fire dynamics ii

Carleton University, 82.583, Fire Dynamics II, Winter 2003, Lecture # 8

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Fire Dynamics IIFire Dynamics II

Lecture # 8Lecture # 8Flame Spread & Burning RatesFlame Spread & Burning Rates

Jim MehaffeyJim Mehaffey

82.58382.583


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Flame Spread & Burning RatesFlame Spread & Burning Rates

OutlineOutline• Models for flame spread on solids (review)Models for flame spread on solids (review)

– wind-aided vs opposed-flow flame spreadwind-aided vs opposed-flow flame spread– in the absence or presence of external radiationin the absence or presence of external radiation

• Burning rates of common itemsBurning rates of common items– in the open (review)in the open (review)– limited by ventilationlimited by ventilation– enhanced by radiationenhanced by radiation


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Factors Affecting Rate of Spread of FlameFactors 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 FlameFactors 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 LiningsSpread of Flame over Wall Linings


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Spread of Flame over Wall LiningsSpread of Flame over Wall Linings


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Room Fire Test - ApparatusRoom Fire Test - Apparatus• ISO 9705 “Fire tests: Full scale room fire tests for ISO 9705 “Fire tests: Full scale room fire tests for

surface products”surface products”


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Room Fire Test - ProcedureRoom Fire Test - Procedure• Line walls and ceiling with productLine walls and ceiling with product

• Burner in back cornerBurner in back corner

– First 10 min: = 100 kW (large wastepaper basket)First 10 min: = 100 kW (large wastepaper basket)

– Last 10 min: = 300 kW (small upholstered chair)Last 10 min: = 300 kW (small upholstered chair)

• Observe time to flashoverObserve time to flashover

• Room experiences flashover when Room experiences flashover when 1,000 kW 1,000 kW

Q

Q

Q


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Room Fire Test - ResultsRoom Fire Test - Results

Wall Lining Ceiling LiningTime to Flashover

(min:sec)Gypsum board Gypsum board

Douglas fir plywood Gypsum board ~ 7:30Douglas fir plywood Douglas fir plywood ~ 3:00Polyurethane foam Polyurethane foam ~ 0:13

Results for CAN/ULC-S102

Gypsum board FSR ~ 15Douglas fir plywood FSR ~ 135Polyurethane foam FSR ~ 500


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Flame Spread Models: ConceptsFlame Spread Models: Concepts• Flame spread = an advancing ignition frontFlame spread = an advancing ignition front• Leading edge of flame is heat source (raising fuel to Leading edge of flame is heat source (raising fuel to

ignition temp) ignition temp) andand the pilot the pilot• Visually flame spread is advancing flame close to solidVisually flame spread is advancing flame close to solid• Two interacting advancing frontsTwo interacting advancing fronts

– flame front in gas phaseflame front in gas phase– pyrolysis front along solid surfacepyrolysis front along solid surface

• Heat transfer from flame Heat transfer from flame pyrolysis front to advance pyrolysis front to advance• Advance of pyrolysis front Advance of pyrolysis front increased release of increased release of

volatiles volatiles advance of flame front advance of flame front• Flame-spread velocity Flame-spread velocity rate of advance of pyrolysis front rate of advance of pyrolysis front


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Wind-aided SpreadWind-aided Spread


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Wind-aided SpreadWind-aided Spread = region of heat transfer from flame & smoke= region of heat transfer from flame & smoke

• For wind-aided spread: 0.1 m For wind-aided spread: 0.1 m 10 m 10 m

• For opposed-flow spread: 1 mm For opposed-flow spread: 1 mm 3 mm 3 mm

• Surface temp in control volume drops from TSurface temp in control volume drops from T ig ig to Tto Tss

• Pyrolysis front moves at speedPyrolysis front moves at speed

• Model for wind-aided flame spread:Model for wind-aided flame spread:

dtdx

v p

n bp

p xxdt

dxv


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Example of Accelerating Flame SpreadExample of Accelerating Flame Spread• Upward turbulent spread on thick PMMAUpward turbulent spread on thick PMMA

• xxbb = 0 and n = 0.94 ~ 1 = 0 and n = 0.94 ~ 1

Eqn (8-Eqn (8-1)1)

• Experiment finding:Experiment finding:When xWhen xpp ~ 1.0 m, v ~ 5.0 mm s ~ 1.0 m, v ~ 5.0 mm s-1-1

p1

p xcdt

dxv


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Example of Constant Flame SpreadExample of Constant Flame Spread• Upward turbulent spread on thin textilesUpward turbulent spread on thin textiles

