PRINCIPLES OF FIRE BEHAVIOR Marc L. Janssens, Ph.D.

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PRINCIPLES OF FIRE BEHAVIORMarc L. Janssens,
Ph.D.

• Fire Protection Engineering Symposium
• Embassy Suites Hotel
• Portland, OR
• November 7-8, 2002

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PRINCIPLES OF FIRE BEHAVIOROutline

• Self-Introductions
• Scientific Notation
• What is Fire?
• Combustion in Natural Fires
• Heat Transfer
• Energy Release Rate
• Fire Plumes
• Compartment Fires
• References and Web Sites

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SELF-INTRODUCTIONSMarc L. Janssens (1 of 2)

• ME degree from University of Ghent in 1980
• Joined Fire Research and Testing Station in 1980
• Moved to the U.S. in 1987 to work for NFoPA at
  NBS
• Transferred to DC headquarters of AFPA in 1990
• Ph.D. degree from University of Ghent in 1991
• Moved to San Antonio, TX in 1996 to work for SwRI
• Joined UNCC ET Department in August 2000
• Director of SwRI Fire Technology since August 2002

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SELF-INTRODUCTIONSMarc L. Janssens (2 of 2)

• Major research interests
• Material flammability testing and modeling
• Heat release rate calorimetry
• Computer hazard assessment and enclosure fire
  modeling
• Author of more than 60 papers and book chapters
• Editorial board member of four journals
• Fire Flammability Bulletin
• Fire Technology

• Journal of Fire and Materials
• Journal of Fire Sciences

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SOUTHWEST RESEARCH INSTITUTE
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PRINCIPLES OF FIRE BEHAVIORScientific Notation
(1 of 5)

• The fire science community has universally
  adopted the International System of Units (SI)
• Length Meter (m)
• Mass Gram (g) or kilogram (kg)
• Temperature Degree Celsius (C) or Kelvin (K)
• Time Second (s)
• Force Newton (1 N 1 kg-m/s²)
• Energy Joule (1 J 1 N-m)
• Power Watt (1 W 1 J/s)

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PRINCIPLES OF FIRE BEHAVIOR Scientific Notation
(2 of 5)

• Alternative Energy Units
• 1 BTU will raise 1 lb of water 1F at 68F
• 1 cal will raise 1 g of water 1C at 20C
• 1 kcal will raise 1 kg of water 1C at 20C
• Conversion factors for units of work and energy
• 1 BTU 1055 J
• 1 kcal 4182 J
• 1 BTU 252 cal
• CONVERT (http//www.joshmadison.com/software)

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PRINCIPLES OF FIRE BEHAVIOR Scientific Notation
(3 of 5)

• Celsius scale is based on water freezing and
  boiling at 0C and 100C, whereas 32F and 212F
  are assigned to the Fahrenheit scale
• Kelvin and Rankine are absolute scales for C and
  F
• Conversion factors for temperature
• T(F) T(C)1.8 32 and T(C) (T(F)
  -32))/1.8
• T(K) T(C) 273.16 and T(R) T(F) 459.69
• Example Convert the average normal human body
  temperature from 98F to C ? (98 32)/1.8
  36.7 C

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PRINCIPLES OF FIRE BEHAVIOR Scientific Notation
(4 of 5)

• Multipliers
• Giga 109
• Mega 106
• Kilo 103
• Centi 10-2
• Milli 10-3
• Micro 10-6
• A dot over a symbol implies rate or per unit
  time
• A double prime means per unit area
• Greek symbols are also common

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PRINCIPLES OF FIRE BEHAVIOR Scientific Notation
(5 of 5)
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PRINCIPLES OF FIRE BEHAVIORWhat is Fire? (1 of 2)

• Scientifically fire and combustion are synonymous
• They are both chemical reactions involving a fuel
  and an oxidizer that release enough energy to be
  sensed
• Conventionally fire and combustion are distinct
• Combustion is controlled or designed
• Fire is generally not controlled or designed
• This leads to the following definition of fire
• An uncontrolled chemical reaction between a fuel
  and an oxidizer producing light and energy
  sufficient to be sensed ,e.g, sufficient to
  damage the skin

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PRINCIPLES OF FIRE BEHAVIORWhat is Fire? (2 of 2)

• Rusting or yellowing of newsprint do not fit the
  definition because of insufficient energy release
• Some fires do not produce light
• Fires may not be very big, but their energy
  release per unit volume is enough to cause local
  burn injury
• Fires typically involve hydrocarbon fuels
• Natural fuels are composed of C, H, and possibly
  O and N
• Man has added Cl, Br, F, etc. which affect the
  fire hazard
• Chemical reactions conserve mass

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COMBUSTION IN NATURAL FIRESTypes of Fires (1 of
2)

• Diffusion Flames
• Predominant category
• Examples Building fire, forest fire, match
• Smoldering
• Can precede or follow diffusion flame
• Examples cigarette ignition of mattress, blown
  out match

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COMBUSTION IN NATURAL FIRESTypes of Fires (2 of
2)

