Fire/Arson Investigations : Fire/Arson Investigations
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Re: If you were a Judge - the hypothesis in full.
Posted by: Russaus (IP Logged)
Date: May 28, 2007 09:12PM
Here is the hypothesis in full. Time for a re-think on commonly held positions?
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Backdraft explosion from ceiling/roof cavity.
Background
Before attempting to understand the phenomenon of a back draft explosion, the basic theory behind combustion must first be briefly examined. Fire combustion is a rapid form of oxidization, known as an exothermic oxidation. The exothermic oxidization produces energy in the form of detectable light and heat. The two types of combustion are flaming and glowing. Flaming is a gaseous combustion in which both the oxidizer and fuel are in a gaseous state. The oxidizer in this case is the oxygen within air. Glowing combustion involves a solid fuel with the gaseous oxidizer (air) (Dehaan, 1997). A typical form of glowing combustion is a smouldering fire in a pile of sawdust or charcoal beads slowly burning away in a barbeque.
Before any of the two forms of combustion can take place and continue to burn the following four conditions must exist: (1) The presence of a combustible fuel, (2) The presence of an oxidizer such as the oxygen in air, (3) The application of energy in die form of heat, and (4) The self-sustaining chain reaction of the fuel and oxidizer. The four conditions and their dependence on each other have been classically symbolized by a four sided geometrical object known as the Fire Tetrahedron.
The propagation of a fire follows the same rule of thermodynamics as the transfer of heat. Heat is transferred in three ways: (1) conduction, (2) convection and (3) radiation.
Conduction is a transfer of heat through a solid object and depends on the materials thermal conductivity. A simple example would be a steal rod that is heated at one end. Eventually the heat transfers through the length of the rod to the other end.
Convection is the transfer of heat energy by continual movement of heated gases or liquids. An example of a liquid convection would be running hot water into a bath of cold water and stirring the water. A gaseous example is a plume layer from a fire spreading across the ceiling of a compartment fire.
Radiation is the transfer of heat from once surface or source to another surface by electromagnetic waves. This can only occur if there are no obstructions between the two surfaces. An example of radiation heat transfer in a fire is the heat from the ceiling's plume layer to the top surface of a table, chair or even the floor.
The development and spread of a fire within an enclosed compartment can develop in many different ways. The development hinges on the enclosure geometry, ventilation, supply of fuel and the surface area. The first stage of any fire is the ignition, of which there are many different ways that this can occur. The cause of the ignition can be attributed to either: deliberate or accidental human involvement, or an unforeseen event such as spontaneous combustion, electrical involvement and chemical reaction to name a few. Regardless of the ignition source, the enclosure has no effect on the early stages of the fire growth. The fire spread and intensity is governed by the fuel loading present. As the fuel progresses, the hot gases are forced upward due to surrounding cooler air (gases).
The result is the development of a plume layer that is trapped by the top section of an enclosure such as a ceiling within a room. The plume layer consists of a mixture of heated combusted and un-combusted gases. The plume layer continues to increase in volume and eventually starts to descend towards the flame region. If there is no avenue for the plume layer to escape and allow the in take of fresh air, the fire becomes starved of oxygen and the energy release rates decreases.
Even though the energy release rate drops, there can still be sufficient heat present to cause pyrolysis of any remaining fuel load (timber), resulting in an accumulation of un-burnt gases. If a window breaks or a door is opened oxygenated air can flow into the enclosed compartment and mix freely with un-burnt pyrolysis products. Any ignition sources such as a glowing ember can then ignite the flammable mixture resulting in explosive gases. The heat created from the rapid combustion causes burning gases to be expelled through the opening under great pressure (Karlsson and Quintiere, 2000). This phenomenon is known as a backdraft explosion. The pressure wave resulting from a backdraft is strong enough to shift entire walls and ceilings by centimetres, and blow out glass windows up to 25 metres away.
In most texts backdraft explosions are associated with a compartment such as a room with a closed door, while the possibility of a backdraft explosion from a ceiling is not often given much consideration.
The typical ceiling structure for most residential dwellings is an enclosed compartment extending from the top of the outer walls to an apex junction. The general purpose of a roof construction in an apex design is to allow the run off of water during rain. The roof line of any house must also extend past and just below the height of the outer walls.
Fire Spread and Backdraft from Roof/Ceiling Cavity
With the top section of a roof/ceiling compartment forming an apex as opposed to the typical flat ceiling compartment the build up of a plume layer would descend more rapidly towards any flaming combustion. Therefore, the oxygen depletion rate would also occur much sooner. The other difference between a typical room compartment as opposed to the roof/ceiling compartment is the lack of walls to slow the transfer of heat and combustion across the house. Due to the lack of any wall restrictions and the apex roof line, the combustion of any timber supporting the ceiling would be rapid and relatively even once a plume layer had established. This would cause the weakening of the ceiling across the entire dwelling, and not just one area in the case of a typical room compartment.