• n ~ 0.6n ~ 0.6

Eqn (8-Eqn (8-2)2)

• After some time, (xAfter some time, (xpp - x - xbb) and v become constant) and v become constant

0.6

bp2

p xxcdt

dxv


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Apartment Fire: Hiroshima, Japan (1996)Apartment Fire: Hiroshima, Japan (1996)• BuildingBuilding - reinforced concrete structure- reinforced concrete structure

- 20 storeys- 20 storeys- height of each storey = 3 m- height of each storey = 3 m- each apartment had a balcony- each apartment had a balcony

• BalconyBalcony - PMMA glazing- PMMA glazing- height of glazing = 1 m - height of glazing = 1 m


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Chronology of FireChronology of Fire• 00:00 Fire commences within apartment 96500:00 Fire commences within apartment 965• 13:00 Outer surface of PMMA glazing (9th storey) ignites13:00 Outer surface of PMMA glazing (9th storey) ignites• 18:00 Outer surface of PMMA glazing (10th storey) ignites18:00 Outer surface of PMMA glazing (10th storey) ignites• 20:00 Outer surface of PMMA glazing (11th storey) ignites20:00 Outer surface of PMMA glazing (11th storey) ignites• 22:00 Outer surface of PMMA glazing (12th storey) ignites22:00 Outer surface of PMMA glazing (12th storey) ignites• 23:00 Outer surface of PMMA glazing (13th storey) ignites23:00 Outer surface of PMMA glazing (13th storey) ignites• 23:30 Outer surface of PMMA glazing (14th storey) ignites 23:30 Outer surface of PMMA glazing (14th storey) ignites • 24:00 Outer surface of PMMA glazing (15th storey) ignites 24:00 Outer surface of PMMA glazing (15th storey) ignites • 24:20 Outer surface of PMMA glazing (16th storey) ignites 24:20 Outer surface of PMMA glazing (16th storey) ignites • 24:40 Outer surface of PMMA glazing (17th storey) ignites 24:40 Outer surface of PMMA glazing (17th storey) ignites • 25:00 Outer surface of PMMA glazing (18th storey) ignites25:00 Outer surface of PMMA glazing (18th storey) ignites• 25:15 Outer surface of PMMA glazing (19th storey) ignites25:15 Outer surface of PMMA glazing (19th storey) ignites• 25:30 Outer surface of PMMA glazing (20th storey) ignites 25:30 Outer surface of PMMA glazing (20th storey) ignites


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- - - - inner surface burning - - - - inner surface burning ——— ——— outer surface burningouter surface burning Ignition of outer side of PMMAIgnition of outer side of PMMAO Burn-out of PMMAO Burn-out of PMMA


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Problem Set 3: Problem 4Problem Set 3: Problem 4

5.5. 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 SpreadOpposed Flow Flame SpreadAbsence of External RadiationAbsence of External Radiation

• Compared to PMMA, a very slow process Compared to PMMA, a very slow process

• Not accelerating, but roughly constant velocity Not accelerating, but roughly constant velocity

• Speed of downward flame spread on PMMASpeed of downward flame spread on PMMAv ~ 0.04 mm sv ~ 0.04 mm s-1-1

v ~ 2.4 mm minv ~ 2.4 mm min-1-1


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Opposed Flow Flame SpreadOpposed Flow Flame SpreadIn Presence of External Radiation (1)In Presence of External Radiation (1)

• Effect of preheating time on rate of downward flame spread on Effect of preheating time on rate of downward flame spread on PMMA exposed to radiant flux (kW mPMMA exposed to radiant flux (kW m-2-2))

• CHF (PMMA) ~ 11 kW mCHF (PMMA) ~ 11 kW m-2-2


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Opposed Flow Spread: Model for Thick MaterialsOpposed Flow Spread: Model for Thick Materials

• Quintiere and Harkleroad, 1985Quintiere and Harkleroad, 1985

Eqn (8-3)Eqn (8-3)

= = flame-heating parameter (kWflame-heating parameter (kW22 m m-3-3) {material property}) {material property}• Provided no dripping, this model holds forProvided no dripping, this model holds for

– downward flame spread (wall)downward flame spread (wall)– lateral flame spread (wall)lateral flame spread (wall)– horizontal flame spread (floor) horizontal flame spread (floor)

, k, kc and Tc and Tigig - measured (LIFT apparatus) - measured (LIFT apparatus)

• TTss - depends on scenario (external flux) - depends on scenario (external flux)

2 Sig TT ck

v


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LIFT Apparatus - Standard TestsLIFT Apparatus - Standard Tests

