On the Significance of a Wall Effect in Enclosures with Growing Fires

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Abstract
This paper studies the significance of a wall effect that has been observed during the growth stage of enclosure fires. Relative to the two-layer phenomenon which tends to develop during such fires, the effect has to do with the near-wall downward injection of hot upper layer gases into the relatively cool uncontaminated lower layer. It is conjectured that these observed wall flows are buoyancy driven, and that they develop because of the relatively cool temperatures of the upper wall whose surfaces are in contact with the hot upper layer gases. For a growing fire (growth proportional to t m ; t being time and m≥0) in an enclosed compartment, an analysis of the conjectured mechanism for the wall flow leads to a time-dependent solution for the ratio of wall flow mass ejection rate from the upper layer, the, to the fire plume mass injection rate to the upper layer, [mdot] p . The solution indicates that in practical fire scenarios [mdot] w /[mdot] p can be of the order of “several tenths” even prior to the time that the upper layer interface has dropped to an elevation midway between the ceiling and fire. In other words, the results of the analysis indicate the importance of taking the wall effect into account in two-layer zonal analyses of enclosure fire phenomena.

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