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Fires and Explosions > Lecture 16 > Flashcards

Lecture 16 Flashcards

1
Q

what is the difference between a flame and a smouldering combustion

A

flame = the region where chemical interactions between gases occur = in gas phase

smouldering combustion = no flame is present, hot materials on the surface where a combustion (oxidation) is occurring = not in gas phase

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2
Q

are all combustions flaming combustions

A

no

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3
Q

give two examples of a smouldering combustion

A

coal
cigarettes

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4
Q

what is meant by pyrolysis

A

the thermal decomposition of molecules via heat - in absence of oxygen

this generates radicals

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5
Q

how do we get from a smouldering combustion to a flaming combustion

A

the products of a smouldering combustion are pyrolyzed to become a flaming combustion

the flammable material itself does not result in a flame it is the gas products of the combustion

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6
Q

what does pyrolysis initiate

A

the radical chain reactions needed to fuel a fire

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7
Q

what can the pyrolysis of plastics be confused with

why is this

A

common arson accelerants

the products are similar
they are also highly toxic

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8
Q

If the concentration of the pyrolysis products is within the flammability range, what can happen (2)

A

they can burn at the surface of the solid

they can be carried in the fire plume and ignite somewhere else

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9
Q

what are the two properties of pyrolysis products

A

they are flammable and volatile

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10
Q

what do the remaining residues of pyrolysis reactions become

A

char (like carbon)

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10
Q

what type of mixture are pyrolysis products

A

complex mixtures

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11
Q

out of solids, liquids and gases which can produce a flame

A

gases only

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12
Q

can paper be on fire

A

the solid paper undergoes a pyrolytic decomposition which forms a flammable gas

this flammable gas can then burn and produce a flame

the paper itself doesn’t flame its the pyrolysis products that cause the flame

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13
Q

does sambuca burn

A

no, the ethanol is volatilised

the gaseous ethanol flaming combusts not the liquid sambuca

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14
Q

what 4 things do we need for a flame to occur

A

fuel
oxygen
heat
gaseous phase

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15
Q

what is needed in the correct ratio for a flame to be seen

A

fuel and oxygen

the heat allows the activation, the fuel is gas in nature and the oxygen is diradical

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16
Q

what is meant by a fuel rich combustion

A

too much fuel

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17
Q

what is meant by a fuel lean combustion

A

too much oxygen

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18
Q

what is meant by the combustion zone of a flame

A

the ideal ratio of fuel vapour to oxygen

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18
Q

what shape of flame is a candle flame

A

a laminar flame

the different zones are well defined and the air flow is smooth

soot is almost totally combusted in the high temperature zone of the flame

this is the idealised flame shape

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19
Q

what type of flames are most seen in forensic settings

A

diffusion flames

here, the gases or vapours supporting the flame diffuse up or out from the surface of the fuel

oxygen diffused towards the fuel from the surrounding air

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20
Q

what determines the colour of a flame

A

the wavelength of light emitted determines flame colour

this is determined by the content of the flame

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21
Q

what can flame colour be useful for (2)

A

identifying the elemental components of a flame

seeing how hot a flame is

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21
Q

what is often seen when we have a pool of liquid as the source/fuel of the fire

where can this be useful

A

the material directly beneath the pool is often undamaged as this is protected from the heat of the fire

