The transitional phase in a fire when the radiant heat ignites the flammable gases within an enclosed structure. This will look like the fire has spread quickly across the thick smoke on the ceiling, and at this point whatever the flame touches catches fire in a matter of seconds. The only warning sign other than the incredible heat at the source of the fire, and the plumes of smoke start to ignite and this is called rollover. These rollovers can exist for a few seconds before flashover.

Firefighters try to prevent a flashover from occurring by cooling the surrounding air with bursts of water mist sprayed into the air above the fire. The burst allows for the most amount of water to be converted into vapor, cooling a wider area than the spray off the nossle alone.

When this phenomenon occurs outside, it's referred to as a firestorm.


Flashover is the near-simultaneous autoignition of all combustible materials in an enclosed space.

A firefighter in full PPE has between two and five seconds to exit a space undergoing flashover before death. This is why, when exploring a burning structure, one never enters an enclosed space more than can be recovered in two to five seconds. Temperatures in a flashing room approach 1000°F; flashover is not a survivable event.



 

Imagination time. Fill a test tube with matches; top the tube with a rubber stopper containing a spout. Hold a torch to the bottom of the tube. Eventually vapor will escape the spout, and if you hold a match to the escaping gases, they ignite.

Only gases burn.

When sufficiently heated, hydrocarbons vaporize. With most hydrocarbons (paper, wood, plastic) this temperature requirement is relatively high, while with others (gasoline, kerosene, propane) it is relatively low. Vaporized hydrocarbons are replete with free radicals, tragic things with empty outer valences.

If you please, Google Image the Periodic Table. Note that the elements run left to right. Among many things, an element's position on the table indicates how many electrons occupy its outermost shell, described alternatively by the term "valence." (I know, quantums and probabils too). Electrons like to travel in even numbers.

Take sodium, on the left. It's on the left because its outer valence holds one electron. This valence is primed to bond with other compounds, thereby evening itself out. When this reaction takes place electrons shift and energy is released as heat. Sodium is so reactive that it hisses when exposed to moisture in air.

A fire chief I took a class under said most firefighters who've been at it a decade or two keep at least one melted helmet on display, as a trophy.





Oxidation is, intuitively, the process by which substances bond with oxygen.

Mitochondria oxidize energy-rich substances. Oxygen introduced into the bloodstream by the lungs combines with glucose retrieved from food by the digestive tract. The chemical reaction produces energy, and the byproducts are expelled through the lungs as carbon dioxide, heat, and water.

In fire, Flick's law draws oxygen into the reaction zone. The heat catalyzes the exchange of electrons between hydrocarbons and oxygen. Fire is a sustained chain-reaction of rapid oxidation. In a perfect burn — like that attempted by your car's engine — the by-products are carbon dioxide, heat, and water.

All things burn imperfectly: this is why there is smoke. An imperfect oxygen/fuel ratio generates incompletely-burned substances, expelled from the fire by the convection column: tars, hydrogen sulfide, carbon monoxide, hydrogen cyanide.

Smoke burns. I toured a facility in Beaumont which disposes of waste by first burning it, then enclosing the smoke in a chamber which recreates the conditions of flashover. The resulting explosion of heat is converted to electrical energy which is then released onto the grid.



Autoignition is the ignition of matter without direct flame involvement. Spontaneous combustion would be a good analogy, but with an actual cause.

The flashpoint of any given material is, therefore, the temperature at which said material will autoignite.

Many of the things we interact with are hydrocarbons. More obviously, gasoline. Less obviously, carpet, blinds, toasters, mattresses. Many of these everyday substances have similar flashpoints.



All things absorb heat before re-radiating heat, vaporizing, and igniting. In hydrocarbons, the temperature at which this happens is largely dependent on the porousness and surface area of the material relative to its mass. So it is that you cannot light a solid log with a match.

But, the log as wood chips--the same quantity of matter with many times more surface area--is, as they say, kindling.

When a fire burns in an enclosed space--say, a bedroom--a certain amount of the heat is absorbed into the walls. Eventually, the walls radiate the heat back into the enclosed space. The smoke itself, thrown off from what was curtain fire, ignites--this is flashover. Free radicals find each other en masse and produce enough heat to soften steel. Firefighters call the licks of flame which signal the beginning of flashover "angel fingers."

 


Excellent video footage of flashover can be found here. As well as the visual stimuli, you will be provided with a good timeline/scale for the stages of an enclosed residential blaze.

Be aware also that the term "rich flashover" is interchangable with "backdraft." Firefighters tend to differentiate flashover and backdraft stubbornly, as backdraft is caused by the sudden introduction of oxygen rather than simultaneous autoignition of everything in an enclosed space.

 

Log in or registerto write something here or to contact authors.