DIESEL ENGINES AND SPRAYS COMBUSTION MODELLING

0 0.1 0.2 0.3 0.4η [−]

0

5

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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

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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 %

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231431!5"6

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60%

9°

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9- !6",

15 %

18% O2 12.6% O2

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1200 %

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2000

6.2

1.7

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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

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Single

0.13

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3.9

1

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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 [-]

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10

20

30

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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

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250

Crank angle [degCA]

AH

RR

[J/d

eg

CA

]

10 5 0 5 10 15 20

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Crank angle [degCA]

AH

RR

[J/d

eg

CA

]

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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