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Flash Over

In the growth of a compartment (room) fire, flashover is defined as the transition between the fire growing by igniting one item and then another (by direct flame contact, radiant heat, convective heating, or immersion in the hot gas layer) to the point where all fuel packages in the compartment are on fire (full involvement) and are burning as fast as air (oxygen) can be made available through window and door openings. This transition may occur throughout a room in just a few seconds in a small, heavily fueled room, over a period of minutes in large rooms, or may never take place due to insufficient size (heat release rate) of the fire. Because it is a transition, defining the "exact moment" is often a problem. 

If the average temperature of the hot gas layer exceeds 6000C (with or without the flaming ignition of the hot smoke called flameover or rollover) the radiant heat from the layer exceeds the minimum igntion radiant heat flux for exposed fuels and those fuels char and then ignite. This is the transition to flashover. If flashover occurs, temperatures throughout the room go to maximum (1000 
0C is not uncommon) as the two-layer environment of the room breaks down and the entire room becomes a turbulently mixed combustion zone--floor to ceiling. The active mixing promotes very effective combustion with oxygen concentrations dropping below 3% and very high temperatures. This environment in turn produces radiant heat fluxes of 120 kW/m2 or higher, ensuring rapid ignition and sustained burning of all exposed fuel surfaces. This includes carpets, flooring, and low-lying fuels like baseboards. Ignition of carpets produces floor-level flames that sweep under chairs, tables, and other surfaces that were protected from the downward radiant heat from the hot gas layer alone. Burning will then proceed throughout the room until the fuel supply is exhausted.