Compartment fire

A compartment fire is a fire whose development is influenced by an enclosure, its boundaries and its openings. Fuel, combustion and heat release interact with hot gas layers, ventilation and heated surfaces. These interactions distinguish compartment behaviour from an otherwise similar fire in the open.

The compartment can amplify heat feedback by retaining hot gases and radiation. It can also restrict oxygen, causing incomplete combustion and fuel-gas accumulation. Fire development therefore depends on the room as well as the fuel package.

Compartment-fire analysis supports estimates of smoke conditions, flashover, structural exposure and spread to adjacent spaces. It requires a time-dependent description of the fire source, plume, openings and boundary heat transfer. The enclosure can therefore be as influential as the initial fuel package in determining when conditions become hazardous.

Fire source, plume and enclosure

The fire source supplies energy, fuel gases and combustion products. Its heat release rate depends on material decomposition, mass loss and combustion efficiency. A fuel inventory alone does not specify this source because heat release rate varies with ignition, spread and ventilation.

Compartment fire as a coupled mass and energy system. Based on Quintiere (2002), pp. 3-162–3-163.

A buoyant plume rises from the source and entrains room air. The plume increases in mass flow while becoming more diluted, then transports mass and energy towards the ceiling. Its behaviour depends on heat release, source geometry and restrictions such as walls.

At the ceiling, the flow turns into a ceiling jet and begins to form an upper layer. The enclosure thus changes a vertical plume into horizontal smoke movement and boundary heating. Free-plume relationships no longer apply unchanged once the plume enters this layer.

Walls, floor and ceiling receive energy by convection and radiation and conduct part of it away. These losses influence gas temperature while heating the construction. Boundary conditions therefore connect compartment fire dynamics with structural and separating performance.

Fire development stages

Fire development stages commonly include growth, fully developed burning and decay. They provide a useful sequence for describing changing fire power and enclosure conditions, but not every fire passes through every stage. Suppression, fuel depletion or unsuitable heat feedback can end development earlier.

During early growth, the fire is often local and fuel controlled. Sufficient oxygen is available, so heat release depends mainly on fuel-gas production and spread. Hot gases accumulate above a cooler lower layer, producing a stratified enclosure.

As additional objects ignite, heat release and upper-layer temperature can increase. Radiation from hot gases and heated boundaries preheats more fuel, producing stronger feedback. This can lead to flashover, although it is not inevitable.

Decay follows fuel depletion or suppression and can include a prolonged period of hot surfaces and smouldering. The end of open flaming does not immediately remove heat or reignition hazards. Compartment conditions therefore remain important after peak power.

Hot gas layer and stratification

The Hot gas layer is the upper region of heated smoke and entrained air fed by the plume. A two-zone model represents it as having uniform temperature and composition above a cooler lower zone. These uniform properties are modelling assumptions rather than exact room measurements.

The layer affects occupants and objects through hot gases, reduced visibility, toxicity and radiation. It also exchanges heat with walls and ceiling and supplies gas to openings. Layer depth, temperature and composition therefore influence several fire-safety outcomes simultaneously.

A stable interface can separate upper and lower regions in the model. Real interfaces can be disturbed by mixing, obstacles, vent flows and plume pulsations. Strong mixing can instead produce approximately homogeneous conditions that are better represented by one zone.

The plume acts as a pump transferring mass from lower to upper zone. Conservation of mass and energy tracks this input alongside vent flow, fuel supply and boundary losses. This system view prevents the layer from being treated as an independent smoke reservoir.

Fuel and ventilation control

A fuel-controlled fire has enough oxygen for its current fuel-gas production, so fuel properties and exposed area dominate heat release. Early local fires commonly have this form. The same applies to outdoor fires in the source's basic description because oxygen is not restricted by an enclosure.

A Ventilation-controlled fire is limited by oxygen availability. Restricted openings can prevent enough fresh air entering and enough smoke leaving, so heat release depends on ventilation flow rather than fuel inventory alone. Unburnt pyrolysis gases can then accumulate.

The transition between these forms depends on fire size and opening conditions. A growing fire can move from fuel control to ventilation control as its oxygen demand increases. Door and window changes can reverse or alter this balance during the incident.

Fuel-controlled and ventilation-controlled fires. Based on Van Rossum (2027), §4.8.

Some volatile gases can leave the compartment and burn outside when they meet fresh air. This external flaming shifts heat release and exposure beyond the enclosure. Total heat generated by the original fuel therefore need not equal heat released within the room.

