# Fire Dynamics II

Mechanics

Feb 22, 2014 (7 years and 8 months ago)

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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

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

)

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