Internal Combustion Engine GroupInternal Combustion Engine Group

OH and NO distributions in combusting diesel sprays

13 June 2006

Romain Demory

Outline

• IntroductionDiesel combustion and nitrogen oxides

• Laser-Induced FluorescenceValidity and limitations

• Results and DiscussionFlame development and nitric oxide formation

• Conclusions

IntroductionIntroduction

Aim: Identify the conditions leading to the formation of NO in combusting diesel sprays

(1) Characterise the combustion in time and space

(2) Acquire spatially and temporally precise distributions of NO

-1

0

1

2

3

4

5

6

7

-180 -120 -60 0 60 120 180

1350

1400

1450

1500

1550

1600

1650

-200 -150 -100 -50 0 50 100 150 200

Instrumented engine Videos of the spray/flame

Combustion marker: OH-LIF

: NO-LIF

2/15

Laser-Induced Fluorescence Correction ModelLaser-Induced Fluorescence Correction Model

),(),()(

21221

2112 TpQWPA

ATpgBTfNIS

iBNOlaserLIF

3/15

1) Population (Boltzmann) distribution

2) Broadening and shift

3) Collisional Quenching

temperature dependent

overlap integral is rather constant over the range of pressure and temperature

can be evaluated if local temperature and molar fractions of colliding species are know

Laser-Induced Fluorescence Correction ModelLaser-Induced Fluorescence Correction Model

4) Model

180°

0.00

0.05

0.10

0.15

0.20

0.25

0.30

0.35

-12.00 -10.00 -8.00 -6.00 -4.00 -2.00 0.00 2.00

time [°c.a. aTDC]

he

at r

ele

ase

ra

te [

MP

a.m

³.s-

1]

total heat release rate pre-mixed mixing-controlled

1) Pre-mixed volume

2) Mixing-controlled volume

Integration of the heat release rate in time gives a local temperature

Combustion is scaled on heat release rate to obtain local χCO2

χH2O χO2

8/15

Laser-Induced Fluorescence Correction ModelLaser-Induced Fluorescence Correction Model

5) Model results

Temperature evolution

Stern-Vollmer (yield) factor Boltzmann (population) factor

1.9 ms 2.3 ms 2.8 ms 3.3 ms 4.0 ms

9/15

Results and discussionResults and discussion

Raw OH-LIF results Laser attenuation is visible (brighter on left side)

All images are for the same timing, different cycles

Injection of 30 mm³ of fuel in a quiescent air at 630 K and 5 MPa

Injection pressure: 100 MPa

OH is found in the flame front

The flame front is heavily rippled and unevenly thick

Even in a quiescent air environment, the spray development and evaporation leads to a rippled diffusion flame

10/15

Results and discussionResults and discussion

1.25 1.5 1.75 2 2.5 3.5 4 4.5 5 5.5 6

OH

flame

NO

3

OH (simulation)

time

[ms aSOI]

laser sheet height

Injection pressure: 100 MPa

Injected volume: 30 mm³

In-cylinder presssure: 5 MPa

In-cylinder temperature: 630 K

Summary

11/15

Results and discussionResults and discussion

Natural flame luminosity + external lights

12/15

Results and discussionResults and discussion

-0.1

0

0.1

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1

1.1

1 2 3 4 5 6 7 8 9 10 11

time [ms aSOI]

aver

age

fluor

esce

nce

/ lu

min

osity

[a.

u.]

-0.1

0

0.1

0.2

0.3

0.4

1 2 3 4 5 6 7 8 9 10 11

heat

rel

ease

rat

e [M

J/s]

NO-LIF signal OH-LIF signal Flame luminosity Heat release rate

injection pressure: 100 MPainjected volume: 30 mm³

in-cylinder pressure: 5 MPain-cylinder temperature: 630 K

Summary

13/15

Conclusion

Pre-mixed phase:•Very short and mostly invisible to the optical techniques employed •The heat release rate remains the best indicator of the beginning of the combustion chemistry

Mixing-controlled phase:•Varying but sometimes very early start of the diffusion flame•Part of the “pre-mixed spike” in the heat release rate could be attributed to the diffusion flame. Whereas the flame front is rather stable downstream of the evaporation zone, the combustion near the tip of the spray is more chaotic and becomes increasingly richer, possibly leading to high concentrations of soot in the late part of the combustion.

Nitric oxide formation:•No fluorescence detected from the pre-mixed combustion (too rich or/and too short) as reported by Dec (1998)•NO distributions moved downstream as the diffusion flame developed, no structured pattern was detected during the stabilised diffusion flame (can be partially explained by the Stern-Vollmer factor and the Boltzmann distribution)

14/15

Thank you 15/15

Questions?