Transcript Fire Dynamics I - Carleton University

Fire Dynamics II
Lecture # 8
Flame Spread & Burning Rates
Jim Mehaffey
82.583
Carleton University, 82.583, Fire
Dynamics II, Winter 2003, Lecture #
1
Flame Spread & Burning Rates
Outline
• Models for flame spread on solids (review)
– wind-aided vs opposed-flow flame spread
– in the absence or presence of external radiation
• Burning rates of common items
– in the open (review)
– limited by ventilation
– enhanced by radiation
Carleton University, 82.583, Fire
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Factors Affecting Rate of Spread of Flame
• Material Factors
Chemical • Composition of fuel
• Presence of fire retardants
Physical
• Initial temperature
• Surface orientation
• Direction of propagation
• Thickness
• Thermal conductivity
• Specific heat
• Density
• Geometry
• Continuity
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Factors Affecting Rate of Spread of Flame
• Environmental Factors
• Composition of atmosphere
• Temperature
• Imposed heat flux
• Air velocity
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Spread of Flame over Wall Linings
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Spread of Flame over Wall Linings
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Dynamics II, Winter 2003, Lecture #
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Room Fire Test - Apparatus
• ISO 9705 “Fire tests: Full scale room fire tests for
surface products”
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Dynamics II, Winter 2003, Lecture #
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Room Fire Test - Procedure
• Line walls and ceiling with product
• Burner in back corner

– First 10 min: Q = 100 kW (large wastepaper basket)

– Last 10 min: Q = 300 kW (small upholstered chair)
• Observe time to flashover

• Room experiences flashover when Q  1,000 kW
Carleton University, 82.583, Fire
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Room Fire Test - Results
Wall Lining
Gypsum board
Douglas fir plywood
Douglas fir plywood
Polyurethane foam
Ceiling Lining
Gypsum board
Gypsum board
Douglas fir plywood
Polyurethane foam
Time to Flashover
(min:sec)

~ 7:30
~ 3:00
~ 0:13
Results for CAN/ULC-S102
Gypsum board
Douglas fir plywood
Polyurethane foam
FSR ~ 15
FSR ~ 135
FSR ~ 500
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Flame Spread Models: Concepts
• Flame spread = an advancing ignition front
• Leading edge of flame is heat source (raising fuel to
ignition temp) and the pilot
• Visually flame spread is advancing flame close to solid
• Two interacting advancing fronts
– flame front in gas phase
– pyrolysis front along solid surface
• Heat transfer from flame  pyrolysis front to advance
• Advance of pyrolysis front  increased release of
volatiles  advance of flame front
• Flame-spread velocity  rate of advance of pyrolysis front
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Dynamics II, Winter 2003, Lecture #
10
Wind-aided Spread
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Wind-aided Spread
•  = region of heat transfer from flame & smoke
• For wind-aided spread: 0.1 m    10 m
• For opposed-flow spread: 1 mm    3 mm
• Surface temp in control volume drops from Tig to Ts
dx p
• Pyrolysis front moves at speed v 
dt
• Model for wind-aided flame spread:
dx p
n
v
 x p  x b 
dt
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Example of Accelerating Flame Spread
• Upward turbulent spread on thick PMMA
• xb = 0 and n = 0.94 ~ 1
dx p
v
 c1 x p
dt
Eqn (8-1)
• Experiment finding:
When xp ~ 1.0 m, v ~ 5.0 mm s-1
Carleton University, 82.583, Fire
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Example of Constant Flame Spread
• Upward turbulent spread on thin textiles
• n ~ 0.6
dx p
0.6
v
 c 2 x p  x b 
dt
Eqn (8-2)
• After some time, (xp - xb) and v become constant
Carleton University, 82.583, Fire
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14
Apartment Fire: Hiroshima, Japan (1996)
• Building - reinforced concrete structure
- 20 storeys
- height of each storey = 3 m
- each apartment had a balcony
• Balcony - PMMA glazing
- height of glazing = 1 m
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00:00
13:00
18:00
20:00
22:00
23:00
23:30
24:00
24:20
24:40
25:00
25:15
25:30
Chronology of Fire
Fire commences within apartment 965
Outer surface of PMMA glazing (9th storey) ignites
Outer surface of PMMA glazing (10th storey) ignites
Outer surface of PMMA glazing (11th storey) ignites
Outer surface of PMMA glazing (12th storey) ignites
Outer surface of PMMA glazing (13th storey) ignites
Outer surface of PMMA glazing (14th storey) ignites
Outer surface of PMMA glazing (15th storey) ignites
Outer surface of PMMA glazing (16th storey) ignites
Outer surface of PMMA glazing (17th storey) ignites
Outer surface of PMMA glazing (18th storey) ignites
Outer surface of PMMA glazing (19th storey) ignites
Outer surface of PMMA glazing (20th storey) ignites
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- - - - inner surface burning ——— outer surface burning
 Ignition of outer side of PMMA
O Burn-out of PMMA
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Problem Set 3: Problem 4
5. In 1975, FMRC studied upward turbulent flame
spread on thick PMMA and found the process
obeyed the model for wind-aided flame spread
presented in class with xb = 0 and n ~ 1. They
found that when the flame extension xp = 1 m,
the upward flame spread velocity V = 5 mm/s.
Calculate the flame extension 2, 4, 6, 8, 10 and
12 minutes later. Compare your predictions
with the observed flame extensions in the
Hiroshima fire by plotting your predictions on
the graph on page 8-17.
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Opposed Flow Flame Spread
Absence of External Radiation
• Compared to PMMA, a very slow process
• Not accelerating, but roughly constant velocity
• Speed of downward flame spread on PMMA
v ~ 0.04 mm s-1
v ~ 2.4 mm min-1
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Opposed Flow Flame Spread
In Presence of External Radiation (1)
• Effect of preheating time on rate of downward flame spread
on PMMA exposed to radiant flux (kW m-2)
• CHF (PMMA) ~ 11 kW m-2
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Opposed Flow Spread: Model for Thick Materials
• Quintiere and Harkleroad, 1985

