Negatively buotant wall flows generated in enclosure fires

Abstract This paper considers wall flows that arise in enclosure fires. Such flows are generated due to the temperature difference between the wall and the adjacent environment as well as due to the downward turning of the fire-plume-driven ceiling jet at the corners of the compartment. At various stages of fire growth and at several locations, the flow is subjected to an opposing buoyancy force. These flows are termed negatively buoyant and the paper investigates in detail the penetration and heat transfer characteristics of flows relevant to enclosure fires. The transport of mass, momentum and energy in wall flows is determined quantitatively, using available analytical results on boundary layer flows. The significance of wall flow effects in a typical compartment fire is studied. It is shown that these effects are important, since they cause additional transport which is comparable to that due to the fire plume or the flow at the opening, and must be included in a mathematical model for an accurate prediction of the changing environment in the enclosure. Negatively buoyant wall and free jets are studied experimentally to obtain the penetration depth, the entrainment into the flow and the wall heat transfer. The penetration of buoyancy-induced wall flows and of negatively buoyant wall jets in a two-layer stably stratified environment is also studied in detail experimentally. The experimental results are presented in the form of correlating equations which can be applied to the existing models for compartment fires. An analytical, integral model for including wall flows is presented, followed by a more accurate treatment based on the experimental results obtained. It is shown that the inclusion of wall flows is important in an accurate prediction of the downward movement of the interface, between the upper and lower zones of a room fire, at the initial stages of the fire and also in the calculation of the transport processes at later stages. Thus, the paper presents the basic information needed for the incorporation of wall flow effects into existing mathematical models for room fires and applies the results to a few typical fires. The trends observed are physically reasonable and agree with earlier work on this problem.

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