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Mechanical smoke exhaust systems are usually required in big atria of the Far East. However, the design criteria are not clearly demonstrated to be worked out through systematic full-scale tests. There are many arguments on sizing the exhaust fans, and whether a high exhaust rate is required in an atrium is a puzzle. Atrium hot smoke tests might be required to be carried out occasionally to evaluate their performance, especially when the design does not comply with the prescriptive code in having a high-enough exhaust rate. Experimental studies on mechanical smoke exhaust systems are necessary and have been carried out. The results are reported in this paper. The measured results are also compared with the empirical equations used in designs, following NFPA-92B derived by a two-layer picture. A simple zone model reported earlier is applied to justify the results. This model is also developed based on a two-layer picture. There are significant deviations of the measured results from those predicted by the equations with a two-layer pattern. Mass exchange through the two layers in an atrium fire is suggested to be the key point. By taking the estimated mass flow rate across the two layers by experiments to be 30% of the exhaust rate under high ventilation conditions, the two-layer zone model is modified. Slightly better argument with the experiments was found for the self-developed two-layer zone model, but still there are large deviations for the NFPA-92B equations.

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Smoke control by a horizontal ceiling vent in an atrium is studied in this paper. A bi-directional airflow moving upward and downward across the vent might be observed and this would affect the efficiency of smoke extraction. Whether the flow is bi-directional or uni-directional depends on the pressure difference across the vent upon activation. Before opening the horizontal ceiling vent, the pressure difference should be higher than a critical value. Thickness of the smoke layer formed under the ceiling before opening the vent is found to be important. The required smoke layer thickness for getting such a critical pressure is discussed in this paper. Two situations of no temperature difference between the inside and outside of the atrium; and the outside temperature is higher than the inside temperature, are considered.

A series of experiments were conducted to study the burning characteristics of wood cribs in a confined space. The results show that burning conditions of wood cribs in a confined space fall into two regimes: the smaller porosity factor burning regime and the larger porosity factor burning regime. The scaled steady mass loss rates of wood cribs burning in the larger porosity factor regime linearly increases with the increase of the porosity factor in a confined space. The concentrations of CO, CO2 and O2 in combustion products, the flame temperature and the mass loss rate exhibit different time-variations or relationships with the porosity factor for wood cribs burning in these two different regimes.