c.n. markides, g. de paola, e. mastorakos( [email protected] ) engm.ac.uk/~em257

Engineering Department University of Cambridge1

Measurements and simulations of mixing and autoignition on an n-heptane plume in a turbulent

flow of heated air

C.N. Markides, G. De Paola,

E. Mastorakos ([email protected])

http://www.eng.cam.ac.uk/~em257/


Introduction

Structure of presentation:

Experimental– Apparatus– Bulk observations

Simulations– The CFD– The CMC model

Results– Ignition lengths– Explanation of trends– Implications

Conclusions & suggestions for the future


Why is autoignition important?

Fuel

Air

Air

0 1

Mixture

0 1

LPP gas turbines:

Premixing for low NOx, but danger of autoignition!

Diesel & HCCI engines:

Fast mixing for low emissions, but need to predict autoignition!


Experiments

1. Apparatus

Air in, hot

grid

Fuel in, cold

Atmospheric pressureAir T up to 1100KBulk velocities up to 30m/s

Fuels: H2/N2, C2H2/N2

C7H16/N2

Techniques:

Hot wire for initial conditions

PLIF of acetone for

2D image of OH* with ICCD

Turbulence intensity boosted by grids.

“Diffusion from point source”.

(Markides & Mastorakos, 2005, Proc. Comb. Inst. 30)


Experiments

2. Visualization

Ignition spot appears and then disappears.

Location of ignition spot is random.

Fuel

Hot air

OH chemiluminescence (0.2 ms exp.):Individual spots, not connected flame

Ignition spot development at 20kHz: nothing, spot, spherical flame, nothing (consistent with DNS!)

C2H2 ignition, natural light (1/125s exp.)


Experiments

2. VisualizationQualitative regimes of operation (for all Ujet/Uair tested between 1 and 5):

T

U

RandomSpots

Flashback

NoIgnition

LiftedFlame

Individual short-lived autoignition kernels

Continuous flame sheet ? Stabilisation in mixture “almost ready to ignite”?This regime more likely at high Ujet/Uair. Similar to “Cabra” burner

Quick propagation back to nozzle

Autoignition not happeningdue to high strain?


Experiments

2. Visualization

Localised autoignition

Statistically-steady

If ignition happens close, then it happens often

They always come in bursts


Experiments

3. Mixing

Mean and variance of mixture fraction as expected

Two-component scalar dissipation measured at Kolmogorov resolution

<> satisfies global conservation (Bilger, 2004)

Data used for validating CFD & CMC model

(Markides & Mastorakos, to appear in Chem Eng Sci)


Calculations

1. CFD – for mixing, neglecting reactions

STAR-CD

k- model

Very good resolution close to nozzle needed

Use experimental initial conditions

Use experimental Cd in model for <>


Calculations

2. CMC model

Conditional Moment Closure equations:

||2

2

wQ

Nt

Q

jturb

jjj

turbj x

QDP

xPx

Q

x

Du

)(

~

)(~1~

~

~~

2

1

0

),,(~

),,(),(~ dtxPtxQtxY

Conditional convection Conditional turbulent flux

Diffusion in -space & chemistry, closed at 1st order


Calculations

2. Formulation of the CMC model for plume

Averaged across plume:

model AMC from |

),(~

2

),(~

|2|

|||

0

0*

2

2**

N

drrPr

drrPNrN

wQ

Nz

Qu

R

R


Calculations

3. Code, chemistry, validation

31-scalar reduced heptane chemistry

(Bikas, PhD Thesis, Aachen)

Ignition times of homogeneous mixtures OK

Ignition times of spray with CMC OK

(Wright, De Paola, Boulouchos, Mastorakos, Comb. Flame, to appear)


Results

1. Mixing: Good agreement

0 10 20 30 40 50 60 70 80 900

0.1

0.4

0.6

0.8

1

Axial Distance [mm]

Mix

ture

Fra

ctio

n [

-]

CFD

Experiment

0 10 20 30 40 50 60 70 80 900

0.02

0.04

0.06

0.08

0.1

Axial Distance [mm]

Mix

ture

Fra

ctio

n V

ari

an

ce [

-]

CFD

Experiment

0 10 20 30 40 50 60 70 80 900

5

10

15

20

25

Axial Distance [mm]

Scal

ar D

issi

pati

on R

ate

[s-1

]

CFD

Experiment

0 0.2 0.4 0.6 0.8 10

2

4

6

8

10

12

14

16

Mixture Fraction [-]

Con

diti

onal

Sca

lar

Dis

sipa

tion

Rat

e [s

-1]

<>

<>

variance

<>


Results

2. Autoignition lengths: reasonably good agreement

1090 1100 1110 1120 1130 1140 11500

50

100

150

200

250

Aut

oig

nit

ion

Len

gth

(m

m)

Tair

(K)

OH LMODE

: U=13.8,=1.05

OH LMIN

: U=13.8,=1.05

LCMC

: U=13.8,=1.05

OH LMODE

: U=17.6,=1.20

OH LMIN

: U=17.6,=1.05

LCMC

: U=17.6,=1.20

Physics: • As U increases, ignition length L increases, but also L/U increases. Hence, not

simply chemistry-controlled!• Trend captured by model

0.875 0.88 0.885 0.89 0.895 0.9 0.905 0.91 0.9150.5

1

1.5

2

2.5

3

3.5

4

ln(A

uto

ign

itio

n T

ime)

(m

s)

1000/Tair

(1/K)

OH MODE

: U=13.8,=1.05

OH MIN

: U=13.8,=1.05

CMC

: U=13.8,=1.05

OH MODE

: U=17.6,=1.05

OH MIN

: U=17.6,=1.05

CMC

: U=17.6,=1.05


Results

2. Conditional statistics

Ignition at the most-reactive mixture fraction, not at stoichiometry.As L increases, P()(-well-mixed).

Ignition time becomes long as P(MR) 0.

0 0.2 0.4 0.6 0.8 11000

1100

1200

1300

1400

1500

1600

1700

1800

1900


Tem

pera

ture

[K

]

0 0.2 0.4 0.6 0.8 11000

1100

1200

1300

1400

1500

1600

1700

1800

1900


Tem

per

atu

re [

K]

Low T: long L High T: short L

MR


Results

3. Discussion

0 10 20 30 40 50 60 70 800

5

10

15

20

25

30

35

40

45

50

Max Conditional Scalar Dissipation Rate <N|> [s -1]

Ign

itio

n D

ela

y T

ime

[ms]

0 20 40 60 80 1000

10

20

30

40

50

60

70

Axial Distance [mm]

Co

nd

itio

na

l S

cala

r D

issi

pa

tio

n R

ate

at

M

R

17.64 m/s

13.79 m/s

Autoignition limit <N|=MR

>

Flamelet or CMC: ignition time increases

as N increases

In our flow: N increases with U

Hence: Ignition time in our flow increases as U increases

Also: N<Ncritical hence 2nd-order CMC not needed


Conclusions

A novel autoignition rig is operational and has produced results for various fuels

Intense turbulence can delay autoignition due to increasing scalar dissipation rate

CMC model can capture all experimental trends

Crucial aspect: modelling of scalar dissipation

Future: Transport equation for <>

2D-CMC to capture spatial diffusion / flashback conditions

LES, PDF calculations

c.n. markides, g. de paola, e. mastorakos( [email protected] ) engm.ac.uk/~em257

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