presumed -PDF 0 0.1 0.2 0.3 0.4 η [−] 250 500 750 1000 1250 1500 1750 temperature [K] Enhancement of soot oxidation rate - order of magnitude enhancement at high EGR (larger soot volume) - caused by additional locally available oxygen - soot evolution of the low-EGR case shows an initial increase due to ad- ded fuel, but subsequent drop due to improved oxidation O2 difference (normalised by the O2 distribution of the single-injection case) shows an accumulation of O2 at the tip of the post-injection being pushed into the main flame. -100 % 0 % -100 % 0 % -100 % -100 % 100 % -100 % 1100 % -100 % 4135 % 231431!5"6 -100 % 0 % -100 % 0% -100 % 15 % -100 % 100 % -100 % 200% -100 % 60% 10° 11° 12° 13° 14° 9- !6", 15 % 18% O2 12.6% O2 -100 % 1200 % 0.07 2000 6.2 1.7 0.5 0.4 0.15 0.13 2200 6.3 1.8 12 12 0.3 Single Post Mixture Fraction Temperature [K] Soot volume fraction [ppmv] Soot formation [1/s] Soot oxidation [1/s] Soot oxidation by O2 [1/s] Soot oxidation by OH [1/s] 0.07 2400 2.5 0.25 1.25 0.4 0.9 Single 0.13 2600 3.9 1 2.5 2.2 0.9 Post 2 2 1 j j j j Q Q Q u N w uy P t x P x j t j Q uy D x , , T w Q Q P 2 t j j j D u u x 2 1 1 0 exp 2 2 1 2 G N erf G P d 0 1 0 S. S. Pandurangi* N. Frapolli D. Farrace M. Bolla Y. M. Wright K. Boulouchos *<[email protected]> Influence of EGR on Post-injection Effectiveness in a Heavy-duty Diesel Engine Introduction Soot particles emitted by diesel engines are dan- gerous for the enviroment and for living organ- isms. Emission legislation around the world is be- coming increasingly stringent, demanding better understanding of the physics of soot formation to design countermeasures. One such measure being heavily researched is the addition of a small fuel post-injection. In this numerical study based on data from a heavy-duty research diesel engine [1], we investigate the effect of post-injection on soot evolution and governing processes, and the influence of exhaust gas recirculation on the in- teraction between post- and main- injections. Soot model: two-equations model [6] CFD model CFD computational setup: Commercial CFD code STAR-CD [2] • 2D grid with 1.0 mm mesh size k-epsilon turbulence model, wall functions Euler-Langrange approach for droplets Coditional Moment Closure (CMC) combus- tion model [3] Reduced mechanism for n-heptane [5] Two-equations soot model [6] Optical-thin soot radiation model [7] Soot differential diffusion effects neglected (Le=1) Experimental setup [1] Solve transport equations for soot mass fraction and soot number density (Le=1): Source term soot mass fraction: Source term soot number density: 21100 4 2 2 ω 10 [ ] T INCP CH e = 12100 3 2 2 ω 6 10 [ ] T SUR GROW soot CH e A = 2 19000 4 2 ω 10 [ ] O T OXID soot O e T A = ω 0.36[ ] OH OXID soot OH T A = ( ) 1 1 11 1 1 2 6 6 2 6 24 6 ω 3 COAG S S S A S R T Y N N ρ ρ πρ =− n nP P (1) Particle inception: (2) Particle surface growth ( ) ( ) ( ) 2 2 2 2 S S CH nC n C H + + + (3) Particle oxidation by O 2 : (4) Particle oxidation by OH: (5) Particle coagulation: ( ) 2 2 2 2 S CH C H + ( ) 2 1 2 S C O CO + ( ) S C OH CO H + + ACETYLENE PRODUCTS Nucleation (1) Coagulation (5) Surface Growth (2) Surface oxidation (3-4) FUEL Reduced n-Heptane mechanism (0) ( ) ( ) 2 2 1 i i i i Q Q Q u N uY P w t x P x α α α α α η η ρ η η η η ρ η ′′ ′′ + + = 2 , , , , S S S S S Y Y inception Y growth Y oxidationO Y oxidationOH w w w w w η η η η η = + + + , , S S S N N inception N coagulation w w w η η η = + Soot