Trench effect - Revision history

Jan: Add 4 watermarked SVG illustrations from the Expert visual plan

2026-11-02T16:18:10Z

Add 4 watermarked SVG illustrations from the Expert visual plan

← Older revision		Revision as of 17:18, 2 November 2026
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	== Origin of the term ==		== Origin of the term ==
	Quintiere describes the behaviour of the flames in the London King's Cross fire, which involved fire spread up a wooden escalator, as a notable orientation effect.<ref name="Q2254" /> He attributes this behaviour to the sidewalls of the escalator, which caused the flames to hug the steps. For this observation he cites a 1992 paper in the Fire Safety Journal by Drysdale, Macmillan and Shilitto.<ref name="Q2254" /><ref name="Q2257" /> The term itself appears in the title of that paper, which presents the work as the experimental verification of the "trench effect" in the King's Cross fire.<ref name="Q2257" />		Quintiere describes the behaviour of the flames in the London King's Cross fire, which involved fire spread up a wooden escalator, as a notable orientation effect.<ref name="Q2254" /> He attributes this behaviour to the sidewalls of the escalator, which caused the flames to hug the steps. For this observation he cites a 1992 paper in the Fire Safety Journal by Drysdale, Macmillan and Shilitto.<ref name="Q2254" /><ref name="Q2257" /> The term itself appears in the title of that paper, which presents the work as the experimental verification of the "trench effect" in the King's Cross fire.<ref name="Q2257" />
			[[File:fse-trench-effect-v1.svg\|thumb\|upright=1.5\|alt=Side view of an inclined escalator between two sidewalls, with flames lying low along the steps, and a cross-section showing the trench shape.\|The configuration of the King's Cross fire: fire spreading up the steps of a wooden escalator between its sidewalls, which caused the flames to hug the steps. Based on Quintiere (2002b), p. 2-254.]]

	Purser refers to the same fire in a review of major fire disasters. A common feature of these disasters was a failure in early detection and effective warning.<ref name="P2142" /> In many cases the fire was found at an early stage, but there was a failure to tell people to leave while the fire was small. There was also a failure to realise that a small fire can grow with exponential speed into a life-threatening one.<ref name="P2142" /> In the King's Cross fire, the escalator fire burned for some time before attempts were made to close the station. Purser concludes that by the time people perceive a fire as a threat to their lives, it may already be too late to escape.<ref name="P2142" />		Purser refers to the same fire in a review of major fire disasters. A common feature of these disasters was a failure in early detection and effective warning.<ref name="P2142" /> In many cases the fire was found at an early stage, but there was a failure to tell people to leave while the fire was small. There was also a failure to realise that a small fire can grow with exponential speed into a life-threatening one.<ref name="P2142" /> In the King's Cross fire, the escalator fire burned for some time before attempts were made to close the station. Purser concludes that by the time people perceive a fire as a threat to their lives, it may already be too late to escape.<ref name="P2142" />
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	== Wind-aided and upward spread ==		== Wind-aided and upward spread ==
	Wind-aided spread can accelerate, so it appears more rapid than opposed-flow spread.<ref name="Q2251" /> In this mode, the length of fuel that the flame heats is of the order of the flame length, typically 0.1 to 10 m. In opposed-flow spread it is only 1 to 3 mm.<ref name="Q2251" /> Quintiere relates the flame length to the energy released by the burning area, so that a growing pyrolysis zone lengthens the flame and speeds up the spread. Depending on how quickly the burnt-out region follows, the spread rate can rise towards a limit or grow without bound.<ref name="Q2252">Quintiere (2002b), p. 2-252.</ref>		Wind-aided spread can accelerate, so it appears more rapid than opposed-flow spread.<ref name="Q2251" /> In this mode, the length of fuel that the flame heats is of the order of the flame length, typically 0.1 to 10 m. In opposed-flow spread it is only 1 to 3 mm.<ref name="Q2251" /> Quintiere relates the flame length to the energy released by the burning area, so that a growing pyrolysis zone lengthens the flame and speeds up the spread. Depending on how quickly the burnt-out region follows, the spread rate can rise towards a limit or grow without bound.<ref name="Q2252">Quintiere (2002b), p. 2-252.</ref>
			[[File:fse-trench-effect-v2.svg\|thumb\|upright=1.5\|alt=Two fuel strips: left, a flame heating only a few millimetres ahead; right, a flame lying along the fuel and heating a long stretch ahead.\|Heated length ahead of a spreading flame. Opposed-flow spread heats about 1 to 3 mm of fuel; in wind-aided spread the heated length is of the order of the flame length, 0.1 to 10 m. Based on Quintiere (2002b), p. 2-251.]]

