Editing Trench effect

{{Short description|Rapid spread of fire up an inclined surface}}
{{no footnotes|date=August 2014}}

The '''trench effect''' is a combination of circumstances that can rush a [[fire]] up an inclined surface. It depends on two well-understood but separate ideas: the [[Coandă effect]] from [[fluid dynamics]] and the [[flashover]] concept from [[fire dynamics]]:

* The Coandă effect is the tendency of a fast stream of gases to bend towards, and adhere to, nearby surfaces. The stream's static pressure tends to decrease, which creates a pressure difference between the surface and areas far from it. This bends the stream towards the surface and tends to keep it attached to that surface.
* Flashover is a sudden widespread fire, which occurs when most surfaces in a space are heated until they emit flammable gases hot enough to auto-ignite. Before flashover, flammable gases may be emitted but are too cool to ignite.

The trench effect occurs when a fire burns beside a steeply inclined surface. The flames lie down along the surface, demonstrating the Coandă effect. The flames heat the material farther up: these emit gases that autoignite in a flashover event. The flames from these areas are themselves subject to the Coandă effect and blow a jet of flame up to the end of the inclined surface. This jet continues until the fuel depletes.

==Background==
The trench effect became known because of the scientific investigation of the [[King's Cross fire]], which occurred on the [[London Underground]] in 1987. The fire started on an escalator (containing wood and years of built-up grease) between the [[Piccadilly line]] platforms and the ticket hall at [[King's Cross St Pancras tube station]]. Many eyewitnesses indicated that early on, the fire in the escalator was of manageable size: officers from the Fire Brigade and British Transport Police indicated that the fire appeared no larger than a large cardboard box burning. Those present were surprised when it suddenly threw a sustained jet of flame into the ticket hall. The Coandă effect alone could not explain the sudden blast that blew some survivors off their feet.

The relatively inconspicuous flames associated with the Coandă effect, hugging the space on and under the inclined trench of the escalator steps, led many travellers to believe they were not in immediate danger. One survivor reported approaching the escalator and noting thick black smoke that suddenly blew up. The main loss of life came from the sudden flashover, rather than the precipitating flames of the 'Coandă effect' fire itself.  The people in the ticket hall at the top of the escalators, whilst avoiding the visible flames, were caught in the path of the flashover. 

In the early stages of the fire, most of the flames were lying down in the escalator trench: only a few visibly protruded above the [[Baluster#Banisters|balustrade]]. The sides of the trench further enhanced the effect by preventing heat loss by radiation to the side, insulating and concentrating the heat along the narrow trench. Emergency services were not prepared for a major fire, due to the lack of visible flames. When the treads of the escalator flashed over, the fire grew dramatically and ignited most of the ticket hall. The [[Health and Safety Executive]]'s [[Health and Safety Executive#HSL|Health and Safety Laboratory]] in [[Buxton]] lit fires in {{frac|10}}-scale and {{frac|3}}-scale models of the escalator and ticket hall, which demonstrated that the trench effect was the main cause of the intensity and rapid spread of the King's Cross fire. The sudden flashover may be attributed to the [[wood gas]] (mainly [[methane]]) emitted from the [[pyrolysis]] of the wooden escalator itself. When the concentration of gas reaches a critical value, the [[lower flammable limit]], the gas suddenly catches fire in the presence of a flame, resulting in an [[explosion]]. 

An episode of the television series ''[[Forensic Files]]'' entitled "Flashover" detailed the King's Cross fire, along with the computer modelling and other analyses which discovered the trench effect. An episode of the television series ''[[Seconds from Disaster]]'' entitled "London's Subway Inferno" detailed both the King's Cross Fire based on eyewitness accounts and the investigation leading up to the discovery of the trench effect.

The trench effect has also been cited as the cause of rapid spread of [[wildfire]]s up hills, which has led on several notorious occasions to the deaths of multiple firefighters.

== See also ==
* [[Wildfire modeling]]

==Further reading==
* {{cite journal|doi=10.1016/0379-7112(92)90045-E |title=The King's Cross fire: Damage assessment and overview of the technical investigation |date=1992 |last1=Moodie |first1=K. |journal=Fire Safety Journal |volume=18 |pages=13–33 }}
* {{cite journal|doi=10.1016/0379-7112(92)90049-I |title=The King's Cross fire: Results and analysis from the scale model tests |date=1992 |last1=Moodie |first1=K. |last2=Jagger |first2=S.F. |journal=Fire Safety Journal |volume=18 |pages=83–103 }}
* {{cite book|url=http://www.hse.gov.uk/research/CRR_htm/|last1=Wu|first1=Y.|last2=Drysdale|first2=D.|title=Study of upward flame spread on inclined surfaces, HSE contract research report no. 122|publisher=[[Health and Safety Executive|HSE]]|year=1996|isbn=0-7176-1289-9}}
* K. Moodie, ''The King's Cross Fire: Damage Assessment and Overview of the Technical Investigation'' Fire Safety Journal, vol 18 (1992) 13-33
* S. Simcox, N.S. Wilkes & I.P. Jones, ''Computer Simulation of the Flows of Hot Gases from the Fire at King's Cross Underground Station'' Fire Safety Journal, vol 18 (1992) 49-73
* K. Moodie & S.F. Jagger, ''Results and analysis from the scale model tests'' Paper presented at I Mech E seminar, ''The King's Cross Underground Fire: fire dynamics and the organisation of safety'' 1 June 1989; {{ISBN|0-85298-705-6}}
* A.F. Roberts, ''The King's Cross Fire: a correlation of the eyewitness accounts and results of the scientific investigation'' Paper presented at I Mech E seminar,''The King's Cross Underground Fire: fire dynamics and the organisation of safety'' 1 June 1989; {{ISBN|0-85298-705-6}}
* [https://web.archive.org/web/20170517011657/https://www.maths.manchester.ac.uk/~jwd/articles/10-TEaEW.pdf The trench effect and eruptive wildfires: lessons from the King’s Cross Underground disaster.] by Jason J. Sharples, A. Malcolm Gill, & John W. Dold. Archived from [http://www.maths.manchester.ac.uk/~jwd/articles/10-TEaEW.pdf the original] on 17 May 2017.
* {{cite book|first1=Desmond|last1=Fennell|title=Investigation into the King's Cross Underground Fire|year=1988|isbn=0-10-104992-7|publisher=[[Department for Transport|Department of Transport]]}} [http://www.railwaysarchive.co.uk/documents/DoT_KX1987.pdf Scan] available online at ''railwaysarchive.co.uk''. Retrieved 27 October 2012.

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[[Category:Types of fire]]
[[Category:1980s neologisms]]