A fire in an enclosure (i.e. a room in a building) can develop in a multitude of different ways, depending on geometry of the enclosure, size/number/location of openings and fuel characteristics (type, amount, and surface area, etc.).
After ignition, the fire grows and produces increasing amounts of energy, mostly due to flame spread. In the early stages the enclosure has no effect on the fire, which then is fuel[1]controlled (the fire is mainly controlled by characteristics of the fuel). Besides releasing energy, a variety of toxic and nontoxic gases and solids are produced.
The hot gases released by the fire will rise upward due to the density difference (buoyancy). This buoyant flow is referred to as a fire plume. This mixture of combustion products (hot gases) will cause a layer of hot gases to be formed below the ceiling. The largest portion of these gases originates from the cool air entrained into the plume as it moves upwards toward the ceiling.
When the plume flow impinges on the ceiling, the gases spread across it and starts to fill up the room with a hot upper layer of fire gases. For a simple geometry, the room becomes divided into two fairly distinct layers: a hot upper layer consisting of a mixture of combustion products and entrained air, and a cold lower layer consisting of air. The larger or more complex the geometry of the enclosure, the more complex this process becomes.
As the hot layer descends and increases in temperature (the fire is releasing energy making the temperature increase). Heat is transferred by radiation and convection from the hot gas layer to the ceiling and walls that are in contact with the hot gases. Heat from the hot layer is also radiated toward the floor and the lower walls. Also, heat is transferred back to the fuel, through radiation from flames as well as from the hot layer and the hot walls and ceiling. This makes the burning rate of the fuel to increase.
If there is an opening to an adjacent room or out to the atmosphere, the fire gases will flow out through it as soon as the hot layer reaches the top of the opening.
If there’s a sufficient amount of fuel, the fire will continue to grow. The temperature in the upper layer increases and as a result of radiation from the hot layer toward other combustible material in the enclosure, there may be a stage where all the combustible material in the enclosure is ignited, with a very rapid increase in energy release rates. This very rapid and sudden transition from a growing fire to a fully developed fire is called flashover.
After a flashover, assuming there’s a sufficient amount of fuel and a sufficient amount of air, the fire becomes fully developed. At this stage, flames extend out through openings and all the combustible material in the enclosure is involved in the fire.
If there are no openings or very small openings in the enclosure, the fire will decay in intensity. There is not enough air in the room and the fire becomes ventilation controlled. Even though the energy release rate decreases, the production of fire gases (pyrolysis) may continue at a relatively high rate, causing the accumulation of unburnt gases in the enclosure. If a window breaks at this point, or if the fire service create an opening, the hot gases will flow out through the top of the opening and cold and fresh air will flow in through its lower part. The fire may then grow toward flashover. As a worst case, the inflowing air may mix up with the unburnt pyrolysis products from the oxygen-starved fire. Any ignition sources, such as a glowing ember, can ignite the resulting flammable mixture. This leads to a very rapid burning of the gases. Due to expansion of the ignited fire gases the combustion may expel the burning gases out through the opening. This phenomenon is known as a backdraft, which can be extremely hazardous.
When unburnt gases from an underventilated fire flow through leakages into an adjacent room, the gases there can mix very well with air to form a combustible gas mixture. If there is an ignition source, such as a small spark, this may cause a smoke gas explosion, which can have very serious consequences.