Statistical Analysis of Spark Ignition of Kerosene-Air Mixtures

1

S. A Coronel1, S. P. M. Bane2, P. A. Boettcher1 and J. E. Shepherd1

Fall Technical Meeting of the Western States Section of the Combustion Institute

University of California, Riverside, CA

Oct 17-18, 2011

1California Institute of Technology, 2Purdue University

Spark Ignition and Minimum Ignition Energy

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•  Risk of accidental ignition in industry and aviation

•  Minimum Ignition Energy (MIE): traditional basis for quantifying ignition hazards

•  Capacitive spark discharge as ignition source

•  Pioneering work – Blanc, Guest, Lewis & von Elbe at Bureau of Mines (1940s)

MIE curves for hydrogen mixtures, Lewis and von Elbe (1961)

High Voltage

R

C Spark Gap

Switch

High Voltage

R

C

Switch

Estored = ½ CV2

55% H2 MIE = 0.5 mJ

Statistical Analysis of Ignition Test Data

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•  New viewpoint – ignition as statistical phenomenon

•  More consistent with test data

•  Little work done on statistics of ignition of other flammable mixtures

•  Can�t assign a probability to historical MIE data

Stoichiometric Methane-Air Kono and Tsue (2009)

Jet A, Lee and Shepherd (1999)

0.00

0.02

0.04

0.06

0.08

0.10

0.12

0 5 10 15 20 25

Ignition

No Ignition

Spa

rk E

nerg

y (J

)

Test number

Spark Breakdown and Spark Channel Formation

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Unpredictable Plasma Instabilities

Localized Ignition

•  Liquid fuel composition usually not known for commercial fuels ! several 100 components

•  composition changes from batch to batch, can be affected by history, transport, etc.

•  composition of liquid not the same as fuel vapor Gas chromatograph (Woodrow, 2000)

Kerosene Tests: Experimental Considerations

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Ignition hazard in aircraft is due to much more complex

fuels, that is a kerosene-based fuel

•  Ignition testing: low vapor pressure must heat significantly or decrease pressure

•  vapor pressure depends on many variables

Fuel vapor pressure vs. temperature and fuel mass loading (Lee and Shepherd, 1999)

Experimental Setup

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22 L, stainless steel, cylindrical combustion vessel

"  Ignition Detection

"  Schlieren visualization

"  Vessel Heating System

•  flame visualization

•  pressure transducer

•  thermocouple

•  high-speed camera (10,000+ frames per second)

•  silicone heaters, 4 zones

•  high-current heater control unit

•  up to ~ 150°C

Results: Kerosene Ignition

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•  1-K kerosene at 45-62°C, 100 kPa

•  fixed 3.3 mm spark gap

•  50 kg/m3 fuel mass loading

•  C ~ 11 – 68 pF, V ~ 6.4 – 11.4 kV ! Espark ~ 0.3 – 2.3 mJ

kerosene-air, 45°C kerosene-air, 55°C

8/22/13 8 kerosene-air, 45°C

8/22/13 9 kerosene-air, 50°C

8/22/13 10 kerosene-air, 55°C

Results: Kerosene-Air at 60°C

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Previous Work (Lee and Shepherd):

Lowest ignition energy

= 40 mJ (52°C, 100 kPa)

2 mJ (56.1°C, 58.5 kPa)

0.65 mJ

0

1

0.00 0.40 0.80 1.20 1.60 2.00

Ignition

No Ignition

Spark Energy (mJ) 1.75 mJ

Results: Kerosene-Air at 60°C

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0 0.1 0.2 0.3 0.4 0.5 0.6 0.7 0.8 0.9

1

0 100 200 300 400 500 600

Spark Energy Density (µJ/mm)

Pro

babi

lity

of Ig

nitio

n

probability distribution

95% confidence intervals

Results: Comparison with Hexane

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0 0.1 0.2 0.3 0.4 0.5 0.6 0.7 0.8 0.9

1

0 200 400 600 800 1000

Spark Energy Density (µJ/mm)

Pro

babi

lity

of Ig

nitio

n

kerosene-air, 60°C

hexane-air, ϕ=1.71

As sensitive as the most sensitive hexane-air mixture

Results: Comparison with H2

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0

0.1

0.2

0.3

0.4

0.5

0.6

0.7

0.8

0.9

1

0 100 200 300 400 500 600 700 800

Spark Energy Density (µJ/mm)

Pro

babi

lity

of Ig

nitio

n

kerosene-air, 60°C 7% H2

5% H2 •  Energy/length used to account for different length sparks

• Normalize spark energies from 5% and 7% H2 tests by spark gap widths used (2 and 1 mm)

•  Normalize spark energies from kerosene tests by 3.3 mm

Results: Varying Kerosene Fuel Temperature

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0

500

1000

1500

2000

2500

40 45 50 55 60 65 70

Fuel temperature (°C)

Spa

rk E

nerg

y (µ

J)

TF = 42oC

4 6 8 10 12 14 16 18 20 220

5

10

15

20

25

carbon #

% m

ole

frac

tion

in li

quid

30.6oC46oC56oC59oC70.6oC71.1oC73.9oC

Parametric Study

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•  Ignition energy model

•  Mixture composition is required

•  Raoult’s Law

liquid composition

saturation pressure

•  Obtain Xi through a distillation curve or gas chromatography

Gas chromatograph (Woodrow, 2000)

Parametric Study

•  Raoult’s law does not take into consideration the fuel mass loading

•  “headspace equation” ! conservation of moles

•  K is the hydrocarbon liquid-vapor distribution coefficient

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initial liquid composition

fuel vapor composition

Parametric Study Results

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0 10 20 30 40 50 60 70 80 90 1005

10

15

20

25

30

fuel mass loading (kg/m3)

pres

sure

(mba

r)

45oC50oC55oC60oC62oC

•  Effect of fuel mass loading on pressure for a kerosene-based fuel

Parametric Study Results

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•  for low fuel mass loadings, the lighter hydrocarbons become depleted

•  fuel vapor pressure is always an increasing function of fuel temperature

•  As the flash point increases, the lighter hydrocarbons are fewer

5 10 15 200

2

4

6

8

10

12

14

16

18

carbon #

% m

ole

frac

tion

in li

quid

fuel

Conclusions

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•  Electrostatic spark ignition tests using kerosene over a range of temperatures at atmospheric pressure.

•  Statistical analysis of probability of ignition vs. spark energy density at 60°C

•  Kerosene-air mixture has a lower ignition energy at 60°C than H2-O2-Ar mixture previously used in aircraft certification (5% H2)

•  Model to obtain fuel vapor composition given changes in flash point, temperature, pressure and fuel mass loading

•  From vapor composition predictions, lighter mass hydrocarbons are more appropriate for low temperature fuel vapor surrogates.

#  The experimental work was carried out in the Explosion Dynamics Laboratory of the California Institute of Technology and was supported by the Boeing Company through a Strategic Research and Development Relationship Agreement CT-BA-GTA-1

Questions

21