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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
Q

what colour do hydrocarbons burn in a plentiful oxygen supply

A

blue

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23
Q

what elemental flame looks very similar to the standard hydrocarbon flame

what colour is this flame

A

CaCl3

an orange flame

24
Q

what is the colour of a hydrocarbon flame related to

when is the classic yellow/orange flame seen

A

the oxygen availability

when incomplete combustion occurs

25
Q

what two things will a material emit if it gets hot enough

A

light and heat

25
Q

what is the fuel in the combustion of a candle

A

the wax and the wick

26
Q

what is another word for a glowing/smouldering combustion

A

a surface oxidation

the presence of very hot materials on the surface where a combustion is proceeding

27
Q

in a smouldering combustion where does the oxygen involved normally come from

A

the material itself

little comes from the surrounding air so it can occur in little O2 concentrations

28
Q

from a smouldering combustion what can occur if more O2 becomes available

where is this a major hazard

A

backdraft

in firefighting

29
Q

what type of substances do smouldering combustions often occur in

A

those that can form char by pyrolysis e.g wood, cellulose fabrics, packaging materials

29
Q

what can char formation do to a fire

A

slow the fire by acting as a physical barrier

preventing oxygen getting to the fuel source

30
Q

what is meant by ventilation induced flash over

A

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
Q

what are three characteristics of a smouldering combustion

A
  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
Q

in what two situations would a flaming combustion return to a smouldering combustion

A

running out of fuel or oxygen

if these are reintroduced the flaming combustion can begin again

33
Q

what is the next limiting factor generally after heat

what is generally the final limiting factor of a fire

A

oxygen

fuel

34
Q

what things does the heat in a fire do (4)

but what can heat be in the early stages of a fire

A
  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
  5. heat can be limiting factor
35
Q

what are the three routes of heat transfer

A

conduction
convection
radiation

36
Q

what is meant by conduction

where is this type of heat transfer most important

A

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
Q

what is the reason as to why fires spread through solid fuels

A

because of conduction

the required activation energy is conducted into an adjacent unreacted area of fuel

38
Q

what types of material heat up most quickly

A

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
Q

why do insulators heat up most rapidly

A

because they don’t let heat dissipate through them

40
Q

in what materials are fires likely to be seen at different locations

A

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
Q

metals have high thermal conductivity what is meant by this

A

high thermal conductivity means the material can dissipate heat away from the material

41
Q

what affects the propagation and spread of the fire

A

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
Q

what is the specific heat capacity

A

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
Q

when do walls and ceilings form pyrolysis products

A

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
Q

what is mean by convection

A

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
Q

what does convection create in the burning of a fire

what do these allow (2)

A

air currents

these can increase ventilation as they draw more oxygen to the base of the fire

dissipating heat around the room

44
Q

what is meant by radiation

A

the emission of heat as electromagnetic radiation (in the infra-red region)

45
Q

how can the radiation of a flame start a fire nearby

A

a fire can be started by radiating heat from the flame onto something else

46
Q

how can radiation be quantified

A

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
Q

what is 1 Watt equivalent to

A

1 Joule per second

48
Q

what is meant by flame over

A

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
Q

what is radiation induced flashover

A

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
Q

what always happens before flashover

A

flameover

49
Q

what is one of the quickest ways a fire can spread

A

through radiation induced flashover

50
Q

what does flashover represent the transition between

A

fire in a room to room on fire

51
Q

when can radiation induced flashover occur

A

when there is a fire enclosed in a space

but it can also occur in a compartment that is open

52
Q

what is direct flame impingement

A

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
Q

what are the ways a fire can spread (3)

A

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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