Fires 5 - Nature and Behaviour of Fire

Question 1

Define flame

Answer 1

the region where chemical interactions between gases occur - the result being the evolution of heat and light.

Question 2

What are the two types of combustion. Explain the difference.

Answer 2

flaming combustion - chemical reaction in gas phase where there is a flame

glowing/smouldering combustion - no flame but hot materials on the surface of which combustion (oxidation) is still occurring.
- not a gas phase reaction

Question 3

Define pyrolysis

Answer 3

decomposition of molecules via heat (oxygen not required)

pyro (heat)
lysis (breaking off)

if you apply enough heat to any material, it will break down eventually because it is going to make radicals

Question 4

Describe the process of pyrolysis?

Answer 4

thermal decomposition of organic/inorganic substances to produce compounds of lower molecular mass without using oxygen

Question 5

What does the process of pyrolysis usually initiate?

Answer 5

pyrolysis (decomposition) usually initiates radical chain reactions

heat is good are making radicals so can produce the reactive radicals

Question 6

What are the properties of products formed?

Answer 6

flammable (easily oxidise)
volatile (easily evaporate)

Question 7

Describe the process of the formation of products during pyrolysis

Answer 7

if the concentration of the products is within the flammability range they can burn at the surface of the solid

if the heat makes small molecules they can burn as they evaporate OR they can also be carried into the fire plume and then ignite elsewhere

remaining residues become char (closer to pure carbon)

this results in complex mixtures of pyrolysis products

Question 8

What can be said about the pyrolysis of plastics?

Answer 8

the products can be highly toxic

the products can resemble arson accelerants so need to be careful if suspect something is arson using detected accelerants

Question 9

what states can be on fire/be a flame?

Answer 9

MUST BE GASEOUS PHASE
gases produce flames so can be on fire

solids and liquids cannot be on fire as they do not make flames

Question 10

Does paper burn?

Answer 10

• the solid paper burns but it is not on fire
• the cellulose (main chemical which makes up paper) undergoes a pyrolytic decomposition which forms a flammable gas
• the flammable gas burns
• the decomposition products of cellulose make up the flame (nothing in paper initially is in the flame)

Question 11

does sambuca burn?

Answer 11

• get pool of ethanol with flame coming off it but liquid ethanol is not on fire
• there is vapour which has come off liquid ethanol so ethanol molecules in the air
• volatilised ethanol (flame) gives off heat when burned and the heat then evaporates more ethanol

Question 12

what processes are happening when a cigarette is burned?

Answer 12

smouldering combustion - glowing embers (smouldering) - hot materials on surface are reacting with are

also pyrolysis - paper is being degraded and it is changing colour

there is no flaming combustion as there is no flame

Question 13

What things do we need for a flame?

Answer 13

fuel and oxygen in the correct ratio

too much oxygen causes a fuel rich combustion

Question 14

Describe the process of a solid fuel burning

Answer 14

• surface pyrolysis is occurring due to heat of fire which makes it volatile
• flammable small molecules are going up in forms of vapours
• when get high enough where ratio of air is correct then you will get flames happening
• flames gradually get incompletely oxidised into carbon monoxide and carbon dioxide
• when molecules fully reacted they will come out top as there is a convective plume
• as the gases rise, more air gets drawn in at bottom
• new air feeds the newly formed pyrolysis products which then go onto combust into fully oxidised products (HCN, H2O CO2) or partially oxidised products (CO, C)

Question 15

Describe the process of a candle burning

Answer 15

candle flame protrudes above the wax as it is not the wax which is burning it is the vapours which are released which then burn

there is an idealised combustion zone where you have the correct ratio of oxygen to fuel

as get further away from wax, the amount of oxygen gets higher but the amount of fuel is not enough for combustion to occur

Question 16

Describe the process of a candle burning in terms of flaming combustion

Answer 16

• there is a solid wax which is melted by heat so it becomes molten wax (liquid)
• liquid is taken up through the wick and it evaporates very easily from wick due to high surface area
• when it is in gaseous state, it sets on fire and causes a flaming combustion
• get a nice clear delineation of temperatures
• temperature closest to fuel is one of the lower ones in the candle because the energy is being taken up by evaporation and there is not as much oxygen in air
• as get further from wick, temp will go up to 1400 degrees Celsius
• in space where have hot temps, have a very fast reaction and plenty of oxygen
• fuel is going to get used up and this will limit extent of the flame
• beyond that the flame will die away and the temperatures will go down again

Question 17

How do the flames persist in a more forensic setting?

