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
8/22/13 3
• 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
8/22/13 4
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
8/22/13 5
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
8/22/13 6
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
8/22/13 11
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
8/22/13 12
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
8/22/13 13
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
8/22/13 16
• 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
8/22/13 17
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
8/22/13 20
• 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
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