Fire Safety Science Digital Archive

If the heating of unburnt fuel ahead of a steadily moving fire front is by radiation from the flames above the fuel bed and by radiation transmitted through the fuel bed there are two possible rates of spread. In one the flames above the fuel bed are essentially "optically thin" and the spread is slow - in the other they are "optically thick" and the spread is fast. Between these two states in an unstable state. Some calculations are made which suggest that only fire fronts larger than of order 10m are capable of fast spread. There is, however, a necessary condition for fast spread involving values of flame properties. These, however, are not sufficiently well enough established quantitively to give a clear answer as to whether fast spread controlled by flame radiation is never possible or possible sometimes.