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Effect of Cabin Pressure Effect of Cabin Pressure on the Piloted
Ignition on the Piloted Ignition of Combustible Solidsof
Combustible SolidsSonia Fereres 1, Chris Lautenberger1, Carlos
Fernandez-Pello 1, David Urban 2, Gary Ruff 2
1University of California at Berkeley
Dept. of Mechanical Engineering
Berkeley, CA
2NASA Glenn Research Center
Cleveland, OH
FISTFISTFISTFISTFISTFIST
6th Triennial International Aircraft Fire and
Cabin Safety Research Conference,
October 28, 2010, Atlantic City, New Jersey
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Motivation
• Fires in pressurized vehicles (aircraft, spacecraft or
submarines) are extremely hazardous ▫ Small compartments▫
Difficulty to escape
• Emphasis on fire prevention:▫ Material flammability▫ Effect of
environmental conditions (oxygen
concentration, pressure, radiant heat flux, etc) on ignition
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Today’s Talk• Understand the physical mechanisms responsible
for
ignition of solid combustibles under low pressure
• Aircraft cabin pressure is typically pressurized to a "cabin
altitude" of 8000 feet or less (~ 75 kPa)
• Are reduced pressure environments a higher fire risk?▫ Piloted
ignition experiments at low P
Forced Ignition and Spread Test (FIST) apparatus at UC Berkeley
to analyze material flammability
▫ Analytical explanation of results
12 psi 6 psi
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Lower ambient pressure can be found at…
• High Altitude
Lhasa, Tibet– 3,650 mpopulation 250,000
Quito, Ecuador- 2,850 mpopulation 2M• Inside Aircraft
• Inside Spacecraft
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Cabin Environments
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Typical Aircraft Cabin
Quito, Ecuador 2,850 m
Lhasa, Tibet 3,650 m
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How does a solid fuel ignite?Piloted ignition process:
1. Solid heating & pyrolysis
2. Mixing of gaseous fuel and air
3. Chemistry: fuel/air mixture reaches lean flammability limit
at high temperature igniter
4. If sufficient pyrolysis gases are generated: a diffusion
flame will anchor on solid (burning) critical mass flux at
ignition
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Possible Fire Scenario
• Heat source: electronic component overheating
• Fuel: polymeric materials used in panels, blocks, covers
• Ignition source: spark from electrical arching
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Forced Ignition and Spread Test (FIST)
Variables:-air flow velocity-incident heat flux-ambient
pressure
Material: PMMA
Ignitery
x
Data logging scale
u
Pressure Chamber
IR heater
Fuelsample holder
q”
Measure :-Tsurface vs. time tig, time to ignite
-Mass vs. time (dm/dt )|tig mass loss rate at ignition
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9
FIST Apparatus
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Video of Test
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0
100
200
300
400
500
600
700
800
‐1
‐0.9
‐0.8
‐0.7
‐0.6
‐0.5
‐0.4
‐0.3
‐0.2
‐0.1
0
0 200 400 600 800
Tempe
rature (C )
Mass Loss (g)
time (s)
m_100kPa
T_100kPa
Mass Loss
Sample Surface T
Experiment Example• 100 kPa (Raw Data)
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Ignition
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Experimental Results• Pressure Comparison : 55, 83 & 100 kPa
(Raw Data)
0
100
200
300
400
500
600
700
800
‐1
‐0.9
‐0.8
‐0.7
‐0.6
‐0.5
‐0.4
‐0.3
‐0.2
‐0.1
0
0 200 400 600 800
Tempe
rature (C )
Mass Loss (g)
time (s)
Heat Flux q"= 16 kW/m2, Air Flow Velocity 0.4 m/s
m_55kPa m_83kPa m_100kPa
T_55kPa T_83kPa T_100kPa
P increase
P increase
Mass Loss
Sample Surface T
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0.0
0.5
1.0
1.5
2.0
2.5
0
200
400
600
800
1000
1200
0 20 40 60 80 100
Mass Loss Rate at Ignition
(g/m
2 s)
Ignition
Delay Tim
e (s)
Ambient Pressure (kPa)
tig (s)m"ig (g/m2s)
No ignition
m''ig= 0.005∙P + 1.483
tig = 4.604∙P + 355.32
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Experimental Results• Ignition Delay & Mass Loss Rate at
Ignition vs. Pressure
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Visual Observations
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• Different surface behavior: bubble formation, size and
bursting characteristics
• Flame establishment over solid surface also different
3 psi (21 kPa) 12 psi (83 kPa)
3 psi (21 kPa) 12 psi (83 kPa)
