The trench effect is a combination of circumstances that can rush a
fire
Fire is the rapid oxidation of a material (the fuel) in the exothermic chemical process of combustion, releasing heat, light, and various reaction Product (chemistry), products.
At a certain point in the combustion reaction, called the ignition ...
up an inclined surface. It depends on two well-understood but separate ideas: the
Coandă effect
The Coandă effect ( or ) is the tendency of a fluid jet to stay attached to a convex surface. ''Merriam-Webster'' describes it as "the tendency of a jet of fluid emerging from an orifice to follow an adjacent flat or curved surface and to en ...
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. The fire started on an escalator (containing wood and years of built-up grease) between the
Piccadilly line
The Piccadilly line is a deep-level London Underground line running from the north to the west of London. It has two branches, which split at Acton Town, and serves 53 stations. The line serves Heathrow Airport, and some of its stations are ...
platforms and the ticket hall at
King's Cross St Pancras tube station
King's Cross St Pancras (also known as King's Cross & St Pancras International) is a London Underground station on Euston Road in the Borough of Camden, Central London. It serves King's Cross and main line stations in fare zone 1, and is an ...
. 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
balustrade
A baluster is an upright support, often a vertical moulded shaft, square, or lathe-turned form found in stairways, parapets, and other architectural features. In furniture construction it is known as a spindle. Common materials used in its ...
. 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 HSE's Health and Safety Laboratory in
Buxton
Buxton is a spa town in the High Peak, Derbyshire, Borough of High Peak, Derbyshire, England. It is England's highest market town, sited at some above sea level.wood gas (mainly
methane
Methane ( , ) is a chemical compound with the chemical formula (one carbon atom bonded to four hydrogen atoms). It is a group-14 hydride, the simplest alkane, and the main constituent of natural gas. The relative abundance of methane on Ear ...
) emitted from the
pyrolysis
The pyrolysis (or devolatilization) process is the thermal decomposition of materials at elevated temperatures, often in an inert atmosphere. It involves a change of chemical composition. The word is coined from the Greek-derived elements ''p ...
of the wooden escalator itself. When the concentration of gas reaches a critical value, the
lower flammable limit Lower flammability limit (LFL), usually expressed in volume per cent, is the lower end of the concentration range over which a flammable mixture of gas or vapour in air can be ignited at a given temperature and pressure. The flammability range is de ...
, the gas suddenly catches fire in the presence of a flame, resulting in an
explosion
An explosion is a rapid expansion in volume associated with an extreme outward release of energy, usually with the generation of high temperatures and release of high-pressure gases. Supersonic explosions created by high explosives are kno ...
.
An episode of the television series ''
Forensic Files
''Forensic Files'', originally known as ''Medical Detectives'', is an American documentary television program that reveals how forensic science is used to solve violent crimes, mysterious accidents, and outbreaks of illness. The show was origi ...
'' 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 wildfires up hills, which has led on several notorious occasions to the deaths of multiple firefighters.
References
*
* 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;
* 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;
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 fro the original on 17 May 2017.