In the early stages of the fire when there are still distinct upper and lower layers and a localized fire plume, the rate of fire growth is usually unaffected by the availability of oxygen. Instead, the fire is fuel-controlled, with its rate of growth limited by the availability of fuel. The fire grows as long as fuel can be vaporized in sufficient quantity and locally mixed with air at a temperature high enough to support combustion in the fire plume. The HRR of the fire at any given time depends on the mass-burning rates and flaming areas of the different fuels that are involved in the fire. These then dictate the amount of heat energy generated by the fire as well as the temperature distribution in the enclosure.

Convection is the primary mode of heat transfer during these early stages of fire growth, since the fire plume is small and radiation heat transfer is limited. The hot gases exchange heat by convection as they flow upwards and across the ceiling, heating it and any other surfaces with which they come into contact.

To understand what can happen later in the fire development, however, it is crucially important to recognize that the hot smoke that is collecting near the ceiling is also emitting thermal radiation to its surroundings throughout the life of the fire. Thermal radiation from the ceiling and walls back into the fire enclosure increases in intensity as the surfaces heat up. This combines with radiation from the upper layer, leading it to preheat, begin to pyrolyze and produce fuel vapours from any combustible items in the enclosure. Radiation therefore has a major impact on the fire environment as the fire continues to grow.

Fire Growth and Spread

As the fire grows, it may spread by ignition of objects via any combination of radiation, convection and conduction (e.g., from the fire plume, hot objects or hot ceiling gases), as shown in Figure 24. One mechanism is through direct impingement of the flame plume onto any secondary flammable materials or objects, causing them to ignite and become involved in the fire. Alternately, the first item ignited (e.g., foams, plastic materials) can melt and then pool, drip or fall down to ignite secondary items. Very often, however, ignition of other objects occurs predominantly through the enhanced radiation, as noted above. This includes both radiation stemming directly from the flame plume, as well as that from heated smoke in the upper layer.

As the fire develops to this state, pockets of flame may begin to form in the hot layer as the first indication that unburned fuel in the hot smoke layer may be coming close to its auto-ignition temperature. If it does ignite, a rollover (travelling flames) occurs in the unburned fuel gases in the hot layer, enhancing the radiation loading down onto the materials below. The flames can also travel along the hot layer and out of the enclosure into any areas in which there is a flammable mixture, sometimes extending into areas well outside the fire enclosure.

Rollover is the condition where unburned fuel from the originating fire has accumulated in the ceiling layer to a sufficient concentration (i.e., at or above the lower flammable limit) so that it ignites and burns; this can occur without ignition of or prior to the ignition of other fuels separate from the origin.

During this stage of fire development, any combination of flame impingement coupled with heat transfer from the fire or other flaming zones (rollover) can play a role in the spread of fire within an enclosure or outside to other areas of the structure.

Flashover: Transition of Fuel-Controlled to Ventilation-Controlled Behaviour

As the fire continues to grow, temperatures continue to increase throughout the enclosure due to ongoing heat transfer and flame spread, as depicted schematically in Figure 25. This can lead to a phenomenon commonly referred to as "flashover". This is the transition of the fire from a growing, fuel-controlled fire to a fully developed, ventilation-controlled fire.

Flashover is a transition phase in the development of a compartment fire in which surfaces exposed to thermal radiation reach ignition temperatures more or less simultaneously and fire spreads rapidly throughout the space, resulting in full room involvement of the compartment or enclosed space.

Flashover can happen in one of two ways: either as a relatively slow transition of the fire to a fully developed fire; or through a very rapid and dangerous change in behaviour of the fire in which the entire enclosure is engulfed in flames virtually instantaneously. In either case, the fire transitions to a fully developed fire characterized by an approximately steady HRR (often the maximum HRR observed during the fire) and uniform temperature (high combustion temperature) throughout the enclosure. Fire resistance ratings and spatial separation between buildings are determined based on the fire temperatures that are expected in such post-flashover fires.

