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Hot gas layer

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Contents
  1. 1Formation by plume and ceiling jet
  2. 2Temperature, density and composition
    1. 2.1Converting layer mass to volume
  3. 3Layer interface and neutral plane
  4. 4Smoke filling and reservoirs
  5. 5Radiation and flashover
  6. 6Tenability and exposure
  7. 7Structural and boundary exposure
  8. 8Modelling and uncertainty
  9. 9See also
  10. 10Notes
  11. 11References

A hot gas layer is an upper region of heated combustion products and entrained air that forms when a fire plume collects beneath a ceiling or other boundary. It can sit above a cooler lower layer, creating a stratified compartment. The layer transports and stores energy and smoke while affecting radiation, vent flow and conditions throughout the enclosure.[1][2]

The layer is fed by plume entrainment and loses mass and energy through openings and boundary heat transfer. Its temperature, depth and composition therefore change with fire growth and ventilation. It is not simply a stationary cloud of products generated directly by the fuel.[3][4]

Hot gas layers are relevant to smoke filling, flashover, structural heating and tenability. They are also a central feature of two-zone fire models, which assign uniform properties to the upper and lower regions as an approximation.[5][6]

Formation by plume and ceiling jet

A fire plume rises because hot gases are less dense than surrounding air. Turbulence entrains additional air, increasing mass flow and diluting the rising combustion products. This process supplies a much larger smoke flow than fuel mass loss alone would produce.[7][3]

A Ceiling jet forms when the plume reaches the ceiling and turns horizontally. The jet spreads hot gases beneath the surface and transfers heat to it and to nearby devices. Its interaction with walls and openings helps establish the upper layer.[3][7]

Plume and ceiling jet feed a hot upper layer above cooler air with wall and vent losses.
Formation of a hot gas layer in a two-zone compartment. Based on Quintiere (2002), pp. 3-162–3-163.

The upper layer grows as plume mass and energy enter it. In a two-zone representation, the plume pumps material from the lower zone into the upper zone. Vent flows and wall losses can remove mass and energy, so layer development depends on the net balance.[2][4]

Free-plume relations no longer apply unchanged once the plume enters the hot layer. Ceiling-jet or zone methods are then required to describe transport and exposure. Treating the layer as an extension of a free plume would mix different regimes.[8]

Temperature, density and composition

Layer temperature results from convective energy input, radiation, boundary losses and mixing with cooler gas. It is not equal to flame temperature. Entrained air lowers temperature while increasing the total mass and volume of smoke.[7][3]

The hot layer is less dense than the cooler gas below, which supports stratification. Temperature and composition influence this density contrast, while pressure differences at openings drive exchange with adjacent spaces. The layer can therefore remain above the lower region while still flowing through vents.[9][10]

Composition includes combustion products and entrained air. Soot, carbon monoxide, carbon dioxide, water vapour and unburnt pyrolysis products can be present, depending on fuel and combustion conditions. Their concentrations affect toxicity, visibility and radiation.[11][12]

Two-zone models assign uniform temperature and composition to each layer. Real layers have gradients and fluctuations, particularly near the plume and room edges. Model outputs therefore represent averages whose usefulness depends on geometry and mixing.[13][2]

Converting layer mass to volume

Van Rossum approximates the density of air at about one atmosphere by dividing 353 by its absolute temperature. This provides a simple connection between the mass supplied by a plume model and the volume occupied by the layer. It treats the smoke mixture as air for the density calculation.[14] ρ≈353T,V=mρ

Symbol Description Unit
ρ Approximate gas density at one atmosphere kg/m³
T Absolute gas temperature K
V Layer volume m³
m Mass in the layer, including entrained air kg

At 20 °C, or 293.15 K, the approximation gives 1.20 kg/m³. At 200 °C, or 473.15 K, it gives 0.746 kg/m³. A given mass therefore occupies more volume when hot, even before additional entrained air is included.[9][14]