• ASTM E1321, “Standard test method for determining ASTM E1321, “Standard test method for determining material ignition and flame spread propertiesmaterial ignition and flame spread properties

• ISO 5668, “Fire tests: Reaction to fire: surface spread ISO 5668, “Fire tests: Reaction to fire: surface spread of flame on building products”of flame on building products”


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LIFT ApparatusLIFT Apparatus


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LIFT Apparatus - ResultsLIFT Apparatus - Results

Material Tig (C) /kc (m K2 s-1)Polyurethane foam 280 82PMMA 378 14Plywood 390 16


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Estimating the Surface Temperature TEstimating the Surface Temperature TSS

• To employ Eqn (8-3) one must estimate TTo employ Eqn (8-3) one must estimate TSS

• Assume the surface is heated by a radiant flux and Assume the surface is heated by a radiant flux and cools by convection h (Tcools by convection h (TSS -T -Too))

• Following pages 5-35 to 5-38 in Fire Dynamics IFollowing pages 5-35 to 5-38 in Fire Dynamics I

Eqn (8-Eqn (8-4) 4)

"q

ckthf

h"qTT oS


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ckth

erfc exp1)f( 2

2)f( 0

lim

1)f( lim


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erfc exp1)f( 2


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Thermal Properties for Ignition, Flame Spread Thermal Properties for Ignition, Flame Spread & Pre-flashover Fires (1)& Pre-flashover Fires (1)


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3.3. Consider a pre-flashover fire in a room 2.4 m x 3.6 m x 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 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 (height)) is open and the interface between the hot layer and cool air is at the mid-height of the door. The 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 fuel in the room is a mixture of wood and plastics and the mean extinction (absorption) coefficient of the the mean extinction (absorption) coefficient of the upper layer is Kupper layer is Kmm ~ 1.0 m ~ 1.0 m-1-1. What is the emissivity of . What is the emissivity of the upper layer? Calculate the radiant flux at the the upper layer? Calculate the radiant flux at the centre of the floor when the layer temperature is centre of the floor when the layer temperature is 300°C, 400°C, 500°C and 600°C.300°C, 400°C, 500°C and 600°C.


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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.


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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 ItemsBurning Rates of Common Items* In the open (review)* In the open (review)* Limited by ventilation* Limited by ventilation* Enhanced by radiation* Enhanced by radiation


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Wooden Cribs (2)Wooden Cribs (2)


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Wooden CribsWooden Cribs• D = stick thickness (m)D = stick thickness (m)

• S = spacing between sticks (m)S = spacing between sticks (m)

• hhcc = height of crib (m) = height of crib (m)

• N = number of rows = hN = number of rows = hcc / D / D

• n = number of sticks per row n = number of sticks per row

• L = length of each stick (m) {L >> D}L = length of each stick (m) {L >> D}

= density of sticks (kg m= density of sticks (kg m-3-3))

• mmoo = initial mass of crib (kg) = N n = initial mass of crib (kg) = N n D D22 L L


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Steady-State Burning of Wooden CribsSteady-State Burning of Wooden Cribs• Fuel surface controlled burning:Fuel surface controlled burning: Stick surfaces burn Stick surfaces burn

freely {S >> D}freely {S >> D}

Eqn (8-Eqn (8-5)5)

• = mass loss rate of crib (kg s= mass loss rate of crib (kg s-1-1))• ttoo = time at which steady burning is established (s)= time at which steady burning is established (s)

• vvpp = surface regression rate (m s = surface regression rate (m s-1-1))

Dtt2v1vm

D4)t(tm OP

POO

m


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Steady-State Burning of Wooden CribsSteady-State Burning of Wooden Cribs• Crib porosity controlled burning:Crib porosity controlled burning: Burning controlled Burning controlled

by rate of flow of air & combustion products through by rate of flow of air & combustion products through holes in crib {S << D}holes in crib {S << D}

Eqn (8-Eqn (8-6)6)

• for t > tfor t > too: is given by lesser of Eqns (8-5) & (8-6): is given by lesser of Eqns (8-5) & (8-6)

Dm

hS10 x 4.4)t(tm O

C

4O

m


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Growth Rates - Burning of Wooden CribsGrowth Rates - Burning of Wooden Cribs• Assume crib is ignited at bottom / centreAssume crib is ignited at bottom / centre

• ttoo = time at which steady burning is established (s)= time at which steady burning is established (s)

• For t < tFor t < too

Eqn (8-Eqn (8-7)7)

