1. Other Important Phenomena
Fire Dynamics II
Lecture # 12
Other Important Phenomena
Jim Mehaffey
82.583
Carleton University, , Fire Dynamics II, Winter 2003, Lecture # 12

2. Other Important Phenomena
Outline
Post-flashover fires in large compartments
Flames issuing through windows
Explosions
Backdrafts
BLEVEs
Carleton University, , Fire Dynamics II, Winter 2003, Lecture # 12

3. Post-flashover Fires in Large Compartments
Gordon Cooke, Tests to determine the behaviour of fully developed natural fires in a large compartment, Fire Note 4, Fire Research Station, British Research Establishment, 1998
9 Post-flashover fires
Basic compartment: 23 m deep, 6 m wide, 3 m high
Objective: simulate an even larger compartment in an open plan office building by allowing no net heat transfer to neighbouring compartments
if only 2 sides of bldg have windows, after flashover there is line of symmetry along centre line of storey
ensure separation walls are well insulated
Carleton University, , Fire Dynamics II, Winter 2003, Lecture # 12

4. Carleton University, 82.583, Fire Dynamics II, Winter 2003, Lecture # 12

5. Ventilation opening in one of the 6 m x 3 m end walls
not glazed (open from outset)
12.5%, 25% 50% or 100% of area of end wall
12.5% simulated fire in basement with ventilation at top
Fuel load: 20 kg m-2 or 40 kg m-2
33 wood cribs: 11 rows of 3 cribs, 1 m apart
D = 50 mm; L = 1.0 m;
1 crib = 155 sticks in 15 layers for 40 kg m-2
1 crib = 75 sticks in 7 layers for 20 kg m-2
6 cribs (every other crib) along centre line on load cell
Carleton University, , Fire Dynamics II, Winter 2003, Lecture # 12

6. Distribution of Cribs
Carleton University, , Fire Dynamics II, Winter 2003, Lecture # 12

7. Temperature measured in two locations:
Room linings:
walls and ceiling: insulating ceramic fibre blanket
floor: layer of dry sand
Temperature measured in two locations:
150 mm below ceiling 6.0 m from rear of compartment
150 mm below ceiling 6.0 m from front of compartment
Ignition sequence in 8 tests: Ignite row of cribs furthest from ventilation opening and observe spread of fire
Carleton University, , Fire Dynamics II, Winter 2003, Lecture # 12

8. Description of Tests
Carleton University, , Fire Dynamics II, Winter 2003, Lecture # 12

9. Mass Loss of Cribs Measured in Test 1
1 = mass loss of central crib in row farthest from opening
11 = mass loss of central crib in row closest to opening
Carleton University, , Fire Dynamics II, Winter 2003, Lecture # 12

10. Temperatures in Test 1
Carleton University, , Fire Dynamics II, Winter 2003, Lecture # 12

11. Temperatures in Test 1
Carleton University, , Fire Dynamics II, Winter 2003, Lecture # 12

12. Analysis of Test 1 Quantity of fuel: Surface area of fuel:
G = 40 kg m-2 x 6 m x 23 m = 5,520 kg
Surface area of fuel:
(Surface area 1 stick) x (no. sticks / crib) x (no. cribs)
Af = (4 x 0.05 m x 1.0 m) x 155 x 33 = 1,023 m2
Ventilation opening:
Duration of fire:
Carleton University, , Fire Dynamics II, Winter 2003, Lecture # 12

13. Model for rate of burning in deep compartments:
W = width of compartment (m)
D = width of compartment (m)
Carleton University, , Fire Dynamics II, Winter 2003, Lecture # 12

14. Analysis of Test 1 W = 6 m D = 23 m
AT = 2 x 6 x x 3 x x 3 x x 6 = 432 m2
tD = 5,520 kg / 1.12 kg s-1 = s = 82 min
Carleton University, , Fire Dynamics II, Winter 2003, Lecture # 12

15. Flames Issuing through Windows
Flame issuing from window of compartment experiencing post-flashover fire is characterised by the flame length
For ventilation-controlled fire with wood cribs
For ventilation-controlled wood-crib post-flashover fire
zf = 0.33 h
Carleton University, , Fire Dynamics II, Winter 2003, Lecture # 12

16. Flames Issuing through Windows
For ventilation-controlled wood-crib fires, we have close to stoichiometric fires (equivalence ratio ~ 0.92)
For other fuels, like gasoline, most plastics, or wood panelling, the mass loss rate is much greater than for a ventilation-controlled wood-crib fire
Not enough air can get into the room to burn the fuel vapours (equivalence ratio > 1) within the room so flaming continues outside the room
Consequently flame length will also be much greater
Carleton University, , Fire Dynamics II, Winter 2003, Lecture # 12

17. Explosions Premixed: Fuel well mixed with air (O2) before burning
Flammability limits: Mixture will only burn if concentration is between LFL and UFL
Minimum ignition energy (MIE) required for ignition
Rate of combustion is high: Governed by chemical kinetics not mixing rate
Deflagration: Combustion propagates through mixture as a flame (below speed of sound)
If mixture is confined, walls & ceiling may not be able to withstand pressure rise  explosion
masonry wall cannot withstand P > atms
Carleton University, , Fire Dynamics II, Winter 2003, Lecture # 12

