Hot gas layer - Revision history

Thomas: Improve Hot gas layer: sourced explanations, calculations and internal links (Codex)

2026-11-02T17:58:23Z

Improve Hot gas layer: sourced explanations, calculations and internal links (Codex)

Sanne: Add diagrams fse-hot-gas-layer-v1.svg, fse-hot-gas-layer-v2.svg, fse-hot-gas-layer-v3.svg (level-1 visual plan)

2026-11-02T07:18:22Z

Add diagrams fse-hot-gas-layer-v1.svg, fse-hot-gas-layer-v2.svg, fse-hot-gas-layer-v3.svg (level-1 visual plan)

← Older revision		Revision as of 08:18, 2 November 2026
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	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.<ref name="IFEG-2.5-2" /><ref name="VR-5.3" />		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.<ref name="IFEG-2.5-2" /><ref name="VR-5.3" />
			[[File:fse-hot-gas-layer-v1.svg\|thumb\|upright=1.5\|alt=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.<ref name="Quintiere-3-162" /><ref name="Quintiere-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.<ref name="Quintiere-3-162" /><ref name="Quintiere-3-163" />
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	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.<ref name="IFEG-2.5-6" />		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.<ref name="IFEG-2.5-6" />
			[[File:fse-hot-gas-layer-v2.svg\|thumb\|upright=1.5\|alt=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.<ref name="IFEG-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.<ref name="IFEG-2.5-6" />
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	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.<ref name="VR-6.8" />		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.<ref name="VR-6.8" />
			[[File:fse-hot-gas-layer-v3.svg\|thumb\|upright=1.5\|alt=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.<ref name="VR-6.8" /><ref name="IFEG-2.5-7" />		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.<ref name="VR-6.8" /><ref name="IFEG-2.5-7" />

Jan: New FSE wiki article: Hot gas layer (written by Kimi Code CLI)

2026-11-01T23:34:09Z

New FSE wiki article: Hot gas layer (written by Kimi Code CLI)

New page

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.<ref name="VR-6.7">Van Rossum (2027), §6.7.</ref><ref name="Quintiere-3-162">Quintiere (2002), p. 3-162.</ref>

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.<ref name="IFEG-2.5-2">Australian Building Codes Board et al. (2005), p. 2.5-2.</ref><ref name="Quintiere-3-163">Quintiere (2002), p. 3-163.</ref>

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.<ref name="Cadorin-Franssen-399">Cadorin and Franssen (2003), pp. 399–400.</ref><ref name="VR-9.1">Van Rossum (2027), §9.1.</ref>

== 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.<ref name="VR-5.3">Van Rossum (2027), §5.3.</ref><ref name="IFEG-2.5-2" />

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.<ref name="IFEG-2.5-2" /><ref name="VR-5.3" />

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.<ref name="Quintiere-3-162" /><ref name="Quintiere-3-163" />

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.<ref name="Heskestad-2-1">Heskestad (2002), p. 2-1.</ref>

== 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.<ref name="VR-5.3" /><ref name="IFEG-2.5-2" />

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.<ref name="VR-3.2">Van Rossum (2027), §3.2.</ref><ref name="IFEG-2.5-6">Australian Building Codes Board et al. (2005), p. 2.5-6.</ref>

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.<ref name="Drysdale-1-95">Drysdale (2002), p. 1-95.</ref><ref name="Tien-1-73">Tien, Lee, and Stretton (2002), p. 1-73.</ref>

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.<ref name="VR-9.2">Van Rossum (2027), §9.2.</ref><ref name="Quintiere-3-162" />

== 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.<ref name="Cadorin-Franssen-399" /><ref name="Quintiere-3-163" />

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.<ref name="IFEG-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.<ref name="IFEG-2.5-6" />

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.<ref name="Cadorin-Franssen-413">Cadorin and Franssen (2003), p. 413.</ref>

== 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.<ref name="VR-9.3">Van Rossum (2027), §9.3.</ref><ref name="IFEG-2.5-2" />

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.<ref name="VR-6.8">Van Rossum (2027), §6.8.</ref><ref name="IFEG-2.5-7">Australian Building Codes Board et al. (2005), p. 2.5-7.</ref>

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.<ref name="VR-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.<ref name="VR-6.8" /><ref name="IFEG-2.5-7" />

