Flashover generalized flash

Mechanism

Heat breaks up the materials (wood, plastics, fabrics…) and produced flammable gases, it is the Pyrolyse. Either the gases burn immediately and feed fire (fire traditional), or they accumulate in a part. If the Air returns regularly in the part, one can have, starting from a certain rate gas/air (Combustible/Comburant), an ignition of all gas. Fire then occupies literally all space, it is the flashover generalized flash.

It is estimated that so that a fire develops in EGE, one needs an initial heat flux from approximately 20 kW/m. One of the fundamental parameters of the heat flux released by the flames is the contribution of air; therefore, ventilation (natural or forced, for example system of air-conditioning, but also operational Ventilation misused) plays at the same time in the production of gas of pyrolysis and in the quantity of combustive available.

On average, a EGE occurs at a temperature of 600°C and it develops a heat flux from approximately 7 M W. The fire of the Interstate Bank with Los Angeles in 1988 released a thermal power of 10 MW. The minimum capacity of a EGE is estimated at 3 MW.

This is to be compared with the power absorptive by a fire hose (diffused jet):

0,5 MW with 40 L/min;
2 MW with 150 L/min;
6 MW with 500 L/min

Intervention

The presence of “rollers of flames” (English roll over ) to the ceiling is the precursory sign of the flashover generalized flash: the air burns there by forming waves. To sprinkle the ceiling with water out of diffused jet informs about the temperature of smoke; if it is formed droplets, smoke is not hot enough so that the flashover generalized flash takes place, on the other hand if water vaporizes, the risk of flashover is important.

To avoid the generalized flashover, it is necessary

to try to evacuate hot smoke by creating a partly high opening (breaking of pane, breaking of roof) or by using the discharge system (trap doors on the roof actionable of bottom) envisaged for this purpose on the recent buildings; watering through the opening from the interior can create a phenomenon of aspiration which accelerates the evacuation of smoke,
: let us note that smoke leaving by the high opening can it also ignite, it thus should be cooled to prevent that fire does not take in roof;
to sprinkle smoke in order to cool them, for example by making Z, O or T with the diffused jet (Technical of the crayonnage);
to control the flow of entering air: to cut ventilation and air-conditioning, to close the doors.

For ventilation, for example, if a rescue operation were to be carried out in urgency without water available to decrease the heat flux, it would be necessary to avoid the EGE controlling the flow of air, for example by charging a fireman with holding the door closed to avoid the air intake.

According to a data-processing simulation of fire in a hotel room (David Birks),

open door of 90 cm: EGE at the end of 2 min 23 S;
carries half-opened of 7,5 cm: EGE at the end of 6 min 58 S;
carries closed: no the EGE.

If the air cannot enter the part on fire (local closed), there is then a different risk, the Explosion of smoke.

Testimony

Here a short extract (translated from English) of testimony '' of the captain Mike Spalding (Indianapolis FD, the United States), survivor of a flashover generalized flash which occurred at the time of the fire of the Indianapolis Athletic Club the February 5th 1992 and during which two firemen found death. (Text reproduces under the terms of the Fair uses .) We arrived on the second floor, smoke was thick and dark. It was really very dark. We bent then down, under smoke, and followed the pipes. As we entered, the weather was a little hot, but nothing insupportable. Whereas we follow the pipes towards the exit, smoke became darker, really sinks. So much so that I had my lit flashlight, in addition to my lamp of helmet, and that did nothing there. The weather was black as in a furnace. And very a blow brutally, the conditions changed. Heat became as in a blast furnace. In the darkness, I could see small sparks oranges around me. It made an incredible heat. Incredible. It was as if my jacket of protection were going to take fire. I intended my ears to fry as in a frying pan. That plated me on the ground. The heat of this generalized flashover was like that of a blast furnace. My mask had melted. My jacket of fire had burned.

EGE in the open air

At the time of the forest fires, it can occur in certain cases a EGE, due to the accumulation of a gas pocket of Pyrolyse; one can thus see more than five hectares blazing up instantaneously. If a strong wind pushes fire quickly, it also dissipates the gas pockets and prevents a EGE; there are thus paradoxically fires which progress very quickly in spite of a moderate wind (about 30 km/h).

