Air Entrainment into Mechanical Smoke Vent on Ceiling

Abstract

Air entrainment ratio to mechanical ceiling vent was investigated by model scale experiments for quiescent two-layers environment. For various combinations of venting velocity and smoke layer temperature, critical smoke layer thickness for onset of entrainment was identified by eye-observation. Air entrainment ratio was measured by the change of carbon dioxide concentration in smoke layer (before entrainment) and in exhaust duct (after entrainment). By summarizing the measured results, the followings were clarified. 1) The critical thickness for onset of entrainment depends on smoke layer temperature and largely on smoke layer thickness. When the smoke layer is thick, large venting velocity is needed to cause air entrainment. In this condition, flow into vent opening can be approximated by potential flow to point sink. As the smoke layer thickness is decreased, relatively small venting velocity can cause air entrainment. In this condition, flow pattern can be approximated by potential flow to line sink or to plane sink depending on aspect ratio. 2) Even at the critical condition determined by eye-observation, certain amount of air is entrained. 3) Beyond the critical condition, air entrainment ratio can be approximated by the fraction of actual smoke layer thickness to critical smoke layer thickness for onset of visible air entrainment.