	For [[upward flame spread]] on a vertical wall, the heat flux from a turbulent flame of moderate size to the fuel ahead of it is about 25 kW/m², nearly independent of the material.<ref name="Q2253" /> Orloff and co-workers found that upward turbulent spread on thick PMMA grew almost exponentially. The speed reached roughly 0.5 cm/s after the flame had spread over about 1 m of the material.<ref name="Q2253" /> Quintiere stresses that the outcome depends on the flame length and on burnout. The flame length is controlled by the configuration and the flow conditions, and the burnout by the material.<ref name="Q2253" />		For [[upward flame spread]] on a vertical wall, the heat flux from a turbulent flame of moderate size to the fuel ahead of it is about 25 kW/m², nearly independent of the material.<ref name="Q2253" /> Orloff and co-workers found that upward turbulent spread on thick PMMA grew almost exponentially. The speed reached roughly 0.5 cm/s after the flame had spread over about 1 m of the material.<ref name="Q2253" /> Quintiere stresses that the outcome depends on the flame length and on burnout. The flame length is controlled by the configuration and the flow conditions, and the burnout by the material.<ref name="Q2253" />

	Wind-aided spread has also been measured on ceiling- and floor-mounted samples under a forced flow. On such PMMA samples 0.3 m long, the spread rate was steady at a given flow speed and increased with the flow speed, over a range of 0.25 to 4.5 m/s.<ref name="Q2253" /> At larger scales, radiation and buoyancy become important, especially for floor-mounted materials. Tests on PMMA 2.4 m and 9.8 m long, at flow speeds of 0.8 to 3.8 m/s, gave spread rates of 1 to 6 cm/s. Smaller samples spread at only about 0.01 to 0.3 cm/s.<ref name="Q2253" />		Wind-aided spread has also been measured on ceiling- and floor-mounted samples under a forced flow. On such PMMA samples 0.3 m long, the spread rate was steady at a given flow speed and increased with the flow speed, over a range of 0.25 to 4.5 m/s.<ref name="Q2253" /> At larger scales, radiation and buoyancy become important, especially for floor-mounted materials. Tests on PMMA 2.4 m and 9.8 m long, at flow speeds of 0.8 to 3.8 m/s, gave spread rates of 1 to 6 cm/s. Smaller samples spread at only about 0.01 to 0.3 cm/s.<ref name="Q2253" />
			[[File:fse-trench-effect-v3.svg\|thumb\|upright=1.5\|alt=Log-scale range bars: lateral or downward near 0.1 cm/s, short samples 0.01 to 0.3, long floor samples 1 to 6, upward spread 1 to 100 cm/s.\|Typical and measured flame spread rates. Spread with the flame lying along the fuel is faster, and on long floor-mounted samples under forced flow it reached 1 to 6 cm/s. Based on Quintiere (2002b), pp. 2-247 and 2-253.]]

	== Effect of walls on flames ==		== Effect of walls on flames ==
	In general, walls change the shape of a flame by limiting the air that it can draw in. According to the fire dynamics tools of the U.S. Nuclear Regulatory Commission, a fire close to a wall or in a corner has a restricted supply of entrained air. This restriction has a significant effect on fire growth and spread.<ref name="IS41">Iqbal and Salley (2004), p. 4-1.</ref> It lengthens the flames and raises the plume temperature at a given height, compared with a fire in the open.<ref name="IS41" />		In general, walls change the shape of a flame by limiting the air that it can draw in. According to the fire dynamics tools of the U.S. Nuclear Regulatory Commission, a fire close to a wall or in a corner has a restricted supply of entrained air. This restriction has a significant effect on fire growth and spread.<ref name="IS41">Iqbal and Salley (2004), p. 4-1.</ref> It lengthens the flames and raises the plume temperature at a given height, compared with a fire in the open.<ref name="IS41" />
			[[File:fse-trench-effect-v4.svg\|thumb\|upright=1.5\|alt=A fire in the open, against a wall and in a corner, with entrained air from all round, from half and from a quarter; the flames get longer.\|Walls restrict the air entrained by a flame: a flame against a wall entrains from half of its perimeter, and walls and corners lengthen the flames. Based on Iqbal and Salley (2004), pp. 4-1 and 4-2.]]

