Heat and Mass Transfer Processes Affecting Smoke Control in Atrium Buildings
Creators: name: Hansell, Graeme O.
Publisher: name: London South Bank University
Year: 1992

description: Modern atrium buildings have suffered the effects of fire, most notably the accumulation of smoke in the atrium, spreading throughout the building. This poses a hazard to life, as escape routes may be affected on many levels simultaneously. Hence there may be a need for a properly designed smoke management system. A comprehensive design guide for smoke control in atrium buildings is necessary if systems are to be satisfactorily designed. There is no fully validated theory describing the flow of smoke from a room with a wide opening, which can allow for a variations in fire size, heat output and room and opening geometry. There is no satisfactory description of the entrainment due to a downstand/balcony combination, which may be one of the most important parameters of a design. There is only limited guidance on adhered plume flows and no simple model exists describing heat transfer from an atrium smoke layer. A series of experiments using a 1/10th scale model atrium have been carried out to examine the mass transfer due to an adhered thermal line plume, which may be many metres in height on the full-scale, in order to assess the validity of the existing theoretical model of Morgan and Marshall. The outflow from the simulated fire room was measured, and the relationship with the coefficient of discharge, gas temperature, aerodynamic disturbances and theoretical horizontal flow models discussed. The entrainment coefficient for an adhered plume is derived, and the practical limitations of this value are given. The same model was fitted with a balcony projection beyond the opening, and simple experiments performed to evaluate the entrainment due to downstands of differing depths, as a precursor to full-scale experiments. The effects of the model geometry upon entrainment are discussed. Full-scale experiments using liquid fuel fires of different heat outputs have been performed in a room with a balcony projection and variable width openings, and the mass flow at the downstand position is measured. The various room-generated entrainment mechanisms and aerodynamic disturbances to the flow are presented and discussed. Unusual velocity profiles in the flow are apparent for shallow downstands, and their possible causes discussed. Buoyant, visible and flow-reversal layer depths are measured and their relationship examined. The horizontal flow theories of Thomas and Morgan are compared and discussed. The coefficients of discharge for the various opening geometries are presented, and shown to vary with downstand depth. A simple empirical correlation is given. The outflow is shown to be enhanced by the plume rising beyond the opening, and a simple empirically-derived flow correction factor provided. Using these empirical corrections to the flow a single engineering equation for flow through a wide vertical opening is given, which is in excellent agreement with the experimental data, and may be used with confidence for engineering design. Measurements of the underbalcony flow were made, and the entrainment due to differing downstand and opening configurations determined. The layer depths as described above were measured and their relationship given. Unusual "vortex" flows within the underbalcony layer were noted and their effect on the outflow is discussed. The horizontal flow equations were compared with the measured flows, and were found to require a Cd of around 1.0. The limitations to the effective use of these equations due to intra-layer flow effects is discussed. The additional air entrained at the rotation region beyond the downstand was measured, and is shown to be a function of the angle of entry of the inflowing airstream, and the angle of outflow of the gas beyond the downstand. A theoretical relationship between the inflowing air velocity and the vertical velocity component of the outflow is derived, for the angular relationship between the flows, and correlates well with the experimental data. This relationship is used to correct the rotational flow model of Morgan and Marshall to satisfactorily calculate the entrainment due to a downstand. A simplified theoretical engineering calculation procedure for estimating the heat loss from a smoke layer is developed for atria with large surface areas of single glazing. This model allows the resultant gas layer temperature, and its effect on the design of a smoke control system, to be determined.