• ttoo = time Eqn (8-7) yields lesser of Eqns (8-5) & (8-6) = time Eqn (8-7) yields lesser of Eqns (8-5) & (8-6)

********************************************************************************************************************************• For a crib ignited at bottom / centre and whose steady-For a crib ignited at bottom / centre and whose steady-

state burning is fuel-surface controlled: tstate burning is fuel-surface controlled: too ~ 15.7 n (s) ~ 15.7 n (s)

Dntvm 0.0254)t(tm

2

2P

OO


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Wooden Cribs - Heat Release RateWooden Cribs - Heat Release Rate• The heat release rate is given by The heat release rate is given by

Eqn (8-8)Eqn (8-8)

• with Hwith Hchch = 12.4 kJ g = 12.4 kJ g-1-1

• Knowing one can also calculate, radiative and Knowing one can also calculate, radiative and convective components of heat release rate, and rates convective components of heat release rate, and rates of generation of CO and soot.of generation of CO and soot.

Q

mH ch

m


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Wooden Cribs in an EnclosureWooden Cribs in an Enclosure• Radiation from upper layer has little impact on Radiation from upper layer has little impact on

because fire is largely “self-contained” with many because fire is largely “self-contained” with many surfaces “seeing” each other.surfaces “seeing” each other.

• If fire is limited by ventilation, will be reduced If fire is limited by ventilation, will be reduced because Hbecause Hchch and are both reduced. and are both reduced.

Q

Q

m


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Post-flashover Fires Involving Wooden CribsPost-flashover Fires Involving Wooden Cribs

• Harmathy (1972) identified two burning regimes for Harmathy (1972) identified two burning regimes for room fires involving wooden cribs: room fires involving wooden cribs: ventilation-controlled & fuel-surface controlledventilation-controlled & fuel-surface controlled

• = mass loss rate of fuel (kg s= mass loss rate of fuel (kg s-1-1))

= ventilation parameter (kg s= ventilation parameter (kg s-1-1))

==

• AAff = exposed surface area of fuel (m = exposed surface area of fuel (m22))

mR

hA 3.76h gA O


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• Post-flashover fire is ventilation-controlled ifPost-flashover fire is ventilation-controlled if

/ A/ Aff < 0.63 kg m < 0.63 kg m-2-2 s s-1-1

Eqn (8-9)Eqn (8-9)

• Fuel mass loss rate isFuel mass loss rate is

Eqn (8-10)Eqn (8-10)

• Least of Eqns (8-5), (8-6), (8-7) or (8-10) appliesLeast of Eqns (8-5), (8-6), (8-7) or (8-10) applies

1/2f m 0.07AhA

1

1

s kg hA 0.09m

s kg 0.0236m


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Wooden Pallets (1)Wooden Pallets (1)


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Wooden PalletsWooden Pallets


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Wooden Pallets - Peak Burning (in the open)Wooden Pallets - Peak Burning (in the open)

Eqn (8-Eqn (8-11)11)

0.12M 50%RH & 20CT @ 0.09M 35%RH & 20CT @

content moistureM(m)height stack h

(kW) rate releaseheat Q

M 0.0271h 2.141 1,000Q

c

c


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Wooden Pallets - Theory vs. ExperimentWooden Pallets - Theory vs. Experiment


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Wooden PalletsWooden Pallets• For non-standard pallet sizes,For non-standard pallet sizes,

Eqn (8-12)Eqn (8-12)

• Heat release rate per unit floor area covered by pallet Heat release rate per unit floor area covered by pallet stackstack

M 0.027 - 1 h 2.14 1 670 " QC

2-mkW "Q


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Wooden Pallets - Mass Loss RateWooden Pallets - Mass Loss Rate• The heat release rate & mass loss rate are related by The heat release rate & mass loss rate are related by

Eqn (8-13)Eqn (8-13)

• Implicitly assumed that HImplicitly assumed that Hchch = 12 kJ g = 12 kJ g-1-1

• Knowing can calculate, radiative and convective Knowing can calculate, radiative and convective components of heat release rate, and rates of components of heat release rate, and rates of generation of CO and soot.generation of CO and soot.

Q

mH ch

m


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Wooden Pallets in an EnclosureWooden Pallets in an Enclosure• Radiation from upper layer has little impact on Radiation from upper layer has little impact on

because fire is largely “self-contained” with many because fire is largely “self-contained” with many surfaces “seeing” each other.surfaces “seeing” each other.

• If fire is limited by ventilation, will be reduced. If fire is limited by ventilation, will be reduced.