	A flame against a single wall can entrain air from only half of its perimeter.<ref name="IS41" /> A fire in a corner is more severe than a fire against one wall, because the two burning walls exchange radiation, although the physical processes that control the two cases are very similar.<ref name="IS42">Iqbal and Salley (2004), p. 4-2.</ref> The same report notes that a fuel burning close to a wall gives higher temperatures and higher flames, because combustion must take place over a greater distance.<ref name="IS63" />		A flame against a single wall can entrain air from only half of its perimeter.<ref name="IS41" /> A fire in a corner is more severe than a fire against one wall, because the two burning walls exchange radiation, although the physical processes that control the two cases are very similar.<ref name="IS42">Iqbal and Salley (2004), p. 4-2.</ref> The same report notes that a fuel burning close to a wall gives higher temperatures and higher flames, because combustion must take place over a greater distance.<ref name="IS63" />

Mark: FSE wiki: corrections after independent fact-check (written by Claude Opus 5.5)

2026-11-01T23:01:24Z

FSE wiki: corrections after independent fact-check (written by Claude Opus 5.5)

Mark: New FSE wiki article: Trench effect (written by Claude Opus 5.5)

2026-11-01T22:32:31Z

New FSE wiki article: Trench effect (written by Claude Opus 5.5)

New page

The '''trench effect''' is an orientation effect in [[flame spread]] that was identified in the London King's Cross fire, in which a fire spread up a wooden escalator.<ref name="Q2254">Quintiere (2002b), p. 2-254.</ref> The sidewalls of the escalator caused the flames to stay close to the steps, or to "hug" them, as the fire spread upward.<ref name="Q2254" /> The name comes from the title of an experimental study of the King's Cross fire, published in 1992, which set out to verify the effect.<ref name="Q2257">Quintiere (2002b), p. 2-257.</ref> The King's Cross fire is also cited as an example of a fire that burned for some time before attempts were made to close the station.<ref name="P2142">Purser (2002), p. 2-142.</ref>

Flame spread theory shows why the position of a flame matters. A flame that lies along unburnt fuel heats a long stretch of it, and the spread can then accelerate, whereas a flame that heats only a few millimetres ahead of its base spreads slowly.<ref name="Q2251">Quintiere (2002b), p. 2-251.</ref><ref name="Q2253">Quintiere (2002b), p. 2-253.</ref> Configuration effects on flame spread, such as spread in corners and on ceilings, have been studied, but they have not yet led to fully established calculation methods.<ref name="Q2247">Quintiere (2002b), p. 2-247.</ref>

== Origin of the term ==
Quintiere describes the trench effect as a notable orientation effect found in the London King's Cross fire, which involved fire spread up a wooden escalator.<ref name="Q2254" /> He attributes the behaviour of the flames to the sidewalls of the escalator, which caused the flames to hug the steps.<ref name="Q2254" /> The term itself appears in the title of a 1992 paper in the Fire Safety Journal by Drysdale, Macmillan and Shilitto. Its title presents the work as the experimental verification of the "trench effect" in the King's Cross fire.<ref name="Q2257" />

Purser refers to the same fire in a review of major fire disasters. A common feature of these disasters was a failure in early detection and effective warning.<ref name="P2142" /> In many cases the fire was found at an early stage, but there was a failure to tell people to leave while the fire was small. There was also a failure to realise that a small fire can grow with exponential speed into a life-threatening one.<ref name="P2142" /> In the King's Cross fire, the escalator fire burned for some time before attempts were made to close the station. Purser concludes that by the time people perceive a fire as a threat to their lives, it may already be too late to escape.<ref name="P2142" />

== Orientation effects in flame spread ==
Flame spread over a solid depends on the direction of the gas flow relative to the spread and on the orientation of the surface. A surface can be vertical or horizontal, and it can face upward or downward, and each of these cases changes the heat transfer from the flame to the fuel.<ref name="Q2247" /> Spread against the flow is called [[opposed-flow flame spread]], and spread in the same direction as the flow is called wind-aided spread.<ref name="Q2247" /> The difference in speed is large. Lateral or downward spread on thick solids has a typical rate of about 0.1 cm/s, while upward spread on thick solids reaches 1 to 100 cm/s.<ref name="Q2247" />

Ito and Kashiwagi measured the heat flux from the flame to the surface ahead of a spreading front on polymethyl methacrylate (PMMA). They tested orientations from vertically upward to vertically downward.<ref name="Q2253" /> For upward spread they found a nearly constant heat flux of about 25 kW/m² over a long distance ahead of the front. For downward spread the flux reached a maximum of about 70 kW/m², but only very close to the front.<ref name="Q2254" />

Orientation also governs fire growth in ordinary rooms. ''Introduction to Fire Safety Engineering'' notes that upward spread along a vertical surface transfers a relatively large amount of heat to the material above the burning part. That material is preheated, so the fire can spread faster.<ref name="VR43">Van Rossum (2027), §4.3.</ref> The fire dynamics tools of the U.S. Nuclear Regulatory Commission give the example of a curtain. A pilot flame at the bottom causes rapid upward spread, while the same flame at the top gives slow downward spread and much slower fire growth.<ref name="IS63">Iqbal and Salley (2004), p. 6-3.</ref> In a standard fire propagation apparatus, upward propagation on PMMA was much faster than downward propagation and accelerated with time.<ref name="T389">Tewarson (2002), p. 3-89.</ref>