The fully developed fire is normally a stage of ventilation-controlled burning. The HRR value is therefore determined by the amount of air available to the fire. There may be flaming on any fuel air interfaces where the air-fuel mixture is correct, and there may also be rolling flames in various locations throughout the enclosure and into the surrounding compartments depending on the quantity of unburned fuel that may be present in the upper layers of smoke, which have migrated throughout the structure.

How a given fire develops from ignition to the point of fully developed behaviour will be linked to the quantity of fuel available as well as the fuel height, orientation and location-all of which impact the HRR of the fire. Fire development is also greatly impacted by the geometry and other characteristics of the enclosure boundaries. The geometry of the enclosure includes the size, shape, area and volume of the fire compartment. The distance to the ceiling of the enclosure, the volume in which hot gases collect in the upper layer, and the geometric relationship between the upper layer and any combustibles will all influence heat transfer and therefore govern how the fire will grow and spread.

The lining materials in an enclosure also have a significant impact on the growth of the fire. The thermal inertia, surface area and thickness of the lining (wall and ceiling) materials affect the retention and/or loss of energy within the enclosure. If the thermal inertia is lower, surface temperatures of compartment walls and ceiling will increase faster, providing accelerating thermal feedback to items within the enclosure. This will lead to higher mass fuel burning rates-and thus heat release rates-of the fire. In this way, lining materials may be a determining factor in whether an enclosure fire will transition through flashover.

Other Factors Affecting Fire Growth

Ambient Temperature

The ambient temperature, including the outside temperature and that of various fuels at their time of ignition, affects how quickly the fire can spread, as well as how heat is transferred in an enclosure fire. Buoyancy forces and movement of the flame and fire plumes are greatly dependent on ambient conditions.

Available Fuel

The fuel sources in the room have a great impact on flashover. Fuel material and height of fuel impact the HRR, temperature of gases in the hot fire plume and hot layer, as well as fire growth rate. Often, fuel sources positioned higher in an enclosure will result in higher temperatures in the ceiling jet when they burn. Alternately, secondary sources of fuel located high in a compartment may absorb more radiant heat from the upper layer and therefore ignite faster than sources that lie closer to the ground.

Geometry

The size, shape, area and volume of a room affect the formation of the upper layer and thus heat transfer within a given fire enclosure. A flashover may occur more slowly in enclosures that have peaked or cathedral ceilings, since these features make it difficult for ceiling jets to form, slowing down the collection of gases in the upper layer and limiting the amount of radiant heat produced and fed back towards the fire and other fuels in the enclosure.

Ventilation

Access to a fresh source of oxygen is essential for continued combustion. As such, limiting the ventilation to an enclosure can slow or stop the growth of the fire, even leading to early decay. However, when this occurs at a time when there has been a large accumulation of hot smoke in the upper layer, extreme care must be taken. This is because if additional ventilation is introduced into the compartment after the fire has entered an under-ventilated state, various rapid fire developments can occur. This extremely dangerous situation is discussed in further detail in Section 4.5.

When the layer of hot smoke within an enclosure grows deep enough to reach an opening, such as a door or window, the smoke flows out through the top of the opening into the adjacent space. The increased volume of the hot gases in the smoke and their accumulation in the upper layer result in a small but noticeable pressure increase inside the compartment. Since gases follow the path of least resistance, the pressure difference between the inside and outside of the compartment drives the flow of smoke through each opening. Ambient air will therefore flow back in through the bottom of the opening to replace the gases that have exited the compartment. The demarcation between flows is often visible in a doorframe or window.

During a fire, flow occurs through openings because of vertical pressure differences in the opening as the upper layer builds. For any position(s) in the opening where the pressure is higher inside than outside the compartment, smoke will flow out of the enclosure (outlet). Conversely, cooler air will flow into the compartment (inlet) for places where the opposite is true; the pressure is higher on the outside than the inside of the compartment. Across a doorway, ambient air will flow back into the fire compartment through the bottom of the opening to replace the gases that have exited it. In this way, the flow of hot smoke in a fire compartment will be affected by natural ventilation (windows, the opening and closing of doors). Moreover, it will be affected by the forced ventilation of any mechanical inlet or exhaust systems in an enclosure.