Flashover

Flashover is the rapid transition in which combustible materials throughout an enclosure ignite or become involved, changing a local fire into a fully developed compartment fire. It is associated with accelerating heat feedback, especially radiation from the hot gas layer and heated boundaries. The transition changes both the scale of burning and the conditions throughout the enclosure.

Before flashover, the fire is generally local and fuel controlled. Afterwards, most or all combustible surfaces can be involved and ventilation commonly restricts combustion. This change can produce abundant pyrolysis gases, external flaming and much higher exposure of boundaries and occupants.

Flashover is not defined by one universal temperature or heat release rate. Criteria based on upper-layer temperature, received heat flux, lower-zone temperature and fire power describe different measurements and models. Their source and location must accompany any threshold used in an assessment.

Local fire to whole-compartment involvement

An early compartment fire can involve one object or a limited number of objects. Heat release depends largely on fuel-gas production while sufficient oxygen remains available. Hot gases rise in a plume and collect near the ceiling, forming an upper layer.

As that layer and surrounding surfaces become hotter, they radiate energy to contents below. Convective heating and direct flame contact can add to this exposure. Additional objects then begin to pyrolyse and ignite, increasing heat release and reinforcing the heating process.

This feedback makes flashover a transition rather than merely one more object igniting. The enclosure becomes the driver of widespread involvement, and a local-fire description no longer represents the dominant conditions. The whole fuel inventory and its exposure become relevant.

Flashover changes local burning into developed compartment fire. Based on Van Rossum (2027), §4.7 and §4.11.

Fire development stages commonly describe this as the change from growth to fully developed fire. The labels are useful for modelling, but the physical transition can be gradual in some scenarios and more abrupt in others. Ignition of every item at exactly the same instant is not required for engineering use of the concept.

Hot gas layer and radiative feedback

The Hot gas layer accumulates combustion products and entrained air transported by the plume. Its temperature and depth influence the radiation received by the floor and lower objects. Heated walls and ceiling add further emission and change the net energy balance.

Van Rossum uses an upper-layer temperature of 500 °C as one criterion, corresponding to radiation of approximately 20 kW/m² in his calculation. Many materials pyrolyse rapidly at that exposure. The example links layer temperature with floor-received heat flux rather than treating temperature as an isolated number.

A second criterion in the same source concerns lower-zone temperatures of about 300 to 350 °C. At those conditions, many combustible materials in the lower layer can become hot enough to ignite. This describes a different measurement location from the upper-layer criterion.

Radiation depends on more than gas temperature. Emissivity, view between surfaces, participating gases and receiver properties affect net exchange. The book's example assumes a view relationship in which upper-layer radiation also reaches floor materials, so the same temperature need not produce the same exposure in every geometry.

Flashover criteria

Flashover criteria are measurable conditions used to identify the transition in a test or model. They can include layer temperature, heat flux to fuel, lower-zone temperature and heat release rate. A criterion is a practical indicator of a complex process, not a separate law governing every compartment.

Cadorin and Franssen report upper-layer temperatures of 450 to 800 °C across collected studies, with most between 600 and 700 °C, and heat fluxes between 15 and 33 kW/m². They also note that 600 °C and about 20 kW/m² are commonly used. These ranges demonstrate uncertainty and differences between experimental definitions.

Collected flashover criteria and common model values. Based on Cadorin and Franssen (2003), p. 412.

Their OZone model uses 500 °C as its default average upper-layer criterion. The authors explain that temperatures near the ceiling can be higher than the volume-averaged layer temperature used by a two-zone model. Comparing a ceiling measurement directly with that average would mix different quantities.

Heat-release criteria require equal caution. Babrauskas gives roughly 1.5 to 1.75 MW as the range where flashover can be reached for room fires ventilated through one normal-sized door. This example is tied to the test configuration and should not be treated as a universal fire-power threshold.