• Fuel mass loss rate isFuel mass loss rate is

Eqn (8-Eqn (8-10)10)

• Smaller of Eqns (8-11) or (8-10) appliesSmaller of Eqns (8-11) or (8-10) applies

Q

Q

1s kg hA 0.09m


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Unusual Nature of Wooden Cribs & PalletsUnusual Nature of Wooden Cribs & Pallets• Early work on enclosure fires used wood cribs to Early work on enclosure fires used wood cribs to

achieve reproducible firesachieve reproducible fires

• However, burning surfaces of wooden cribs & pallets However, burning surfaces of wooden cribs & pallets are shielded from environment within the enclosureare shielded from environment within the enclosure

• Consequently rate of burning is relatively insenstive to Consequently rate of burning is relatively insenstive to the thermal environmentthe thermal environment

• When wood is present as wall lining, however, there is When wood is present as wall lining, however, there is a large exposed area that is sensitive to the thermal a large exposed area that is sensitive to the thermal environmentenvironment


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Diffusion Flames (in the open)Diffusion Flames (in the open)


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Rate of Burning (in the open)Rate of Burning (in the open)

Eqn (8-Eqn (8-14)14) v

LF

L"q"q"m

(solids)on gasificati ofheat (liquids)n evaporatio ofheat latent

)g (kJ fuel vaporizeheat toL

)m(kW surface fuel fromflux heat "q

)m(kW fuel toflame fromflux heat "q

)sm (g area / rate loss mass"m

1V

2L

2F

12


53

Heat Release Rates (in the open)Heat Release Rates (in the open)• Fires burning in the open are well-ventilatedFires burning in the open are well-ventilated

• Actual (chemical) heat release rate / unit area isActual (chemical) heat release rate / unit area is

(kW m(kW m-2-2)) Eqn (8- Eqn (8-15)15)

HHchch = Actual ( = Actual (chemicalchemical) heat of combustion (kJ / g) ) heat of combustion (kJ / g)

"mH ch

" Q


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Consider a material burning in an enclosure Consider a material burning in an enclosure but getting sufficient air for combustion?but getting sufficient air for combustion?


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Rate of Burning (Mass Loss Rate)Rate of Burning (Mass Loss Rate)

Eqn (8-3)Eqn (8-3)

Eqn (8-4)Eqn (8-4)

• 2nd term can be estimated by open burning models2nd term can be estimated by open burning models

v

LFE

L"q"q"q"m

)m(kW fuel layer tohot fromflux heat "q 2E

V

LF

V

E

L

""

L

""m qqq


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Example: Study of effect of trapping heat on rate of Example: Study of effect of trapping heat on rate of burning of slab of PMMA (0.76m x 0.76 m) (*)burning of slab of PMMA (0.76m x 0.76 m) (*)


57

ObservationsObservations• Trapping of heat (radiation from hot layer) Trapping of heat (radiation from hot layer)

increases steady-state burning rate of PMMAincreases steady-state burning rate of PMMA

• Trapping of heat (radiation from hot layer) Trapping of heat (radiation from hot layer) reduces time to steady-state burning reduces time to steady-state burning rate of rate of flame spread across PMMA also increases flame spread across PMMA also increases


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• Burning rate in post-flashover fires involving fuels with Burning rate in post-flashover fires involving fuels with exposed surfaces is enhanced by radiationexposed surfaces is enhanced by radiation

• Large burning rates inhibit inflow of air so increase Large burning rates inhibit inflow of air so increase equivalence ratio equivalence ratio reduced heat release (inside) reduced heat release (inside)

• Heat release rate still can be ventilation-controlledHeat release rate still can be ventilation-controlled


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• Burning rate as function of radiant intensity at ceilingBurning rate as function of radiant intensity at ceiling


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Burning rate as function of radiant intensity at ceilingBurning rate as function of radiant intensity at ceiling

ethanol (Lethanol (LVV = 850 J g = 850 J g-1-1))

PMMA pool (LPMMA pool (LVV = 1,600 J g = 1,600 J g-1-1))

polyethylene (Lpolyethylene (LVV = 22,00 J g = 22,00 J g-1-1))

wood (Lwood (LVV = 1,340 J g = 1,340 J g-1-1))

PMMA cribPMMA crib

— — ethanol in openethanol in open


61

• Pool fire burning rates in the open & in enclosuresPool fire burning rates in the open & in enclosures• ffexex = 1 - 1/ = 1 - 1/ (excess fuel factor)(some fuel burns outside)(excess fuel factor)(some fuel burns outside)


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ReferencesReferences• 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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