Fire Growth

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Fire Growth
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• One of the key objectives of fire dynamics is to
  develop an understanding of how a fire develops,
  grows and spreads.
• Fire growth depends on many parameters, but
  begins with the basics of the fire triangle
  heat, fuel, and oxygen.
• Heat is our ignition source.
• In order for a fire to grow, the heat source must
  transfer enough energy to the fuel to vaporize or
  pyrolyze the fuel and have enough energy to
  ignite the combustible vapors.

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• The burning vapors in turn continue to heat other
  areas of the fuel and continue to spread the
  flames
• provided that the burning vapors do not lose so
  much heat to the environment or
• to a non-combustible surface that they do not
  have enough energy to pyrolyze the fuel or
• the unburned fuel is not in the thermal path of
  the flames,
• in which case, the fire growth stops.

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• The attributes of the fuel also play a
  significant role in fire growth.
• Fuel type (chemical properties),
• Quantity of fuel,
• Geometry or arrangement,
• Surface texture and
• Density of the fuel (physical properties),
• Position relative to other fuels,
• Fuel shielding,
• Enclosure and compartmentation of the fuels.
• These are some of the factors relative to the
  fuel that control fire growth.

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• Last but not least, the fire needs to breathe.
• It needs oxygen.
• In this regard we can have one of two cases
• a ventilation limited fire,
• which is fire growth that is controlled by the
  amount of oxygen available (fuel rich)
• or a fuel limited fire,
• which has an excess amount of oxygen available
  but the amount of fuel available is governing the
  fire growth.
• How the fire entrains the oxygen will in many
  cases direct the path of fire growth.

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FIRE GROWTH OUTDOORS

• Outdoor fires typically have all the oxygen they
  need to burn, therefore the fuel is usually the
  limiting factor.
• However if a large area is burning the combustion
  zone near the perimeter may be fuel limited
• But the combustion area near the center of the
  burning area may be ventilation limited
• Due to the oxygen being consumed by the fire near
  the outer perimeter before it can reach the
  center of burning area.

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FIRE GROWTH OUTDOORS

• In addition to the items listed, outdoor fire
  growth is driven by the following factors
• slope of the ground the fuel is positioned on
• the density of fuels
• position of fuels
• mixture of fuel types
• moisture content of the fuels
• weather conditions.

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Fuel Position, density

• An experiment
• Place four wood matches in a straight line about
  an inch apart.
• Use clay to stand the matches up on a
  non-combustible surface.
• Light the match at one of the ends of the row.
• Does the fire spread and light the other matches?
• With four new matches, hold up one end of your
  non-combustible surface to form a steep incline.
• Light the match at the bottom of the row.
• Does the fire spread?

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Fuel Position, density

• While the discussion so far has pointed to fires
  in a rural sitting,
• outdoor fires in urban areas are also susceptible
  to wind or slope
• providing a favorable chain of fuels in the form
  of closely spaced homes or
• a hill as a mechanism to spread a running
  gasoline fire down a slope.

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Weather conditions

• Wind and low relative humidity are essential for
  rapid fire growth and spread in outdoor fires.
• Once a fire has started, high wind conditions
  will accelerate the fire spread through adjacent
  fuels by pushing the flame front over an unburned
  surface and increasing the heat transfer due to
  radiation or convection and radiation combined,
  sometimes referred to as direct flame impingement.

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Weather conditions

• Windy conditions or the fire-induced winds from
  large area fires, can loft burning brands or
  embers and move then to an area remote from the
  original fire area where secondary ignitions may
  occur.
• The low relative humidity is usually the
  contributing factor to reducing the moisture
  content of the fuels, enabling easier ignition
  and promoting flame propagation.
• Looking at a history of wildland/urban interface
  conflagrations in Southern California, the trend
  of low relative humidity and high winds becomes
  obvious.

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Inside structures

• Fire inside structures have a complex synergy
  between the fuels, the products of combustion and
  the structure itself, which may also be composed
  of or lined with fuel.
• In many cases, at some point in the growth of a
  fire in a structure, the geometry and openings in
  a room or building (or lack thereof) will force
  the fire into a ventilation limited condition.

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Inside structures

• When the fire is burning in an environment that
  only supports inefficient combustion, the
  production of toxic gases such as carbon monoxide
  tends to increase.
• Remembering that many of the products of
  combustion contain carbon, which is a fuel,
  confining the fire gasses in a burning room until
  they heat up enough to ignite, can lead to
  dramatic and rapid growth.
• These rapid fire growth transitions are called
  flashover and backdraft.

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Factors affecting fire growth Again back to the
basics heat, fuel, oxygen

• HEAT
• As the ignition fire gives off heat some will be
  lost to heating the fuel adjacent to the flame,
  but as the hot gases rise they begin to collect
  under the ceiling of the room.
• This is different from the outdoor case where the
  plume was unconfined and the thermal energy in
  the plume dissipated quickly as the products of
  combustion moved away from the fire.
• The hot gases in the compartment are now
  transferring heat to the ceiling.

