FIELD MODELLING OF CEILING MOUNTED SMOKE VENTS.
Ceiling mounted ventilation systems (horizontal vents) are an accepted fire safety device, and it is therefore essential that we fully understand and are able to predict their behaviour and performance in all likely configurations and circumstances. Through the use of field modelling techniques, the Fire Safety Engineering Group is attempting to provide greater understanding of these operating characteristics.
The function of ceiling ventilators is to reduce the impact of the growing hot gas layer which develops beneath the ceiling of a fire compartment. This is achieved by allowing the hot gases to escape to the outside, thereby reducing the growth rate of the smoke layer depth and its temperature. This in turn eases the occupants evacuation, reduces smoke and fire damage to the structure and assists the fire fighters in the location and extinguishment of the fire
The basic principles of ceiling vent operation in idealised situations were investigated in the early 1960's. While these studies have enabled engineers to design practical smoke extraction systems, many questions still remain concerning their design and efficient operation. This is particularly true in situations where only a single vent located in the ceiling exists. Unlike the better understood vertical vent problem (compartments with a door or window opening) where the flow is stable and essentially steady, horizontal vents may produce flows which are unstable with irregular oscillatory behaviour. The resulting flow behaviour within the compartment will have an influence on vent efficiency and possibly the flash-over process, there are also implications to sprinkler and fire detector location and operation.
Using field models, the Fire Safety Engineering Group are investigating fires in compartments which are vented to the outside by a single ceiling vent.
Model predictions suggest that the flow through the vent produces oscillatory behaviour in vent temperatures with puffs of hot gases emerging from the fire compartment in regular cyclic fashion. The vent flow is observed to oscillate between three modes of bi-directional flow.
The models are also being used to undertake a parametric study to consider the effects on vent temperature oscillations (n) of varying the heat release rate (Q0). These results suggest that a relationship exists between the frequency of vent temperature oscillation (n) and the heat release rate (Q0) of the type n Q00.29 similar to that observed for compartments with two horizontal vents.
Another important observation to emerge from this work is that the vent discharge coefficient is not fixed but varies in an oscillatory manner. While its mean value of 0.59 is close to the 0.6 recommended for use in zone and empirical models it can vary by as much as 16%. This may have a pronounced effect on the flow rates predicted by zone and empirical models.
Reference:
A Two-Dimensional Numerical Simulation of the Oscillatory Flow Behaviour in Fire Compartments with a Single Horizontal Ceiling Vent Authors: L Kerrison, E R Galea and M K Patel. Paper No.95/IM/02, CMS Press, ISBN 1 899991 01 8.