== Flames lying along the fuel ==
Wind-aided spread can accelerate, so it appears more rapid than opposed-flow spread.<ref name="Q2251" /> In this mode, the length of fuel that the flame heats is of the order of the flame length, typically 0.1 to 10 m. In opposed-flow spread it is only 1 to 3 mm.<ref name="Q2251" /> Quintiere relates the flame length to the energy released by the burning area, so that a growing pyrolysis zone lengthens the flame and speeds up the spread. Depending on how quickly the burnt-out region follows, the spread rate can rise towards a limit or grow without bound.<ref name="Q2252">Quintiere (2002b), p. 2-252.</ref>

For [[upward flame spread]] on a vertical wall, the heat flux from a turbulent flame of moderate size to the fuel ahead of it is about 25 kW/m², nearly independent of the material.<ref name="Q2253" /> Orloff and co-workers found that upward turbulent spread on thick PMMA grew almost exponentially. The speed reached roughly 0.5 cm/s after the flame had spread over about 1 m of the material.<ref name="Q2253" /> Quintiere stresses that the outcome depends on the flame length and on burnout. The flame length is controlled by the configuration and the flow conditions, and the burnout by the material.<ref name="Q2253" />

Wind-aided spread has also been measured on floor-mounted samples under a forced flow. On PMMA samples 0.3 m long, the spread rate was steady at a given flow speed and increased with the flow speed, over a range of 0.25 to 4.5 m/s.<ref name="Q2253" /> At larger scales, radiation and buoyancy become important, especially for floor-mounted materials. Tests on PMMA 2.4 m and 9.8 m long, at flow speeds of 0.8 to 3.8 m/s, gave spread rates of 1 to 6 cm/s. Smaller samples spread at only about 0.01 to 0.3 cm/s.<ref name="Q2253" />

== Effect of walls on flames ==
Walls change the shape of a flame by limiting the air that it can draw in. According to the fire dynamics tools of the U.S. Nuclear Regulatory Commission, a fire close to a wall or in a corner has a restricted supply of entrained air. This restriction has a significant effect on fire growth and spread.<ref name="IS41">Iqbal and Salley (2004), p. 4-1.</ref> It lengthens the flames and raises the plume temperature at a given height, compared with a fire in the open.<ref name="IS41" />

A flame against a single wall can entrain air from only half of its perimeter.<ref name="IS41" /> A fire in a corner is more severe than a fire against one wall, because the two burning walls exchange radiation, although the physical processes that control the two cases are very similar.<ref name="IS42">Iqbal and Salley (2004), p. 4-2.</ref> The same report notes that a fuel burning close to a wall gives higher temperatures and higher flames, because combustion must take place over a greater distance.<ref name="IS63" />

== Consequences for fire growth ==
Fast spread over combustible surfaces can carry a room fire to [[flashover]], the stage at which the whole room suddenly becomes involved in the fire.<ref name="VR47">Van Rossum (2027), §4.7.</ref><ref name="ABDS49">Australian Building Codes Board (2020a), Data Sheet B4, p. 49.</ref> ''Introduction to Fire Safety Engineering'' names a hot smoke layer above 500 °C as one condition for flashover. Such a layer radiates about 20 kW/m² to the floor, a level at which many materials pyrolyse rapidly.<ref name="VR47" />

In Australia, the data sheets of the Fire Safety Verification Method (print version 1.1, 2020) link the spread behaviour of linings to flashover directly. Wall and ceiling linings are grouped by the time to flashover in the ISO 9705 [[room corner test|room-corner test]], and Group 1 applies when flashover does not occur within the 20-minute test.<ref name="ABDS49" /> A Group 3 lining that reaches a heat release rate of 1 MW within 5 minutes in this test is given a fast design growth rate.<ref name="ABDS50">Australian Building Codes Board (2020a), Data Sheet B4, p. 50.</ref> If it takes 5 minutes or more, a medium rate applies, and Group 4 linings are assigned an ultra-fast growth rate.<ref name="ABDS50" /> The room-corner test was developed as a realistic test for interior finish materials, and its results have been predicted from cone calorimeter data and flame spread equations.<ref name="Q2254" />

== Prediction and limits ==
Quintiere's framework for wind-aided spread can be applied to a particular problem only when the flame heat flux and the flame length can be expressed for that configuration.<ref name="Q2252" /> Attempts to include configuration effects, such as spread in vertical corners and on ceilings, have not led to fully established methods.<ref name="Q2247" /> Quintiere concludes that flame spread in enclosures, mines, ducts and buildings presents a complex array of conditions. Flame spread on materials therefore has to be evaluated in the context of their use.<ref name="Q2256">Quintiere (2002b), p. 2-256.</ref>