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Factors affecting fire growthAgain back to the
basics heat, fuel, oxygen

• HEAT
• As the ceiling gets hotter, less energy is
  transferred from the hot gasses to the ceiling,
  hence more heat energy stays in the hot gas
  layer.
• The hot gas layer transfers heat via thermal
  radiation back to the fuels enhancing the flame
  spread and fire growth.
• However if the hot gas layer fills the room, such
  that it is lower than the lowest level that the
  flames can entrain oxygen from fresh air then
  the oxygen depleted hot gases will smother the
  fire and slow or even stop fire growth.

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Factors affecting fire growth
Factors affecting fire growthAgain back to the
basics heat, fuel, oxygen

• FUEL
• The fuel affects fire growth based on its
• Chemical properties
• Physical properties,
• Quantity,
• Loading density and
• Position in the room.

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Factors affecting fire growth
Factors affecting fire growthAgain back to the
basics heat, fuel, oxygen

• OXYGEN
• The amount of air available,
• The areas of entry into the compartment and
• Speed of entry of the air containing oxygen will
  all impact fire growth in a room or in a
  building.
• Wind conditions outside the building can also
  have a dramatic effect on the fire spread within
  the structure.
• A developing fire in a compartment goes through
  several stages
• ignition,
• plume and ceiling jet development,
• hot gas layer development,
• flame extension to the ceiling or flameover.

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Factors affecting fire growth
Factors affecting fire growthAgain back to the
basics heat, fuel, oxygen

• OXYGEN
• If the fire continues to grow, flashover may
  occur leading to full room involvement or a fully
  developed fire.
• The room may continue to burn this way for some
  time depending on the fuel load and ventilation
  conditions, but as the fuel is consumed the fire
  will begin the decay stage.

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Fuel load and geometry

• Fuel load is a term given to indicate the
  possible severity of a fire.
• Fuel load can be characterized by kilograms of
  fuel per square meter (kg/m2) of floor area.
• The heat energy contained in the fuel load is
  expressed in terms of kilo-Joules (kJ)
• This measurement assumes the total heat available
  if all the fuel were to burn
• it does predict how fast a fire will develop once
  the fire starts.

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Fuel load and geometry

• The total fuel load in a room does not correlate
  to the rate of growth of any given fire in a
  preflashover condition.
• During the development of a fire, the rate of
  growth can be characterized by the heat release
  rate (HRR) of the materials that are burning.
• The chemical and physical properties of the fuel
  and the size and shape of the fuel will determine
  the initial rate.
• HRR is expressed in terms of kilo-Watts (kW).
• The fuels environment will also impact fire
  growth.

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Fuel load and geometry

• Once flashover has occurred the HRR will be
  determined by both physical and chemical
  properties as well as the amount of air available
  during the fire and the exposed combustible
  surfaces.
• Different materials obviously burn at a different
  rate.
• A solid piece of wood will take longer to burn
  than the same mass of wood shavings.
• Plastics have much higher HRR than wood and
  urethane foam is much higher than cotton.
• Therefore the physical and chemical properties
  will determine the HRR.

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Fuel load and geometry

• The density of materials also changes the burn
  time as well as the HRR.
• Low-density materials will burn faster than
  high-density.
• Soft wood burns faster than hard woods,
  lightweight foams burn faster than dense, hard
  plastic.
• There are many contributing factors to the
  development of the fire, which include the fuel
  load and the environment.
• In addition, the build up of heat in the room and
  well as opening or lack there of for ventilation
  will also effect the fire development.

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Compartment size and ventilation

• The output of heat from a fire in a compartment
  which is confined by both walls and ceiling will
  be determined by the proximity of the ceiling,
  floor and walls provided that the fire has
  adequate ventilation.
• For example, the smaller the compartment, the
  more rapidly the fire will develop due to less
  heat loss and increased re-radiation between the
  walls and the fuels.

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Compartment size and ventilation

• To illustrate this point test fires were
  conducted with similar office workstations.
• The fuel load was maintained with the exception
  of additional sides, which added a third side to
  the workstation in one case and a partial fourth
  side to another.
• The fire in workstation with only an entry way on
  one side, (four sided) developed faster with a
  peak HRR that is twice that of the two-sided
  workstation and the peak occurs at 6 minutes
  after ignition as opposed to 8 minutes after
  ignition.