For a rectangular room with a horizontal layer, depth is layer volume divided by floor area. Van Rossum's shop example has a 20 m by 15 m floor and a 4 m ceiling. A 1.5 m deep upper layer occupies 450 m³, leaving the idealised interface 2.5 m above the floor.[15]

The 2.5 m result follows from geometry; it is not a tenability limit or an evacuation-time result. The calculation does not specify layer temperature, toxic-gas concentrations or the time needed to accumulate that volume. Those quantities require the fire history, plume flow, exhaust and heat transfer.[15][16][14]

The density approximation also has a defined scope. Large departures from atmospheric pressure or a composition unlike air require a more appropriate gas relation. In a changing fire, temperature and mass evolve together, so constant mass or constant density cannot be assumed without a corresponding model condition.[9][4]

Layer interface and neutral plane

The layer interface is the conceptual boundary between upper hot smoke and the cooler lower region. In a stable two-zone model, limited mixing occurs across it except through defined flows. Its height changes as smoke enters and leaves the upper layer.[5][4]

The Neutral plane is instead the height around which pressure-driven flow through an opening changes direction. Hot gas can leave above it while cooler air enters below. The 2005 guidelines note that neutral-plane height is often near the smoke-layer interface but is not identical to it.[10]

Smoke interface and neutral plane at different heights with hot outflow and cool inflow.
Layer interface and neutral plane are related but not identical. Based on Australian Building Codes Board et al. (2005), p. 2.5-6.

This distinction prevents a smoke-concentration boundary from being used as a pressure boundary without checking. Their positions respond to different physical variables, although they influence one another. Opening flow should be evaluated using the appropriate pressure and density description.[10]

A descending interface can expose more of the room to smoke and increase the layer's contact with walls and contents. If it approaches the lower region or floor, a two-zone approximation can become less meaningful. OZone uses an interface-height criterion among its possible switch conditions.[17]

Smoke filling and reservoirs

Smoke filling is the accumulation of smoke in an enclosure as plume input exceeds removal. The layer descends when more mass enters than leaves, while sufficient exhaust can stabilise or reverse that movement. Fire growth and vent operation therefore shape the time history.[14][3]

A Smoke reservoir is a volume in which hot smoke collects, such as an upper region of a compartment or a larger atrium-like space. Its size affects how much smoke can accumulate before conditions at lower levels change. The plume and exhaust must still be analysed rather than assuming unlimited capacity.[18][19]

Excessive extraction at one point can cause plug holing, where clean air from below the smoke layer is drawn through the outlet. This wastes part of the exhaust capacity and is easier with a thin layer. Van Rossum describes multiple smaller outlets, a thicker layer or altered flow direction as ways to address the effect.[18]

Left, exhaust removes hot layer; right, plug holing draws clean air from below.
Plug holing reduces effective smoke extraction. Based on Van Rossum (2027), §6.8.

Natural and mechanical exhaust also change the pressure field and make-up-air requirement. An exhaust rate without a compatible air-supply path cannot be evaluated properly as a smoke-layer balance. The complete enclosure and openings must be included.[18][19]

Radiation and flashover

The hot layer radiates to lower objects and room surfaces. Its temperature, emissivity, geometry and participating gases influence the heat flux received below. Heated walls and ceiling can add further radiation, strengthening compartment-wide feedback.[20][12]

Flashover is associated with this widespread heating and subsequent ignition of combustible materials. Van Rossum uses an upper-layer temperature of 500 °C as one example criterion, corresponding to approximately 20 kW/m² in his stated calculation. Many materials pyrolyse rapidly under that exposure.[21]

Collected studies show broader criteria. Cadorin and Franssen report upper-layer temperatures of 450 to 800 °C and heat fluxes of 15 to 33 kW/m², with most temperatures between 600 and 700 °C. Their OZone default of 500 °C is an average-layer model value rather than a near-ceiling measurement.[22]

