That is the purpose of this chapter. The fuel for most unwanted fires is organic material, e.g., the wooden frame of a house, an item of furniture, or gas leaking from a heater. The reaction is rapid, usually taking a few hundredths of a second. The speed of burning and hence the intensity of the fire are usually governed by the rate at which fuel vapor and air enter the flame, where temperatures are high enough to initiate their reaction. The important transport processes in a diffusion flame are shown in Figure 2-1.
Fuel vapor is produced when heat from the flame radiates back to the fuel surface.
The hot vapor rises, mixes with air entering near the base of the flame, and ignites. This buoyant expansion in turn creates turbulence, which causes more air to be entrained. A sizable fraction of the heat produced by combustion appears as radiant energy, some of which is absorbed by the fuel surface beneath, so the evolution of fuel vapors continues. If the fuel is a volatile liquid, very little energy is required to vaporize it. The surface of a burning solid usually either melts or chars.