Lecture 16 Flashcards
what is the difference between a flame and a smouldering combustion
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
are all combustions flaming combustions
no
give two examples of a smouldering combustion
coal
cigarettes
what is meant by pyrolysis
the thermal decomposition of molecules via heat - in absence of oxygen
this generates radicals
how do we get from a smouldering combustion to a flaming combustion
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
what does pyrolysis initiate
the radical chain reactions needed to fuel a fire
what can the pyrolysis of plastics be confused with
why is this
common arson accelerants
the products are similar
they are also highly toxic
If the concentration of the pyrolysis products is within the flammability range, what can happen (2)
they can burn at the surface of the solid
they can be carried in the fire plume and ignite somewhere else
what are the two properties of pyrolysis products
they are flammable and volatile
what do the remaining residues of pyrolysis reactions become
char (like carbon)
what type of mixture are pyrolysis products
complex mixtures
out of solids, liquids and gases which can produce a flame
gases only
can paper be on fire
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
does sambuca burn
no, the ethanol is volatilised
the gaseous ethanol flaming combusts not the liquid sambuca
what 4 things do we need for a flame to occur
fuel
oxygen
heat
gaseous phase
what is needed in the correct ratio for a flame to be seen
fuel and oxygen
the heat allows the activation, the fuel is gas in nature and the oxygen is diradical
what is meant by a fuel rich combustion
too much fuel
what is meant by a fuel lean combustion
too much oxygen
what is meant by the combustion zone of a flame
the ideal ratio of fuel vapour to oxygen
what shape of flame is a candle flame
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
what type of flames are most seen in forensic settings
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
what determines the colour of a flame
the wavelength of light emitted determines flame colour
this is determined by the content of the flame
what can flame colour be useful for (2)
identifying the elemental components of a flame
seeing how hot a flame is
what is often seen when we have a pool of liquid as the source/fuel of the fire
where can this be useful
the material directly beneath the pool is often undamaged as this is protected from the heat of the fire
in fire scenes
what colour do hydrocarbons burn in a plentiful oxygen supply
blue
what elemental flame looks very similar to the standard hydrocarbon flame
what colour is this flame
CaCl3
an orange flame
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
what two things will a material emit if it gets hot enough
light and heat
what is the fuel in the combustion of a candle
the wax and the wick
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
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
from a smouldering combustion what can occur if more O2 becomes available
where is this a major hazard
backdraft
in firefighting
what type of substances do smouldering combustions often occur in
those that can form char by pyrolysis e.g wood, cellulose fabrics, packaging materials
what can char formation do to a fire
slow the fire by acting as a physical barrier
preventing oxygen getting to the fuel source
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
what are three characteristics of a smouldering combustion
- they have small amounts of direct damage associated with them but can produce large amounts of poisonous CO2
- they are usually the first and/or last stage of a fire incident
- a slow smouldering fire can persist for prolonged periods of time before producing a flame
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
what is the next limiting factor generally after heat
what is generally the final limiting factor of a fire
oxygen
fuel
what things does the heat in a fire do (4)
but what can heat be in the early stages of a fire
- generates radicals, this can be enough to start a fire
- heat drives and accelerates fires
- heat spreads fires
- heat causes damage
- heat can be limiting factor
what are the three routes of heat transfer
conduction
convection
radiation
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
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
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
why do insulators heat up most rapidly
because they don’t let heat dissipate through them
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
metals have high thermal conductivity what is meant by this
high thermal conductivity means the material can dissipate heat away from the material
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
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
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
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
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
what is meant by radiation
the emission of heat as electromagnetic radiation (in the infra-red region)
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
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
what is 1 Watt equivalent to
1 Joule per second
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
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
what always happens before flashover
flameover
what is one of the quickest ways a fire can spread
through radiation induced flashover
what does flashover represent the transition between
fire in a room to room on fire
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
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
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