Field models based on [[computational fluid dynamics]] (CFD) can describe fires in complex geometries and include a wide variety of physical processes.<ref name="FDS2">McGrattan et al. (2026), p. 2.</ref> Their ability to predict flame spread is limited, however. The technical guide of the Fire Dynamics Simulator notes that modelling the physical and chemical changes of real materials as they burn is still in its infancy.<ref name="FDS1">McGrattan et al. (2026), p. 1.</ref> In the validation of the same model for nuclear power plant applications, even a 10 cm grid was not necessarily fine enough to predict the heat release rate or the flame spread along cable trays. Coarser grids still gave reasonable predictions of quantities tied directly to the conservation of mass and energy, such as average temperatures and pressures.<ref name="MG52">McGrattan (2007), p. 5-2.</ref>

== See also ==
* [[Fire spread]]
* [[Critical heat flux for ignition]]
* [[Time to ignition]]

== Notes ==
<references />

== References ==
* Australian Building Codes Board. (2020a). ''Data sheets: Fire Safety Verification Method'' (Handbook annex, Print version 1.1).
* Iqbal, N., & Salley, M. H. (2004). ''Fire Dynamics Tools (FDTs): Quantitative fire hazard analysis methods for the U.S. Nuclear Regulatory Commission Fire Protection Inspection Program'' (NUREG-1805). U.S. Nuclear Regulatory Commission, Office of Nuclear Reactor Regulation.
* McGrattan, K. (2007). ''Verification and validation of selected fire models for nuclear power plant applications: Vol. 7. Fire Dynamics Simulator'' (NUREG-1824; EPRI 1011999). U.S. Nuclear Regulatory Commission, Office of Nuclear Regulatory Research; Electric Power Research Institute.
* McGrattan, K., Hostikka, S., Floyd, J., McDermott, R., Vanella, M., Mueller, E., & Paul, C. (2026). ''Fire Dynamics Simulator technical reference guide: Vol. 1. Mathematical model'' (6th ed., NIST Special Publication 1018-1). National Institute of Standards and Technology. https://doi.org/10.6028/NIST.SP.1018
* Purser, D. A. (2002). Toxicity assessment of combustion products. In P. J. DiNenno, D. Drysdale, C. L. Beyler, W. D. Walton, R. L. P. Custer, J. R. Hall Jr., & J. M. Watts Jr. (Eds.), ''SFPE handbook of fire protection engineering'' (3rd ed., pp. 2-83–2-171). National Fire Protection Association; Society of Fire Protection Engineers.
* Quintiere, J. G. (2002b). Surface flame spread. In P. J. DiNenno, D. Drysdale, C. L. Beyler, W. D. Walton, R. L. P. Custer, J. R. Hall Jr., & J. M. Watts Jr. (Eds.), ''SFPE handbook of fire protection engineering'' (3rd ed., pp. 2-246–2-257). National Fire Protection Association; Society of Fire Protection Engineers.
* Tewarson, A. (2002). Generation of heat and chemical compounds in fires. In P. J. DiNenno, D. Drysdale, C. L. Beyler, W. D. Walton, R. L. P. Custer, J. R. Hall Jr., & J. M. Watts Jr. (Eds.), ''SFPE handbook of fire protection engineering'' (3rd ed., pp. 3-82–3-161). National Fire Protection Association; Society of Fire Protection Engineers.
* Van Rossum, E. (2027). ''Introduction to Fire Safety Engineering''. Fire Safety Academy.

[[Category:Ignition & Flame Spread]]

← Older revision		Revision as of 00:01, 2 November 2026
Line 1:		Line 1:
	The '''trench effect''' is an orientation effect in [[flame spread]] that was identified in the London King's Cross fire, in which a fire spread up a wooden escalator.<ref name="Q2254">Quintiere (2002b), p. 2-254.</ref> The sidewalls of the escalator caused the flames to stay close to the steps, or to "hug" them, as the fire spread upward.<ref name="Q2254" /> The ~~name comes from~~ the title of an experimental study of the King's Cross fire~~, published in 1992, which set out to verify the effect~~.<ref name="Q2257">Quintiere (2002b), p. 2-257.</ref> The King's Cross fire is also cited as an example of a fire that burned for some time before attempts were made to close the station.<ref name="P2142">Purser (2002), p. 2-142.</ref>		The '''trench effect''' is an orientation effect in [[flame spread]] that was identified in the London King's Cross fire, in which a fire spread up a wooden escalator.<ref name="Q2254">Quintiere (2002b), p. 2-254.</ref> The sidewalls of the escalator caused the flames to stay close to the steps, or to "hug" them, as the fire spread upward.<ref name="Q2254" /> The term appears in the title of a 1992 experimental study of the King's Cross fire.<ref name="Q2257">Quintiere (2002b), p. 2-257.</ref> The King's Cross fire is also cited as an example of a fire that burned for some time before attempts were made to close the station.<ref name="P2142">Purser (2002), p. 2-142.</ref>

