Trench effect

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.

Background

The trench effect became known because of the scientific investigation of the King's Cross fire. The fire started on an escalator (containing combustible wood) 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. This trench effect does not explain the sudden explosions that blew some survivors off their feet.

The relatively inconspicuous flames associated with the trench effect, hugging the space on and under the inclined trench of the escalator steps, meant that many travellers watched a fire growing without thinking it might be a good idea to leave the area. One survivor reports approaching the escalator and noting thick black smoke that suddenly blew up - showing that the main loss of life came from the sudden superheated explosion of gases in the flashover, rather than the precipitating flames of the 'trench effect' fire itself, which fueled the release and heating of those gases. The people in the ticket hall at the top of the escalators, while avoiding the visible flames, were unwittingly waiting in the path of the eventual sudden flashover explosion fueled by the fire.

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 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. 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. The HSE's Health and Safety Laboratory in 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. 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 flammability limit), the gas suddenly catches fire in the presence of a flame.

An episode of TV series Forensic Files entitled "Flashover" detailed the King's Cross fire, along with the computer modeling and other analyses which discovered the trench effect.

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