Carleton University, 82.583, Fire Dynamics II, Winter 2003, Lecture # 12 1 Fire Dynamics II Lecture...

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

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Fire Dynamics IIFire Dynamics II

Lecture # 12Lecture # 12

Other Important PhenomenaOther Important PhenomenaJim MehaffeyJim Mehaffey

82.58382.583


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Other Important PhenomenaOther Important Phenomena

OutlineOutline

• Post-flashover fires in large compartmentsPost-flashover fires in large compartments

• Flames issuing through windowsFlames issuing through windows

• ExplosionsExplosions

• BackdraftsBackdrafts

• BLEVEsBLEVEs


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Post-flashover Fires in Large CompartmentsPost-flashover Fires in Large Compartments• Gordon Cooke, Gordon Cooke, Tests to determine the behaviour of fully Tests to determine the behaviour of fully

developed natural fires in a large compartmentdeveloped natural fires in a large compartment, Fire Note 4, , Fire Note 4, Fire Research Station, British Research Establishment, 1998Fire Research Station, British Research Establishment, 1998

• 9 Post-flashover fires9 Post-flashover fires

• Basic compartment: 23 m deep, 6 m wide, 3 m highBasic compartment: 23 m deep, 6 m wide, 3 m high

• Objective: simulate an even larger compartment in an Objective: simulate an even larger compartment in an open plan office building by allowing no net heat open plan office building by allowing no net heat transfer to neighbouring compartmentstransfer to neighbouring compartments– if only 2 sides of bldg have windows, after flashover if only 2 sides of bldg have windows, after flashover

there is line of symmetry along centre line of storeythere is line of symmetry along centre line of storey– ensure separation walls are well insulatedensure separation walls are well insulated


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• Ventilation opening in one of the 6 m x 3 m end wallsVentilation opening in one of the 6 m x 3 m end walls– not glazed (open from outset) not glazed (open from outset) – 12.5%, 25% 50% or 100% of area of end wall12.5%, 25% 50% or 100% of area of end wall– 12.5% simulated fire in basement with ventilation at top12.5% simulated fire in basement with ventilation at top

• Fuel load: 20 kg mFuel load: 20 kg m-2-2 or 40 kg m or 40 kg m-2-2 – 33 wood cribs: 11 rows of 3 cribs, 1 m apart33 wood cribs: 11 rows of 3 cribs, 1 m apart– D = 50 mm; L = 1.0 m; D = 50 mm; L = 1.0 m; – 1 crib = 155 sticks in 15 layers for 40 kg m1 crib = 155 sticks in 15 layers for 40 kg m-2-2 – 1 crib = 75 sticks in 7 layers for 20 kg m1 crib = 75 sticks in 7 layers for 20 kg m-2-2 – 6 cribs (every other crib) along centre line on load cell6 cribs (every other crib) along centre line on load cell


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Distribution of CribsDistribution of Cribs


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• Room linings:Room linings:– walls and ceiling: insulating ceramic fibre blanketwalls and ceiling: insulating ceramic fibre blanket– floor: layer of dry sandfloor: layer of dry sand

• Temperature measured in two locations:Temperature measured in two locations:– 150 mm below ceiling 6.0 m from rear of compartment150 mm below ceiling 6.0 m from rear of compartment– 150 mm below ceiling 6.0 m from front of compartment150 mm below ceiling 6.0 m from front of compartment

• Ignition sequence in 8 tests: Ignite row of cribs furthest Ignition sequence in 8 tests: Ignite row of cribs furthest from ventilation opening and observe spread of firefrom ventilation opening and observe spread of fire


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Description of TestsDescription of Tests


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Mass Loss of Cribs Measured in Test 1Mass Loss of Cribs Measured in Test 1• 1 = mass loss of central crib in row farthest from opening1 = mass loss of central crib in row farthest from opening• 11 = mass loss of central crib in row closest to opening11 = mass loss of central crib in row closest to opening


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Temperatures in Test 1Temperatures in Test 1


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Temperatures in Test 1Temperatures in Test 1


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Analysis of Test 1Analysis of Test 1

• Quantity of fuel: Quantity of fuel: – G = 40 kg mG = 40 kg m-2-2 x 6 m x 23 m = 5,520 kg x 6 m x 23 m = 5,520 kg

• Surface area of fuel:Surface area of fuel:– (Surface area 1 stick) x (no. sticks / crib) x (no. cribs)(Surface area 1 stick) x (no. sticks / crib) x (no. cribs)

– AAf f = (4 x 0.05 m x 1.0 m) x 155 x 33 = 1,023 m= (4 x 0.05 m x 1.0 m) x 155 x 33 = 1,023 m22

• Ventilation opening:Ventilation opening:–

• Duration of fire: Duration of fire:

–

5/2m 31.23m x 6m x 3mhA

min 32.8s 1966s kg 31.2 x 0.09

kg 5,520

hA 0.09

A L t 1-

FD


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• Model for rate of burning in deep compartments:Model for rate of burning in deep compartments:

W = width of compartment (m)W = width of compartment (m)

D = width of compartment (m)D = width of compartment (m)

hA

AT

) 0.036exp(1 DWhA180m

.