Fire growth and heat release

Heat release rate influences how quickly the plume and upper layer develop. Increasing power can raise gas temperature and radiative feedback, which then accelerates additional ignition. Fire growth and enclosure heating therefore form a coupled process.

Design fires sometimes represent growth with a t-squared curve followed by fully developed burning and decay. These simplified curves are modelling choices for evaluation, not predictions of when flashover must occur. Incubation, object-to-object spread and changing combustion efficiency can alter the real sequence.

Fuel arrangement affects the transition. Thin or easily ignited objects can become involved earlier, while thick objects can absorb energy internally. Separation and orientation change received radiation and convective exposure, so identical total fire load can produce different development.

Suppression during growth can prevent the developed stage. Van Rossum notes that active systems commonly intervene before flashover and can interrupt increasing heat release. Flashover is therefore not an inevitable stage of every compartment fire.

Rollover and backdraft

Rollover is a separate enclosure-fire term and should not be used interchangeably with flashover. Flashover describes widespread involvement of combustible materials, whereas the present article reserves backdraft for rapid ignition of accumulated combustible gases after oxygen enters. These labels address different mechanisms.

Backdraft requires a fuel-rich atmosphere created under oxygen restriction and a subsequent ventilation and mixing event. Flashover instead concerns the transition to widespread fuel involvement through enclosure heating. One can influence the conditions for the other, but their definitions remain distinct.

A fully developed fire can have abundant pyrolysis gases and external burning, which makes these distinctions especially important. External flames alone do not identify whether the enclosure underwent flashover, backdraft or another process. The preceding history is needed.

Post-flashover conditions

A Post-flashover fire involves the whole compartment environment and is often ventilation controlled. Limited air supply can restrict internal combustion while pyrolysis continues. Unburnt gases can then burn outside openings after mixing with fresh air.

External flaming shifts part of the heat release to facades, adjacent spaces and neighbouring objects. It also means that heat release measured inside the compartment need not equal total heat generated by fuel originating there. System boundaries must be stated in the analysis.

Post-flashover combustion can occur inside and outside the room. Based on Van Rossum (2027), §4.11 and Drysdale (2002), p. 1-95.

Gas conditions can become more uniform, motivating one-zone rather than two-zone models. OZone switches between these representations while conserving mass and energy. The switch is a modelling approximation and does not mean that real compartment properties become perfectly uniform.

High gas and surface temperatures can threaten structural elements and separating constructions. Sustained exposure after the transition can be more important for structural assessment than the short growth period. Fire resistance therefore needs the post-flashover temperature-time history.

Modelling and testing

Zone models represent early development with hot and cool layers and developed fire with one mixed zone. Their results depend on the chosen flashover or switch criterion. Different criteria can change when the model transitions and what exposure it predicts.

Room-fire tests include enclosure effects that open-burning calorimetry does not capture. Hot-layer radiation and ventilation can change heat release rate significantly. Test enclosure size, openings and fuel arrangement must therefore accompany any reported flashover observation.

The Room corner test provides a standardised enclosure arrangement for evaluating fire development. Results from such tests belong to their method and cannot be transferred to every room without assessing scale, ventilation and fuel configuration. Flashover criteria should be identified within the test report.

Glazing and openings can change during the transition. The 2005 International Fire Engineering Guidelines allow time of glass breakage to be estimated from fire severity or, in a simplified method, linked to flashover. Breakage affects both spread and ventilation, so it should not be treated as an isolated visual observation.

Limits and common misunderstandings

Flashover is not a synonym for ignition. Ignition marks the start of self-sustaining combustion, while flashover is a later enclosure-scale transition. A room can have a sustained local fire for a substantial period without reaching flashover.

It is also not the same as a backdraft or smoke explosion. Those events involve rapid combustion of a combustible gas atmosphere, while flashover concerns widespread involvement of fuel through heating. Their timing and required conditions differ.

No single temperature defines flashover in every enclosure. Average layer temperature, ceiling temperature, lower-zone temperature and received heat flux are different measurements. Reported thresholds must retain their source and measurement location.