	~~Flame spread theory shows why~~ the ~~position~~ of a flame ~~matters~~. A flame that lies along unburnt fuel heats a long stretch of it, ~~and~~ the spread can ~~then~~ accelerate~~, whereas a~~ flame that heats only a few millimetres ahead of its base spreads slowly.<ref name="Q2251">Quintiere (2002b), p. 2-251.</ref><ref name="Q2253">Quintiere (2002b), p. 2-253.</ref> Configuration effects on flame spread, such as spread in corners and on ceilings, have been studied, but they have not yet led to fully established calculation methods.<ref name="Q2247">Quintiere (2002b), p. 2-247.</ref>		The handbook account that reports the effect attributes it to the sidewalls of the escalator, and none of the literature cited here explains its mechanism.<ref name="Q2254" /> The later sections therefore describe general effects of surface orientation and of walls on flame spread, not an explanation of the trench effect itself. In general flame spread theory, a flame that lies along unburnt fuel heats a long stretch of it, so that the spread can accelerate. A flame that heats only a few millimetres ahead of its base spreads slowly.<ref name="Q2251">Quintiere (2002b), p. 2-251.</ref><ref name="Q2253">Quintiere (2002b), p. 2-253.</ref> Configuration effects on flame spread, such as spread in corners and on ceilings, have been studied, but they have not yet led to fully established calculation methods.<ref name="Q2247">Quintiere (2002b), p. 2-247.</ref>

	== Origin of the term ==		== Origin of the term ==
	Quintiere describes the ~~trench effect as a notable orientation effect found~~ in the London King's Cross fire, which involved fire spread up a wooden escalator.<ref name="Q2254" /> He attributes ~~the~~ behaviour ~~of the flames~~ to the sidewalls of the escalator, which caused the flames to hug the steps.<ref name="Q2254" /> The term itself appears in the title of ~~a 1992~~ paper ~~in the Fire Safety Journal by Drysdale~~, ~~Macmillan and Shilitto. Its title~~ presents the work as the experimental verification of the "trench effect" in the King's Cross fire.<ref name="Q2257" />		Quintiere describes the behaviour of the flames in the London King's Cross fire, which involved fire spread up a wooden escalator, as a notable orientation effect.<ref name="Q2254" /> He attributes this behaviour to the sidewalls of the escalator, which caused the flames to hug the steps. For this observation he cites a 1992 paper in the Fire Safety Journal by Drysdale, Macmillan and Shilitto.<ref name="Q2254" /><ref name="Q2257" /> The term itself appears in the title of that paper, which presents the work as the experimental verification of the "trench effect" in the King's Cross fire.<ref name="Q2257" />

	Purser refers to the same fire in a review of major fire disasters. A common feature of these disasters was a failure in early detection and effective warning.<ref name="P2142" /> In many cases the fire was found at an early stage, but there was a failure to tell people to leave while the fire was small. There was also a failure to realise that a small fire can grow with exponential speed into a life-threatening one.<ref name="P2142" /> In the King's Cross fire, the escalator fire burned for some time before attempts were made to close the station. Purser concludes that by the time people perceive a fire as a threat to their lives, it may already be too late to escape.<ref name="P2142" />		Purser refers to the same fire in a review of major fire disasters. A common feature of these disasters was a failure in early detection and effective warning.<ref name="P2142" /> In many cases the fire was found at an early stage, but there was a failure to tell people to leave while the fire was small. There was also a failure to realise that a small fire can grow with exponential speed into a life-threatening one.<ref name="P2142" /> In the King's Cross fire, the escalator fire burned for some time before attempts were made to close the station. Purser concludes that by the time people perceive a fire as a threat to their lives, it may already be too late to escape.<ref name="P2142" />
Line 15:		Line 15:
	Orientation also governs fire growth in ordinary rooms. ''Introduction to Fire Safety Engineering'' notes that upward spread along a vertical surface transfers a relatively large amount of heat to the material above the burning part. That material is preheated, so the fire can spread faster.<ref name="VR43">Van Rossum (2027), §4.3.</ref> The fire dynamics tools of the U.S. Nuclear Regulatory Commission give the example of a curtain. A pilot flame at the bottom causes rapid upward spread, while the same flame at the top gives slow downward spread and much slower fire growth.<ref name="IS63">Iqbal and Salley (2004), p. 6-3.</ref> In a standard fire propagation apparatus, upward propagation on PMMA was much faster than downward propagation and accelerated with time.<ref name="T389">Tewarson (2002), p. 3-89.</ref>		Orientation also governs fire growth in ordinary rooms. ''Introduction to Fire Safety Engineering'' notes that upward spread along a vertical surface transfers a relatively large amount of heat to the material above the burning part. That material is preheated, so the fire can spread faster.<ref name="VR43">Van Rossum (2027), §4.3.</ref> The fire dynamics tools of the U.S. Nuclear Regulatory Commission give the example of a curtain. A pilot flame at the bottom causes rapid upward spread, while the same flame at the top gives slow downward spread and much slower fire growth.<ref name="IS63">Iqbal and Salley (2004), p. 6-3.</ref> In a standard fire propagation apparatus, upward propagation on PMMA was much faster than downward propagation and accelerated with time.<ref name="T389">Tewarson (2002), p. 3-89.</ref>

