Definition Flame spread over liquid pools

Report
Flame Spread
Over Liquid Pools
Brendan Paxton
Dr. Peter Disimile
Aviation Fire Dynamics
April 5th 2013
Agenda
• Definition
• Impact
• Research
– Fluid dynamics
•
•
•
Review
Regimes of flame propagation
Environmental disturbances
– Suppression
Definition
• Flame spread over liquid pools:
the result of a combustion process heating, vaporizing,
and igniting proximal liquid fuel
• Governing factors
– Initial pool temperature and the flash point of the fuel
– Environmental conditions
Impact
• Why research this phenomenon?
– Fire presents danger to living beings and structures
– Flame propagation over liquid fuels enlarges and intensifies a fire
•
•
•
Greater heat generation, leading to rapid vaporization of surrounding liquid fuel
Greater volume of toxic gases
Greater difficulty in extinguishment
– Traveling flames may spread to fire-sensitive locations
• Knowledge is power
– Research is necessary for controlling fire propagation, isolating the pool’s
source, and ultimately extinguishing the flames
Impact
• Aircraft crashes
• Oil spills
– Storage facilities
– Tankers – marine, railway, road vessels
– Marine drill sites
• Solvent handling
– Ethanol
– Methanol
– n-Propanol
“…it looked like the sun exploded.”
- Witness of an ethanol fire caused by the July 2012 train derailment in Columbus, Ohio
Railcar Derailment
Columbus, Ohio, July 2012
Effect: Catastrophic liquid ethanol fire
http://www.youtube.com/embed/2tY2HzWCvhw?rel=0
China Airlines Flight 120
Okinawa, Japan, August 2007
Cause: Punctured fuel tank
Buncefield Oil Depot Explosions
Hertfordshire, England, December 2005
Cause: Liquid gasoline spill
Buncefield Oil Depot Explosions
Hertfordshire, England, December 2005
Cause: Liquid gasoline spill
Buncefield Oil Depot Explosions
Hertfordshire, England, December 2005
Cause: Liquid gasoline spill
Buncefield Oil Depot Explosions
Hertfordshire, England, December 2005
Cause: Liquid gasoline spill
Research – Fluid Dynamics
• Characteristic regimes of flame propagation
1.
2.
3.
4.
5.
Pseudo-uniform, subflash
Pulsating
Transition
Uniform, near-flash
Uniform, superflash
• Complex thermophysical interactions
– Liquid convection, eddies, thermal currents
– Pre-mixed gas-phase interactions
Research – Fluid Dynamics
• Characteristic regimes of flame propagation
1.
2.
3.
4.
5.
Pseudo-uniform, subflash
Pulsating
Transition
Uniform, near-flash
Uniform, superflash
• Complex thermophysical interactions
– Liquid convection, eddies, thermal currents
– Pre-mixed gas-phase interactions
Review
• What is the flash point?
– The lowest temperature at which a substance can vaporize enough of its volume
to form an ignitable mixture in air (this volume concentration is roughly the LFL)
– Lower than the boiling point
• What is vaporization?
– Vaporization is the phase transition from liquid to gas
•
•
Evaporization – vaporization near the surface (at temperatures below boiling point)
Boiling – vaporization below the surface (at temperatures equal to and above boiling point)
– Boiling occurs when the pressure of the atmosphere can no longer hold the
molecules of substance in a liquid state
– This occurs at the boiling point, the temperature at which the vapor pressure
reaches the environmental pressure
• What is vapor pressure?
– The pressure exerted by a vapor on the
environment
– Dependent on the substance’s chemical
properties and temperature
Pv = vapor pressure
T = fluid temperature
A, B, C = chemical properties
T → Pv → mv
• Buoyancy effects
– The density of a cold, unburned, gaseous fuel-air mixture
is greater than the densities of its combustion products
•
•
In an unconfined environment, a combustion process can be
considered isobaric (constant pressure)
Temperature changes lead to density changes
– Buoyancy effects are seen when the lighter combustion
products pass upward above heavier, unburned reactants
– As the combustion products leave, cold air and vaporized
fuel are entrained, or drawn toward the base of the flame
Torch
Torch
Regimes of flame propagation
1.
2.
3.
4.
5.
