Convection This only occurs in liquids and gases, including smoke.
A good example is the process of convection when a pan of water is heated. A pan full of water is heated from the base of the pan and the water warms up, it expands and therefore becomes less dense and so a given volume is lighter. Because the heated liquid is buoyant, it rises, then colder, denser fluid takes its place at the bottom, this then becomes heated and so a circulation is set up. Updraft in chimneys is caused by convection and by the same principle when a fire occurs in a building. Convection currents can convey hot gases produced upwards through stairwells, open lifts, and service shafts, thereby spreading the fire to the upper parts of buildings. If the hot gas products escape from the upper levels, cool air must enter at low levels to replace them. This will help to maintain the burning as a fresh supply of oxygen will be drawn into supply the fire. Radiation Heat is transmitted by a means, which is neither conduction nor convection nor requires an intervening medium. An everyday example of this is a radiant heater. When placed at a high level in a room the heat can be felt at lower levels, where neither conduction nor convection can carry it. This method of heat transmission is called radiation and does not involve any contact between the bodies that are providing and accepting the heat. For all intents and purposes, it behaves in the same way as light (‘visible radiation’). Light travels in straight lines and will be transmitted through some materials and not others. When radiant energy (heat in this context) falls upon any material, there are 3 potential outcomes: • Transmission. Many fires have been caused by radiation. One of the most common is clothing being ignited by being placed too close to a source of radiation, as sometimes happens when people air clothes placed too close to a fire. Radiant heat from the sun passing through a glass window has sometimes acted as a lens, having been concentrated by an object inside the house, such as a magnifying glass or a shaving mirror. Direct burning One of the main methods of fire spread is by physical flame contact. As a material burns, it has the potential for the flames from the combustion process to touch and ignite other materials nearby. • Absorption. • Reflection.
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