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30 sec 100 kW
180 sec 350 kW
2-Sided Workstation
300 sec 1.4 MW
360 sec 2.2 MW
420 sec 3.0 MW
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60 sec 90 kW
180 sec 280 kW
3-Sided Workstation
330 sec 1.7 MW
360 sec 2.3 MW
480 sec 3.8 MW
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60 sec 140 kW
4-Sided Workstation
150 sec 400 kW
180 sec 700 MW
240 sec 1.5 MW
360 sec 6.9 MW
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Heat release rate curves for two, three and
four-sided work station configurations.
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Type of construction

• The type of materials used to build a structure
  and how these materials are used within the
  structure can dramatically impact the rate of
  fire growth in that structure.
• A structure built with concrete and block will
  absorb a tremendous amount of heat from a fire
  and not provide any additional fuel to the fire.

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Type of construction

• Whereas a wood frame structure with wood paneling
  covering the interior walls and a combustible
  ceiling surface will remove less heat from the
  fire and will contribute to the fuel load and the
  fire growth in the structure significantly.
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• Windows, mechanical ventilation and fire
  sprinkler systems can also have a major impact on
  fire growth and development.

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Type of construction

• During the growth stage of a fire, if the windows
  do not fail or self-vent, the fire may not have
  enough oxygen.
• This will prevent the fire from generating enough
  energy for a flashover, effectively smothering
  itself.
• Conversely, if the windows fail too early in the
  fire growth stage, too much heat energy may be
  vented out the window.

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Type of construction

• This prevents the development of a hot gas layer
  and limits the fuels in the compartment that are
  being pyrolyzed.
• Hence flashover may not occur due to a fuel lean
  condition in this case. 
• Needless to say automatic fire sprinkler systems
  are designed to control the fire by cooling the
  fuels and the compartment gases in order to
  prevent flashover and fire spread.

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Flashover Backdraft

• Flashover and Backdraft are two different fire
  phenomena. Both terms describe a rapid and
  dramatic change in the fire conditions in a
  compartment.
• Flashover Definition
• a transition phase in the development of a
  contained fire in which surfaces exposed to
  thermal radiation reach ignition temperature more
  or less simultaneously and fire spreads rapidly
  throughout the space

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Flashover Backdraft

• Prior to flashover, the growing fire in the
  compartment, produces products of combustion that
  form a hot gas layer below the ceiling.
• This hot gas layer contains unburned fuel, soot
  particles and other gases.
• The hot gas layer continues to increase in
  thickness under the ceiling of the compartment.

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Flashover Backdraft

• The temperature of the hot gas layer is also
  increasing.
• The heat flux from the layer to the floor of the
  compartment exceeds 20 kW/m2 at the floor when
  flashover begins.
• This level of radiant heating on most fuel
  surfaces found in a home or office will cause the
  materials to pyrolyze, thereby adding more fuel
  vapor to the hot gas layer.

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Flashover Backdraft

• Typically when the hot gas layer exceeds
  approximately 600 ?C (1100 ?F), which is
  approximately the ignition temperature of carbon
  monoxide, the hot gas layer ignites.
• This further increases the radiation on adjacent
  surfaces causing them to auto-ignite.
• Once the compartment has become fully involved in
  fire, flashover is complete and the fire is in
  the post-flashover or fully developed stage.

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Flashover Backdraft

• Backdraft Definition
• an explosion resulting from the sudden
  introduction of air (i.e. oxygen) into a confined
  space containing oxygen-deficient superheated
  products of incomplete combustion.
• A backdraft can occur when a fire has filled a
  room with hot gases, that are above their
  ignition temperature in an unventilated space or
  an oxygen-depleted space in an under-ventilated
  space.

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Flashover Backdraft

• While a backdraft releases a lot of energy in a
  very short period of time it may not cause the
  level of thermal damage that a flashover will
  cause.
• Given the delicate balance of fuel, temperature
  and the required rapid introduction of fresh
  oxygen, backdrafts tend to occur less frequently
  than flashovers.

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Controlling Factors

• As with the developing fires, controlling factors
  for both flashovers and backdrafts include
• Fuel properties
• Compartment size
• Ventilation
• Building construction

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Resulting Conditions

• Post-flashover usually everything in the fire
  room has significant thermal damage, floor to
  ceiling and wall to wall.
• If the post-flashover full room burning period is
  short due to lack oxygen or due to suppression,
  pyrolysis patterns and charring of fuels in the
  lower part of the room should have distinctive
  shadow patterns caused by the direction nature of
  the radiant heating from the upper hot gas layer.
  
• If the fire burned vigorously in the
  post-flashover period, fire damage may be so
  complete that few if any patterns showing the
  directionality of the fire flow may exist.

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Resulting Conditions

• Under heavy post flashover burning conditions,
  areas with the most damage do not necessarily
  coincide with the location of the fire origin.
• The damage just indicates that there was a
  significant amount of heat energy in that area.
• That could be due to a significant fuel load in
  the area, the ventilation in the room increasing
  the burning of combustion gases in that area or
  it could be the area that burning the longest.
• None of these reasons automatically equal that
  is where the fire started