in fire scenes

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22
what colour do hydrocarbons burn in a plentiful oxygen supply
blue
23
what elemental flame looks very similar to the standard hydrocarbon flame what colour is this flame
CaCl3 an orange flame
24
what is the colour of a hydrocarbon flame related to when is the classic yellow/orange flame seen
the oxygen availability when incomplete combustion occurs
25
what two things will a material emit if it gets hot enough
light and heat
25
what is the fuel in the combustion of a candle
the wax and the wick
26
what is another word for a glowing/smouldering combustion
a surface oxidation the presence of very hot materials on the surface where a combustion is proceeding
27
in a smouldering combustion where does the oxygen involved normally come from
the material itself little comes from the surrounding air so it can occur in little O2 concentrations
28
from a smouldering combustion what can occur if more O2 becomes available where is this a major hazard
backdraft in firefighting
29
what type of substances do smouldering combustions often occur in
those that can form char by pyrolysis e.g wood, cellulose fabrics, packaging materials
29
what can char formation do to a fire
slow the fire by acting as a physical barrier preventing oxygen getting to the fuel source
30
what is meant by ventilation induced flash over
when O2 restriction causes a fire to smoulder and then O2 is reintroduced and there is a slight delay before a flaming combustion is seen again
31
what are three characteristics of a smouldering combustion
1. they have small amounts of direct damage associated with them but can produce large amounts of poisonous CO2 2. they are usually the first and/or last stage of a fire incident 3. a slow smouldering fire can persist for prolonged periods of time before producing a flame
32
in what two situations would a flaming combustion return to a smouldering combustion
running out of fuel or oxygen if these are reintroduced the flaming combustion can begin again
33
what is the next limiting factor generally after heat what is generally the final limiting factor of a fire
oxygen fuel
34
what things does the heat in a fire do (4) but what can heat be in the early stages of a fire
1. generates radicals, this can be enough to start a fire 2. heat drives and accelerates fires 3. heat spreads fires 4. heat causes damage 1. heat can be limiting factor
35
what are the three routes of heat transfer
conduction convection radiation
36
what is meant by conduction where is this type of heat transfer most important
the transfer of heat through a material by direct atomic or molecular contact in solids as they atoms are in direct contact with each other
37
what is the reason as to why fires spread through solid fuels
because of conduction the required activation energy is conducted into an adjacent unreacted area of fuel
38
what types of material heat up most quickly
those with low thermal conductivity = insulators e.g wood due to this they can quickly reach the required temperature for pyrolysis and then ignition to occur as the heat is stored in the material it can then undergo pyrolysis
39
why do insulators heat up most rapidly
because they don't let heat dissipate through them
40
in what materials are fires likely to be seen at different locations
when heat is conducted through materials with high thermal conductivity the heat can be spread throughout the material and if something is near the material that is easy to set alight then a new fire or multiple ones may be started
40
metals have high thermal conductivity what is meant by this
high thermal conductivity means the material can dissipate heat away from the material
41
what affects the propagation and spread of the fire
the material of the fuel of the fire - how easily it transfers heat and the specific heat capacity heat transfer processes = convection, conduction and radiation
42
what is the specific heat capacity
the quantity of heat energy which a unit mass of a substance requires to raise its temperature by 1 degree water has a high heat capacity but other substances like brick's don't
42
when do walls and ceilings form pyrolysis products
when they become dried and heated the flame may not have to directly touch the wall /ceiling but the heat can cause a fire by it being hot enough
43
what is mean by convection
the transfer of heat in a gas or a liquid by the circulation of molecules caused by temperature differences - higher temp regions are less dense so rise up
43
what does convection create in the burning of a fire what do these allow (2)
air currents these can increase ventilation as they draw more oxygen to the base of the fire dissipating heat around the room
44
what is meant by radiation
the emission of heat as electromagnetic radiation (in the infra-red region)
45
how can the radiation of a flame start a fire nearby
a fire can be started by radiating heat from the flame onto something else
46
how can radiation be quantified
by the intensity = radiation flux this is measured in Watt per m^2 or kW per m^2 equal to how much power is being 'shone' over an area
47
what is 1 Watt equivalent to
1 Joule per second
48
what is meant by flame over
if a fire plume can not escape from a compartment or a room then it will spread a layer of hot gases underneath the ceiling which are flammable these spread horizontally at a fast speed so everything is likely to get hot and ignite very fast
49
what is radiation induced flashover
the downward spread of a fire where flames radiate downward can result from flameover - the heat from this can cause flammable fuels and gases in the space to decompose and when they reach their ignition temps they will catch fire
49
what always happens before flashover
flameover
49
what is one of the quickest ways a fire can spread
through radiation induced flashover
50
what does flashover represent the transition between
fire in a room to room on fire
51
when can radiation induced flashover occur
when there is a fire enclosed in a space but it can also occur in a compartment that is open
52
what is direct flame impingement
a combination of both convective and radiative mechanisms holding a flame against something at it sets alight plume of hot gases rise and transfer heat to new fuels the new fuel pyrolyses and generates flammable gases that are ignited by flames of the plume
53
what are the ways a fire can spread (3)
radiation induced flash over heat transfer (conduction convection, radiaition) resulting in flameover and flashover direct flame impingement

Fires and Explosions (20 decks)

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