Trench effect
Encyclopedia
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 air to deflect to nearby surfaces. The stream's static pressure tends to decrease, which creates a pressure difference between the wall and areas far from the wall. 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 self-ignite. Prior to flashover, flammable gases may be emitted but are too cool to self-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 self-ignite, demonstrating flashover theory. 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.
. The fire started on an escalator 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.
What seems clear is that in the early stages of the fire, the flames visible to anyone not standing on the burning escalator were a small part of the full story. Most of the flames were lying down in the escalator trench; only a few visibly protruded above the balustrade. The lack of visible flames lulled the emergency services into a false sense of security. When the treads of the escalator flashed over, the fire grew dramatically and ignited most of the ticket hall. HSL Buxton lit fires in 1/10-scale and 1/3 scale models of the escalator and ticket hall to prove that the trench effect was the main cause of the King's Cross fire.
An episode of TV series Extreme Evidence
entitled "Flashover" detailed the King's Cross fire, along with the computer modeling and other analyses which discovered the trench effect. Outside of the United States, the show airs as an episode of Forensic Files
.
Fire
Fire is the rapid oxidation of a material in the chemical process of combustion, releasing heat, light, and various reaction products. Slower oxidative processes like rusting or digestion are not included by this definition....
up an inclined surface. It depends on two well-understood but separate ideas: the Coandă effect
Coanda effect
The Coandă effect is the tendency of a fluid jet to be attracted to a nearby surface. The principle was named after Romanian aerodynamics pioneer Henri Coandă, who was the first to recognize the practical application of the phenomenon in aircraft development....
from fluid dynamics and the flashover
Flashover
A flashover is the near simultaneous ignition of most of the directly exposed combustible material in an enclosed area. When certain organic materials are heated they undergo thermal decomposition and release flammable gases...
concept from fire dynamics.
The Coandă effect is the tendency of a fast stream of air to deflect to nearby surfaces. The stream's static pressure tends to decrease, which creates a pressure difference between the wall and areas far from the wall. 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 self-ignite. Prior to flashover, flammable gases may be emitted but are too cool to self-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 self-ignite, demonstrating flashover theory. 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 fireKing's Cross fire
The King's Cross St. Pancras tube station fire was a fatal fire on the London Underground. It broke out at approximately 19:30 on 18 November 1987, and killed 31 people....
. The fire started on an escalator between the Piccadilly Line
Piccadilly Line
The Piccadilly line is a line of the London Underground, coloured dark blue on the Tube map. It is the fifth busiest line on the Underground network judged by the number of passengers transported per year. It is mainly a deep-level line, running from the north to the west of London via Zone 1, with...
platforms and the ticket hall at King's Cross St. Pancras tube station
King's Cross St. Pancras tube station
King's Cross St. Pancras is a tube station in the London Borough of Camden, on the London Underground network, serving both King's Cross and main line stations. It is in Travelcard Zone 1. It is the fourth busiest station on the system and serves more lines than any other...
. 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.
What seems clear is that in the early stages of the fire, the flames visible to anyone not standing on the burning escalator were a small part of the full story. Most of the flames were lying down in the escalator trench; only a few visibly protruded above the balustrade. The lack of visible flames lulled the emergency services into a false sense of security. When the treads of the escalator flashed over, the fire grew dramatically and ignited most of the ticket hall. HSL Buxton lit fires in 1/10-scale and 1/3 scale models of the escalator and ticket hall to prove that the trench effect was the main cause of the King's Cross fire.
An episode of TV series Extreme Evidence
Extreme Evidence
Extreme Evidence is an American docudrama television series that premiered on Court TV on September 29, 2003. Each half-hour episode attempts to use forensic science to uncover the cause of a catastrophic event...
entitled "Flashover" detailed the King's Cross fire, along with the computer modeling and other analyses which discovered the trench effect. Outside of the United States, the show airs as an episode of Forensic Files
Forensic Files
Forensic Files is an American documentary-style series which reveals how forensic science is used to solve violent crimes, mysterious accidents, and even outbreaks of illness. The show is broadcast on truTV, narrated by Peter Thomas, and produced by Medstar Television, in association with truTV...
.