What is fire
A rapid, persistent chemical that releases heat and light and is generally accompanied by a flame
• Oxidative combustive process which gives rise to heat and light
• Multiple mechanisms at play simultaneously
• CH4 + 2O2 +CO2 +H2O
what kind of reaction is fire
• Flame or fire is a gaseous oxidative reaction
what is fuel converted into in fires
gaseous form
liquid fuels vaporise to…
form a combustible vapour
solid fuels decompose to..
produce a combustible vapour
within the flame
- Fuel vapour ripped apart due to chemical conditions
- Products combine with oxygen releasing energy
combustion releases what
energy thereby increasing fuel temp promoting further vapour production
what is combustion an example of
positive feedback/chain reaction
limitation in fire
- Limited by external factors which is why not every fire turns into raging inferno
complete chain reactions don’t always occur due to what limitations
- Ventilation – O2 concentration
- Ventilation – rate of O2 diffusion
- Amount of fuel available
- Rate of fuel vaporisation
- Rate of heat loss
limitations can lead to what
a glowing or smouldering fire
- Normally O2 limited
- Can be extremely destructive nevertheless
- May become flaming if limitations are removed
the model flame
• May form distinct undisturbed zones
- At the bottom, solid fuel forms combustible vapours
- This vapour is decomposed into simpler molecules such as carbon and hydrogen which combust readily
- Heated fuels diffuse upwards and outwards by convection drawing in air allowing combustion proper
- Efficient combustion occurs only in the zone where fuel concentration and air concentration within appropriate bounds
combustion zones
• Zones relate to fuel-air ration and are more of a continuum
fuel rich region
bottom
- Fuel vich vapour – combustion may not be possible
stoichiometric region
middle
- Fuel-air ration ideal – combustion most efficient and flame at its hottest
air rich region
top
- Air rich vapour – combustion less efficient and may not be possible
