Fire Dynamics II
Lecture # 12
Other Important Phenomena
Jim Mehaffey
82.583
Carleton University, 82.583, Fire
Dynamics II, Winter 2003, Lecture #
1
Other Important Phenomena
Outline
•
•
•
•
•
Post-flashover fires in large compartments
Flames issuing through windows
Explosions
Backdrafts
BLEVEs
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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
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• 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
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Distribution of Cribs
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• 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
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Description of Tests
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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
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Temperatures in Test 1
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Temperatures in Test 1
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Analysis of Test 1
• Quantity 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:
– A h  3m x 6m x 3m  31.2 m5/2
• Duration of fire:
L AF
5,520 kg

 1966 s  32.8 min
– tD 
-1
0.09 A h 0.09 x 31.2 kg s
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• Model for rate of burning in deep compartments:

m  0.18 A h W
D
1  exp(0.036 )
W = width of compartment (m)
D = width of compartment (m)
AT

A h
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Analysis of Test 1
W=6m
D = 23 m
AT = 2 x 6 x 23 + 2 x 3 x 23 + 2 x 3 x 6 - 3 x 6 = 432 m2
AT
432 m 2
1/2



13.8
m
A h 31.2 m 5/2

m  1.12 kg s
1
tD = 5,520 kg / 1.12 kg s-1 = 4929 s = 82 min
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Flames Issuing through Windows
• Flame issuing from window of compartment experiencing
post-flashover fire is characterised by the flame length

 


m

  1

z f  h 16
1/ 2

A ( h g ) 






2/3
• For ventilation-controlled fire with wood cribs

m  0.09 A h
A gh  3.76 A gh
• For ventilation-controlled wood-crib post-flashover fire
zf = 0.33 h
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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
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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 > 0.035 atms
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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 ~ 45-50 g m-3
• For dust (< 100 m): LFL ~ 30-60 g m-3
– usually a two-event phenomenon
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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
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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
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Pressure Considerations
• Assume gas obeys the ideal gas law
PV=nRT
• 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
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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:
 dP  1/3
K  V
 dt 
K = deflagration index
V = volume of enclosure
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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 ~ 10-12 atm; K ~ 200-300 atm m/s
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Deflagration Venting
• Low strength enclosures cannot withstand P > 0.1
atm. Gas or mist deflagrations can be vented with
vents with combined area
C AS
AV 
Pred
AV = vent area (m2)
AS = internal surface area of enclosure (m2)
C = venting constant (for methane = 0.037 atm1/2)
Pred = maximum P permitted (2/3 enclosure strength, atm)
• Expansion through vent causes fireball outside
enclosure. Must be considered when placing vents
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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
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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
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5.6 s after opening the vent
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7.1 s after opening the vent
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8.0 s after opening the vent
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Backdraft
Schematic of temperature
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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
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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
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900
800
Temperature (°C)
700
600
500
400
300
200
100
0
0
5
10
15
20
25
30
40
35
Time (minutes)
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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
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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.
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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
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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
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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
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Behaviour of liquified gas metal container
(carbon steel) when exposed to fire
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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
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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
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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)
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A Fireball
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Protection against a BLEVE
• Insulate the container
• Apply water: Create a film of water coating portions of
container not in internal contact with liquid
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