# The LWF Blog

## Fire Safety Engineering for Design – Fire Growth – Part 88

July 4, 2022 10:48 am

LWF’s Fire Safety Engineering blog series is written for Architects, building designers and others in the construction industry to highlight and promote discussion on all topics around fire engineering. In part 87, LWF discussed the effect of sprinklers on fire size, before considering smoke plumes. In part 88, we talk about smoke plume entrainment and the types of plume.

The volume of smoke produced by a plume is equal to the amount of air entrained into the plume. The contents of the smoke in terms of the concentration of smoke particles and presence of toxic products depends upon the type of fuel being burned and the ventilation rate of the surrounding area.

The entrainment of the plume depends on the overall plume height. At a small plume height, the entrainment depends on the geometry of the source of the fire. At a given height, it depends on the heat output, at a large plume height, entrainment is equivalent to that above a point source.

The plume may be located in the room containing the fire, directly above, or it may be outside the area, having emerged from an open door or window. Outside factors can affect placement of the plume too, for instance the wind blowing in through an aperture can push the plume in a different direction. A simplified version of plume movement behaviour in a compartment can be illustrated with the use of a lit cigarette or an incense stick.

A plume which rises directly above the fire source is known as an axisymmetric plume and one which is outside a room, through an aperture is known as a spill plume.

### Axisymmetric Plumes

It is most common to see axisymmetric plumes when a fire is sited on a floor away from the walls. This allows the air to be entrained from all sides of the fire and along the entire height of the plume. This continues until the plume is submerged in the smoke layer that develops below the ceiling height.

CIBSE Guide E gives examples of calculations for axisymmetric plumes including for ‘far field’ and ‘near field’.