18. Examples Methane CH4 at T=25ºC & P=1 atm
LFL = 5% (by vol); UFL = 15% (by vol); MIE = 0.26 mJ
Propane C3H8 at T=25ºC & P=1 atm
LFL = 2.1% (by vol); UFL = 9.5% (by vol); MIE = 0.25 mJ
****************************************************************
For alkanes (gaseous): LFL ~ 48 g m-3
For aerosol or droplet suspension: LFL ~ g m-3
For dust (< 100 m): LFL ~ g m-3
usually a two-event phenomenon
Carleton University, , Fire Dynamics II, Winter 2003, Lecture # 12

19. Deflagration Mitigation
Prevention:
Reduction of concentration of flammables (by ventilation for vapours or housekeeping for dusts)
Control potential ignition sources (mechanical sparks, hot surfaces, electrical equipment)
Rapid suppression: terminate combustion by very rapid introduction of inert gas or chemical inhibitor
Protection:
Venting
Carleton University, , Fire Dynamics II, Winter 2003, Lecture # 12

20. Deflagration Venting
Objective: Design vents to relieve pressures developed by a deflagration
NFPA 68: Guide for Venting of Deflagrations
Rate of pressure rise is used in design of deflagration venting for high strength enclosures.
Rapid rate of rise means short time available to vent
Rapid rate of rise requires greater area for venting
Pred = maximum pressure attained during venting is commonly set at 2/3 of enclosure strength
Pred is used in design of deflagration venting for low strength enclosures
Carleton University, , Fire Dynamics II, Winter 2003, Lecture # 12

21. Pressure Considerations
Assume gas obeys the ideal gas law
P V = n R T
Fire Dynamics I: Adiabatic flame temperature of a stoichiometric mixture of propane in air: T ~ 2462 K
In enclosure without vents, volume is constant
P2 / P1 = (n2T2) / (n1T1)
n2 / n1 ~ 1
T2 / T1 ~ 2462 K / 293 K ~ 8.4
P2 / P1 ~ 8.4
Carleton University, , Fire Dynamics II, Winter 2003, Lecture # 12

22. Pressure Considerations
Maximum deflagration pressure and rate of pressure rise dP/dt are determined by test
For most fuels maximum pressure rise is 6 to 10 times pressure before ignition
Fundamental basis for deflagration venting theory is the cubic law:
K = deflagration index
V = volume of enclosure
Carleton University, , Fire Dynamics II, Winter 2003, Lecture # 12

23. Examples (at optimal concentrations)
Methane CH4
Pmax ~ 7.1 atm; K ~ 55 atm m/s)
Propane C3H8
Pmax ~ 7.9 atm; K ~ 100 atm m/s
Dusts
Pmax ~ atm; K ~ atm m/s
Carleton University, , Fire Dynamics II, Winter 2003, Lecture # 12

24. Deflagration Venting AV = vent area (m2)
Low strength enclosures cannot withstand P > 0.1 atm. Gas or mist deflagrations can be vented with vents with combined area
AV = vent area (m2)
AS = internal surface area of enclosure (m2)
C = venting constant (for methane = atm1/2)
Pred = maximum P permitted (2/3 enclosure strength, atm)
Expansion through vent causes fireball outside enclosure. Must be considered when placing vents
Carleton University, , Fire Dynamics II, Winter 2003, Lecture # 12

25. Backdrafts
Limited ventilation  large quantity of unburnt “gas” (products of pyrolysis or incomplete combustion) generated
When opening suddenly introduced, inflowing air mixes with “gas” creating flammable mixture
Ignition source (smouldering material) ignites flammable mixture, resulting in extremely rapid burning
Expansion due to heat released expels burning “gas” through opening & causes fireball outside enclosure
Backdrafts extremely hazardous for firefighters
Backdraft of short duration. Flashover often follows
Carleton University, , Fire Dynamics II, Winter 2003, Lecture # 12

26. Backdraft Experiments: Fleischmann
70 kW methane flame burned in a small “sealed” chamber
Flame eventually self-extinguished due to oxygen starvation
Vent opened, air enters
Continuous ignition source present near back of chamber
Observed a backdraft
Carleton University, , Fire Dynamics II, Winter 2003, Lecture # 12

27. 5.6 s after opening the vent
Carleton University, , Fire Dynamics II, Winter 2003, Lecture # 12

28. 7.1 s after opening the vent
Carleton University, , Fire Dynamics II, Winter 2003, Lecture # 12

29. 8.0 s after opening the vent
Carleton University, , Fire Dynamics II, Winter 2003, Lecture # 12

30. Schematic of temperature
Backdraft
Schematic of temperature
Carleton University, , Fire Dynamics II, Winter 2003, Lecture # 12

