(Review)
Heat transfer
An understanding of heat (or energy) transfer is the key to an understanding of fire behaviour and fire processes. The subject deserves careful study. There are many excellent texts to which one may turn (Welty, Wilson and Wicks 1976; DiNenno 1988), but for the present purposes it is necessary only to draw attention to the three mechanisms: conduction, convection and radiation.
The basic equations for steady-state heat transfer ( ) are:
Conduction:
Convection:
Radiation:
Conduction is relevant to heat transfer through solids; (k is a material property known as thermal conductivity (kW/mK ) and l is the distance (m) over which the temperature falls from T1 to T2 (in degrees Kelvin). Convection in this context refers to the transfer of heat from a fluid (in this case, air, flames or fire products) to a surface (solid or liquid); h is the convective heat transfer coefficient kW/m2K) and depends on the configuration of the surface and nature of the flow of fluid past that surface. Radiation is similar to visible light (but with a longer wavelength) and requires no intervening medium (it can traverse a vacuum); £` is the emissivity (efficiency by which a surface can radiate), £m is the Stefan-Boltzman constant (56.7 x 10-12 kW/m2K4). Thermal radiation travels at the speed of light (3 x 108 m/s) and an intervening solid object will cast a shadow.