At the time of this flashover flash, the speed of the face of flame is about 5 to 40 m/s, the temperature rises with 1 500 even 2 000 °C. Combustion typically consumes ten million cubic meters of air; the released hot gas creates an extremely powerful ascending wind and can even lead to a local depression.

The EGE appear in Talweg, peaks or on plates. The harbingers are:

turbid vision;
deafened sounds;
breathing difficulties (one does not carry a breathing apparatus in a forest fire);
pyrolysis of the foliage by radiated heat.

The English-speaking indicate this phenomenon by the terms “flames ascending” ( updraft in opposition to the Backdraft ) or “storm of fire” ( firestorm ).

In the case of a hot area, such as for example the Forest the Mediterranean nne, the phenomenon is worsened by the following mechanism: to be protected from heat, part of the water of the ground is evaporated by the plants (by the Feuille S: evapotranspiration). In the event of Dryness, this evapotranspiration is supplemented by the vaporization of made up organics known as “volatile” (COV): primarily of the Isoprene for the holm oaks and the kermes, Terpene, α- Pinene for the aromatic plants (Rosemary, Thyme) and the pine S. These compounds is flammable, and with sufficiently high contents (about voluminal 1% in the air), they can ignite. With the approach of a fire, the temperature rises and the plants, to fight this rise in temperature, emit even more COV: with 170 °C, rosemary emits 55 times more terpene than to 50 °C. This temperature of 170 °C is a critical temperature which involves a rise in the content of sufficient COV to allow the flashover generalized flash.

In addition, at the time of the fire, these organic compounds mix with gases of pyrolysis, and mixes it gas to pyrolyse/COV/air can quickly reach the lower Limite of explosiveness (DREGS).

The relief has a complex influence. A “closed” relief, confined (of the small valley type, reads dry river) increases the heating and thus the emission of COV, in particular for the Romarin, the Ciste and the Pin of Alep. Contrary, the kermes oak emits more COV in open medium (standard plain, plate).

In addition to the relief, the other conditions supporting the appearance of EGE are an important temperature of the air, about 35 °C in the shade (except fire), a hygroscopy lower than 30% (which increases the hydrous stress of the plants, therefore supports the emission of COV, the evapotranspiration not being more sufficient), and a moderate wind or weak.

But these parameters are not constant, and the analyzed cases do not make it possible to list objective factors of predictibility: one observes as much the phenomenon by strong wind or weak, of day like night, with marked reliefs or on plates.

One generally distinguishes the types of following EGE:

thermal bubble: in a content of small valley rich in fuel, the fuel gas takes the shape of a bubble which cannot mix with the air because its temperature is too high; this bubble is pushed by the wind and moves by chance;
carpet of fire: in an open deep small valley, one attends the instantaneous flashover of all the small valley;
containment by layer of cold air: a strong wind and cold prevent gases of pyrolysis from escaping (phenomenon of type Couche of inversion), which leads to the explosive situation;
pyrolysis of the opposite slope: fire descends a slope, but radiated heat causes the pyrolysis of the plants of the opposite slope which blazes up “spontaneously”;
bottom of small valley: the gases accumulate in a bed of dry brook, the arrival of fire closes the triangle of fire and the bottom of small valley blazes up.

Currently, considering the weakness of the applied research in this field, various tracks are possible:

to try to model in laboratory the phenomenon starting from models representing the places of accidents, to include/understand initially empirically what occurs and to direct, to even validate a model;
to gauge the model in situ , by series of measurements including during the fires, in order to approach the validation of the model;
finally, to seek tracers (predictive parameters) appears a major axis in order to indicate that of them, which, measurable, will be the subject of an individualized detection and will make it possible to give alarm.

Military context

Another type of generalized flashover, of different nature, can be caused by Bombardement S solid masses using incendiary bombs. This phenomenon, known under the name of Feuersturm in the Military context , took place in particular during the bombardments of Dresden and Hamburg in Germany, and of Tokyo and Kobe with the Japan, at the time of the Second world war.

See too

Fire control
so known Explosion of smoke under the name of Backdraft
Fire
Forest fire
Fireman

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