Video Transcript
Fire can be self-propagating through direct flame contact. Thermal energy is also transmitted by
- conduction,
- convection and
- radiation.
Convection
Convection occurs only in liquids and gases and involves the movement of heated liquid or gas molecules from the source of the heat to cooler areas allowing cooler molecules to replace them before being heated and establishing a circulation of convection currents.
Fire spread by convection is the most dangerous and causes the largest number of injuries and deaths.
When fires start in enclosed spaces such as buildings, the smoke rising from the fire gets trapped by the ceiling and then spreads in all directions to form an ever-deepening layer over the entire room space.
The smoke will pass through any holes or gaps in the walls, ceiling and floor into other parts of the building.
The heat from the fire gets trapped in the building and the temperature rises.
Conduction
Conduction may occur in solids, liquids or gases, although it is most clearly present in solids. In conduction, heat energy is passed between molecules with heat flowing away from the source of heat towards low temperature regions.
Some materials, such as metal shutters and ducting, can absorb heat and transmit it to the next room, where it can set fire to combustible items that are in contact with the heated material.
Radiation
Heat may also be transmitted by radiation which does not require an intervening medium.
Radiation heats the air in the same way as an electric bar heater heats a room. Any material close to a fire will absorb the heat until the item starts to smoulder and then burn.