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Analysis of Test 1Analysis of Test 1

W = 6 mW = 6 m

D = 23 mD = 23 m

AATT = = 2 x 6 x 23 + 2 x 3 x 23 + 2 x 3 x 6 - 3 x 62 x 6 x 23 + 2 x 3 x 23 + 2 x 3 x 6 - 3 x 6 = 432 m = 432 m22

ttDD = 5,520 kg / 1.12 kg s = 5,520 kg / 1.12 kg s-1-1 = 4929 s = 82 min = 4929 s = 82 min

1/2

5/2

2

T m 13.8m 31.2

m 432

hA

A

1s kg .121m


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Flames Issuing through WindowsFlames Issuing through Windows

• Flame issuing from window of compartment experiencing Flame issuing from window of compartment experiencing

post-flashover fire is characterised by the flame lengthpost-flashover fire is characterised by the flame length

• For ventilation-controlled fire with wood cribsFor ventilation-controlled fire with wood cribs

• For ventilation-controlled wood-crib post-flashover fireFor ventilation-controlled wood-crib post-flashover fire

zzff = 0.33 h = 0.33 h

1)(

16

3/2

2/1ghA

mhz

f

hA 0.09m

ghAghA 76.3


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Flames Issuing through WindowsFlames Issuing through Windows

• For ventilation-controlled wood-crib fires, we have For ventilation-controlled wood-crib fires, we have close to stoichiometric fires (equivalence ratio ~ 0.92) close to stoichiometric fires (equivalence ratio ~ 0.92)

• For other fuels, like gasoline, most plastics, or wood For other fuels, like gasoline, most plastics, or wood panelling, the mass loss rate is much greater than for panelling, the mass loss rate is much greater than for a ventilation-controlled wood-crib firea ventilation-controlled wood-crib fire

• Not enough air can get into the room to burn the fuel Not enough air can get into the room to burn the fuel vapours (equivalence ratio > 1) within the room so vapours (equivalence ratio > 1) within the room so flaming continues outside the roomflaming continues outside the room

• Consequently flame length will also be much greaterConsequently flame length will also be much greater


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ExplosionsExplosions

• Premixed: Fuel well mixed with air (OPremixed: Fuel well mixed with air (O22) before burning) before burning

• Flammability limits: Mixture will only burn if Flammability limits: Mixture will only burn if concentration is between LFL and UFLconcentration is between LFL and UFL

• Minimum ignition energy (MIE) required for ignitionMinimum ignition energy (MIE) required for ignition• Rate of combustion is high: Governed by chemical Rate of combustion is high: Governed by chemical

kinetics kinetics notnot mixing rate mixing rate• Deflagration: Combustion propagates through mixture Deflagration: Combustion propagates through mixture

as a flame (below speed of sound)as a flame (below speed of sound)• If mixture is confined, walls & ceiling may not be able If mixture is confined, walls & ceiling may not be able

to withstand pressure rise to withstand pressure rise explosion explosion– masonry wall cannot withstand masonry wall cannot withstand P > 0.035 atmsP > 0.035 atms


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ExamplesExamples

• Methane CHMethane CH44 at T=25ºC & P=1 atm at T=25ºC & P=1 atm

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

• Propane CPropane C33HH88 at T=25ºC & P=1 atm at T=25ºC & P=1 atm

LFL = 2.1% (by vol); UFL = 9.5% (by vol); MIE = 0.25 mJLFL = 2.1% (by vol); UFL = 9.5% (by vol); MIE = 0.25 mJ

********************************************************************************************************************************• For alkanes (gaseous): LFL ~ 48 g mFor alkanes (gaseous): LFL ~ 48 g m-3-3

• For aerosol or droplet suspension: LFL ~ 45-50 g mFor aerosol or droplet suspension: LFL ~ 45-50 g m -3-3

• For dust (< 100 For dust (< 100 m): LFL ~ 30-60 g mm): LFL ~ 30-60 g m-3-3

– usually a two-event phenomenonusually a two-event phenomenon


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Deflagration MitigationDeflagration Mitigation

