statistical nature of spark ignition -...
TRANSCRIPT
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Statistical Analysis of Electrostatic Spark Ignition of Lean H2 -O2 -Ar Mixtures
Statistical Analysis of Electrostatic Spark Ignition of Lean H2 -O2 -Ar Mixtures
Sally P. M. Bane1, Joseph E. Shepherd1
Eddie Kwon2, Art C. Day2
1California Institute of Technology, Pasadena, CA2Boeing Research & Technology, Seattle, WA
3rd
International Conference on Hydrogen SafetyAjaccio, Corsica, FranceSeptember 16-18, 2009
2
Spark Ignition & Minimum Ignition Energy
•
determining risk of accidental ignition extremely important in industry and aviation safety
•
Minimum Ignition Energy (MIE) –
traditional basis for quantifying ignition hazards
•
experimental work using capacitive spark, tabulations of MIE values
Percent Hydrogen
Min
imum
Igni
tion
Ener
gy
MIE curves, Lewis and von Elbe, 1961
0 5 10 15 20 250
0.02
0.04
0.06
0.08
0.1
0.12
Test Number
Spa
rk E
nerg
y (J
)
GONO GO
Jet A ignition test data, Lee and Shepherd, 1999
•
New viewpoint –
ignition as statistical
phenomenon
•
previous statistical analysis of Jet A ignition, hot surface ignition
•
little work done on statistics of ignition of hydrogen
Data overlap region
3
Statistical View of Ignition
GOAL: isolate and examine statistical nature of ignition with respect to spark energy
Must quantify and minimize other sources of experimental variability
1) uncertainties in mixture composition
2) ignition detection method
3) turbulence
4) spark energy measurement
Small changes in composition lead to large change in combustion characteristics, MIE
false positives or negatives
effect on spark channel formation, flame propagation
SOME SOURCES OF VARIABILITY:
Previous work done to assess these sources,
improve experimental
design
Objective: examine statistical nature of lean hydrogen aviation test mixtures
4
Statistical Analysis of Ignition Test Data
Goal: probability distribution for ignition versus stimulus level (spark energy)
logistic distribution –
often used to analyze binary “failure”
data
Example: Jet A spark ignition (Lee and Shepherd, 1999)
0.0
0.3
0.5
0.8
1.0
0 0.02 0.04 0.06 0.08 0.1 0.12 0.14 0.16
Spark Energy (mJ)
Prob
abili
ty o
f Ign
ition
Logistic probability distribution
95% confidence envelope
50th
percentile (50% probability of
ignition)
test data points
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Short, Fixed Spark Ignition Testing: Spark Ignition System
SS relay (opens
HV relay)
variable vacuum capacitor (~3-30 pF)
50 G
resistors
HV relay
current transformer
HV power supply input
acrylic tube filled
with dry air conical tungsten electrode tips
fixed length spark gap
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Short, Fixed Spark Ignition Testing: Estimating Spark Energy
...21
22
RiEMRshockresidualthermalstored EEEEECVE
sparkE estimate neglect
residualstoredspark EEE
2
21
breakdownstored CVE
CQE residual
residual
2
21
dttiCVQQQ sparkbreakdownsparkstoredresidual
high-speed current transformer
Calculate the charge left in the capacitor
using the spark current
7
Experimental Setup
static pressure
gauge
fan mixer
gas fill line
vacuum line
thermocouple
dynamic pressure
transducer
spark circuit
spark gap
window for schlieren
Composition control
Reliable ignition detection
Turbulence
• vacuum chamber
• fill by partial pressures
•
static pressure measurement with 0.01 kPa
precision
• pressure transducer
• thermocouple
•
high-speed schlieren
visualization
• fan mixer
• wait time after turn-off
8
Flame Visualization
high-speed flame visualization using schlieren
optics
high-speed camera (1000 fps)
5% hydrogen test mixture (ARP), 2 other mixtures with 1% more H2
→
5% H2
–12% O2
–83% Ar, 6% H2
–12% O2
–82% Ar, 7% H2
–12% O2
–81% Ar
5% H2 6% H2 7% H2
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Pressure Measurement
1.0
1.5
2.0
2.5
3.0
3.5
4.0
0.0 0.5 1.0 1.5 2.0 2.5 3.0 3.5 4.0
time (s)
pres
sure
(bar
)
7% H2
6% H2
5% H2
1.0
1.5
2.0
2.5
3.0
3.5
4.0
0.0 0.5 1.0 1.5 2.0 2.5 3.0 3.5 4.0
time (s)
pres
sure
(bar
)
7% H2
6% H2
5% H2
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Short, Fixed Spark Ignition Testing: Ignition Probability
ignition tests in three H2
test mixtures
fixed spark gap length (1-2 mm), range of spark energies (vary capacitance)
0
0.2
0.4
0.6
0.8
1
0 300 600 900 1200 1500
Spark Energy (J)
Prob
abili
ty o
f Ign
ition Probability
95% ConfidenceData Points
Ignition –
a “go”
Binary Result = 1
No Ignition –
a “no go”
Binary Result = 0
5% H2 Mixture
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Short, Fixed Spark Ignition Testing: Ignition Probability (cont.)