model accouts for simultaneous soot particle inception, surface growth, oxidation by O 2 and OH and coagulation Governing equations [3] 4 4 4 RAD soot Wall w T T η σα η =− First order closure for source terms Linear model for conditional velocity Gradient flux AMC model for scalar dissipation rate Optically-thin radiation model [7] 1 2370 SOOT soot V f T mK α = References Acknowledgements [1] O‘Connor, J. and M. Musculus. International Journal of Engine Research, 2013. [2] CD-adapco, STAR-CD v4.16, 2010. [3] A. Y. Klimenko and R. W. Bilger. Progress in Energy and Combustion Science, vol. 25, pp. 595-687, 1999. [4] Y. M. Wright, et al. Combustion and Flame, vol. 143, pp. 402-419, 2005. [5] S. L. Liu, et al., Combustion and Flame, vol. 137, pp. 320-339, 2004. [6] K. M. Leung, et al. Combustion and Flame, vol. 87, pp. 289-305, 1991. [7] J. F. Widmann. Combustion Science and Technology, vol. 175, pp. 2299-2308, 2003. [8] Bolla, M., et al. International Multidimensional En- gine Modeling Users‘ Group Meeting 2014 Funding Competence Center for Energy and Mobility (CCEM-CH) project “In-cylinder emission re- duction for large diesel engines” • Swiss Federal Office of Energy (BfE) Engine heat release and flowfield validation Soot formation and oxidation processes Engine parameters bore: 140 mm (VH=2.34 L) CR=11.2(geom.), 16(sim.) swirl ratio: 0.5 injector: 8 x 0.131 mm fuel: n-heptane Measurement techniques pressure & AHRR 2D natural luminosity planar LII Multiple Injections [8] 10 5 0 5 10 15 20 0 50 100 150 200 250 300 Crank angle [degCA] AHRR [J/degCA] 15 10 5 0 5 10 15 20 25 0 50 100 150 200 250 Crank angle [degCA] AHRR [J/degCA] 10 5 0 5 10 15 20 0 50 100 150 200 250 300 Crank angle [degCA] AHRR [J/degCA] 10 5 0 5 10 15 20 0 50 100 150 200 250 Crank angle [degCA] AHRR [J/degCA] 21% O2 18% O2 12.6% O2 15% O2 +12 °CA +16 °CA +15 °CA +14 °CA +13 °CA Expt. Soot NL CFD Integrated FvS Pandurangi, Frapolli, Bolla, Boulouchos & Wright. Influence of EGR on Post-Injection Effectiveness in a Heavy-Duty Diesel Engine Fuelled with n-Heptane. SAE 2014-01-2633. Relative oxygen field for the post-injection case compared to the single-injection case Soot-quantities for single- and post-injection at high EGR (12.5% O2) at low EGR (18% O2) -10 -5 0 5 10 15 20 25 30 35 40 Crank angle aTDC [deg] 0 0.08 0.16 0.24 0.32 0.4 Soot mass in cylinder [mg] Single Post Soot evolution at low EGR (18% O2) very good accuracy maintained considering the large dilution spread • overestimations of AHRR of the main injection strongly influence the ignition and heat release from the ‹sensitive› post-injection excellent prediction of the positi- on of the post-injection soot with respect to penetration and orien- tation impingement on the wall and subsequent curling qualitatively well captured Interfacing of CFD and CMC code [4]: Multiple injections using a single total mixture fraction (MFT) as the conditioning scalar. re-initialisation of conditional tempe- rature and composition in every CMC cell at the time of the first appea- rance of MF from post injection (MF2) constraint: MF1 + MF2 = MFT <= 1.0 Number of subsequent injections is not limited with this approach! At low ambient oxygen levels, the oxidation rate by O2 is found to rise strongly and is the major contributor to overall oxidation. At high ambient oxygen the contribution of O2-oxidation decreases due to the higher activity of OH caused by the increase in flame temperature.

