Trench effect

The trench effect describes a set of conditions that can cause a fire to rapidly accelerate up an inclined surface. It arises from the interaction of two established principles: the coanda effect from fluid dynamics and the concept of flashover from the physics of fire behaviour.

The coanda effect refers to the tendency of a fast-moving stream of gas to curve toward and cling to nearby surfaces. As the stream moves, its static pressure drops, creating a pressure difference that pulls it toward the surface and keeps it attached. In the case of fire spread this refers to where rather than moving directly vertically in a fire gases may move toward and cling to nearby surfaces such as external walls.

Flashover is a sudden, intense ignition of all combustible materials in a space. It occurs when surfaces become so hot that they release flammable gases, which spontaneously ignite once they reach a critical temperature. Prior to flashover, these gases may be present but remain too cool to ignite.

As a scenario a fire burning next to a steep slope causes the flames to follow the surface (coanda effect ), the flames heat the material further upslope, which with the heat emit flammable gases and ignite creating a flashover. The new flames, from the flashover in turn, follow the slope, creating more gases and rapid spread of fire, continuing upward until the any available fuel is exhausted.

A particular case in point example of the trench effect occurring with tragic consequences was the King’s Cross Underground fire on 18 November, 1987, which claimed 31 lives and injured commuters and emergency personnel. Initially the fire was judged as manageable, but the fire unexpectedly intensified, rapidly spreading up a wooden escalator and into the ticket hall. The post-incident investigations identified the trench effect as a key factor in the escalation of the fire. As the flames and hot gases were funnelled beneath the escalator the fire rapidly intensified as it went up to the hall space of the station. The fire was studied with experimental scale models and computer simulations, which revealed how fire dynamics in trench-like structures encourage this phenomenon and can lead to sudden, catastrophic flare-ups, particularly significant and of danger when occurring in confined spaces such as was the case with the Kings Cross fire.

For detailed information about the trench effect see "Fires in Inclined Trenches: the Effects of Trench and Burner Geometry on the Critical Angle" by PJ. Woodburn and D.D.Drysdale

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