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

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

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

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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FIST Apparatus

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

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

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)

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

21

PrRe

1PrRe Grh +∝

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31

21

PrRe∝h

41

41

PrGrh ∝

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

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

Simplified AnalysisSpecies Concentration

• Reduced pressure leads to a thicker species boundary layer

δyx

δc

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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

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

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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

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

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