The two-zone model is one of the most common, simplified models for the development of the early stages of an enclosure fire. N.D. Fowkes first advanced the model in 1974 in an attempt to better understand the development of enclosure fires as they grew to flashover. Over the years, the two-zone concept of early fire behaviour has led to the development of computer software that can be used to better understand fire growth and severity within an enclosure.

The fundamentals of the two-zone model are shown schematically in Figure 15. The fire is defined as a fire plume of specified size within an enclosure. As the fire burns, it is assumed that two separate zones, each well-mixed and therefore at a uniform temperature, develop within the enclosure. The lower zone consists of a volume of cool, fresh air. The upper zone consists of a volume of hot smoke generated by the fire plume. The size and temperatures of the different volumes will vary depending on the fire HRR (fuel and fire size), the ventilation pathways, heat losses to the enclosure surfaces and energy losses from the fire enclosure due to outflows of hot smoke.

Following this model, each feature of the fire is discussed separately in the following sections.

Flame Plume

The steadily-growing fire that is established in the early stages of an enclosure fire often involves the burning of only a single fuel item. At this stage, the fire is in a fuel-controlled state. Local to the burning fuel surface is the flame plume, which is a distinct local volume of flaming combustion.

The type of fuel, as well as the diameter and height of the flame plume, are linked to the HRR, rate of growth and potential spread of the fire. Energy from the flame plume preheats nearby materials (fuels) by radiation heat transfer in this early stage of fire growth, but because the flaming region is quite small, this is not the primary means of heat transfer at this stage in the fire.

Fire Plume and Hot Layer

The full fire plume in the earliest stages of fuel-controlled fire growth includes the area from the base of the flame plume (fuel surface) back to a location where the smoke plume returns to ambient atmospheric conditions. Therefore, it includes both the flame plume and the visible smoke plume comprised of hot gases mixed with airborne solid particulates and liquid aerosols.

As the original fuel vapour and air mix and react in the fire plume, the combustion products increase in temperature. This causes the gases in the flaming zones and smoke to increase in volume and become less dense than the surrounding air. They thus rise upward due to buoyancy in what is commonly referred to as the "thermal plume" or "convection column" of the fire plume.

The smoke in the fire plume accumulates in the hot upper layer of the enclosure. At this stage in the fire, the primary mode of heat transfer is through convection to the surrounding areas from the hot gases flowing along ceiling and upper wall surfaces. The cool lower layer near the floor of the enclosure contains fresh air. The natural formation of a volume of hot smoke at the ceiling and cooler fresh air near the floor has led to the common two-zone description and model of early fire development.

The flame and fire plumes are the basis for many correlations for flame height and HRR, as well as velocity and temperature of hot gases accumulating within an enclosure in the early stages of a fire. These form the background for design of sprinklers and smoke detection systems.

Ceiling Jet and Smoke Spread

As the plume of hot smoke in the fire plume encounters the ceiling of the enclosure, it has to change direction and travel across the ceiling in what is known as a ceiling jet.

Ceiling Jet: A relatively thin layer of flowing hot gases that develops under a horizontal surface (e.g., a ceiling) as a result of plume impingement and the flowing gas being forced to move horizontally.

Upper Layer and Smoke Spread

Over time, the hot smoke collects in a thick, relatively uniform layer throughout the upper levels of the enclosure. This is commonly referred to as an "upper layer."

Upper Layer: Buoyant smoke and gases collected by the ceiling walls of an enclosure that begin to form a relatively uniform layer throughout the upper layers of a compartment.

Once the upper hot layer is formed, the additional flow of gases from the fire continues to collect along the ceiling, forcing the bottom edge of the upper layer volume to descend downward into the room as the fire HRR and upper layer temperatures increase. Decreasing the HRR of the fire within the enclosure will decrease the rate of formation of hot gases that collect in the upper layer, as well as their temperature. If the HRR is reduced for some time, this can result in a contraction of the upper layer volume, a decrease in the depth of the hot layer and therefore an increase in the height of smoke within the enclosure. Directly decreasing the temperature of the upper layer (e.g., through application of water) will also result in a contraction of the upper layer volume as a result of the hot smoke cooling.

Lower Layer

Lower layer: The lower zone of a two-zone model consisting primarily of ambient air that is entrained into a fire.

The cold lower layer consists of air, at temperature and humidity levels close to ambient conditions, drawn in through ventilation openings in the enclosure. As the hot gases rise in the buoyant plume, the cooler air is pulled into the enclosure towards the fire plume. This drives the air entrainment that feeds air into the fire.

Interface Layer and Interface Height

The zone along the bottom of the upper layer and top of the lower layer is called the "interface layer." This layer can be represented as a sharp transition from the hot smoke in the upper layer to the cool ambient air of the lower layer (determined by temperature measurements, for example). In reality, the interface layer can be it's own zone due to turbulent mixing between the two zones and not simply a boundary between the hot and cold zones.

Interface height: The vertical distance from the fire source to the lower boundary of the upper layer.

Alternatively, this height can also be referenced from the floor of the enclosure.

Neutral Plane

The flow through an opening is described as either unidirectional or bidirectional. Unidirectional flow occurs when the flow through the opening is either entirely into or out of the enclosure. Bidirectional flow occurs when an opening acts as both an inlet and an outlet for flows at the same time.

Neutral plane: The horizontal line along the ventilation opening where no flow occurs due to the equality of internal and external pressures.

The volume of hot smoke generated by the fire largely determines the pressure distribution in an enclosure under fire conditions. As the hot smoke rises and accumulates, the pressure at the ceiling will rise. Similarly, the pressure at the floor will be lower, as cool air is entrained into the fire plume. Above the neutral plane, the pressure inside the enclosure will be higher than the exterior pressure, and the direction of flow will be outwards. Below the neutral plane, the direction will be inwards.

The horizontal line between the inflow and outflow at a ventilation opening (i.e., at the plane where there is zero pressure difference and thus zero flow in or out of the opening) is known as the neutral plane. The position of the neutral plane can often be observed during a fire, given that the outflow often consists of visible smoke (due to aerosols, particulates or vapours). In contrast, the inflow consists of clear air:

a

b

Figure 20: Descending neutral plane in a test fire

In Figure 20a, the neutral plane can be observed high up, near the top of the opening. As the fire grows and smoke fills the upper layer, it flows out of the opening and the neutral plane descends lower and lower into the opening. After the room flashes over to a fully developed fire (Figure 20b), the neutral plane is located at ⅓ to ½ of the opening height. The neutral plane in an opening is often around this height when the interface layer has dropped to the floor of the enclosure.

Knowledge Check What two factors cause a fire to enter the decay stage? What is the interface layer? What is the neutral plane? The flame plume + smoke plume form what? As the fire grows, the neutral plane does what?