Pseudo-uniform, subflash
Pulsating
Transition
Uniform, near-flash
Uniform, superflash
Pseudo-uniform, subflash regime
• Slow propagation over cold liquid fuel
– (Subflash temperature)
– Laminar air entrainment with turbulent gas-phase mixing at flame front
Pseudo-uniform spread
• Spread appears uniform and steady
• Long, slow pulsations
– (a) and (b) represent diffusion burning
•
•
Heating of fuel downstream of the flame front
Slow and smooth flame propagation
– (c) and (d) represent premixed burning
•
•
•
•
Once the fuel becomes sufficiently heated, it vaporizes
Vaporized air-fuel mixing creates a flammable layer
The flammable gas-phase layer is ignited after reaching a
sufficiently mixed state
Rapid pre-mixed consumption
• After burning quickly through the premixed
vapors, the overall flame spread returns to
reduced speeds, termed “crawl”
Pseudo-uniform spread
• Flames that extend through the premixed
flammable layer may quench after arrival
– Termed “precursor” flames
– The liquid fuel at their new location might not be
readily heated enough to sustain combustion
– The flame front advances, but does not reach the
location where the precursor flame quenches
• Experiment, high-speed video
– Capture: 300 fps, duration: 3 seconds
– Playback: 30 fps, duration: 30 seconds
Experiment performed at the University of Cincinnati Fire Test Center
Assistants: Dr. Samir Tambe and Derick Endicott
Video not attached due to size
Regimes of flame propagation
1.
2.
3.
4.
5.
Pseudo-uniform, subflash
Pulsating
Transition
Uniform, near-flash
Uniform, superflash
Pulsating regime
• Characterizations
– Behavior similar to pseudo-uniform spread
– Pulsations function with higher frequency and
reduced wavelength (premixed propagation
covers shorter distances during each pulsation)
• High-speed video of precursor flame
pulsations
– Capture: 1000 fps, duration: 1.8 seconds
– Playback: 30 fps, duration: 1 minute
• Normal video of pseudo-uniform and
pulsating spread
–
–
Capture: 30 fps, duration: 1 minute
Playback: 30 fps, duration: 1 minute
Video not attached due to size
Video not attached due to size
Experiment notes and conclusions
• Observations
–
–
–
–
–
Cold ambient temperature, 40°F, and low-volatility fuel, flash point = 100-151°F
Spread acceleration (pulsations) even under concurrent and opposed air flow
Average flame spread rate did not exceed 3.0 cm/s
Boiling occurred after 2:30 beneath the flames, boiling point = 293-575°F
Fuel began readily ejecting from the pan, even without wind flow
• Personal thoughts
– Experiment could only visualize subflash flame regimes
– Served as realistic scenario of an ignited Jet-A pool
•
•
Flame height for 16-inch diameter pool easily exceeded 20 inches
Approximately 2.5 oz of Jet-A required 3 minutes to be fully consumed
• Empirical support
• Empirical support
–
–
–
–
A = Pseudo-uniform, subflash
B = Pulsating
C = Uniform, near-flash
D = Uniform, superflash
– Methanol flash-point = 52°F
– Ethanol flash-point = 56°F
– n-Propanol flash-point = 72°F
– Methanol pulsating spread rate = 6.0 cm/s
– Ethanol pulsating spread rate = 5.5 cm/s
– n-Propanol pulsating spread rate = 4.8 cm/s
•
Methanol pulsations reach over 15 cm/s, averaging around 6 cm/s
Regimes of flame propagation
1.
2.
3.
4.
5.
Pseudo-uniform, subflash
Pulsating
Transition
Uniform, near-flash
Uniform, superflash
Transition regime
• Characterizations
– Very high pulsation frequencies, if pulsations even occur
– Increased flame spread rate
– Decreased liquid flow velocity relative to flame spread rate
Regimes of flame propagation
1.
2.
3.
4.
5.
Pseudo-uniform, subflash
Pulsating
Transition
Uniform, near-flash
Uniform, superflash
Uniform, near-flash regime
• Characterizations
– Uniform spread, pulsations eliminated
– Further reductions in liquid flow velocity
– Primary heat transfer mechanisms
•
•
•
Gas-phase radiation
Gas-phase convection
Liquid-phase conduction
– Minimized liquid-phase convection away from the flame front because the flame
spread rate now exceeds the liquid flow velocity
Regimes of flame propagation
1.
2.
3.
4.
5.
Pseudo-uniform, subflash
Pulsating
Transition
Uniform, near-flash
Uniform, superflash
Uniform, superflash regime
• Characterizations
–
–
–
–
Uniform, fast flame spread
Liquid fuel consumption maximized
Mostly occurs if the liquid fuel is heated above its flash point prior to ignition
Rarely occurs after flames have spread through cooler regimes
•
•
Example: gasoline combustion at room temperature
High volumes of vapors ignited under superflash conditions will appear explosive
– Often yields a “triple flame,” a premixed flame followed by a diffusion flame
•
•
Air is entrained behind the premixed flame front
This air undergoes diffusion combustion with the rapidly vaporizing fuel
http://www.youtube.com/embed/yoDhfB5DyBQ?rel=0
• If T0 > Tst the flame propagation rate is 4-5 times faster
than the stoichiometric laminar flame speed, Sl
Uniform, superflash regime
• 4-5 times faster than stoichiometric laminar flame speed?
