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Why is it Important to Recognize Flashover in a Room Fire?

by John D. DeHaan
California Institute of Criminalistics

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 got 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. It can be likened to defining the moment when one has crossed a bridge: is it when one sets foot on the bridge, one gets halfway, or sets foot on the other side? It is all part of the process, and the time it takes for completion can vary. Up to flashover, the highest average temperatures are found in the upper or "hot gas" layer. Temperatures in the lower or "normal" zone are at or close to ambient. This produces a distinctive area of thermal damage on the upper walls and ceiling of the room due to radiant and convective transfer of heat from the fire gases. Extensions from lower portions of the room (V-patterns) into this layer may then give clues as to the location and involvement of fuel packages in the room.

If the average temperature of the hot gas layer exceeds 600 degrees Celsius (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 degrees Celsius is not uncommon) as the two-layer environment of the room breaks down and the entire room becomes a turbulently mixed combusyion 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 or some attempt is made at extinguishment.

Unfortunately, post-flashover burning in a room produces numerous effects that were once thought to be produced only by accelerated (arson) fires involving flammable liquids such as gasoline. Wall scorched or charred from floor to ceiling can result from post-flashover burning no matter how the fire actually began as well as from the fireball of gasoline vapors. The combustion of carpets and underlying pads once considered fairly unusual in accidental fires, is quite common in rooms filled with today's high heat release materials--polyurethane foam, synthetic (thermoplastic) fabrics in upholstery, draperies, and carpets. This problem is exacerbated now with the increasing use of very combustible fibers like polypropylene for the face yarns of economical carpets as well as in the backings of nearly all wall-to-wall carpets. These carpets now melt, shrink and ignite under radiant heat flux exposing the combustible urethane foam pad underneath to the flames. Fire damage under tables and chairs (once thought to be the result of flammable liquids burning at floor level) can be produced by the flaming fire of the carpet as it notes during flashover. High temperatures and total room involvement once linked to flammable liquid accelerants are produced during post-flashover burning. Localized patterns of smoke- and fire-damage that can help locate fuel packages and characterize their flame heights and reveal direction of flame spread can be obliterated with prolonged post-flashoever burning, making the reconstruction of the fire very difficult.

The fire investigator, then, should be aware of the possibility of flashover in modern room fires and what effects post-flashover burning can have. Interviews of first-in fire fighters may reveal if a room was fully inolved floor-to-ceiling when entry was first made. Investigators must be aware of what conditions can lead to flashover and how to recognize when it has and has not occurred. A thorough understanding of heat release rates and fire spread characteristics of modern furniture will be of great assistance.

Reprinted with permission from the author.

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