Flashover can change the layer representation itself. A two-zone stratified model can give way to a one-zone mixed model, but the transition does not make real properties perfectly uniform. The model criterion and its averaging basis must be reported.[23][6]

Tenability and exposure

Tenability criteria describe conditions used to assess whether people can remain or escape through a space. Hot-layer temperature, radiation, visibility and toxic gases all contribute. Exposure duration and whether people are below or within the smoke are also relevant.[16]

The height of the layer interface is only one part of this assessment. A person below the layer may still receive radiation from it, while local mixing can introduce smoke into the lower region. A clear geometric space beneath an idealised layer therefore does not establish acceptable exposure.[16][13]

Radiation depends on both layer temperature and the properties of the emitting region. Van Rossum illustrates conservative calculations using unit emissivity and a full view of the hot layer. Those assumptions define an idealised exposure and do not give one temperature limit applicable to every enclosure.[16][20]

When the smoke becomes mixed throughout the occupied region, visibility and inhaled-gas exposure become especially relevant. Criteria selected for a stratified two-layer situation cannot automatically represent that mixed atmosphere. The applicable assessment method defines the exposure quantities and their acceptance conditions.[16]

Structural and boundary exposure

The layer heats ceilings, walls and structural members through convection and radiation. Conduction then distributes energy through those solids, affecting unexposed surfaces and material properties. Layer temperature alone does not determine the member's response.[24][23]

Boundary losses also affect the fire. Energy entering the construction reduces the energy remaining in the gas, while hot boundaries can later radiate back to the compartment. This two-way exchange belongs in the layer energy balance.[25][23]

Openings at different heights expose different layer regions to adjacent spaces. Outflow can remove hot smoke, while inflow can supply cooler air and alter stratification. The layer therefore connects local compartment heating with building-wide smoke and heat spread.[10][26]

Modelling and uncertainty

Two-zone models calculate average layer properties using conservation of mass and energy. Submodels provide plume input, vent exchange, combustion and wall losses. This structure makes the causes of layer growth and cooling explicit.[2][4]

Dynamic models update these properties as fire power and openings change, while stationary models evaluate selected conditions. A stationary result can be useful for a particular assessment time but does not show how the layer formed. The choice should match the required decision.[14]

Common limitations include uniform layers, negligible plume volume and transport time, simple geometry and restricted treatment of wind or mechanical flow. These assumptions can be poor in large, cluttered or strongly ventilated enclosures. Model review should include them alongside input uncertainty.[4][13]

Results should not be given more precision than the fire and layer data justify. Heat release rate, emissivity, interface height and vent coefficients can all be uncertain. Sensitivity analysis and safe-direction rounding help keep those limitations visible.[27]

A layer balance begins with the plume input and ends with accumulation and outflow. Fuel mass, entrained air, vent transport and boundary energy losses must remain consistent with the chosen zone structure. This is a useful check when an estimated layer grows without a corresponding smoke source.[4][3]

Layer-average properties should be distinguished from local measurements. Gas near the ceiling, plume centreline or an opening can differ from the average assigned to a two-zone model. Using one local reading as the whole-layer value can distort radiation and vent-flow calculations.[13][22]

The distinction between a measured ceiling temperature and a modelled layer-average temperature also affects radiation estimates. A local hot region near the plume can differ from the rest of the upper layer. Using it as a uniform layer temperature changes the represented emitting region and thermal exposure.[2][13]

Plug holing illustrates why outlet position and layer depth matter. If clean air reaches the exhaust from below, part of the system's capacity is spent without removing the intended smoke volume. A review should therefore check the flow field as well as the fan's nominal volumetric rating.[18]

Neutral-plane analysis adds a separate pressure check. Outflow above and inflow below the neutral plane can exchange mass without changing the room's total opening area. Confusing this plane with the smoke interface can lead to an incorrect explanation of the flow direction and layer development.[10]

Structural assessment needs the complete layer history rather than only its hottest average. Early stratification, flashover and later cooling produce different boundary exposures, and conduction continues after gas temperature declines. These stages should remain linked in a Compartment fire analysis.[24][23]