Answer 17

not such a tidy flame - will have turbulent flames

• airflow is being drawn in by heat which is going up
• air is is being drawn in at such a rate that the oxygen supply is no longer even
• this makes the vapours diffuse away from the vertical fuel (solid or liquid) which creates a fuel of vapours
• the flame happens at some distance from the surface where there is the optimum ratio of oxygen and fuel

Question 18

What are the majority of flames?

Explain this type.

Why do these occur?

What can occur with this type of flame?

Answer 18

diffusion flames

this is where the gases or vapours supporting the flame diffuse upwards or outwards from surface of fuel

because heat makes the vapours come out and oxygen diffuses towards the fuel from the surrounding air

a pool of liquid fuel can insulate and protect the underlying surface from the heat of the fire which means this surface will be less badly burnt than the surrounding region

Question 19

Why are flames different colours?

Answer 19

flame colour is determined by the wavelengths of light emitted which depends on the flame contents

Question 20

If have complete combustion, what does the flame colour be important for?

Answer 20

it can identify elemental components of flame

if have HCs doing complete combustion - will get blue flame

Question 21

What type of combustion is most likely to occur in fire investigations?

Answer 21

incomplete - characteristic of classic yellow flames which are caused by soot or other liquid/solid products raised to incandescent temperatures

Question 22

what is the fuel in this combustion (candle burning)?

Answer 22

wax is the fuel but the fire doesn’t happen where the fuel is located

Question 23

What is smouldering combustion?

Give an example

Answer 23

when have surface oxidation processes happening

it is normally found with the absence of a flame

coal fires - there is presence of very hot materials on surface of which combustion is proceeding
- the colour is from incandescence
- blue flame on coal fire as coal is making CO which is then burning in air

Question 24

What substances can smouldering combustion occur on?

Answer 24

can occur on substances that can form a char by pyrolysis (things which form solids which still contain oxidisable compounds but not gaseous e.g. wood cellulosic fabrics and packaging materials)

Question 25

Where does smouldering combustion take place?

Answer 25

at surface of material and not in gas phase

Question 26

What can be said about the ratio of fuel and oxygen in smouldering combustions?

Answer 26

not so important therefore it can occur at very low O2 concentrations especially if oxygen already in fuel e.g. carbohydrates like cellulose

Question 27

What can char formation do?

Answer 27

can slow a fire by acting as a physical barrier to flaming combustion spreading

Question 28

What is the danger associated with more oxygen suddenly becoming available?

Answer 28

flames will occur - backdraft (ventilation induced flashover)

if room runs out of oxygen, fire will die down to smouldering combustion but when firefighters open door this lets in lots of oxygen and causes loads of flames to form

Question 29

What are the effects associated with smouldering combustions forensically?

Answer 29

small associated amount of direct damage as slow and doesn’t spread so easily

however can produce large amounts of poisonous CO

Question 30

What stage of fire incidents are smouldering combustions usually?

Answer 30

first and or last stage of fire incident

Question 31

What can be said about the length of time a smouldering fire can persist for?

Answer 31

slow smouldering fire can persist for prolonged periods of time before producing flames

Question 32

What do all methods by which fires are ignited involve?

Answer 32

application of heat

under the right circumstances the addition of heat is enough to cause a fire to start by the generation of radicals

Question 33

Other than a driving force, what other purpose does heat have in a fire?

Answer 33

accelerates them, spreads them and is what causes major damage and injuries

Question 34

What is limiting factor in early stages of fire?

What is the next limiting factor?

What is the final limiting factor?

Answer 34

Heat as it is lost rapidly to surroundings
- normally only have small flame to begin with but reaction rates double with every additional 10 degrees so once something starts its going to accelerate

next is oxygen - a closed room will deplete oxygen and result in smouldering fire

last is fuel - the fire will burn until entire structure is destroyed

Question 35

What are the three classical categories of heat transfer?