v
2
kc Tig  TS 
Eqn (8-3)
•  = flame-heating parameter (kW2 m-3) {material property}
• Provided no dripping, this model holds for
– downward flame spread (wall)
– lateral flame spread (wall)
– horizontal flame spread (floor)
• , kc and Tig - measured (LIFT apparatus)
• Ts - depends on scenario (external flux)
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LIFT Apparatus - Standard Tests
• ASTM E1321, “Standard test method for determining
material ignition and flame spread properties
• ISO 5668, “Fire tests: Reaction to fire: surface spread
of flame on building products”
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Dynamics II, Winter 2003, Lecture #
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LIFT Apparatus
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LIFT Apparatus - Results
Material
Tig (C)
Polyurethane foam
280
PMMA
378
Plywood
390
2
-1
/kc (m K s )
82
14
16
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Estimating the Surface Temperature TS
• To employ Eqn (8-3) one must estimate TS

• Assume the surface is heated by a radiant flux q" and
cools by convection h (TS -To)
• Following pages 5-35 to 5-38 in Fire Dynamics I

q"  h t 

TS  To  f 
h  kc 
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Eqn (8-4)
25
h t

kc
f( )  1  exp 2  erfc 
lim
2
f(  ) 

 0

lim
f( )  1
 
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f( )  1  exp  erfc 
2
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Thermal Properties for Ignition, Flame Spread
& Pre-flashover Fires (1)
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Problem Set 3: Problem 3
3. Consider a pre-flashover fire in a room 2.4 m x 3.6 m x
2.4 m (height). The door to the room (0.8 m x 2.0 m
(height)) is open and the interface between the hot
layer and cool air is at the mid-height of the door. The
fuel in the room is a mixture of wood and plastics and
the mean extinction (absorption) coefficient of the
upper layer is Km ~ 1.0 m-1. What is the emissivity of
the upper layer? Calculate the radiant flux at the
centre of the floor when the layer temperature is
300°C, 400°C, 500°C and 600°C.
Carleton University, 82.583, Fire
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Problem Set 3: Problem 4
4. Calculate the time to piloted ignition of a wood floor
and a polyurethane cushion at floor level for the four
upper layer temperatures considered in Problem 3.
Use Tewarson’s model assuming for the wooden floor
that CHF = 10 kW m-2 and TRP = 134 kW s1/2 m-2, and
for the polyurethane cushion CHF = 11 kW m-2 and
TRP = 55 kW s1/2 m-2.
Carleton University, 82.583, Fire
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Problem Set 3: Problem 6
6. Consider the room of Problem 3. For upper layer
temperatures of 300°C and 400°C, calculate the flame
velocity on a wooden floor and on a polyurethane
cushion 30 seconds and 1 minute after the flux is
applied. (Assume that the convective cooling is
governed by h = 9.0 W m-2 K-1).
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Burning Rates of Common Items
* In the open (review)
* Limited by ventilation
* Enhanced by radiation
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Wooden Cribs (2)
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Wooden Cribs
• D = stick thickness (m)
• S = spacing between sticks (m)
• hc = height of crib (m)
• N = number of rows = hc / D
• n = number of sticks per row
• L = length of each stick (m) {L >> D}
•  = density of sticks (kg m-3)
• mo = initial mass of crib (kg) = N n  D2 L
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Steady-State Burning of Wooden Cribs
• Fuel surface controlled burning: Stick surfaces burn
freely {S >> D}