• Prevention:Prevention:– Reduction of concentration of flammables (by Reduction of concentration of flammables (by

ventilation for vapours or housekeeping for dusts)ventilation for vapours or housekeeping for dusts)– Control potential ignition sources (mechanical sparks, Control potential ignition sources (mechanical sparks,

hot surfaces, electrical equipment)hot surfaces, electrical equipment)– Rapid suppression: terminate combustion by very rapid Rapid suppression: terminate combustion by very rapid

introduction of inert gas or chemical inhibitorintroduction of inert gas or chemical inhibitor

• Protection:Protection:– VentingVenting


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Deflagration VentingDeflagration Venting

• Objective: Design vents to relieve pressures Objective: Design vents to relieve pressures developed by a deflagrationdeveloped by a deflagration

• NFPA 68: NFPA 68: Guide for Venting of DeflagrationsGuide for Venting of Deflagrations

• Rate of pressure rise is used in design of deflagration Rate of pressure rise is used in design of deflagration venting for high strength enclosures.venting for high strength enclosures.– Rapid rate of rise means short time available to ventRapid rate of rise means short time available to vent– Rapid rate of rise requires greater area for ventingRapid rate of rise requires greater area for venting

• PPredred = maximum pressure attained during venting is = maximum pressure attained during venting is

commonly set at 2/3 of enclosure strengthcommonly set at 2/3 of enclosure strength

• PPredred is used in design of deflagration venting for low is used in design of deflagration venting for low

strength enclosuresstrength enclosures


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Pressure ConsiderationsPressure Considerations

• Assume gas obeys the ideal gas lawAssume gas obeys the ideal gas law

P V = n R TP V = n R T

• Fire Dynamics I: Adiabatic flame temperature of a Fire Dynamics I: Adiabatic flame temperature of a stoichiometric mixture of propane in air: T ~ 2462 Kstoichiometric mixture of propane in air: T ~ 2462 K

• In enclosure without vents, volume is constantIn enclosure without vents, volume is constant

PP2 2 / P/ P1 1 = (n= (n22TT22) / (n) / (n11TT11))

nn22 / n / n11 ~ 1 ~ 1

TT22 / T / T11 ~ 2462 K / 293 K ~ 8.4 ~ 2462 K / 293 K ~ 8.4

PP2 2 / P/ P11 ~ 8.4 ~ 8.4


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Pressure ConsiderationsPressure Considerations

• Maximum deflagration pressure and rate of pressure Maximum deflagration pressure and rate of pressure rise dP/dt are determined by testrise dP/dt are determined by test

• For most fuels maximum pressure rise is 6 to 10 times For most fuels maximum pressure rise is 6 to 10 times pressure before ignitionpressure before ignition

• Fundamental basis for deflagration venting theory is Fundamental basis for deflagration venting theory is the cubic law:the cubic law:

K = deflagration indexK = deflagration index

V = volume of enclosureV = volume of enclosure

1/3V dt

dPK


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Examples (at optimal concentrations)Examples (at optimal concentrations)

• Methane CHMethane CH44

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

• Propane CPropane C33HH88

PPmaxmax ~ 7.9 atm; K ~ 100 atm m/s ~ 7.9 atm; K ~ 100 atm m/s

• DustsDusts

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


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Deflagration VentingDeflagration Venting• Low strength enclosures cannot withstand Low strength enclosures cannot withstand P > 0.1 P > 0.1

atm. Gas or mist deflagrations can be vented with atm. Gas or mist deflagrations can be vented with vents with combined area vents with combined area

AAVV = vent area (m = vent area (m22))

AASS = internal surface area of enclosure (m = internal surface area of enclosure (m22))

C = venting constant (for methane = 0.037 atmC = venting constant (for methane = 0.037 atm1/21/2))

PPredred = maximum = maximum P permitted (2/3 enclosure strength, atm)P permitted (2/3 enclosure strength, atm)

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

red

S

V P

A CA


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BackdraftsBackdrafts

• Limited ventilation Limited ventilation large quantity of unburnt “gas” large quantity of unburnt “gas” (products of pyrolysis or incomplete combustion)(products of pyrolysis or incomplete combustion) generated generated

• When opening suddenly introduced, inflowing air When opening suddenly introduced, inflowing air mixes with “gas” creating flammable mixturemixes with “gas” creating flammable mixture

• Ignition source Ignition source (smouldering material)(smouldering material) ignites flammable ignites flammable mixture, resulting in extremely rapid burningmixture, resulting in extremely rapid burning