Answer 35

radiation, conduction, convection

Question 36

define conduction

what is heat energy

what state is this most important in?

why do fires spread through this fuel state

what type of thermal conductivity material heats up quickest? and what does this mean for them?

give an example

Answer 36

transfer of heat through a material by direct atomic or molecular contact

heat energy is atomic vibration

most important in solids where atoms in direct contact with each other

fires spread through solid fuel due to conduction
- required activation energy is conducted into adjacent unreacted areas of fuels

low thermal conductivity materials (insulators) heat up quickest because don’t let heat dissipate through them
- therefore insulators can be flammable - they quickly reach the required temp for pyrolysis (and ignition)

wood is excellent example of thermal insulator

Question 37

What type of material has high thermal conductivity?

What consequence does this have? (hint: structures)

Answer 37

metals
as a result dissipate heat which means heat gets conducted away

• metals can transfer large quantities of heat energy to other locations
• if the other end of that metal (e.g. steel girder) is near something which is easily set alight then it might start a fire in another place in building

Question 38

define specific heat capacity

how does specific heat capacity value effect flammability?

Answer 38

the quantity of heat energy which a unit mass of a substance requires to raise its temperature by 1 degree

higher value e.g. water isn’t flammable but lower value e.g. polyurethane foam will get hotter faster and then it is more likely to get pyrolysis temperatures

Question 39

define convection

how does convection cause heat transfer in fires?

Answer 39

transfer of heat in a gas or a liquid by the circulation of molecules which is caused by temperature differences

• regions of high temperature are less dense so rise upwards
• atoms have more relative motion so must spread out which makes them less dense
• in a burning fire this creates air currents
• air currents draw more oxygen to the base of fire increasing ventilation
• this also dissipates heat around the room which can heat up other things in room
• walls and ceilings are dried and heated by fire plume (hot gases rising from a fire) so even if flames do not reach them directly they can auto ignite if hot enough

Question 40

define radiation

how can radiation cause heat transfer in fires?

Answer 40

the emission of heat as electromagnetic radiation (infra-red region)
- it doesn’t involve the transfer of heat through molecules

radiation from flames plays a major role in the spread of fire - can start fires nearby just by radiating the heat of one flame onto it

Question 41

what is the intensity of the radiation proportional to?

what is the radiation energy proportional to?

Answer 41

fourth power of the absolute temperature (Stefan-Boltzmann law)

it will increase really rapidly in intensity when the temp increases
E proportional T^4

radiation energy falls off as the inverse square of the distance from the source

Question 42

How can we quantify the propagation and spread of fire?

Give some relative values

Answer 42

as radiation flux (intensity) in W m-2 or kW m-2

this is how much power is shining over a certain area

1 - direct summer sun
6 - pain after 8 seconds skin exposure
12 - charring of wood
20 - some cellulosic’s ignite within 60 seconds
30 - most flammable vapours and pyrolysis gases reach auto-ignition temp
50 - auto-ignition of nearly all materials within 5 seconds

Question 43

What happens if a fire plume cannot escape from a compartment?

Answer 43

it will spread a layer of hot flammable gases underneath the ceiling

flames can spread horizontally under the ceiling at great speed - flameover

when the plume cannot escape, there is lots of radiant heat energy which goes downwards from these flames
- even in the absence of flameover, the gas flammable layer under ceiling at 600 degrees can produce 20 kWm-2 radiant heat at floor level so going to ignite cellulosic’s quickly
- flames at 750-850 across ceiling can produce 25-30 kWm-2 radiant heat at floor level at which all flammable fuels and gases in room begin to decompose
- within seconds they reach ignition temp and catch fire
- called radiation induced flashover

Question 44

define radiation induced flashover

Answer 44

represents downward spread of fire because flames are at top and they radiate downwards

flashover represents transition from ‘fire in room’ to ‘room on fire’

• hot gases go to ceiling which then ignite and then fire in ceiling
• radiates back downwards and ignites anything else not yet ignited

Question 45

What exceptional circumstance can flashover still occur in?

Answer 45

in a compartment that is open on 3 sides e.g. Bradford City football stadium where 56 people died in 1985

radiation induced flashover is one of quickest ways for fires to spread

Question 46

Define direct flame impingement

explain the process

Answer 46

combination of both convective and radiative mechanism - classically what you do when you hold one flame against something and that thing sets alight

• plume of hot gases rises by buoyant flow (they are light) and transfers heat to new fuels by convective transfer and radiation
• new fuel pyrolyzes and generates flammable gases which are ignited by flames of plume because you have temperature there