Fire Safety Science Digital Archive

The flow of smoke and hot gases in an enclosed shopping mall is discussed in general terms in another note. The present note gives the theoretical background of that discussion which is based to a large extent on the theory of "gravity currents" investigated by Benjamin. Formulae have been derived for calculating the rate of spread of a layer of hot gases beneath the ceiling and the depth of the layer. The depth of the spreading layer cannot exceed half the height of the mall but smoke can mix into the cold air flowing towards the fire thus effectively deepening the depth of smoke. The formulae show encouraging agreement with the results of a large scale experiment on smoke spread carried out in Japan but more experimental work is required to establish their validity. If the theory is correct it would be necessary to install a ventilation system to ensure that occupants can escape from a mall before being overtaken by smoke in the event of fire, even if a sprinkler system is fitted in the shops bordering on the mall. Should the sprinkler system fail, smoke could spread rapidly along the mall (e.g. 100 m in 30 s)