Trench effects

The trench effect is a combination of circumstances that can cause a fire to climb rapidly 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-moving stream of air to deflect towards nearby surfaces. The fast-moving stream tends to experience a decrease in static pressure, which creates a pressure difference between those areas far from the wall and the wall itself. This bends the fast-moving stream towards the surface and tends to keep it attached to that surface.

Flashover is a sudden widespread spreading of fire, which occurs when the majority of surfaces in a space are heated to the point at which they give off flammable gases hot enough to ignite themselves. Prior to flashover, flammable gases may be given off but are not hot enough to ignite themselves.

The trench effect occurs when a fire burns next to a steeply-inclined surface. The flames lie down along the surface, in accordance with the Coandă effect. The flames heat the material farther up: these emit gases, reach their auto-ignition temperature and then start burning, in accordance with 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 is sustained until the fuel is exhausted.