Radiation
|
Radiation: Heat transfer by way of electromagnetic energy. |
It is this radiative heat transfer that prompts people to move back when they are standing too close to a fire or other source of heat.
The fire fighter in Figure 25 will feel higher levels of heat transfer by radiation as he approaches the fire. Radiative heat transfer does not require an intervening medium such as a solid, liquid or gas; however, radiative heat transfer requires that there is a line of sight between the target and the radiator. Heat is radiated in all directions from the combustion zone as well as from any hot smoke (above ambient temperature) in the fire plume. The heat from the fire plume, combined with that from the upper hot gas layers during a compartment fire, is one of the major causes for flashover, the rapid ignition of all combustibles within view of the lower layer.
The equation for the radiative heat transfer from an emitter is:
Equation 2: Radiative heat transfer Q= εσ T4,
where ε is the emissivity of the emitter, σ is the Stephan-Boltzmann constant, and T is the temperature of the emitter in Kelvin. The emissivity of an element is a measure of how well it will absorb radiation. Objects that are darker and less polished will have a higher emissivity and will both be better emitters and absorbers of thermal radiation. The radiant energy emitted is proportional to the fourth power of the emitter's absolute temperature, therefore, as the temperature of a material increases, the radiated heat increases significantly.
|
Think about it this way… If a baking tray's temperature is doubled from room temperature to 327°C (600K), about the temperature of an oven, it will emit 24 = 16 times more radiant heat than the same object at just above room temperature (27°C or 300K). If the same baking tray was heated instead to 927°C (1200K), the radiant heat emitted would be 44 = 256 times more radiant heat. |
All solids, liquids and gases emit and absorb radiant energy. An object will be an emitter if it is hotter than its environment and will be an absorber if it is cooler. The amount of radiant heat transferred between an emitter and an absorber therefore depends on their relative temperatures, their size, shape and orientation, as well as the distance between them. As the distance between the emitter and absorber increases, the amount of radiation hitting the target decreases considerably. Figure 25 highlights that the temperature and size of the fire, as well as the distance between the fire and the firefighter, are important in determining appropriate separation distance for firefighting operations. A small fraction of radiation is emitted as visible light if the temperature of an object is high enough. In the case of a fire, the main source of yellow-orange radiation is the hot soot particles in the fire plume. Visible radiation increases in intensity and changes in colour as the temperature increases. As the fire burns hotter, the soot also becomes hotter and the overall colour changes from a deep orange to a lighter yellowish-orange colour.