	== ~~Flames lying along the fuel~~ ==		== Wind-aided and upward spread ==
	Wind-aided spread can accelerate, so it appears more rapid than opposed-flow spread.<ref name="Q2251" /> In this mode, the length of fuel that the flame heats is of the order of the flame length, typically 0.1 to 10 m. In opposed-flow spread it is only 1 to 3 mm.<ref name="Q2251" /> Quintiere relates the flame length to the energy released by the burning area, so that a growing pyrolysis zone lengthens the flame and speeds up the spread. Depending on how quickly the burnt-out region follows, the spread rate can rise towards a limit or grow without bound.<ref name="Q2252">Quintiere (2002b), p. 2-252.</ref>		Wind-aided spread can accelerate, so it appears more rapid than opposed-flow spread.<ref name="Q2251" /> In this mode, the length of fuel that the flame heats is of the order of the flame length, typically 0.1 to 10 m. In opposed-flow spread it is only 1 to 3 mm.<ref name="Q2251" /> Quintiere relates the flame length to the energy released by the burning area, so that a growing pyrolysis zone lengthens the flame and speeds up the spread. Depending on how quickly the burnt-out region follows, the spread rate can rise towards a limit or grow without bound.<ref name="Q2252">Quintiere (2002b), p. 2-252.</ref>

	For [[upward flame spread]] on a vertical wall, the heat flux from a turbulent flame of moderate size to the fuel ahead of it is about 25 kW/m², nearly independent of the material.<ref name="Q2253" /> Orloff and co-workers found that upward turbulent spread on thick PMMA grew almost exponentially. The speed reached roughly 0.5 cm/s after the flame had spread over about 1 m of the material.<ref name="Q2253" /> Quintiere stresses that the outcome depends on the flame length and on burnout. The flame length is controlled by the configuration and the flow conditions, and the burnout by the material.<ref name="Q2253" />		For [[upward flame spread]] on a vertical wall, the heat flux from a turbulent flame of moderate size to the fuel ahead of it is about 25 kW/m², nearly independent of the material.<ref name="Q2253" /> Orloff and co-workers found that upward turbulent spread on thick PMMA grew almost exponentially. The speed reached roughly 0.5 cm/s after the flame had spread over about 1 m of the material.<ref name="Q2253" /> Quintiere stresses that the outcome depends on the flame length and on burnout. The flame length is controlled by the configuration and the flow conditions, and the burnout by the material.<ref name="Q2253" />

	Wind-aided spread has also been measured on floor-mounted samples under a forced flow. On PMMA samples 0.3 m long, the spread rate was steady at a given flow speed and increased with the flow speed, over a range of 0.25 to 4.5 m/s.<ref name="Q2253" /> At larger scales, radiation and buoyancy become important, especially for floor-mounted materials. Tests on PMMA 2.4 m and 9.8 m long, at flow speeds of 0.8 to 3.8 m/s, gave spread rates of 1 to 6 cm/s. Smaller samples spread at only about 0.01 to 0.3 cm/s.<ref name="Q2253" />		Wind-aided spread has also been measured on ceiling- and floor-mounted samples under a forced flow. On such PMMA samples 0.3 m long, the spread rate was steady at a given flow speed and increased with the flow speed, over a range of 0.25 to 4.5 m/s.<ref name="Q2253" /> At larger scales, radiation and buoyancy become important, especially for floor-mounted materials. Tests on PMMA 2.4 m and 9.8 m long, at flow speeds of 0.8 to 3.8 m/s, gave spread rates of 1 to 6 cm/s. Smaller samples spread at only about 0.01 to 0.3 cm/s.<ref name="Q2253" />