• Expansion due to heat released expels burning “gas” Expansion due to heat released expels burning “gas” through opening & causes fireball outside enclosurethrough opening & causes fireball outside enclosure

• Backdrafts extremely hazardous for firefightersBackdrafts extremely hazardous for firefighters

• Backdraft of short duration. Flashover often followsBackdraft of short duration. Flashover often follows


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Backdraft Experiments: FleischmannBackdraft Experiments: Fleischmann

• 70 kW methane flame burned in a small “sealed” 70 kW methane flame burned in a small “sealed” chamberchamber

• Flame eventually self-extinguished due to oxygen Flame eventually self-extinguished due to oxygen starvationstarvation

• Vent opened, air entersVent opened, air enters

• Continuous ignition source present near back of Continuous ignition source present near back of chamberchamber

• Observed a backdraftObserved a backdraft


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5.6 s after opening the vent5.6 s after opening the vent


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BackdraftBackdraft

Schematic of temperatureSchematic of temperature


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Kemano: Fire in Basement Recreation RoomKemano: Fire in Basement Recreation Room

• Room dimensions: 3.25 m x 3.44 m x 2.2 m (height)Room dimensions: 3.25 m x 3.44 m x 2.2 m (height)

• Walls: 2 gypsum board // 2 (6 mm) wood panelling Walls: 2 gypsum board // 2 (6 mm) wood panelling

• Ceiling: gypsum boardCeiling: gypsum board

• Floor: carpet over concreteFloor: carpet over concrete

• Furnishings: couch / coffee table / TV on wood deskFurnishings: couch / coffee table / TV on wood desk

• Ventilation: no window / hollow-core wood door closedVentilation: no window / hollow-core wood door closed


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Temperatures in Basement FireTemperatures in Basement Fire

• Temperature predictions from Lecture 3 for leaky Temperature predictions from Lecture 3 for leaky enclosures (based on oxygen depletion):enclosures (based on oxygen depletion):

• For a heat loss fraction For a heat loss fraction 11= 0.9, = 0.9, TTg,limg,lim = 120 K = 120 K

• For a heat loss fraction For a heat loss fraction 11= 0.6, = 0.6, TTg,limg,lim = 480 K = 480 K

11 = 0.6 appropriate for spaces with smooth ceilings & = 0.6 appropriate for spaces with smooth ceilings &

large ceiling area to height ratioslarge ceiling area to height ratios

11 = 0.9 appropriate for spaces with irregular ceiling = 0.9 appropriate for spaces with irregular ceiling

shapes, small ceiling area to height ratios & where shapes, small ceiling area to height ratios & where fires are located against wallsfires are located against walls


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Basement

0

100

200

300

400

500

600

700

800

900

1000

0 5 10 15 20 25 30 35 40 45

Time (minutes)

Te

mp

era

ture

(°C

)


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BLEVE: Boiling Liquid Expanding Vapour ExplosionBLEVE: Boiling Liquid Expanding Vapour Explosion

– Propane is a gas under atmospheric conditionsPropane is a gas under atmospheric conditions– Liquified by application of pressure & stored in tank Liquified by application of pressure & stored in tank – In tank, liquid & vapour at equilibrium, with vapour In tank, liquid & vapour at equilibrium, with vapour

at high pressureat high pressure– If tank immersed in fire, heat causes pressure of If tank immersed in fire, heat causes pressure of

vapour to risevapour to rise– Activates relief valve (turbulent jet flame)Activates relief valve (turbulent jet flame)– Pressure still high & fire may weaken metal casingPressure still high & fire may weaken metal casing– Tank ruptures Tank ruptures BLEVE BLEVE


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What is a Liquified Gas?What is a Liquified Gas?• Gas = a substance that exist in the gaseous state at Gas = a substance that exist in the gaseous state at

standard temperature (20°C) and pressure (101 kPa)standard temperature (20°C) and pressure (101 kPa)

• Economic necessity and ease of usage Economic necessity and ease of usage gas stored gas stored in containers containing as much gas as practicalin containers containing as much gas as practical

• Compressed gas = stored in a container under Compressed gas = stored in a container under pressure but remains gaseous at 20°C. Typical pressure but remains gaseous at 20°C. Typical pressure range is 3 to 240 atmpressure range is 3 to 240 atm

• Liquified gas = stored in a container under pressure Liquified gas = stored in a container under pressure and exists partly in liquid and partly in gaseous state. and exists partly in liquid and partly in gaseous state. Pressure depends on temperature of liquid.Pressure depends on temperature of liquid.