–
Flame front is very curved, causing a significant pressure gradient across its surface
–
–
–
–
Gas-phase motion ahead of flame front
Gas-phase motion behind flame front, low-density products expanding
This expansion displaces unburned gas layers ahead of the flame front
Flame propagation rate increases
Disturbances from the environment
• Airflow, gusts of wind
– Concurrent airflow, in the direction of flame propagation
– Opposed airflow, against the direction of flame propagation
– Considering uniform, superflash flame spread
•
•
If the concurrent airflow exceeds the static flame spread rate, the flame spread rate will match the
magnitude of the concurrent airflow
Opposed airflow very gradually decreases the flame spread rate
Strong opposed airflow
Weak concurrent airflow
Strong concurrent airflow
Disturbances from the environment
• Obstacles
– Beds of solid material have been studied to simulate liquid pools that encounter
porous surfaces or obstacles
•
•
Solid material will reduce convective heat transfer
Conductive heat transfer through the pool’s constituents will come into play
– Balls or beads made out of sand, glass, and metal were tested
– Flame propagation rates were reduced by factors between 30 and 50
– The slow flame speeds compared more closely with solid than with liquid fuels
•
•
Obstacles only affect near-flash and preflash flame spread regimes
Gas-phase flame spread will not “see” the solids
• Airflow and obstacles
– Opposed airflow greatly cuts flame propagation over a pool filled with solids
– Considerable flow circulation and turbulent interactions around obstacles can
even lead to instability and blow-off
Disturbances from the environment
• Contamination
– Contaminants
•
•
Water (by rain or condensation)
Combustion products (soot, vapors)
– Changes the thermocapillarity of the fuel, diminishing convective pre-heating
Suppression
• Class B fire extinguishants
– Dry chemical (powders)
•
•
•
•
•
Pro – Separation of the fire tetrahedron
Pro – Interference with the chain reaction, through reacting
Pro – Halts production of fire-sustaining free-radicals
Pro – Slows fire propagation
Con – Difficulty in cooling the fuel, potential for flashback
– Liquid-solid foams (frothy blanket)
•
•
•
Pro – Some dry chemical properties, smothering
Pro – Cooling of the fuel
Con – Does not interfere with the chain reaction, takes longer
– Water, wet chemical (soapy foam)
•
•
Pro – Cooling of the fuel
Con – Mixes with the fuel, does not effectively displace oxygen
– Clean agents (gases)
•
•
•
•
Pro – Clean interaction, little residue
Pro – Interrupts chemical reaction
Pro – Displaces oxygen
Con – Hazardous toxins (Halon)
–
Halon replacement Bromo–trifluoro–propene (BTP) generates hydrogen fluoride
Water mist
BTP
5% BTP
10% BTP
Suppression
• Dry chemical
http://www.youtube.com/embed/LbExsN4jdcA
• Liquid-solid foams
http://www.youtube.com/embed/eHzUhm-3HX8
• Water, wet chemical
http://www.youtube.com/embed/tavsFhniNVY
Prevention – concluding thoughts
• Gravel or sand could be useful in preventing liquid pool fire
propagation at ground-based oil storage sites
• Water mist could be useful in preventing liquid pool fires if it was
sprayed/injected when a leak is detected, before ignition
Questions?
References
[1] Guo, J., Lu, S., Li, M., and Wang, C., “Flame spread over aviation kerosene with an obstacle in liquid
phase,” Journal of Thermal Science, 20, 6, 543-547, 2011.
(http://link.springer.com.proxy.libraries.uc.edu/content/pdf/10.1007%2Fs11630-011-0508-z)
[2] Hirano, T., and Suzuki, T., “Flame propagation across liquids—A review of gas phase phenomena,” Fire
Safety Journal, 21, 3, 207-229, 1993. (http://www.sciencedirect.com/science/article/pii/037971129390028O)
[3] Ishida, H., “Flame tip traveling in boundary layer flow with flammable mixture along fuel-soaked ground,”
Journal of Fire Sciences, 30, 1, 17-27. (http://jfs.sagepub.com.proxy.libraries.uc.edu/content/29/2/99)
[4] Remick, E. M., “Flame spread over liquid fuels,” Cornell University, 1981.
(http://search.proquest.com/docview/303016167?accountid=2909)
[5] Zhou, J., Chen, G., Li, P., Chen, B., Wang, C., and Lu, S., “Analysis of flame spread over aviation
kerosene,” Chinese Science Bulletin, 55, 17, 1822-1827, 2009.
(http://link.springer.com/article/10.1007/s11434-010-3014-x)
[6] Xiaomin Ni, W.K. Chow, Performance evaluation of water mist with bromofluoropropene in suppressing gasoline
pool fires, Applied Thermal Engineering, Volume 31, Issues 17–18, December 2011, Pages 3864-3870, ISSN 13594311, 10.1016/j.applthermaleng.2011.07.034. (http://www.sciencedirect.com/science/article/pii/S1359431111003930)

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