31. Kemano: Fire in Basement Recreation Room
Room dimensions: 3.25 m x 3.44 m x 2.2 m (height)
Walls: 2 gypsum board // 2 (6 mm) wood panelling
Ceiling: gypsum board
Floor: carpet over concrete
Furnishings: couch / coffee table / TV on wood desk
Ventilation: no window / hollow-core wood door closed
Carleton University, , Fire Dynamics II, Winter 2003, Lecture # 12

32. Temperatures in Basement Fire
Temperature predictions from Lecture 3 for leaky enclosures (based on oxygen depletion):
For a heat loss fraction 1= 0.9, Tg,lim = 120 K
For a heat loss fraction 1= 0.6, Tg,lim = 480 K
1 = 0.6 appropriate for spaces with smooth ceilings & large ceiling area to height ratios
1 = 0.9 appropriate for spaces with irregular ceiling shapes, small ceiling area to height ratios & where fires are located against walls
Carleton University, , Fire Dynamics II, Winter 2003, Lecture # 12

33. Carleton University, 82.583, Fire Dynamics II, Winter 2003, Lecture # 12

34. BLEVE: Boiling Liquid Expanding Vapour Explosion
Propane is a gas under atmospheric conditions
Liquified by application of pressure & stored in tank
In tank, liquid & vapour at equilibrium, with vapour at high pressure
If tank immersed in fire, heat causes pressure of vapour to rise
Activates relief valve (turbulent jet flame)
Pressure still high & fire may weaken metal casing
Tank ruptures  BLEVE
Carleton University, , Fire Dynamics II, Winter 2003, Lecture # 12

35. What is a Liquified Gas?
Gas = a substance that exist in the gaseous state at standard temperature (20°C) and pressure (101 kPa)
Economic necessity and ease of usage  gas stored in containers containing as much gas as practical
Compressed gas = stored in a container under pressure but remains gaseous at 20°C. Typical pressure range is 3 to 240 atm
Liquified gas = stored in a container under pressure and exists partly in liquid and partly in gaseous state. Pressure depends on temperature of liquid.
Carleton University, , Fire Dynamics II, Winter 2003, Lecture # 12

36. Heating of a Container Containing Compressed Gas
Compressed gas obeys ideal gas law
PV = nRT
V & n are constant so pressure rises according to
P2 = P1 T2 / T1
Carleton University, , Fire Dynamics II, Winter 2003, Lecture # 12

37. Heating of a Container Containing Liquified Gas
Liquified gas exhibits more complex behaviour because net effect is a combination of three effects
Gas phase is subject to same effect as compressed gas
Liquid attempts to expand, compressing vapour
Vapour pressure increases as temperature of liquid increases
Combined result: an increase in pressure
Carleton University, , Fire Dynamics II, Winter 2003, Lecture # 12

38. Overpressure Relief Devices
Spring-loaded pressure-relief valves, bursting discs or fusible plugs (small containers) used to limit pressure to a level the container can safely withstand
P(activation) > P(operating) >> P(atmospheric)
Relieving capacity (gas flow rate through device) is based on maximum heat input rates resulting from fire exposure
Gas discharge is in the form of a turbulent jet and if the gas is flammable, it will be a turbulent jet flame
Carleton University, , Fire Dynamics II, Winter 2003, Lecture # 12

39. Behaviour of liquified gas metal container
(carbon steel) when exposed to fire
Carleton University, , Fire Dynamics II, Winter 2003, Lecture # 12

40. Failure of Container
Precise curves a little different for other steels, but loss of strength is significant as temperature climbs
Spring-loaded relief valve only reduces pressure to activation pressure
Pressure remains high in container
container stressed in tension
Liquid always at temp > normal boiling point
When exposed to fire, metal in contact with vapour phase heats up, may stretch and a rupture develop
Before rupture relieves pressure, it propagates and container fails catastrophically
Carleton University, , Fire Dynamics II, Winter 2003, Lecture # 12

41. Potential for Rapid Vaporization of Liquid
Liquified gases are stored at high pressure, in containers at temperature (~ 20°C) > boiling point at atmospheric pressure (101 kPa)
e.g. boiling point at 1 atm of propane (C3H8) = - 42°C
Pressure drop to 1 atmosphere (failure of container) causes very rapid vaporization of a portion of liquid
Fraction vaporized depends on temperature difference between liquid at failure and its normal boiling point
For fire induced failure about 1/2 of liquid is vaporized
Carleton University, , Fire Dynamics II, Winter 2003, Lecture # 12

42. After Failure of the Container: A BLEVE
Pressure difference, inside to outside, propels pieces of the container at high velocity for some distance (up to 1.0 km)
Liquid vaporizes and vapour expands rapidly
Rapid turbulent mixing of vapour and air
If vapour is flammable, observe a huge fireball (diameter up to 150 m)
Carleton University, , Fire Dynamics II, Winter 2003, Lecture # 12

43. A Fireball
Carleton University, , Fire Dynamics II, Winter 2003, Lecture # 12

44. Protection against a BLEVE
Insulate the container
Apply water: Create a film of water coating portions of container not in internal contact with liquid
Carleton University, , Fire Dynamics II, Winter 2003, Lecture # 12