Model outputs can be tested for sensitivity to heat release rate, interface height, vent coefficient and emissivity. These inputs affect both the layer itself and the criteria evaluated against it. A narrow result should not be presented without showing how those choices influence it.[27][4]

tenability and structural criteria should not be mixed. A layer condition used to assess escape is not automatically a structural failure criterion, and a flashover indicator is not automatically a safe-escape threshold. Each comparison needs its own stated purpose, exposure duration and jurisdiction.[16][28]

Measurements used for comparison should share the same averaging basis. A thermocouple near the ceiling, an optical layer measurement and a model-layer average can respond to different aspects of the hot region. Reporting their locations and response characteristics is necessary before declaring agreement or disagreement.[22][13]

A traceable layer result therefore includes the fire source, plume model, zone definition, opening history and selected criterion. It also states whether properties are local or averaged and which stage is being evaluated. These details allow the hot gas layer to be used consistently across smoke, structural and tenability assessments.[4][14][16]

See also

Notes

  1. ↑ Van Rossum (2027), §6.7.
  2. ↑ 2.0 2.1 2.2 2.3 2.4 Quintiere (2002), p. 3-162.
  3. ↑ 3.0 3.1 3.2 3.3 3.4 3.5 Australian Building Codes Board et al. (2005), p. 2.5-2.
  4. ↑ 4.0 4.1 4.2 4.3 4.4 4.5 4.6 4.7 4.8 Quintiere (2002), p. 3-163.
  5. ↑ 5.0 5.1 Cadorin and Franssen (2003), pp. 399–400.
  6. ↑ 6.0 6.1 Van Rossum (2027), §9.1.
  7. ↑ 7.0 7.1 7.2 Van Rossum (2027), §5.3.
  8. ↑ Heskestad (2002), p. 2-1.
  9. ↑ 9.0 9.1 9.2 Van Rossum (2027), §3.2.
  10. ↑ 10.0 10.1 10.2 10.3 10.4 Australian Building Codes Board et al. (2005), p. 2.5-6.
  11. ↑ Drysdale (2002), p. 1-95.
  12. ↑ 12.0 12.1 Tien, Lee, and Stretton (2002), p. 1-73.
  13. ↑ 13.0 13.1 13.2 13.3 13.4 13.5 Van Rossum (2027), §9.2.
  14. ↑ 14.0 14.1 14.2 14.3 14.4 14.5 Van Rossum (2027), §9.3.
  15. ↑ 15.0 15.1 Van Rossum (2027), §6.5.
  16. ↑ 16.0 16.1 16.2 16.3 16.4 16.5 16.6 Van Rossum (2027), §7.7.
  17. ↑ Cadorin and Franssen (2003), p. 413.
  18. ↑ 18.0 18.1 18.2 18.3 Van Rossum (2027), §6.8.
  19. ↑ 19.0 19.1 Australian Building Codes Board et al. (2005), p. 2.5-7.
  20. ↑ 20.0 20.1 Van Rossum (2027), §5.4.
  21. ↑ Van Rossum (2027), §4.7.
  22. ↑ 22.0 22.1 22.2 Cadorin and Franssen (2003), p. 412.
  23. ↑ 23.0 23.1 23.2 23.3 Cadorin and Franssen (2003), pp. 400–401.
  24. ↑ 24.0 24.1 Rockett and Milke (2002), p. 1-29.
  25. ↑ Van Rossum (2027), §5.1.
  26. ↑ Australian Building Codes Board et al. (2005), p. 1.6-7.
  27. ↑ 27.0 27.1 Van Rossum (2027), §1.7.
  28. ↑ U.S. Nuclear Regulatory Commission (2005), p. F-45.

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.
  • 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.
  • 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.
  • 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).
  • Van Rossum, E. (2027). Introduction to Fire Safety Engineering. Fire Safety Academy.
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