Fire Dynamics II

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

1

Fire Dynamics II

Lecture # 12

Other Important Phenomena

Jim Mehaffey


82.583



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

6

Distribution of Cribs






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

7



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, 82.583, Fire
Dynamics II, Winter 2003, Lecture #
12

8

Description of Tests





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

10

Temperatures in Test 1







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

11

Temperatures in Test 1







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

12

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)


A
f
= (4 x 0.05 m x 1.0 m) x 155 x 33 = 1,023 m
2



Ventilation opening:






Duration of fire:









5/2
m

31.2
3m
x
6m
x
3m
h
A


min

32.8
s

1966
s

kg

31.2
x
0.09
kg

5,520

h
A

0.09
A

L



t
1
-
F
D




Carleton University, 82.583, 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)










h
A
A
T




)

0.036
exp(
1

D
W
h
A
18
0
m






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

14

Analysis of Test 1



W = 6 m



D = 23 m



A
T

=
2 x 6 x 23 + 2 x 3 x 23 + 2 x 3 x 6
-

3 x 6

= 432 m
2










t
D

= 5,520 kg / 1.12 kg s
-
1

= 4929 s = 82 min



1/2
5/2
2
T
m

13.8
m

31.2
m

432
h
A
A





1
s

kg

.12
1
m



Carleton University, 82.583, 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

z
f

= 0.33 h




















1
)
(
16
3
/
2
2
/
1
g
h
A
m
h
z
f


h
A

0.09
m


gh
A
gh
A
76
.
3


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

17

Explosions



Premixed: Fuel well mixed with air (O
2
) 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

倠㸠⸰㌵⁡瑭s


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

18

Examples



Methane CH
4

at T=25ºC & P=1 atm


LFL = 5% (by vol); UFL = 15% (by vol); MIE = 0.26 mJ



Propane C
3
H
8

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






Carleton University, 82.583, 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, 82.583, 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


P
red

= maximum pressure attained during venting is
commonly set at 2/3 of enclosure strength


P
red

is used in design of deflagration venting for low
strength enclosures

Carleton University, 82.583, 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


P
2
/ P
1
= (n
2
T
2
) / (n
1
T
1
)


n
2

/ n
1

~ 1


T
2

/ T
1

~ 2462 K / 293 K ~ 8.4


P
2
/ P
1

~ 8.4





Carleton University, 82.583, 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


1/3
V

dt
dP
K







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

23

Examples (at optimal concentrations)



Methane CH
4



P
max

~ 7.1 atm; K ~ 55 atm m/s)



Propane C
3
H
8


P
max

~ 7.9 atm; K ~ 100 atm m/s



Dusts


P
max

~ 10
-
12 atm; K ~ 200
-
300 atm m/s






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

24

Deflagration Venting


Low strength enclosures
cannot withstand

P > 0.1
atm. G
as or mist deflagrations can be vented with
vents with combined area





A
V

= vent area (m
2
)


A
S

= internal surface area of enclosure (m
2
)


C = venting constant (for methane = 0.037 atm
1/2
)


P
red

= maximum



灥牭楴瑥搠⠲⼳⁥(捬潳畲攠獴牥湧瑨Ⱐ慴c
)



Expansion through vent causes fireball outside
enclosure. Must be considered when placing vents



red
S
V
P
A

C
A

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

27

5.6 s after opening the vent



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

28

7.1 s after opening the vent



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

29

8.0 s after opening the vent



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

30

Backdraft


Schematic of temperature



Carleton University, 82.583, 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, 82.583, 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,

T
g,lim

= 120 K



For a heat loss fraction

1
= 0.6,

T
g,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, 82.583, Fire
Dynamics II, Winter 2003, Lecture #
12

33

Basement
0
100
200
300
400
500
600
700
800
900
1000
0
5
10
15
20
25
30
35
40
45
Time (minutes)
Temperature (°C)
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, 82.583, 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, 82.583, 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

P
2

= P
1
T
2
/ T
1


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

39

Behaviour of liquified gas metal container

(carbon steel) when exposed to fire


Carleton University, 82.583, 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, 82.583, 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 (C
3
H
8
) =
-

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, 82.583, 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, 82.583, Fire
Dynamics II, Winter 2003, Lecture #
12

43

A Fireball



Carleton University, 82.583, 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