An opening-based power estimate

For a ventilation-controlled fire, Drysdale estimates the incoming air mass flow from the effective opening area and height. If all oxygen in that air is consumed inside, the internal heat release follows from the energy released per unit mass of air. This is a restricted natural-ventilation approximation, not a general prediction for arbitrary openings or mechanical systems. $${\dot{m}}_a\approx 0.52A_v\sqrt{h_v},{\dot{Q}}_{\mathrm{i}\mathrm{n}\mathrm{s}\mathrm{i}\mathrm{d}\mathrm{e}}\approx {\dot{m}}_a{H}_a$$

Symbol	Description	Unit
${\dot{m}}_a$	Incoming air mass flow	kg/s
$A_v$	Effective ventilation opening area	m²
$h_v$	Opening height	m
$H_a$	Heat released per mass of air consumed; approximately 3,000	kJ/kg
${\dot{Q}}_{\mathrm{i}\mathrm{n}\mathrm{s}\mathrm{i}\mathrm{d}\mathrm{e}}$	Estimated heat release inside the compartment	kW

Babrauskas describes a room-fire test doorway 0.8 m wide and 2.0 m high. Using that geometry as an illustration gives an opening area of 1.6 m². The air-flow approximation gives 0.52 × 1.6 × √2 = 1.18 kg/s, and the corresponding internal power is approximately 3.53 MW.

The calculation assumes enough fuel is available to consume the incoming oxygen. It can overestimate a fuel-controlled fire, where excess air passes through without supporting additional combustion. It also leaves out heat released by fuel gases burning beyond the compartment opening.

This 3.53 MW result is not the power required for flashover. Flashover concerns enclosure-wide heating and ignition, whereas the calculation estimates a ventilation-limited burning rate. Boundary heat losses, fuel placement and thermal feedback remain necessary to describe the transition and the gas temperature.

Flashover and post-flashover fire

Flashover is the rapid transition towards ignition of combustible materials throughout the compartment. Radiation from the upper layer and heated boundaries can drive widespread pyrolysis and ignition. The enclosure then moves from local to fully developed involvement.

The transition is associated with criteria such as upper-layer temperature and received heat flux, but no single universal threshold exists. Cadorin and Franssen report broad experimental ranges and distinguish near-ceiling measurements from layer averages. Model criteria must be interpreted accordingly.

A Post-flashover fire commonly involves abundant pyrolysis gases and ventilation-limited combustion. External flaming can be prominent, and boundaries experience sustained high temperatures. This stage is particularly relevant to structural fire resistance and fire service exposure.

Local and developed compartment-fire representations. Based on Cadorin and Franssen (2003), pp. 399–401 and Van Rossum (2027), §4.11.

One-zone models approximate this developed state as a uniform hot compartment, while two-zone models represent earlier stratification. Switching between them is a modelling choice that should conserve mass and energy. Real rooms do not necessarily become perfectly mixed at the same instant.

Compartmentation and spread

Fire compartmentation uses enclosing constructions to limit fire and smoke spread between spaces. Its performance depends on barrier integrity, stability, conduction and openings. A compartment fire tests all of these functions through heat, pressure and combustion products.

Conduction can expose combustibles on the unexposed side without visible barrier failure. Cracks and failed firestopping can allow flames and hot gases through, while collapse creates larger openings. These routes require separate timing and assessment.

Openings also control the fire within the original compartment. Doors and windows change oxygen supply, pressure and external flaming, so their state affects both development and spread. Evacuation and fire service actions can therefore alter the fire scenario.

Adjacent spaces can become new compartments with their own plumes and layers. The time of spread determines when their design fires begin and when evacuation or intervention conditions change. Compartment analysis is thus part of a building-wide sequence rather than a single-room calculation.

Fire dynamics and modelling

Fire dynamics supplies the physical framework for compartment analysis. Fuel chemistry and decomposition provide the source, heat transfer provides feedback, and buoyancy and pressure drive transport. A compartment model combines these rather than treating any one as the complete fire.

Zone models divide the enclosure into uniform regions and apply conservation laws to each. Submodels describe plume entrainment, combustion, vents and wall heat transfer. They can efficiently estimate average conditions when the assumed zones and geometry remain appropriate.

Many zone models neglect plume volume and transport time, treat contents heat capacity as negligible and use simplified pressure and friction assumptions. These choices can matter in large, cluttered or rapidly changing compartments. The user must assess their effect rather than treating model output as direct observation.

OZone combines two-zone and one-zone representations with coupled wall calculations and several combustion options. It illustrates how a model can cover localised and developed fires, but its default criteria are informative rather than universal. Application still requires scenario-specific inputs and review.

Conservation provides a first review of any compartment model. Fuel and entrained air entering the system should be balanced by accumulation and outflow, while combustion energy should appear in gases, boundaries, radiation or other modelled losses. An impossible inventory can indicate an inappropriate vent or plume assumption.

