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Unit 10 Aim B - Explore the cause, phases and behaviour of fire
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DISTINCTION awarded assignment. Includes evaluation and referencing. Part of unit 10 - FORENSIC FIRE INVESTIGATION
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1Unit 10: Forensic Fire Inves ga on
Aim B: Explore the cause, phases and behaviour of re
Assignment report: the behaviour of re
There are many reasons and factors involved in a re’s origin, be it a re that’s natural, deliberate or
accidental. O en these factors can in uence the propaga on (spread) of the re in its characteris cs and
speed.
Accidental res and natural res: Non-deliberate res may include electrical res, children ligh ng
ammable substances with matches, chip pan res or leaving an oven switched on for a long period of me.
Taking chip pan res as an example, when oil is le on the heat for too long or at too high a temperature, it
can reach its auto igni on temperature - the lowest temperature at which it will spontaneously combust.
Spontaneous combus on occurs when a combus ble material is heated to a temperature at which it will
ignite under normal ambient condi ons, without an external ini ator such as a ame or spark. This is due to
a chemical reac on that occurs involving oxida on of the combus ble material, crea ng a heat that can not
be dissipated, and so builds up in the material un l combus on. Spontaneous combus on is o en a cause
of many natural res, another example being dry grass and bushes exposed to high temperatures from the
sun. Therefore oil used in cooking can auto ignite given the correct condi ons. The chance of a material
spontaneously combus ng is increased if the surrounding air is dry and warm. In fact, chip-pan res cause
one- h of all accidental dwelling res in the UK each year, spontaneously combus ng at 406°C. As well as
the damage they can cause to people’s homes, these res also injure around 4,000 people every year (from
CLEAPSS Guide, L195: ’Safer Chemicals, Safer Reac ons’, sec on 9.5).
Deliberate res & suspicious circumstances: There are various mo ves for arson. These include crime
concealment (such as murder or burglary), revenge for perceived injus ce, monetary gain (e.g pro t from
insurance cover or pu ng a compe tor out of business), malicious vandalism or even as a result of mental
illness. However there are mul ple indicators of a deliberate re. Firstly, many arson scenes have charred
ooring, indica ng an accelerant has been used. V pa ers (to be expanded on later) also indicate the use of
an accelerant. Furthermore, the scene may include the presence of lots of combus bles in one area to
increase combus on rate, mul ple points of origin, blocked windows and doors and obvious odours such as
gas or kerosene. Furthermore, a sign of a deliberate re speci cally may be a lack of personal items in the
burned area.
In addi on, indicators of ammable liquid use in par cular involve unnatural ame movement (e.g
downward or too rapid), gapping of wooden oors caused by liquid pooling, ooring beneath appliances
and furniture being burnt and inverted cone shaped burns on ver cal surfaces
The stages of re & heat
release rate
Heat release rate (HRR) is
a measure of the rate at
which thermal energy is
released over me. It is
measured in joules/
second.
This can be shown in on a
graph (Figure 1). Below
each sec on of the graph
is explained along side
pictures personally taken
in the lab. Figure 1 -
h ps://guides. redynamicstraining.ca/g/fd203-enclosure- res-sd/
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, 2
1) Incipient:
once combus on begins, certain characteris cs of the re will
be dependent on the fuel involved, but oxygen will always be
supplied from the air. Radiant heat will heat adjacent fuel and
the process of pyrolysis will con nue. The overall temperature
of the room will begin to increase as hot gases and energy begin
to rise, transferring energy to surroundings. As this plume
reaches the ceiling of a room, hot gases will spread horizontally,
covering the area. At this stage, smoke will be limited and the
upper layer of hot gases will not be well de ned. Smoke is
generally lighter in colour in the incipient stage. Minimal
condensa on would be visible on windows, and the
temperature of the room (although increased) will not be
par cularly high. The ames would be lower than ceiling hight
and generally con ned to a smaller area. All of these factors are
re ected in gure 1; the incipient stage has the lowest heat
release rate, although it does increase slightly over me from
the point go ini al combus on.