== 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.<ref name="VR-5.4">Van Rossum (2027), §5.4.</ref><ref name="Tien-1-73" />

[[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.<ref name="VR-4.7">Van Rossum (2027), §4.7.</ref>

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.<ref name="Cadorin-Franssen-412">Cadorin and Franssen (2003), p. 412.</ref>

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.<ref name="Cadorin-Franssen-400">Cadorin and Franssen (2003), pp. 400–401.</ref><ref name="VR-9.1" />

== 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.<ref name="VR-7.7">Van Rossum (2027), §7.7.</ref>

Dutch guidance cited by Van Rossum uses 2.5 m of smoke-free height above the highest part of an escape route and radiation below the layer of no more than 2 kW/m² for a non-enclosed space. These values belong to the Netherlands' Bbl explanatory material and should not be presented as universal limits.<ref name="VR-7.7" />

The same section explains that 2.5 kW/m² is often used for escape in practice, while exposure time must also be considered. Under highly conservative blackbody and full-view assumptions, this corresponds to a layer temperature of about 200 °C for the stated calculation. A 10 °C increase already exceeds that flux in the example.<ref name="VR-7.7" />

For mixed smoke conditions, the Dutch explanatory values cited include no more than 45 °C and visibility of at least 30 m for luminous objects. Van Rossum also notes stricter and less strict choices in other contexts, including 8 to 10 m in BS 9999. The applicable jurisdiction and space geometry must therefore be stated.<ref name="VR-7.7" />

== 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.<ref name="Rockett-Milke-1-29">Rockett and Milke (2002), p. 1-29.</ref><ref name="Cadorin-Franssen-400" />

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.<ref name="VR-5.1">Van Rossum (2027), §5.1.</ref><ref name="Cadorin-Franssen-400" />

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.<ref name="IFEG-2.5-6" /><ref name="IFEG-1.6-7">Australian Building Codes Board et al. (2005), p. 1.6-7.</ref>

== 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.<ref name="Quintiere-3-162" /><ref name="Quintiere-3-163" />

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.<ref name="VR-9.3" />

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.<ref name="Quintiere-3-163" /><ref name="VR-9.2" />

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.<ref name="VR-1.7">Van Rossum (2027), §1.7.</ref>

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.<ref name="Quintiere-3-163" /><ref name="IFEG-2.5-2" />

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.<ref name="VR-9.2" /><ref name="Cadorin-Franssen-412" />

Radiation calculations should retain their assumptions about view and emissivity. The Dutch tenability example deliberately uses full view and unit emissivity to obtain a conservative relationship between layer temperature and received flux. Another room geometry or receiver position requires a new assessment rather than the same 200 °C result.<ref name="VR-7.7" /><ref name="VR-5.4" />

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.<ref name="VR-6.8" />

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.<ref name="IFEG-2.5-6" />

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.<ref name="Rockett-Milke-1-29" /><ref name="Cadorin-Franssen-400" />

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.<ref name="VR-1.7" /><ref name="Quintiere-3-163" />

Finally, 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.<ref name="VR-7.7" /><ref name="NUREG-1805-F45">U.S. Nuclear Regulatory Commission (2005), p. F-45.</ref>

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.<ref name="Cadorin-Franssen-412" /><ref name="VR-9.2" />

The layer's role in different analyses should also remain explicit. Smoke filling may focus on interface descent, structural analysis on boundary heat flux and tenability on the environment encountered by a person. Each use requires different output and acceptance conditions from the same layer model.<ref name="IFEG-2.5-2" /><ref name="VR-7.7" />

A final review should ask whether the layer is expected to persist, deepen, mix or be exhausted. Those alternatives change which model and criterion are appropriate. Keeping them visible prevents a stable two-zone layer from being assumed after ventilation or fire growth has invalidated it.<ref name="VR-6.8" /><ref name="Cadorin-Franssen-400" />

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.<ref name="Quintiere-3-163" /><ref name="VR-9.3" /><ref name="VR-7.7" />

== See also ==

* [[Backlayering]]
* [[Flashover criteria]]
* [[Plugholing]]
* [[Smoke stratification]]
* [[Zone model]]

== Notes ==

<references />

== 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). Author.
* Van Rossum, E. (2027). ''Introduction to Fire Safety Engineering''. Fire Safety Academy.

[[Category:Enclosure Fires]]