A critical study on fire-induced aerodynamics and design on smoke management system for atria

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A critical study on fire-induced aerodynamics and design on smoke management system for atria

 

Author: Li, Junmei
Title: A critical study on fire-induced aerodynamics and design on smoke management system for atria
Degree: Ph.D.
Year: 2004
Subject: Hong Kong Polytechnic University -- Dissertations
Buildings -- Smoke control systems -- Design and construction
Atrium buildings -- Fires and fire prevention
Aerodynamics
Department: Dept. of Building Services Engineering
Pages: 1 v. (various pagings) : ill. ; 30 cm
Language: English
InnoPac Record: http://library.polyu.edu.hk/record=b1762024
URI: http://theses.lib.polyu.edu.hk/handle/200/3270
Abstract: As atrium fires are not yet well understood, it is doubtful whether the fire safety provisions would work as expected. Of all the key fire aspects, fire-induced aerodynamics and smoke management systems are very important and studying these two aspects becomes the objective of this thesis. It is still not clear about the scenarios to be considered in designing smoke management systems, leading to arguments on using an axisymmetric plume, a balcony spill plume or a window plume. Some fire safety designs in an open atrium void are conflicting with each other. Natural smoke filling in atrium space and spreading throughout the building should be studied for solving that problem. This relies on a good understanding of fire-induced aerodynamics. A comprehensive design guide for smoke control in atrium buildings cannot be developed without in-depth investigations. This thesis starts with a critical review on atrium smoke management systems. Design guides and practices were reviewed and the key scientific aspects were identified. In designing smoke management systems, smoke movement in the atrium under typical fire scenarios should be understood first. Scenarios involving a fire located on the atrium floor to give an axisymmetric plume; and a fire in a room adjacent to the atrium to give a balcony spill plume were then analyzed. As there are few research works on spill plumes, there are still many uncertainties regarding this kind of plume. The dynamics of axisymmetric plumes and balcony spill plumes was studied in this thesis by Computational Fluid Dynamics (CFD) with the Reynolds Averaging Navier Stokes (RANS) approach. Different empirical plume models were evaluated by the predicted results. Selection of appropriate plume models for smoke control design is outlined. Smoke movement in an atrium was then studied by a zone modelling approach with an axisymmetric plume. Smoke filling was also studied with CFD. More practical scenarios such as balcony spill plumes were also investigated by zone models. A two-layer zone model, CL-Atrium was developed for studying the smoke filling process in an atrium due to a balcony spill plume. There, four spill plume models were compared under different fire conditions. Using this model, the smoke layer temperature and the rate of descending could be predicted to get the available evacuation time. All these four models are possible choices in CL-Atrium. This is recommended to the industry as a fire engineering tool for predicting the smoke filling process for a fire located in a compartment adjacent to the atrium. Operating the air-conditioning system or installing glazing roof would give a strong vertical density gradient in the atrium, thus affecting the smoke movement. Upward movement of a line plume under a constant negative density gradient environment was studied numerically. Thermal stratification of hot air below the ceiling is found to be very significant in the upper regions above the fire. The plume might not be able to reach the ceiling in a building with a large indoor vertical temperature gradient. These results are useful for designing smoke control systems in atria. Static smoke exhaust system in an atrium was also studied. Scenarios with an axisymmetric plume or a balcony spill plume were considered. Wind effect on the system performance was investigated. It is found that negative wind pressure above the vent would enhance the smoke extraction rate, while positive wind pressure tends to reduce the vent efficiency. The critical wind speed which might result in zero net flow or plugholing was also discussed. Because of the large volume space, the conditions of having 'near-zero' pressure drop across a horizontal ceiling vent should be considered carefully. Bidirectional vent flow across the horizontal vent might be resulted. The critical pressure difference across the vent giving the unidirectional or bidirectional vent flows would be specified. For a vent with a small pressure difference, the standard vent flow model is found to be not suitable for calculating the vent area. Evacuation pattern should be analyzed with the smoke management system. The total evacuation time predicted by different evacuation models was investigated by the software buildingEXODUS. Main factors affecting the evacuation performance were identified by using the predicted results of total evacuation time under different scenarios. Partitioning in the atrium floor was identified to be a concern in the evacuation process. Finally, a time constant was proposed to describe the smoke filling process in an atrium. It is a parameter with the atrium geometry and the design fire taken into account. Based on the empirical smoke filling equations, simple correlations between the smoke filling time and the time constant were derived. These correlations are recommended for setting up design criteria for smoke management systems.

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