THE VARIABLES OF FLASHOVER
Flashover can be described as the instantaneous ignition of a large area of combustible materials. This “area” may be a single room, a large indoor area, or even a narrow canyon. The important aspect of flashover is that the space must be contained enough to accumulate heat through any combination of convection, conduction, and radiation for the combustible materials within to reach their respective flash points (in the presence of an ignition source) or ignition temperatures. Fire within a space convects heat and combustible gases to the upper spaces that in turn radiate heat to nonburning contents within the space. This cycle repeats until the point of flashover when all the combustibles will ignite simultaneously.
Upper space gas temperature and the rate of heat release within the space (by either the burning objects themselves or radiated from the convection column gases) are believed to be the two principal variables determining flashover. First, temperatures of 930° to 1,100° F must be achieved within the confined upper space before flashover can be expected; second, the minimum rate of heat release is 20 kilow atts of pow er per square meter.
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