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  • DIESEL ENGINES AND SPRAYS COMBUSTION MODELLING

    0 0.1 0.2 0.3 0.4η [−]

    0

    5

    10

    15

    20

    β−P

    DF

    [−]

    823K, 50bar, Bikas, initial turbulence 2

    , k, ,

    ,PP,t

    n

    i ii 1

    h h T Y

    STAR-CD

    CMC code

    solve flow fieldand compute CMC

    “parameters”:

    2P P , ,

    solve species and enthalpy equations in physical and conserved scalar space

    1

    0i iY Y P d

    return mean values by weighting with presumed -PDF

    0 0.1 0.2 0.3 0.4η [−]

    250

    500

    750

    1000

    1250

    1500

    1750

    tem

    pera

    ture

    [K]

    823K, Bikas, with initial turbulence

    Enhancement of soot oxidation rate - order of magnitude enhancement at high EGR (larger soot volume) - caused by additional locally available oxygen - soot evolution of the low-EGR case shows an initial increase due to ad-

    ded fuel, but subsequent drop due to improved oxidation

    O2 difference (normalised by the O2 distribution of the single-injection case) shows an accumulation of O2 at the tip of the post-injection being pushed into the main flame.

    -100 %

    0 %

    -100 %

    0 %

    -100 %

    -100 %

    100 %

    -100 %

    1100 %

    -100 %

    4135 %

    231431!5"6

    -100 %

    0 %

    -100 %

    0 %

    -100 %

    15 %

    -100 %

    100 %

    -100 %

    200%

    -100 %

    60%

    10°

    11°

    12°

    13°

    14°

    9- !6",

    15 %

    18% O2 12.6% O2

    -100 %

    1200 %

    0.07

    2000

    6.2

    1.7

    0.5

    0.4

    0.15

    0.13

    2200

    6.3

    1.8

    12

    12

    0.3

    Single Post

    MixtureFraction

    Temperature[K]

    Soot volume fraction [ppmv]

    Soot formation[1/s]

    Soot oxidation[1/s]

    Soot oxidationby O2 [1/s]

    Soot oxidationby OH [1/s]

    0.07

    2400

    2.5

    0.25

    1.25

    0.4

    0.9

    Single

    0.13

    2600

    3.9

    1

    2.5

    2.2

    0.9

    Post

    MixtureFraction

    Temperature[K]

    Soot volume fraction [ppmv]

    Soot formation[1/s]

    Soot oxidation[1/s]

    Soot oxidationby O2 [1/s]

    Soot oxidationby OH [1/s]

    2

    2

    1j j

    j j

    Q Q Qu N w u y Pt x P x

    j tj

    Qu y Dx

    , ,Tw Q Q P

    2t

    j jj

    Du ux

    Gradient flux

    Linear model for conditional velocities

    First order closure for the source terms

    211

    0

    exp 2 2 12 G

    N erfG P d

    AMC model for thescalar dissipation rate

    0 0.2 0.4 0.6 0.8 1eta [-]

    0

    10

    20

    30

    40

    50

    CH

    I [1/

    s]

    AMC model

    S. S. Pandurangi* N. Frapolli D. Farrace M. Bolla Y. M. Wright K. Boulouchos*

    Influence of EGR on Post-injection Effectiveness in a Heavy-duty Diesel Engine

    IntroductionSoot particles emitted by diesel engines are dan-gerous for the enviroment and for living organ-isms. Emission legislation around the world is be-coming increasingly stringent, demanding better understanding of the physics of soot formation to design countermeasures. One such measure being heavily researched is the addition of a small fuel post-injection. In this numerical study based on data from a heavy-duty research diesel engine [1], we investigate the effect of post-injection on soot evolution and governing processes, and the influence of exhaust gas recirculation on the in-teraction between post- and main- injections.

    Soot model: two-equations model [6]

    CFD modelCFD computational setup:

    • Commercial CFD code STAR-CD [2]• 2D grid with 1.0 mm mesh size• k-epsilon turbulence model, wall functions• Euler-Langrange approach for droplets• Coditional Moment Closure (CMC) combus-

    tion model [3]• Reduced mechanism for n-heptane [5]• Two-equations soot model [6]• Optical-thin soot radiation model [7]• Soot differential diffusion effects neglected

    (Le=1)

    Experimental setup [1]

    Solve transport equations for soot mass fraction and soot number density (Le=1):

    Source term soot mass fraction:

    Source term soot number density:

    211004

    2 2ω 10 [ ] TINCP C H e−

    =

    121003

    2 2ω 6 10 [ ] TSUR GROW sootC H e A−

    = ⋅ ⋅

    2

    190004

    2ω 10 [ ]OT

    OXID sootO e T A−

    = ⋅ ⋅

    ω 0.36[ ]OHOXID soot

    OH T A= ⋅ ⋅

    ( )1 1

    111 12 662 6

    24 6ω 3COAG S SS A S

    R T Y NN

    ρρ πρ

    = −

    nnP P→

    (1) Particle inception:

    (2) Particle surface growth( ) ( ) ( )2 2 22S SC H nC n C H+ → + +

    (3) Particle oxidation by O2:

    (4) Particle oxidation by OH:

    (5) Particle coagulation:

    ( )2 2 22 SC H C H→ +

    ( ) 21

    2SC O CO+ →

    ( )SC OH CO H+ → +

    ACETYLENE

    PRODUCTS

    Nucleation (1)

    Coagulation (5)SurfaceGrowth

    (2)

    Surface oxidation (3-4)

    FUEL

    Reduced n-Heptanemechanism (0)