4
 2v P t  t O  
m(t  t O )  mO v P 1 

Eqn (8-5)
D
D



• m = mass loss rate of crib (kg s-1)
• to = time at which steady burning is established (s)
• vp = surface regression rate (m s-1)
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Steady-State Burning of Wooden Cribs
• Crib porosity controlled burning: Burning controlled
by rate of flow of air & combustion products through
holes in crib {S << D}
 S  m O 
m(t  t O )  4.4 x 10  
 Eqn (8-6)
 h C  D 

4

• for t > to: m is given by lesser of Eqns (8-5) & (8-6)
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Growth Rates - Burning of Wooden Cribs
• Assume crib is ignited at bottom / centre
• to = time at which steady burning is established (s)
• For t < to
2
 vP t 
m(t  t O )  0.0254 mO  2 
n D

Eqn (8-7)
• to = time Eqn (8-7) yields lesser of Eqns (8-5) & (8-6)
****************************************************************
• For a crib ignited at bottom / centre and whose steadystate burning is fuel-surface controlled: to ~ 15.7 n (s)
Carleton University, 82.583, Fire
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Wooden Cribs - Heat Release Rate
• The heat release rate is given by


Q  Hch m
Eqn (8-8)
• with Hch = 12.4 kJ g-1

• Knowing m one can also calculate, radiative and
convective components of heat release rate, and rates
of generation of CO and soot.
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Wooden Cribs in an Enclosure

• Radiation from upper layer has little impact on Q
because fire is largely “self-contained” with many
surfaces “seeing” each other.

Q will be reduced
• If fire is limited by ventilation,

because Hch and m are both reduced.
Carleton University, 82.583, Fire
Dynamics II, Winter 2003, Lecture #
39
Post-flashover Fires Involving Wooden Cribs
• Harmathy (1972) identified two burning regimes for
room fires involving wooden cribs:
ventilation-controlled & fuel-surface controlled

• R  m = mass loss rate of fuel (kg s-1)
•  = ventilation parameter (kg s-1)
=  O A g h  3.76 A h
• Af = exposed surface area of fuel (m2)
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Dynamics II, Winter 2003, Lecture #
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Post-flashover Fires Involving Wooden Cribs
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Post-flashover Fires Involving Wooden Cribs
• Post-flashover fire is ventilation-controlled if
 / Af < 0.63 kg m-2 s-1
A h A f  0.07 m
1/2
Eqn (8-9)
• Fuel mass loss rate is

m  0.0236  kg s
1

m  0.09 A h kg s 1
Eqn (8-10)
• Least of Eqns (8-5), (8-6), (8-7) or (8-10) applies
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42
Wooden Pallets (1)
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Wooden Pallets
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Wooden Pallets - Peak Burning (in the open)

Q  1,0001  2.14h c 1  0.027M 
Eqn (8-11)

Q  heat release rat e(kW)
h c  st ack height (m)
M  moist urecont ent
@ T  20C & RH  35% M  0.09
@ T  20C & RH  50% M  0.12
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Wooden Pallets - Theory vs. Experiment
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Wooden Pallets
• For non-standard pallet sizes, Q"kW m-2 

Q" 670 1  2.14 h C 1 - 0.027 M
Eqn (8-12)

