Fires 5 - Nature and Behaviour of Fire Flashcards
Define flame
the region where chemical interactions between gases occur - the result being the evolution of heat and light.
What are the two types of combustion. Explain the difference.
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
Define pyrolysis
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
Describe the process of pyrolysis?
thermal decomposition of organic/inorganic substances to produce compounds of lower molecular mass without using oxygen
What does the process of pyrolysis usually initiate?
pyrolysis (decomposition) usually initiates radical chain reactions
heat is good are making radicals so can produce the reactive radicals
What are the properties of products formed?
flammable (easily oxidise)
volatile (easily evaporate)
Describe the process of the formation of products during pyrolysis
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
What can be said about the pyrolysis of plastics?
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
what states can be on fire/be a flame?
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
Does paper burn?
- 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)
does sambuca burn?
- 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
what processes are happening when a cigarette is burned?
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
What things do we need for a flame?
fuel and oxygen in the correct ratio
too much oxygen causes a fuel rich combustion
Describe the process of a solid fuel burning
- 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)
Describe the process of a candle burning
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
Describe the process of a candle burning in terms of flaming combustion
- 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
How do the flames persist in a more forensic setting?
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
What are the majority of flames?
Explain this type.
Why do these occur?
What can occur with this type of flame?
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
Why are flames different colours?
flame colour is determined by the wavelengths of light emitted which depends on the flame contents
If have complete combustion, what does the flame colour be important for?
it can identify elemental components of flame
if have HCs doing complete combustion - will get blue flame
What type of combustion is most likely to occur in fire investigations?
incomplete - characteristic of classic yellow flames which are caused by soot or other liquid/solid products raised to incandescent temperatures
what is the fuel in this combustion (candle burning)?
wax is the fuel but the fire doesn’t happen where the fuel is located
What is smouldering combustion?
Give an example
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
What substances can smouldering combustion occur on?
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)
Where does smouldering combustion take place?
at surface of material and not in gas phase
What can be said about the ratio of fuel and oxygen in smouldering combustions?
not so important therefore it can occur at very low O2 concentrations especially if oxygen already in fuel e.g. carbohydrates like cellulose
What can char formation do?
can slow a fire by acting as a physical barrier to flaming combustion spreading
What is the danger associated with more oxygen suddenly becoming available?
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
What are the effects associated with smouldering combustions forensically?
small associated amount of direct damage as slow and doesn’t spread so easily
however can produce large amounts of poisonous CO
What stage of fire incidents are smouldering combustions usually?
first and or last stage of fire incident
What can be said about the length of time a smouldering fire can persist for?
slow smouldering fire can persist for prolonged periods of time before producing flames
What do all methods by which fires are ignited involve?
application of heat
under the right circumstances the addition of heat is enough to cause a fire to start by the generation of radicals
Other than a driving force, what other purpose does heat have in a fire?
accelerates them, spreads them and is what causes major damage and injuries
What is limiting factor in early stages of fire?
What is the next limiting factor?
What is the final limiting factor?
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
What are the three classical categories of heat transfer?
radiation, conduction, convection
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
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
What type of material has high thermal conductivity?
What consequence does this have? (hint: structures)
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
define specific heat capacity
how does specific heat capacity value effect flammability?
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
define convection
how does convection cause heat transfer in fires?
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
define radiation
how can radiation cause heat transfer in fires?
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
what is the intensity of the radiation proportional to?
what is the radiation energy proportional to?
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
How can we quantify the propagation and spread of fire?
Give some relative values
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
What happens if a fire plume cannot escape from a compartment?
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
define radiation induced flashover
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
What exceptional circumstance can flashover still occur in?
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
Define direct flame impingement
explain the process
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