Once a section of ceiling has weakened enough to collapse, the resultant opening would allow for fresh air (oxygenation) into the roof/ceiling cavity causing rapid and intense burning throughout the ceiling opening producing a backdraft explosion.
With the backdraft explosion emanating from the roof/ceiling cavity it would also place pressure downward on an already weakened ceiling. As a backdraft explosion would normally move a wall structure, it would also do the same to a ceiling. The difference between a ceiling and wall is the added continuous force of gravity acting downward on the ceiling. In the event of a backdraft of this nature, the likelihood of a ceiling collapse throughout the entire house, either during or soon after the backdraft explosion is virtually certain.
The Effect on Contents and Occupants.
In the case of any fire, the spread and heat transfer is always in an upward direction first before descending. Therefore, with a fire originating within a ceiling cavity, the indication of the fire presence from within the dwelling would be masked until the ceiling is breached. Then typical smoke detectors placed on the ceiling would not activate until the ceiling was breached by the fire. The smoke alarms also have a high probability of falling from the ceiling due to the melting of any form of plastic aided attachment. Therefore if occupants were sleeping during this time of the fire, they would have no audible warning.
In the event of a backdraft explosion from a ceiling, the pressure wave that emanates from the opening of the ceiling cavity would also bring with it a flame front with a temperature in the order of 700 to 1200.C through out the house. This would also be followed, if not at the same time, the collapse of the ceiling and its contents onto any occupants within the house. Within seconds of the backdraft explosion and ceiling collapse, all combustible material within the house would ignite.
The time taken for a fire within a ceiling cavity to progress to the stage of ceiling collapse is dependant on the following factors: fuel loading, geometry and size of the roof/ceiling cavity and construction materials. Research in compartment fires by Dehaan has recorded times ranging from several minutes for a compartment fire to reach flash over (everything within the compartment has ignited) and up to 8 or more hours for a smouldering fire within a compartment resulting in a backdraft explosion once an opening is created.
References
DeHaan, J. Kirk's Fire Investigation, Sib Edition, Prentice HaIl, New Jersey, 2002.
Karlsson, Band Quintiere, J. Enclosure Fire Dynamics CRC Press, London, 2000.
NFPA 921, Guide/or Fire and Explosion Investigations. 2001 Edition. USA.
_____________________________________________________________________
Backdraft explosion from ceiling/roof cavity.
Background
Before attempting to understand the phenomenon of a back draft explosion, the basic theory behind combustion must first be briefly examined. Fire combustion is a rapid form of oxidization, known as an exothermic oxidation. The exothermic oxidization produces energy in the form of detectable light and heat. The two types of combustion are flaming and glowing. Flaming is a gaseous combustion in which both the oxidizer and fuel are in a gaseous state. The oxidizer in this case is the oxygen within air. Glowing combustion involves a solid fuel with the gaseous oxidizer (air) (Dehaan, 1997). A typical form of glowing combustion is a smouldering fire in a pile of sawdust or charcoal beads slowly burning away in a barbeque.
Before any of the two forms of combustion can take place and continue to burn the following four conditions must exist: (1) The presence of a combustible fuel, (2) The presence of an oxidizer such as the oxygen in air, (3) The application of energy in die form of heat, and (4) The self-sustaining chain reaction of the fuel and oxidizer. The four conditions and their dependence on each other have been classically symbolized by a four sided geometrical object known as the Fire Tetrahedron.
The propagation of a fire follows the same rule of thermodynamics as the transfer of heat. Heat is transferred in three ways: (1) conduction, (2) convection and (3) radiation.
Conduction is a transfer of heat through a solid object and depends on the materials thermal conductivity. A simple example would be a steal rod that is heated at one end. Eventually the heat transfers through the length of the rod to the other end.
Convection is the transfer of heat energy by continual movement of heated gases or liquids. An example of a liquid convection would be running hot water into a bath of cold water and stirring the water. A gaseous example is a plume layer from a fire spreading across the ceiling of a compartment fire.
Radiation is the transfer of heat from once surface or source to another surface by electromagnetic waves. This can only occur if there are no obstructions between the two surfaces. An example of radiation heat transfer in a fire is the heat from the ceiling's plume layer to the top surface of a table, chair or even the floor.