• Heat release rate per unit floor area covered by pallet
stack
Carleton University, 82.583, Fire
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Wooden Pallets - Mass Loss Rate
• The heat release rate & mass loss rate are related by


Q  Hch m
Eqn (8-13)
• Implicitly assumed that Hch = 12 kJ g-1

• Knowing m can calculate, radiative and convective
components of heat release rate, and rates of
generation of CO and soot.
Carleton University, 82.583, Fire
Dynamics II, Winter 2003, Lecture #
48
Wooden Pallets in an Enclosure

• Radiation from upper layer has little impact on Q
because fire is largely “self-contained” with many
surfaces “seeing” each other.

• If fire is limited by ventilation, Q will be reduced.
• Fuel mass loss rate is

m  0.09 A h kg s 1
Eqn (8-10)
• Smaller of Eqns (8-11) or (8-10) applies
Carleton University, 82.583, Fire
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Unusual Nature of Wooden Cribs & Pallets
• Early work on enclosure fires used wood cribs to
achieve reproducible fires
• However, burning surfaces of wooden cribs & pallets
are shielded from environment within the enclosure
• Consequently rate of burning is relatively insenstive to
the thermal environment
• When wood is present as wall lining, however, there is
a large exposed area that is sensitive to the thermal
environment
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Diffusion Flames (in the open)
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Rate of Burning (in the open)

q"F  q"L
m" 
Lv



Eqn (8-14)
 2 1
m"  mass loss rate / area (g m s )

q"F  heat flux fromflame to fuel (kW m  2 )

q"L  heat flux fromfuel surface (kW m 2 )
L V  heat tovaporizefuel (kJ g 1 )
 latentheat of evaporatio
n (liquids)
 heat of gasification (solids)
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Heat Release Rates (in the open)
• Fires burning in the open are well-ventilated
• Actual (chemical) heat release rate / unit area is


Q "  Hch m"
(kW m-2)
Eqn (8-15)
Hch = Actual (chemical) heat of combustion (kJ / g)
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Consider a material burning in an enclosure
but getting sufficient air for combustion?
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Rate of Burning (Mass Loss Rate)



q"E  q"F  q"L
m" 
Lv

Eqn (8-3)

q"E  heat flux from hot layer to fuel (kW m2 )


m" 
q "
E
LV



q " q "
F
L
Eqn (8-4)
LV
• 2nd term can be estimated by open burning models
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Example: Study of effect of trapping heat on rate of
burning of slab of PMMA (0.76m x 0.76 m) (*)
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Observations
• Trapping of heat (radiation from hot layer)
increases steady-state burning rate of PMMA
• Trapping of heat (radiation from hot layer)
reduces time to steady-state burning  rate of
flame spread across PMMA also increases
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• Burning rate in post-flashover fires involving fuels with
exposed surfaces is enhanced by radiation
• Large burning rates inhibit inflow of air so increase
equivalence ratio  reduced heat release (inside)
• Heat release rate still can be ventilation-controlled
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• Burning rate as function of radiant intensity at ceiling
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Burning rate as function of radiant intensity at ceiling
 ethanol (LV = 850 J g-1)
 PMMA pool (LV = 1,600 J g-1)
 polyethylene (LV = 22,00 J g-1)
 wood (LV = 1,340 J g-1)
 PMMA crib
— ethanol in open
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• Pool fire burning rates in the open & in enclosures
• fex = 1 - 1/ (excess fuel factor)(some fuel burns outside)
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References
• D. Drysdale, An Introduction to Fire Dynamics,Wiley, 1999, Chap 1
• F.W. Billmeyer, Textbook of Polymer Science, Wiley, 1984, Chap 1
• Donald R. Askeland, Science and Engineering of Materials, Chapman
& Hall, 1990, Chapter 15
• C.F. Cullis and M.M. Hirschler, The Combustion of Organic
Polymers, Oxford Science Publications, 1981, Chapter 1
• C.L. Beyler and M.M. Hirschler, "Thermal Decomposition of
Polymers" Section 1 / Chapter 7, SFPE Handbook, 2nd Ed. (1995)
• C.F. Cullis and M.M. Hirschler, The Combustion of Organic
Polymers, Oxford Science Publications, 1981, Chapter 1
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