Flame Spread Over Liquid Pools

1. Flame Spread Over Liquid Pools Brendan Paxton Dr. Peter Disimile Aviation Fire Dynamics April 5th 2013

2. Agenda • Definition • Impact • Research • Fluid dynamics • Review • Regimes of flame propagation • Environmental disturbances • Suppression

3. Definition • Governing factors • Initial pool temperature and the flash point of the fuel • Environmental conditions • Flame spread over liquid pools: • the result of a combustion process heating, vaporizing, • and igniting proximal liquid fuel

4. 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

5. 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

6. Railcar Derailment Columbus, Ohio, July 2012 Effect: Catastrophic liquid ethanol fire

7. China Airlines Flight 120 Okinawa, Japan, August 2007 Cause: Punctured fuel tank http://www.youtube.com/embed/2tY2HzWCvhw?rel=0

8. Buncefield Oil Depot Explosions Hertfordshire, England, December 2005 Cause: Liquid gasoline spill

9. Buncefield Oil Depot Explosions Hertfordshire, England, December 2005 Cause: Liquid gasoline spill

10. Buncefield Oil Depot Explosions Hertfordshire, England, December 2005 Cause: Liquid gasoline spill

11. Buncefield Oil Depot Explosions Hertfordshire, England, December 2005 Cause: Liquid gasoline spill

12. Research – Fluid Dynamics • Characteristic regimes of flame propagation 1. Pseudo-uniform, subflash 2. Pulsating 3. Transition 4. Uniform, near-flash 5. Uniform, superflash • Complex thermophysical interactions • Liquid convection, eddies, thermal currents • Pre-mixed gas-phase interactions

13. Research – Fluid Dynamics • Characteristic regimes of flame propagation 1. Pseudo-uniform, subflash 2. Pulsating 3. Transition 4. Uniform, near-flash 5. Uniform, superflash • Complex thermophysical interactions • Liquid convection, eddies, thermal currents • Pre-mixed gas-phase interactions

14. 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

15. 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

16. 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

18. Torch Torch

21. Regimes of flame propagation 1. Pseudo-uniform, subflash 2. Pulsating 3. Transition 4. Uniform, near-flash 5. Uniform, superflash

22. Pseudo-uniform, subflash regime • Slow propagation over cold liquid fuel • (Subflash temperature) • Laminar air entrainment with turbulent gas-phase mixing at flame front

24. 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”

25. 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

26. Video not attached due to size

27. Regimes of flame propagation 1. Pseudo-uniform, subflash 2. Pulsating 3. Transition 4. Uniform, near-flash 5. Uniform, superflash

28. 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

29. Video not attached due to size

30. Video not attached due to size

31. 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

32. 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

33. Methanol pulsations reach over 15 cm/s, averaging around 6 cm/s

34. Regimes of flame propagation 1. Pseudo-uniform, subflash 2. Pulsating 3. Transition 4. Uniform, near-flash 5. Uniform, superflash

35. Transition regime • Characterizations • Very high pulsation frequencies, if pulsations even occur • Increased flame spread rate • Decreased liquid flow velocity relative to flame spread rate

36. Regimes of flame propagation 1. Pseudo-uniform, subflash 2. Pulsating 3. Transition 4. Uniform, near-flash 5. Uniform, superflash

37. 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

40. Regimes of flame propagation 1. Pseudo-uniform, subflash 2. Pulsating 3. Transition 4. Uniform, near-flash 5. Uniform, superflash

41. 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 • If T0 > Tstthe flame propagation rate is 4-5 times faster than the stoichiometric laminar flame speed, Sl http://www.youtube.com/embed/yoDhfB5DyBQ?rel=0

42. 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

44. 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

46. Strong opposed airflow Weak concurrent airflow Strong concurrent airflow

48. 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

50. Disturbances from the environment • Contamination • Contaminants • Water (by rain or condensation) • Combustion products (soot, vapors) • Changes the thermocapillarityof the fuel, diminishing convective pre-heating