2) Growth:
From the point of “established burning” (Figure 1), heat release rate rapidly and exponen ally increases (if
adequate oxygen and fuel is present). The gas will now exist in two layers; a ho er layer extending down
from the ceiling and a cooler layer towards the oor. Convec on
occurs as a result of the plume, radiant heat from the re and
other hot par culates in the smoke. As the one layer of gas is
heated, they expand and increase in pressure (as they are
con ned to an extent). This high pressure will cause the gases
to push through any openings. However the pressure of the
cooler gases present is lower, and this results in inward
movement of air from outside of the compartment in which the
re is contained. At the point where these two layers meet, the
pressure is neutral (some mes refereed to as the neutral
plane). The re can con nue to grow through ame spread or
by igni on of other fuels within the compartment. The
ammable products of pyrolysis or byproducts of incomplete
combus on in the hot gas layer will ignite and con nue the
horizontal extension across the ceiling. As the re moves
further into the growth stage and heat release rate increases,
the dominant heat transfer mechanism shi s from convec on
to radia on. It is likely at this point that smoke will be visible
outside of the compartment. More condensa on would be seen
on windows (displaying products of pyrolysis), and other heat
indicators include cracking of window glass or even doors of
certain materials. The ames of the re will extend well past the
object(s) of origin and likely reach ceiling height, at which they will bend and, like the gases, extend
horizontally. It is possible that isolated ames may be observed in the hot gas layer away from the
immediate area of ames (indica ng ven la on controlled condi ons).
3) Fully developed:
There is a “ ashover” ( gure 1) transi on into a fully developed re. Flashover refers to there sudden
transi on from the growth stage into a fully developed re, when a rapid shi to a state of total surface
involvement of all combus ble materials occurs. There are mul ple condi ons that make up the process of
flfi fltifi tititi ti flfi ti ti fifl flflfiti tititi titi fi tifl ti ftfititiflfifl titi fififi ti ti tifiti ti ft tt ti
, 3
ashover. In general, ceiling temperature in the compartment
must reach 500 - 600 °C, and/or heat ux (a measure of heat
transfer) to the oor must reach 15 - 20 kW/m^2. When
ashover occurs, burning gases will force out of openings at a
signi cant speed. It is important to note that ashover does
not always occur; there must be su cient fuel and oxygen. If
there is not su cient energy from the combus on or this
energy is not released rapidly enough (heat release rate),
ashover will not occur. Post- ashover, the re will be in the
fully developed stage. Energy release is at its greatest (but s ll
limited by ven la on. Unburned gases will accumulate at the
ceiling and o en burn as they leave the compartment,
resul ng in ames showing from doors and windows. The gas
temperature at this stage can range from 700 - 1200 °C.
Smoke will darken to grey, black or brown, and the volume of
smoke present will increase. Depending on the size of the
compartment, the hot gas layer can expand towards the oor.
The fully developed re will also have a strong and well
de ned air track, and the velocity of smoke and air movement
will be high. Windows will o en turn to blackened and
frequently cracked.
4) Decay:
A re may enter the decay stage as the available fuel is consumed or due to limited oxygen. Fuel that does
not contain su cient energy or does not have a su cient heat release rate to bring an area to ashover will
pass through each of the stages of re development. On a larger scale, without interven on an en re
structure may reach full involvement and as fuel is consumed,
move into the decay stage. However, there is another way in
which the re can move into the decay stage. When the
ven la on of the compartment or building does not provide
su cient oxygen, the re may move into this stage. Heat
release rate decreases as oxygen concentra on drops,
however, temperature may con nue to rise for some me.
This presents a signi cant threat as the involved area and
combus bles may contain a high concentra on of hot,
pyrolized fuel, and ammable gaseous products of
combus on.
Ven la on Controlled Fires
Under ven la on controlled condi ons and ammable
products of combus on present in smoke are a signi cant
hazard to re ghters. While fuel, heat, and oxygen are
present in propor on to support combus on where the re is
burning, the heat of the re is pyrolyzing more fuel vapour
than the re can consume. In addi on, incomplete
combus on results in produc on of ammable gases such as
carbon monoxide. The speed of re development is limited
by the availability of atmospheric oxygen provided by the
current ven la on pro le of the compartment or building.
Pa erns of re
Fire pa erns are made by markings created by re as it burns, depending on the re’s characteris cs. These
will be analysed in slightly di erent ways and can be indicators of di erent origins such as accelerants or
poolings. These pa erns can also be dependent on the compartment in which they are contained, as these
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