	== Effect of walls on flames ==		== Effect of walls on flames ==
	~~Walls~~ change the shape of a flame by limiting the air that it can draw in. According to the fire dynamics tools of the U.S. Nuclear Regulatory Commission, a fire close to a wall or in a corner has a restricted supply of entrained air. This restriction has a significant effect on fire growth and spread.<ref name="IS41">Iqbal and Salley (2004), p. 4-1.</ref> It lengthens the flames and raises the plume temperature at a given height, compared with a fire in the open.<ref name="IS41" />		In general, walls change the shape of a flame by limiting the air that it can draw in. According to the fire dynamics tools of the U.S. Nuclear Regulatory Commission, a fire close to a wall or in a corner has a restricted supply of entrained air. This restriction has a significant effect on fire growth and spread.<ref name="IS41">Iqbal and Salley (2004), p. 4-1.</ref> It lengthens the flames and raises the plume temperature at a given height, compared with a fire in the open.<ref name="IS41" />

	A flame against a single wall can entrain air from only half of its perimeter.<ref name="IS41" /> A fire in a corner is more severe than a fire against one wall, because the two burning walls exchange radiation, although the physical processes that control the two cases are very similar.<ref name="IS42">Iqbal and Salley (2004), p. 4-2.</ref> The same report notes that a fuel burning close to a wall gives higher temperatures and higher flames, because combustion must take place over a greater distance.<ref name="IS63" />		A flame against a single wall can entrain air from only half of its perimeter.<ref name="IS41" /> A fire in a corner is more severe than a fire against one wall, because the two burning walls exchange radiation, although the physical processes that control the two cases are very similar.<ref name="IS42">Iqbal and Salley (2004), p. 4-2.</ref> The same report notes that a fuel burning close to a wall gives higher temperatures and higher flames, because combustion must take place over a greater distance.<ref name="IS63" />
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	Fast spread over combustible surfaces can carry a room fire to [[flashover]], the stage at which the whole room suddenly becomes involved in the fire.<ref name="VR47">Van Rossum (2027), §4.7.</ref><ref name="ABDS49">Australian Building Codes Board (2020a), Data Sheet B4, p. 49.</ref> ''Introduction to Fire Safety Engineering'' names a hot smoke layer above 500 °C as one condition for flashover. Such a layer radiates about 20 kW/m² to the floor, a level at which many materials pyrolyse rapidly.<ref name="VR47" />		Fast spread over combustible surfaces can carry a room fire to [[flashover]], the stage at which the whole room suddenly becomes involved in the fire.<ref name="VR47">Van Rossum (2027), §4.7.</ref><ref name="ABDS49">Australian Building Codes Board (2020a), Data Sheet B4, p. 49.</ref> ''Introduction to Fire Safety Engineering'' names a hot smoke layer above 500 °C as one condition for flashover. Such a layer radiates about 20 kW/m² to the floor, a level at which many materials pyrolyse rapidly.<ref name="VR47" />

	In Australia, the data sheets of the Fire Safety Verification Method (print version 1.1, 2020) ~~link the spread behaviour of linings to flashover directly. Wall~~ and ceiling linings ~~are grouped~~ by ~~the~~ time to flashover in the ISO 9705 [[room corner test\|room-corner test]]~~, and~~ Group 1 applies when flashover does not occur within the 20-minute test.<ref name="ABDS49" /> A Group 3 lining that reaches a heat release rate of 1 MW within 5 minutes in this test is given a fast design growth rate.<ref name="ABDS50">Australian Building Codes Board (2020a), Data Sheet B4, p. 50.</ref> If it takes 5 minutes or more, a medium rate applies, and Group 4 linings are assigned an ultra-fast growth rate.<ref name="ABDS50" /> The room-corner test was developed as a realistic test for interior finish materials, and its results have been predicted from cone calorimeter data and flame spread equations.<ref name="Q2254" />		In Australia, the data sheets of the Fire Safety Verification Method (print version 1.1, 2020) group wall and ceiling linings by their time to flashover in the ISO 9705 [[room corner test\|room-corner test]]. Group 1 applies when flashover does not occur within the 20-minute test.<ref name="ABDS49" /> A Group 3 lining that reaches a heat release rate of 1 MW within 5 minutes in this test is given a fast design growth rate.<ref name="ABDS50">Australian Building Codes Board (2020a), Data Sheet B4, p. 50.</ref> If it takes 5 minutes or more, a medium rate applies, and Group 4 linings are assigned an ultra-fast growth rate.<ref name="ABDS50" /> The room-corner test was developed as a realistic test for interior finish materials, and its results have been predicted from cone calorimeter data and flame spread equations.<ref name="Q2254" />

	== Prediction and limits ==		== Prediction and limits ==