• Spontaneous combustion
• Start of chemical reaction leading to smoldering
  or flaming
• Examples oily cotton rags, wet haystack, pile of
  wood chips
• Premixed flames
• Incipient flame in ignition of solids or liquids
  before diffusion flame emerges
• Examples Gasoline engine (spark ignition),
  Diesel engine (autoignition)

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COMBUSTION IN NATURAL FIRESDiffusion Flames (1
of 5)

• Diffusion flame Combustion process in which the
  fuel and oxygen are transported (diffused) from
  opposite sides of the reaction zone (flame)
• Diffusion Process of species transport from high
  to low concentration (Ficks law)
• Species Distinct chemical compound in a mixture

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COMBUSTION IN NATURAL FIRESDiffusion Flames (2
of 5)

• Liquid fuels evaporate to feed a diffusion flame
• Solid fuels thermally decompose or pyrolyze
• Pyrolysis Process of breaking up a substance
  into other molecules as a result of heating
• Small diffusion flames are typically laminar
  (candle)
• Laminar refers to orderly and unfluctuating
  fluid motion
• Large flames ( 1 ft) are turbulent (pool fire)
• Turbulent refers to randomly fluctuating fluid
  motion around a mean flow

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COMBUSTION IN NATURAL FIRESDiffusion Flames (4
of 5)
30 W
300 kW
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COMBUSTION IN NATURAL FIRESDiffusion Flames (5
of 5)

• Buoyancy affects the shape of diffusion flames
• Buoyancy Effective force on a fluid due to
  density or temperature differences in a
  gravitational field
• Diffusion flame shapes
• Jet negligible buoyancy
• Liquid spill v 0.01 m/s
• Forest air from above

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HEAT TRANSFERDefinitions and Concepts (1 of 2)

• Energy a state of matter representative of its
  ability to do work or transfer heat
• Symbol Q
• Units Joule (J) or kiloJoule (kJ)
• Example it takes 4.182 J to raise 1 g of water
  1C
• Work energy needed to displace a mass over a
  distance (force x distance ? units N x m ? J)
• Thermal or Internal Energy energy directly
  related to the temperature of an object or system

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HEAT TRANSFERDefinitions and Concepts (2 of 2)

• Heat Form of energy that is transferred from a
  hot to a cold region or system
• Heat Flow Rate rate at which heat is transferred
• Symbol q
• Units Watt (1 W 1 J/s) or kiloWatt (1 kW 1
  kJ/s)

.
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HEAT TRANSFERModes of Heat Transfer

• Conduction heat transfer due to molecular energy
  transfer (non-metals) or drift of electrons
  (metals)
• Convection conduction heat transfer from a
  moving fluid to a solid surface
• Radiation heat transfer due to electromagnetic
  waves

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HEAT TRANSFERConduction (1 of 2)

• The law of heat conduction was formulated by the
  French scientist Joseph Fourier in the early
  1800s
• Practical problems are more complex (unsteady)

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HEAT TRANSFERConduction (2 of 2)

• Thermal conductivity (k) is a material property
• Units W/mK
• Thermal conductivity may be a function of
  temperature
• Order of magnitude
• Metals 10-400 W/mK
• Building products 0.1-2 W/mK
• Insulation 0.02-0.2 W/mK
• Gases 0.01-0.03 W/mK
• Rl/(kA) is often used to quantify thermal
  resistance

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HEAT TRANSFERConvection (1 of 2)

• Thermal Boundary Layer region close to the solid
  where the temperature changes from Tf to Ts
• h is the convective heat transfer coefficient
  (W/m2?K)

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HEAT TRANSFERConvection (2 of 2)
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HEAT TRANSFERRadiation (1 of 8)

• All bodies continuously emit energy in the form
  of electromagnetic waves
• Electromagnetic waves are characterized by their
  wavelength ? (m) or frequency ? (1/s or Hz)

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HEAT TRANSFERRadiation (2 of 8)

• A body is heated by incident radiation at
  wavelengths between 0.1 and 100 ?m ? Thermal
  Radiation
• Ultraviolet 0.01 0.4 ?m
• Visible light 0.4 - 0.7 ?m
• Infrared 0.7 - 1000 ?m
• Wiens displacement law ?max? 1/T
• Surface becomes visible (dull red) at
  approximately 900 K
• Brightness increases as temperature goes up
• Infrared and night vision cameras detect IR
  radiation

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HEAT TRANSFERRadiation (3 of 8)
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HEAT TRANSFERRadiation (4 of 8)

• Blackbody a radiator emitting the maximum
  possible radiation
• Real objects emit a fraction of a blackbody
  radiation
• The emissivity of solid and liquid surfaces
  typically ranges from 0.6 to 1.0

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HEAT TRANSFERRadiation (5 of 8)