Model selection should follow the physical state. Two zones can represent a stratified local fire, while one zone can represent a more uniform developed fire. Switching because a criterion is convenient rather than physically appropriate can change layer temperature and exposure without a valid reason.

Opening data require care because geometric and effective flow areas differ. Van Rossum describes contraction and obstacles reducing the effective opening through which gas passes. Using the measured open area without the relevant coefficient can therefore overestimate the available ventilation path.

Fire-source data should be matched to the enclosure question. Open-burning tests omit room feedback, while room tests include it and can produce different heat release. A compartment model should not silently use one as though it were the other.

Sensitivity analysis can test heat release rate, opening size, layer criterion and boundary properties. These inputs affect both the timing of developed conditions and the resulting exposure. A conclusion that reverses after a plausible change should be reported with that dependence.

The output should then be compared with the appropriate performance measure. Smoke-layer height and composition support tenability assessment, gas and wall temperatures support structural assessment, and spread timing supports compartmentation review. These outputs should not be substituted for one another.

Validation is strongest when the comparison uses the same physical arrangement and measured quantity. A near-ceiling temperature is not directly comparable with a layer average, and an open-calorimeter curve is not directly comparable with a room-fire curve. Matching these definitions is part of model evaluation.

Smoke assessments use compartment models to estimate layer height, temperature, composition and vent flow. Detector and sprinkler studies use plume and ceiling-jet exposure, while structural studies use gas and boundary temperatures. These outputs serve different decisions and require suitable time resolution.

Evacuation analysis compares changing compartment conditions with the time occupants need to respond and escape. Structural analysis may instead focus on sustained developed exposure. A single design time or peak value cannot represent every purpose without checking its relevance.

Comparisons with tests can reveal missing physics or inappropriate data. Cadorin and Franssen compare OZone with full-scale tests to evaluate its compartment model, while noting the assumptions behind its submodels. Validation is therefore tied to scenario and submodel quality.

Limits and common misunderstandings

A compartment fire is not just a larger open fire placed indoors. The enclosure changes heat retention, oxygen supply and the location of combustion. The same fuel can consequently produce different heat release and exposure histories.

A hot upper layer is not perfectly uniform in reality. Temperature and composition vary between plume and room edges, and interfaces can mix. Uniform zones are useful approximations whose suitability depends on the compartment.

Flashover is not guaranteed to occur, and a fully developed fire is not always fuel limited. Fuel can be abundant while oxygen controls heat release. Describing the fire only by inventory misses this distinction.

external flames do not mean the original compartment fire has ended. They can be part of its developed stage, fed by gases produced inside. The whole connected system must be assessed.

Documentation should make the model boundaries visible. The fire source, gas zones, vents and wall surfaces each have inputs and outputs, and external combustion may require another region. Identifying those boundaries prevents internal heat release, total fuel energy and adjacent-space exposure from being combined incorrectly.

See also

• Fuel-controlled fire
• Fire-induced pressure
• Zone model

References

• Australian Building Codes Board, National Research Council of Canada, International Code Council, & Department of Building and Housing New Zealand. (2005). International fire engineering guidelines (2005 ed.). Australian Building Codes Board.
• Babrauskas, V. (2002). Heat release rates. In P. J. DiNenno (Ed.), SFPE handbook of fire protection engineering (3rd ed.). National Fire Protection Association.
• Cadorin, J.-F., & Franssen, J.-M. (2003). A tool to design steel elements submitted to compartment fires: OZone V2. Part 1: Pre- and post-flashover compartment fire model. Fire Safety Journal, 38, 395–427.
• Drysdale, D. D. (2002). Thermochemistry. In P. J. DiNenno (Ed.), SFPE handbook of fire protection engineering (3rd ed.). National Fire Protection Association.
• Heskestad, G. (2002). Fire plumes, flame height, and air entrainment. In P. J. DiNenno (Ed.), SFPE handbook of fire protection engineering (3rd ed.). National Fire Protection Association.
• Ohlemiller, T. J. (2002). Smoldering combustion. In P. J. DiNenno (Ed.), SFPE handbook of fire protection engineering (3rd ed.). National Fire Protection Association.
• Quintiere, J. G. (2002). Compartment fire modeling. In P. J. DiNenno (Ed.), SFPE handbook of fire protection engineering (3rd ed.). National Fire Protection Association.
• Rockett, J. A., & Milke, J. A. (2002). Conduction of heat in solids. In P. J. DiNenno (Ed.), SFPE handbook of fire protection engineering (3rd ed.). National Fire Protection Association.
• U.S. Nuclear Regulatory Commission. (2005). Fire dynamics tools: Quantitative fire hazard analysis methods for the U.S. nuclear regulatory commission fire protection inspection program (NUREG-1805).
• Van Rossum, E. (2027). Introduction to Fire Safety Engineering. Fire Safety Academy.