Fire Plumes

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Fire Plumes

• Up until now, we have been focusing on the
  flame.
  
• In addition to flame development after ignition,
  the hot gases and products from incomplete
  combustion begin to rise by buoyant convection
  and develop airflow by drawing in surrounding air
  and its oxygen into the flame.
• This cycle continues until the fuel runs out or
  the oxygen level in the air drops below what is
  required for combustion.

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INTRODUCTION To Fire Plumes

• To visualize the extremes of the plume
  development, picture a forest fire and a small
  fire in a sealed container.
  
• In the forest fire scenario, there is virtually
  an unlimited air source, and the sky above.

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INTRODUCTION
INTRODUCTION To Fire Plumes

• The plume will rise until it dissipates in the
  atmosphere and fresh air will continue to be
  supplied at the base of the fire.
  
• In the sealed container the plume will rise to
  the top of the container then spread across the
  top and begin to fill the container from the top
  downward.

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INTRODUCTION
INTRODUCTION To Fire Plumes

• It will continue to replace the fresh air until
  the oxygen level in the air has been depleted and
  the fire discontinues to combust.
  
• The entire plume and associated air currents
  produce the fire patterns on its surroundings.
  
• The dynamics of the plume are necessary for the
  investigator to understand in order to interpret
  the patterns the plume leaves behind.

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INTRODUCTION
INTRODUCTION To Fire Plumes

• A fire plume is described as the column of hot
  gases, flames, and smoke rising above the fire.
  (Ref NFPA 921, Ch1.3.92)
  
• The flame and its column of hot combustion
  products rise because of a buoyancy force that is
  caused by the differences in density or
  temperature.

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INTRODUCTION
INTRODUCTION To Fire Plumes

• If the hot gases cool to ambient temperature, the
  buoyancy force becomes zero and the plume will no
  longer rise.
  
• This way we see smoke stratifying in the outdoors
  or from a small fire in a room.

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INTRODUCTION
INTRODUCTION To Fire Plumes

• The air being drawn in at the bottom of the flame
  is called entrainment.
  
• The speed of this entrainment flow will have a
  pronounced effect on the flame height.
  
• The faster the fresh air source is moving into
  the flame, the higher it will rise.

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INTRODUCTION
INTRODUCTION To Fire Plumes

• In Lesson 4 (Combustion), we discussed diffusion
  flames being laminar or turbulent.
  
• As you saw with the candle experiment, the
  laminar flame becomes turbulent above the flame.

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INTRODUCTION
INTRODUCTION To Fire Plumes

• With all the different air flows within the
  plume, eddies or rotating regions begin to form.
  
• These currents also have an affect on the height
  of the flame.
  
• This also is the reason why you see fluctuating
  heights in the flame.

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INTRODUCTION
INTRODUCTION To Fire Plumes

• Temperatures within a fire plume will decrease in
  a predictable pattern from the hottest area just
  above the fuel package upward to the tip of the
  flame and continues losing heat as it rises
  vertically and horizontally as it travels from
  the plumes centerline.
• Objects above the flame will generally only
  ignite with flame contact or radiant heat from a
  large fuel package.

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Unconfined/Unobstructed Fire Plumes

• In a fire outdoors, like a forest fire, structure
  fire that has vented or just a plain campfire,
  there will be a plume that will rise vertically
  until the smoke and hot gases cool to the ambient
  air temperature.
• When this occurs the smoke will stratify and
  diffuse in the air.

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Unconfined/Unobstructed Fire Plumes

• A similar condition will occur inside a structure
  that has a high ceiling, or when the plume is
  small.
  
• Resultant affects of an unconined fire plume.

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Confined

• In a confined fire the fire plume will rise until
  it interacts with the ceiling above it.
  
• The hot gases and other products of combustion
  will flow away from the centerline of the plume
  until it meets another obstruction, such as a
  wall or object.
• After this occurs the hot gases and products of
  combustion will begin filling the area from the
  top downward.
  

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Confined

• Any holes or other ventilation leaks in the
  structure will determine the rate that the
  confined area will fill.
  
• It will continue to descend at some rate
  depending on how much is lost through ventilation
  leaks and rate of fuel package production.
  
• If the plume is close to any combustible product,
  the radiated and convective heat will transfer to
  the new fuel and it will begin to pyrolyze and
  generate more gaseous fuels which can be ignited
  by the plume flame.

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Confined

• As the fire develops further and larger plume
  develops and more gases and products of
  combustion rise above the flame.
  
• These gases and products of combustion will also
  loose heat as they rise by mixing with
  surrounding air and by convective transfer and by
  radiant loss.
• The confined area however, is raising in
  temperature thus the heat in the plume gases will
  not transfer as much heat.

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Confined

• The lower level will begin to increase in
  temperature and the radiant heat from it will
  begin to heat any unignited target fuels.
  
• Many times this heat will be sufficient to ignite
  airborne fuel gases.
  
• This is referred to as rollover.
• The flames will travel along the ceiling and exit
  the confined area in the same direction as the
  smoke and other products of combustion.

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CONCLUSION

• Fire plumes leave behind significant patterns
  that the investigator can observe to assist in
  determining the flow of the fire
  
• such as smoke and heat patterns left behind on
  the structure and the surrounding landscape.
  
• In addition, many fires that an investigator
  would classify as unusual can be explained by
  verifying whether or not an unconfined or
  confined condition occurred.