The physics of the outflow from the opening of an enclosure fire and re-examination of Yokoi's correlation

A new length scale is proposed for correlating, flame heights, plume temperatures and heat fluxes on external façades from flames in enclosures. Yokoi proposed a length scale ro (equal to the radius of a circle having half the area of the opening) and a dimensionless temperature Θ (from thermal plume analysis) that are widely used in correlating the temperature distribution in the hot gases issuing from the opening outside the enclosure. This paper revisits the physics of façade flames from enclosures by discussing the applicability of Yokoi's proposals and suggesting that two length scales are needed to describe the physics in over-ventilated and under-ventilated fires: one length scale is related to the thermal outflow at the opening and the other one represents the length after which the flow turns from horizontal to vertical owing to buoyancy. These new length scales correlate experimental data better than using Yokoi's length scale. Moreover the analysis and experiments show that the local plume density in the dimensionless Yokoi's temperature Θ has also to be replaced by the ambient density in conformity with an improved understanding of the physics of entrainment in buoyant plumes.

► Two length scales have been discovered to characterize the development of the flow from an opening outside a burning enclosure.
► One length scale represents the flow of the convective heat at the enclosure opening.
► The other represents the length scale after which the flow becomes vertical from horizontal.
► With these length scales a dimensionless temperature distribution outside the enclosure was developed.