Editing Trench effect (section)

{{Short description|Rapid spread of fire up an inclined surface}}
{{no footnotes|date=August 2014}}

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 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.