Finally, flashover is not inevitable. Fuel depletion, suppression, insufficient heat feedback or unsuitable geometry can prevent it. Design scenarios should demonstrate why the transition is expected rather than inserting it automatically.

A criterion map can clarify what a model or test actually measures. Upper-layer average temperature, near-ceiling temperature, lower-zone temperature, received heat flux and total heat release rate occupy different places in the analysis. Treating them as interchangeable thresholds would confuse the evidence for the transition.

The averaging distinction is particularly relevant to two-zone models. Their upper-layer value is an average over the whole layer, not the hottest gas immediately beneath the ceiling. A criterion taken from a ceiling measurement should therefore be evaluated before being applied to that average.

Heat-flux criteria also depend on the receiver. View factor, surface orientation, emissivity and the temperature of surrounding surfaces affect net radiation at fuel. Two rooms with the same upper-layer temperature can therefore produce different ignition responses at their contents.

Fire-power criteria belong to their ventilation arrangement. Babrauskas's approximate 1.5 to 1.75 MW example concerns room flashover with one normal-sized door. Larger or smaller openings can change oxygen supply and external burning, so the same power has different implications elsewhere.

Test selection should reflect the stage being examined. Open-burning data can describe early item behaviour without room feedback, while room tests are needed when layer radiation and ventilation alter heat release. Using only open data for a developed Compartment fire would omit a defining part of the problem.

Post-flashover analysis should state where combustion occurs. Fuel volatiles can burn inside, outside or in both regions, and external flames can expose facades and neighbouring objects. The model's heat-release boundary must include these locations consistently with the scenario.

Opening changes can link several stages. Glass failure can create a new vent and flame path, alter oxygen supply and mark a change in external exposure. Using flashover as a simplified proxy for breakage is a modelling choice from the 2005 guidelines, not a universal physical equivalence.

A final sensitivity review can vary layer criterion, fire growth, ventilation and fuel arrangement. This reveals whether the predicted transition is controlled by one assumption and whether the selected model remains valid after it occurs. The resulting time range is more informative than an unjustified single flashover time.

The review should also state whether flashover is an input or an output. A model can be told to switch at a selected criterion, while another analysis estimates when that criterion is reached. Confusing these approaches can make a chosen transition look like an independent prediction.

Evidence before and after the transition should remain separated. Pre-flashover plume and layer assumptions support local-fire analysis, while post-flashover conditions support developed exposure and external burning. Combining data from both without identifying the transition creates an inconsistent fire history.

A clear report therefore gives the selected criterion, its measurement basis and the resulting post-flashover treatment. It also states whether the criterion was varied and what happened to the outcome. This allows flashover to be assessed as an uncertain physical transition rather than an automatic stage label.

Flashover should finally be interpreted as one transition within a wider compartment assessment. Earlier growth determines how the criterion is reached, and later combustion determines the exposure that follows. Analysing only the threshold without those stages would omit the timing and consequences that make the concept useful.

Where the transition cannot be supported by the available fire, enclosure or test data, it should remain an open scenario rather than an asserted outcome. This preserves the difference between a physically justified flashover analysis and a design assumption introduced only to produce a severe condition.

See also

• Fire dynamics
• Fire spread
• Glass breakage in fire
• Thermal radiation
• Trench effect

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.
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• Quintiere, J. G. (2002). Compartment fire modeling. In P. J. DiNenno (Ed.), SFPE handbook of fire protection engineering (3rd ed.). National Fire Protection Association.
• Quintiere, J. G. (2002). Surface flame spread. In P. J. DiNenno (Ed.), SFPE handbook of fire protection engineering (3rd ed.). National Fire Protection Association.
• Tien, C. L., Lee, K. Y., & Stretton, A. J. (2002). Radiation heat transfer. 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). Author.
• Van Rossum, E. (2027). Introduction to Fire Safety Engineering. Fire Safety Academy.