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.
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