International Journal on Engineering Performance-Based Fire Codes, Volume 7, Number 1, p.8-14, 2005 8 NOTES ON SIZING OF HORIZONTAL CEILING VENTS WITH TRADITIONAL FLOW MODEL W.K. Chow and J. Li Department of Building Services Engineering, The Hong Kong Polytechnic University, Hong Kong, China (Received 5 July 2003; Accepted 20 August 2003) ABSTRACT Traditional flow models for smoke exhaust through horizontal ceiling vent in an atrium fire will be reviewed. There, buoyancy of the smoke layer is the driving force for extraction. Key equations on calculating the smoke exhaust rates and required vent area are derived. An atrium is taken as an example to calculate the vent areas required. Two scenarios for a fire at the atrium floor to give an axisymmetric plume; and a fire at a shop adjacent to the atrium to give a balcony spill plume are considered. It is found that a balcony spill plume will give a much higher smoke production rate and so vent with larger area is required for the same design fire in comparing with an axisymmetric plume. 1. INTRODUCTION Natural vents are commonly installed in large atria for removing smoke [e.g. 1-4]. This is also known as static smoke exhaust system in some fire codes [e.g. 5]. Most of them are horizontal ceiling vents installed at roof. This is because many atria are located in the central core of a building, so relatively easier to allocate roof spaces than vertical walls. The driving forces for natural ventilation [e.g. 2] are stack effect due to temperature differences between indoor and outdoor; wind- induced action; and buoyancy of smoke. In areas with low temperature difference between indoor and outdoor, stack effect is low except in tall lift shafts or staircases with high aspect ratio of height to length (or width) as demonstrated [6]. Wind-induced air flow is a transient phenomenon depending on the ambient conditions. Buoyancy of the hot smoke layer is rather strong in an atrium fire, especially at later stage of the fire. Therefore, natural vent design was based on removing smoke by taking buoyancy as the driving force. But for very tall atria, the smoke might be cooled down while moving up. Buoyancy of smoke will then be reduced to give lower extraction rate. Putting in sprinkler would also affect the system performance. All these should be considered in designing static smoke exhaust systems. In this paper, traditional flow models [e.g. 2-4] based on buoyancy for horizontal ceiling vent was firstly reviewed. Smoke exhaust rate was studied to understand how the required vent area was estimated. Both axisymmetric plume [e.g. 12] due to a fire at the atrium floor and balcony spill plume [e.g. 13] due to a fire in a shop adjacent to the atrium will be studied. 2. FLOW ACROSS A VENT DUE TO BUOYANCY A typical description of an elevated smoke layer in an atrium with natural vent is shown in Fig. 1a. This is the physical basis of two-layer zone models and some design guides for smoke management systems. The ceiling jet is assumed to be completely immersed in the smoke layer in most of the zone models. Circulation within the layer is not considered, giving a stagnant environment at a uniform temperature. Mixing between the smoke layer and the cool air underneath is inhibited by the density difference, and neglected in many simulations. Following analysis of Rayleigh- Taylor instability [14], lighter fluid placed above a dense fluid with an acceleration acting perpendicular towards their intersection plane will give a stable situation. Assuming the smoke layer is effectively stagnant and thick enough to give a length scale bigger than the linear dimension of the vent. Applying Bernoulli’s theorem between points A and C [e.g. 2-4,10] with pressure P A and P C for an atrium with height H: 2 A a A 0 v 2 1 P P ρ + = (1) 00 2 C g C P v 2 1 P + ρ = (2) gH P P a 0 00 ρ − = (3) g a g g A C gH ) H H ( g P P ρ − − ρ − = (4) In the above equations, ρ a and T a are the ambient air density and temperature, ρ g and T g are the