• Flame emissivity can be estimated from
• Absorption Coefficient (?, kappa) pertains to
  the amount of radiation absorbed per unit length
  (m-1)
• fuel vapors and gaseous combustion products
  absorb/emit radiation in discrete wavelength
  bands
• soot particles absorb/emit radiation over the
  full spectrum
• ? for turbulent flames typically ranges from 0.1
  1 m-1
• l is the mean path or beam length for radiation,
  also referred to as the flame thickness (m)

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HEAT TRANSFERRadiation (6 of 8)

• Configuration Factor fraction of radiation
  received by a target compared to the total
  emitted by the source

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HEAT TRANSFERHeat Flux (1 of 2)

• Heat fluxes cause objects to get hot and possibly
  damaged or ignited
• The heat flux from the sun is 1 kW/m2 at most
• Threshold values for damage (minutes of exposure)
• Pain to bare skin 1 kW/m2
• Burn to bare skin 4 kW/m2
• Ignition of objects 10 to 20 kW/m2
• Thresholds are higher for shorter exposure times

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HEAT TRANSFERHeat Flux (2 of 2)

• Heat fluxes from wood and plastic crib flames are
  proportional to the heat release rate ? ?r?
  constant
• Flashover an event in which flames suddenly fill
  a room
• Heat flux to the floor ? 20 kW/m2
• Average hot layer temperature ? 500 - 600C
• Consistency of flashover criteria suggest that
  the radiation from the hot layer is the main
  contributor to the floor heat flux

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ENERGY RELEASE RATEIntroduction

• Energy release rate energy produced by a fire
  per unit of time, that is, fire power
• Symbol
• Units W or kW
• represents size and damage potential of the
  fire
• Flame height for a given diameter is a function
  of
• Radiant heat flux to the surroundings is
  determined by
• Fire growth and flashover potential are related
  to
• Energy release rate is also called heat release
  rate

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ENERGY RELEASE RATEPredictions (1 of 4)

• The energy release rate is estimated from
• Heat of gasification net heat flux required to
  pyrolyze a mass unit of fuel
• Effective heat of combustion amount of energy
  released by the fire per unit mass of fuel burned
• The effective heat of combustion is used instead
  of the theoretical oxygen bomb calorimeter value

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ENERGY RELEASE RATEPredictions (2 of 4)
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ENERGY RELEASE RATEPredictions (4 of 4)

• Equation has limited practical use due to the
  difficulty in specifying the heat flux term
• The heat flux depends on the type, orientation,
  and configuration of the fuel
• Approximate heat flux values are known for some
  specific cases and burning rate can be predicted
  (Example liquid pool fires)
• Specific experimental measurements are needed to
  estimate for other configurations

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ENERGY RELEASE RATEMeasurements (1 of 3)
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ENERGY RELEASE RATEMeasurements (2 of 3)
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ENERGY RELEASE RATEMeasurements (3 of 3)
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ENERGY RELEASE RATEDesign Fires (1 of 2)

• The fire growth rate of items of furniture and
  many commodities can be represented by
• ? (1/t1)² with t1 equal to the time to reach 1
  MW
• NFPA 72B and NFPA 92B classify growth times as
• Slow t1 600 s
• Medium t1 300 s
• Fast t1 150 s
• Ultrafast t1 75 s

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FIRE PLUMESFire Plume Temperatures (2 of 2)
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COMPARTMENT FIRESStages of Fire Development (1
of 9)

• Developing fire the early stage of growth in a
  room fire before flashover and full room
  involvement
• May involve more than one burning item
• The fire behaves as if it is burning in the open
  for most of this stage
• Heat feedback from hot smoke layer and upper
  walls and ceiling is low
• A developing fire is usually fuel-limited, i.e.,
  the air supply is sufficient to maintain
  combustion of all fuel

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COMPARTMENT FIRESStages of Fire Development (3
of 9)

• Flashover a rapid change in a developing room
  fire to full room involvement
• Rapid ignition and flame spread due to increased
  heat flux
• Sudden eruption of fire in a compartment due to
  the introduction of fresh air (backdraft)
• Increase in the burning rate and the sudden
  extension of flames through the room
• Flashover usually causes a fire to reach its
  fully developed state in which all of the
  available fuel becomes involved

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COMPARTMENT FIRESStages of Fire Development (6
of 9)

• Fully developed fire state of a compartment fire
  during which flames fill the room involving all
  combustibles
• Temperatures are typically in the range of 800C
  to 1000C
• Heat fluxes can cause structural damage (?150
  kW/m2)
• Often ventilation-limited or ventilation-controlle
  d
• Flames emerge from doors and windows
• Oxygen concentration close to zero
• High concentrations of products of incomplete
  combustion

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COMPARTMENT FIRESStages of Fire Development (9
of 9)
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PRINCIPLES OF FIRE BEHAVIORReferences and Web
Sites

• References
• Quintiere, J., Principles of Fire Behavior,
  Delmar, 1998
• Friedman, R., Principles of Fire Protection
  Chemistry and Physics, NFPA, 1998
• SFPE Handbook of Fire Protection Engineering,
  NFPA, 2002
• Web Sites
• http//www.fire.swri.org
• http//fire.nist.gov
• http//www.doctorfire.com