    ( ) ( )2

    2

    1i i

    i i

    Q Q Qu N u Y P wt x P xα α α

    α αη η ρ η η ηη ρ η∂ ∂ ∂ ∂ ′′ ′′+ − + = ∂ ∂ ∂ ∂

    2, , , ,S S S S SY Y inception Y growth Y oxidationO Y oxidationOHw w w w wη η η η η= + + +

    , ,S S SN N inception N coagulationw w wη η η= +

    Soot model accouts for simultaneous soot particle inception, surface growth, oxidation by O2 and OH and coagulation

    Governing equations [3]

    4 44RAD soot Wallw T Tη σα η = − −

    First order closure for source terms

    Linear model for conditional velocity

    Gradient flux

    AMC model for scalar dissipation rate

    Optically-thin radiation model [7]12370SOOTsoot V

    f TmK

    α = ⋅ ⋅

    References

    Acknowledgements

    [1] O‘Connor, J. and M. Musculus. International Journal of Engine Research, 2013.

    [2] CD-adapco, STAR-CD v4.16, 2010.[3] A. Y. Klimenko and R. W. Bilger. Progress in Energy

    and Combustion Science, vol. 25, pp. 595-687, 1999.[4] Y. M. Wright, et al. Combustion and Flame, vol. 143,

    pp. 402-419, 2005.[5] S. L. Liu, et al., Combustion and Flame, vol. 137, pp.

    320-339, 2004.[6] K. M. Leung, et al. Combustion and Flame, vol. 87,

    pp. 289-305, 1991.[7] J. F. Widmann. Combustion Science and Technology,

    vol. 175, pp. 2299-2308, 2003.[8] Bolla, M., et al. International Multidimensional En-

    gine Modeling Users‘ Group Meeting 2014

    Funding• Competence Center for Energy and Mobility

    (CCEM-CH) project “In-cylinder emission re-duction for large diesel engines”

    • Swiss Federal Office of Energy (BfE)

    Engine heat release and flowfield validation

    Soot formation and oxidation processes

    Engine parametersbore: 140 mm (VH=2.34 L)CR=11.2(geom.), 16(sim.)swirl ratio: 0.5injector: 8 x 0.131 mmfuel: n-heptaneMeasurement techniquespressure & AHRR2D natural luminosityplanar LII

    Multiple Injections [8]10 5 0 5 10 15 20

    0

    50

    100

    150

    200

    250

    300

    Crank angle [degCA]

    AH

    RR

    [J/d

    eg

    CA

    ]

    15 10 5 0 5 10 15 20 25

    0

    50

    100

    150

    200

    250

    Crank angle [degCA]

    AH

    RR

    [J/d

    eg

    CA

    ]

    10 5 0 5 10 15 20

    0

    50

    100

    150

    200

    250

    300

    Crank angle [degCA]

    AH

    RR

    [J/d

    eg

    CA

    ]

    10 5 0 5 10 15 20

    0

    50

    100

    150

    200

    250

    Crank angle [degCA]

    AH

    RR

    [J/d

    eg

    CA

    ]

    21% O2 18% O2

    12.6% O215% O2

    +12 °CA

    +16 °CA

    +15 °CA

    +14 °CA

    +13 °CA

    Expt. Soot NL CFD Integrated FvS

    Pandurangi, Frapolli, Bolla, Boulouchos & Wright. Influence of EGR on Post-Injection Effectiveness in a Heavy-Duty Diesel Engine Fuelled with n-Heptane. SAE 2014-01-2633.

    Relative oxygen field for the post-injection case compared to the single-injection case

    Soot-quantities for single- and post-injection

    at high EGR (12.5% O2) at low EGR (18% O2)

    -10 -5 0 5 10 15 20 25 30 35 40Crank angle aTDC [deg]

    0

    0.08

    0.16

    0.24

    0.32

    0.4

    Soo

    t mas

    s in

    cyl

    inde

    r [m

    g] SinglePost

    Soot evolution at low EGR (18% O2)

    • very good accuracy maintained considering the large dilution spread

    • overestimations of AHRR of the main injection strongly influence the ignition and heat release from the ‹sensitive› post-injection

    • excellent prediction of the positi-on of the post-injection soot with respect to penetration and orien-tation

    • impingement on the wall and subsequent curling qualitatively well captured

    Interfacing of CFD and CMC code [4]:

    Multiple injections using a single total mixture fraction (MFT) as the conditioning scalar.re-initialisation of conditional tempe-rature and composition in every CMC cell at the time of the first appea-rance of MF from post injection (MF2) constraint: MF1 + MF2 = MFT