The development and spread of a fire within an enclosed compartment can develop in many different ways. The development hinges on the enclosure geometry, ventilation, supply of fuel and the surface area. The first stage of any fire is the ignition, of which there are many different ways that this can occur. The cause of the ignition can be attributed to either: deliberate or accidental human involvement, or an unforeseen event such as spontaneous combustion, electrical involvement and chemical reaction to name a few. Regardless of the ignition source, the enclosure has no effect on the early stages of the fire growth. The fire spread and intensity is governed by the fuel loading present. As the fuel progresses, the hot gases are forced upward due to surrounding cooler air (gases).
The result is the development of a plume layer that is trapped by the top section of an enclosure such as a ceiling within a room. The plume layer consists of a mixture of heated combusted and un-combusted gases. The plume layer continues to increase in volume and eventually starts to descend towards the flame region. If there is no avenue for the plume layer to escape and allow the in take of fresh air, the fire becomes starved of oxygen and the energy release rates decreases.
Even though the energy release rate drops, there can still be sufficient heat present to cause pyrolysis of any remaining fuel load (timber), resulting in an accumulation of un-burnt gases. If a window breaks or a door is opened oxygenated air can flow into the enclosed compartment and mix freely with un-burnt pyrolysis products. Any ignition sources such as a glowing ember can then ignite the flammable mixture resulting in explosive gases. The heat created from the rapid combustion causes burning gases to be expelled through the opening under great pressure (Karlsson and Quintiere, 2000). This phenomenon is known as a backdraft explosion. The pressure wave resulting from a backdraft is strong enough to shift entire walls and ceilings by centimetres, and blow out glass windows up to 25 metres away.
In most texts backdraft explosions are associated with a compartment such as a room with a closed door, while the possibility of a backdraft explosion from a ceiling is not often given much consideration.
The typical ceiling structure for most residential dwellings is an enclosed compartment extending from the top of the outer walls to an apex junction. The general purpose of a roof construction in an apex design is to allow the run off of water during rain. The roof line of any house must also extend past and just below the height of the outer walls.
Fire Spread and Backdraft from Roof/Ceiling Cavity
With the top section of a roof/ceiling compartment forming an apex as opposed to the typical flat ceiling compartment the build up of a plume layer would descend more rapidly towards any flaming combustion. Therefore, the oxygen depletion rate would also occur much sooner. The other difference between a typical room compartment as opposed to the roof/ceiling compartment is the lack of walls to slow the transfer of heat and combustion across the house. Due to the lack of any wall restrictions and the apex roof line, the combustion of any timber supporting the ceiling would be rapid and relatively even once a plume layer had established. This would cause the weakening of the ceiling across the entire dwelling, and not just one area in the case of a typical room compartment.
Once a section of ceiling has weakened enough to collapse, the resultant opening would allow for fresh air (oxygenation) into the roof/ceiling cavity causing rapid and intense burning throughout the ceiling opening producing a backdraft explosion.
With the backdraft explosion emanating from the roof/ceiling cavity it would also place pressure downward on an already weakened ceiling. As a backdraft explosion would normally move a wall structure, it would also do the same to a ceiling. The difference between a ceiling and wall is the added continuous force of gravity acting downward on the ceiling. In the event of a backdraft of this nature, the likelihood of a ceiling collapse throughout the entire house, either during or soon after the backdraft explosion is virtually certain.
The Effect on Contents and Occupants.
In the case of any fire, the spread and heat transfer is always in an upward direction first before descending. Therefore, with a fire originating within a ceiling cavity, the indication of the fire presence from within the dwelling would be masked until the ceiling is breached. Then typical smoke detectors placed on the ceiling would not activate until the ceiling was breached by the fire. The smoke alarms also have a high probability of falling from the ceiling due to the melting of any form of plastic aided attachment. Therefore if occupants were sleeping during this time of the fire, they would have no audible warning.
In the event of a backdraft explosion from a ceiling, the pressure wave that emanates from the opening of the ceiling cavity would also bring with it a flame front with a temperature in the order of 700 to 1200.C through out the house. This would also be followed, if not at the same time, the collapse of the ceiling and its contents onto any occupants within the house. Within seconds of the backdraft explosion and ceiling collapse, all combustible material within the house would ignite.
The time taken for a fire within a ceiling cavity to progress to the stage of ceiling collapse is dependant on the following factors: fuel loading, geometry and size of the roof/ceiling cavity and construction materials. Research in compartment fires by Dehaan has recorded times ranging from several minutes for a compartment fire to reach flash over (everything within the compartment has ignited) and up to 8 or more hours for a smouldering fire within a compartment resulting in a backdraft explosion once an opening is created.
References
DeHaan, J. Kirk's Fire Investigation, Sib Edition, Prentice HaIl, New Jersey, 2002.
Karlsson, Band Quintiere, J. Enclosure Fire Dynamics CRC Press, London, 2000.
NFPA 921, Guide/or Fire and Explosion Investigations. 2001 Edition. USA.
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