0
0.2
0.4
0.6
0.8
1
0 200 400 600 800 1000 1200 1400
Spark Energy (J)
Prob
abili
ty o
f Ign
ition
7% H2 6% H2 5% H2
Data Overlap Region
0
0.2
0.4
0.6
0.8
1
0 200 400 600 800 1000 1200 1400
Spark Energy (J)
Prob
abili
ty o
f Ign
ition
0
0.2
0.4
0.6
0.8
1
0 200 400 600 800 1000 1200 1400
Spark Energy (J)
Prob
abili
ty o
f Ign
ition
7% H2 6% H2 5% H2
Data Overlap Region
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Short, Fixed Spark Ignition Testing: Ignition Probability (cont.)
0
0.2
0.4
0.6
0.8
1
0 200 400 600 800 1000 1200 1400
Spark Energy (J)
Prob
abili
ty o
f Ign
ition
Prob
abili
ty o
f Ign
ition
Comparison with original MIE data (Lewis & von Elbe 1961):
952 J
(50% probability of ignition) determined from experiments
MIE for 5% H2
Mixture ~ 200 J
(from extrapolation)
5% H27% H2
MIE for 7% H2
Mixture = 100 J
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Long, Variable Spark Ignition TestingQUESTION: In addition to the spark energy, is the spark length important too?
Is spark energy density (spark energy/spark length) a more appropriate parameter?
Teflon tube
Isolated capacitor plate, charged (-)
Charged electrode
Movable grounded electrode
Motor-driven linear stage
Movable electrode
Current transformer
developed ignition system to vary both spark energy and spark length
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Long Spark Ignition Testing: Results (cont.)
6 mm
3 mm
NO IGNITIONSpark Energy: 1000 J
Spark Length: 6 mm
Energy Density = 167 J/mm
IGNITIONSpark Energy: 740 J
Spark Length: 3 mm
Energy Density = 247 J/mm
0
1
0 500 1000 1500 2000Spark Energy (J)
Test
Res
ult
Long Spark Data Points
MIE for Fixed 1.5mm Spark
Median Energy for Long Spark
351 J at 1.5 mm 741 J at 3.0 mm
0
1
0 500 1000 1500 2000Spark Energy (J)
Test
Res
ult
Long Spark Data Points
MIE for Fixed 1.5mm Spark
Median Energy for Long Spark
351 J at 1.5 mm 741 J at 3.0 mm
Test
Res
ult
Spark Energy (J)
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Long Spark Ignition Testing: Results
0
0.2
0.4
0.6
0.8
1
0 100 200 300 400 500
Energy Density (J/mm)
Prob
abili
ty
95% ConfidenceData PointsProbabilityFixed 1.5mm Spark
50% probability of ignition using fixed 1.5 mm spark
0
0.2
0.4
0.6
0.8
1
0 100 200 300 400 500
Energy Density (J/mm)
Prob
abili
ty
95% ConfidenceData PointsProbabilityFixed 1.5mm Spark
50% probability of ignition using fixed 1.5 mm spark
second set of tests in 6% H2
mixture
vary both spark energy and length →
range of energy densities
50% Probability of Ignition:
Short, Fixed Length Sparks:
351 J
at 1.5 mm
234 J/mm
Longer, Variable Length Sparks:
154 J/mm
reduced effect of quenching
instability of spark channel
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Conclusions
developed low-energy capacitive spark system to produce short, fixed length sparks
used to perform ignition tests in 3 hydrogen-based aviation test mixtures
analyzed statistically –
probability distributions for ignition vs. spark energy
• results statistical in nature –
contradict traditional MIE view
• small change in composition → large change in flame propagation, MIE
second spark ignition system –
sparks of variable energy and lengths
ignition tests in 6% H2
mixture, varying spark energy density
probability distributions for ignition vs. spark energy density
• longer sparks = more energy to ignite
• spark length must be considered, NOT just energy
• spark energy density is more appropriate parameter