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Effect of Pressure (1)• Ignition delay time, tig:▫ tig= theating
+ tmixing/transport +tinduction
• Heating time: convective heat loss over flat plate▫ Forced
flow:
▫ Natural convection:
▫ Mixed flow
Re= ρUL/μ , Re~ P Pr≠f(P) h~P1/2
Ideal gas: Gr ~ P2
As pressure decreases, convective heat loss of material to
surroundings is lower heats more rapidly
4 32
31
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PrRe
1PrRe Grh +∝
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31
21
PrRe∝h
41
41
PrGrh ∝
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Effect of Pressure (2)•
• Mixing/transport time: Mass loss rate at which a flammable
concentration (LFL) is obtained at the pilotSimplified Analysis :
Boundary Layer – Integral Method
3rd order polynomials for velocity, temperature and species
profiles:
Integrate BL Eqns. analytical expressions for hydrodynamic,
thermal and concentration BL thicknesses:
δyxδc
160.000
0.005
0.010
0.015
0.020
0.025
0.030
0.035
0.00 0.05 0.10 0.15 0.20 0.25 0.30 0.35 0.40
boun
dary layer thickne
ss (m
)
longitudinal distance, x (m)
Velocity and Concentration Boundary Layers
d_ 100 kPa
d_85 kPa
d_69 kPa
d_55 kPa
dc_100 kPa
dc_85 kPa
dc_69 kPa
dc_55 kPa
P ↑ P ↑
δ
δc
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Simplified AnalysisHeat Transfer Coefficient
• h ≈k/δT
• At the sample location, h decreases by 13% when the pressure
is reduced from 100 kPa to 75 kPa
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0
1
2
3
4
5
6
7
8
0.0 0.1 0.2 0.3 0.4
h (W
/m2 K)
Longitudinal Distance (m)
28 kPa
41 kPa
55 kPa
69 kPa
83 kPa
100 kPasamplelocation
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Simplified AnalysisSpecies Concentration
• Reduced pressure leads to a thicker species boundary layer
δyx
δc
18
0.000
0.002
0.004
0.006
0.008
0.010
0.012
0.014
0.0 0.2 0.4 0.6 0.8 1.0
Vertical distance (m
)
Fuel Mass Fraction Profile YF/YF0
28 kPa
41 kPa
55 kPa
69 kPa
83 kPa
100 kPa
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Simplified Analysis
• To determine mass loss rate:
• At lower P, required mass flow rate of fuel to reach lean
flammability limit at igniter location is reduced
19
0.00
0.02
0.04
0.06
0.08
0.10
0.0 0.5 1.0 1.5 2.0 2.5
Fuel M
ole Fraction
, XF
m0" (g/m2s)
28 kPa41 kPa55 kPa69 kPa83 kPa100 kPaMMA LFL
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Comparison of Trends• Mass Loss Rate at Ignition vs.
Pressure
0.0
0.5
1.0
1.5
2.0
2.5
0 20 40 60 80 100
Mass Flow
Rate at Ignitio
n (g/sm
2)
Ambient Pressure (kPa)
Experiments (Burning)
Analysis (Flash Point)
m''ig = 0.005∙P + 1.483
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Current Work
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Fire Dynamics Simulator (FDS) 2D model▫ PMMA is irradiated under
a prescribed heat flux the solid
decomposes and the products of the pyrolysis ignite in the gas
phase.
HRR Temperature
Premixed flame appears in the gas phase
Flame ‘jumps’on to solid fuel surface
Diffusion flame anchored on solid surface travels
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Current Work• Fire Dynamics Simulator (FDS) 2D model
▫ Heat release rate/volume:
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Summary & Conclusions• Experimental results from piloted
ignition show that
tig & m”ig decrease with pressure:
▫ At 75 kPa , Δtig = - 15%Δmig”= -7%
• A theoretical explanation provides insight on the effect of
pressure on:▫ Heat transfer coefficient▫ Mass loss rate required to
reach a flammable mixture
• Next steps include developing a numerical model using FDS to
compare to experiments
• Overall, a reduction in ambient pressure leads to an increased
fire risk
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Acknowledgements• The research at University of California at
Berkeley
was supported by NASA on grant NNX08BA77A
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Effect of Cabin Pressure on the Piloted Ignition of Combustible
SolidsMotivationToday’s TalkLower ambient pressure �can be found
at…Cabin EnvironmentsHow does a solid fuel ignite?Forced Ignition
and Spread Test (FIST)Video of TestExperiment ExampleExperimental
ResultsVisual ObservationsEffect of Pressure (1)Effect of Pressure
(2)Simplified Analysis�Heat Transfer CoefficientSimplified
Analysis�Species ConcentrationSimplified AnalysisComparison of
TrendsCurrent WorkCurrent WorkSummary &
ConclusionsAcknowledgements