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Heating of a Container Containing Compressed GasHeating of a Container Containing Compressed Gas

• Compressed gas obeys ideal gas law Compressed gas obeys ideal gas law

PV = nRTPV = nRT

• V & n are constant so pressure rises according toV & n are constant so pressure rises according to

PP22 = P = P1 1 TT2 2 / T/ T11


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Heating of a Container Containing Liquified GasHeating of a Container Containing Liquified Gas

• Liquified gas exhibits more complex behaviour Liquified gas exhibits more complex behaviour because net effect is a combination of three effectsbecause net effect is a combination of three effects– Gas phase is subject to same effect as compressed Gas phase is subject to same effect as compressed

gasgas– Liquid attempts to expand, compressing vapourLiquid attempts to expand, compressing vapour– Vapour pressure increases as temperature of liquid Vapour pressure increases as temperature of liquid

increases increases

• Combined result: an increase in pressureCombined result: an increase in pressure


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Overpressure Relief DevicesOverpressure Relief Devices

• Spring-loaded pressure-relief valves, bursting discs or Spring-loaded pressure-relief valves, bursting discs or fusible plugs (small containers) used to limit pressure fusible plugs (small containers) used to limit pressure to a level the container can safely withstandto a level the container can safely withstand

P(activation) > P(operating) >> P(atmospheric)P(activation) > P(operating) >> P(atmospheric)

• Relieving capacity (gas flow rate through device) is Relieving capacity (gas flow rate through device) is based on maximum heat input rates resulting from fire based on maximum heat input rates resulting from fire exposureexposure

• Gas discharge is in the form of a turbulent jet and if Gas discharge is in the form of a turbulent jet and if the gas is flammable, it will be a turbulent jet flamethe gas is flammable, it will be a turbulent jet flame


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Behaviour of liquified gas metal containerBehaviour of liquified gas metal container

(carbon steel) when exposed to fire(carbon steel) when exposed to fire


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Failure of ContainerFailure of Container

• Precise curves a little different for other steels, but Precise curves a little different for other steels, but loss of strength is significant as temperature climbsloss of strength is significant as temperature climbs

• Spring-loaded relief valve only reduces pressure to Spring-loaded relief valve only reduces pressure to activation pressureactivation pressure– Pressure remains high in containerPressure remains high in container– container stressed in tensioncontainer stressed in tension– Liquid always at temp > normal boiling pointLiquid always at temp > normal boiling point

• When exposed to fire, metal in contact with vapour When exposed to fire, metal in contact with vapour phase heats up, may stretch and a rupture developphase heats up, may stretch and a rupture develop

• Before rupture relieves pressure, it propagates and Before rupture relieves pressure, it propagates and container fails catastrophicallycontainer fails catastrophically


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Potential for Rapid Vaporization of LiquidPotential for Rapid Vaporization of Liquid

• Liquified gases are stored at high pressure, in Liquified gases are stored at high pressure, in containers at temperature (~ 20°C) > boiling point at containers at temperature (~ 20°C) > boiling point at atmospheric pressure (101 kPa)atmospheric pressure (101 kPa)– e.g. boiling point at 1 atm of propane (Ce.g. boiling point at 1 atm of propane (C33HH88) = - 42°C) = - 42°C

• Pressure drop to 1 atmosphere (failure of container) Pressure drop to 1 atmosphere (failure of container) causes very rapid vaporization of a portion of liquidcauses very rapid vaporization of a portion of liquid

• Fraction vaporized depends on temperature difference Fraction vaporized depends on temperature difference between liquid at failure and its normal boiling pointbetween liquid at failure and its normal boiling point

• For fire induced failure about 1/2 of liquid is vaporized For fire induced failure about 1/2 of liquid is vaporized


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After Failure of the Container: A BLEVEAfter Failure of the Container: A BLEVE

• Pressure difference, inside to outside, propels pieces Pressure difference, inside to outside, propels pieces of the container at high velocity for some distance of the container at high velocity for some distance (up to 1.0 km)(up to 1.0 km)

• Liquid vaporizes and vapour expands rapidlyLiquid vaporizes and vapour expands rapidly

• Rapid turbulent mixing of vapour and airRapid turbulent mixing of vapour and air

• If vapour is flammable, observe a huge fireball If vapour is flammable, observe a huge fireball (diameter up to 150 m)(diameter up to 150 m)


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A FireballA Fireball


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Protection against a BLEVEProtection against a BLEVE

• Insulate the containerInsulate the container

• Apply water: Create a film of water coating portions of Apply water: Create a film of water coating portions of container not in internal contact with liquidcontainer not in internal contact with liquid

Carleton University, 82.583, Fire Dynamics II, Winter 2003, Lecture # 12 1 Fire Dynamics II Lecture...

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