Trench Effect (Trail Graphics)
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 Coanda effect from fluid dynamics and the flashover concept from fire dynamics.
The Coanda 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.
Flashover is a sudden broad 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 a surface that has a steep incline. The flames lie down across the inclined surface, as described by the Coanda effect. The flames heat the surface along the length of the incline. These surfaces release gases, and as the reach their ignition point, they start burning. This is the flashover theory.
These flames, according to the Coanda effect, blow a blast of flames up to the end of the sloped surface. This blast of flames continues until the surface is entirely consumed.
King's Cross Investigation.
The Trench Effect has become well known as a result of the scientific and forensic investigation of the King's Cross fire. The fire started under an escalator between the Piccadilly Line platforms and the King's Cross St. ticket hall tube station.
According to eyewitness testimony, at the early stages of the fire under the escalator, it was small and very manageable. Fire Brigade officers testified that the fire appeared ti be about the size of a large box.
TMBA provided these accident recreations based on the investigation report from the London Fire Brigade for the series "Extreme Evidence" (Court TV).
For more information call (212) 789-9077
or go here- http://www.tmba.tv/trial-graphics/arson-investigation/london-underground/
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