|Title:||A study on enclosure fires with mechanical ventilation system|
|Subject:||Hong Kong Polytechnic University -- Dissertations|
Buildings -- Performance
Ventilation -- Fires and fire prevention
|Department:||Department of Building Services Engineering|
|Pages:||1 v. (various pagings) : ill. ; 30 cm|
|Abstract:||Enclosure fires with forced ventilation were studied in this thesis with three stages. Stage one is on reviewing fire codes in China, Hong Kong, USA and other countries for identifying the fire safety aspects in enclosures with mechanical ventilation. Theoretical background of airflow induced by mechanical ventilation system appeared in the literature was studied. New concepts on performance-based fire codes were discussed. Stage two is on evaluation of different system designs. Possible fire hazard scenarios in relation to ventilation requirements were identified. Consequences of those hazard scenarios on the occupants, the building and its contents were assessed by fire models. Results are useful for designing appropriate fire services systems to provide better fire safety. Stage three is on developing a practical air flow model for forced ventilated fire. Cabin design, a safety concept commonly used in big halls in Hong Kong, was selected for a more detailed evaluation. This was taken as an example with provision of mechanical ventilation system. Concepts of zone models with some fire plume equations available in the literature were studied. Models were then applied for scenario analysis. The principles of ventilation system design and aspects of smoke movement in a chamber were studied. In the zone model simulations, both natural ventilation and forced ventilation conditions in a cabin were considered. The associated developed equations were solved by symbolic mathematics which have greater flexibility in changing the parameters concerned; and more transparent to the users. The two-layer zone model ASET was used for enclosures without openings. 'Bare cabin' fires with operation of smoke extraction system were solved by FIREWIND program. The Computational Fluid Dynamics (CFD) package PHOENICS was also applied to simulate several cases of cabin fire. In verifying the model, CFD results are compared with those experimental results reported in the literature including the ISO room-corner fire test and other compartment fires. Simulations with different geometrical aspects of the chamber, fire sizes, zone models, and plume models were carried out. Results are useful to identify the possible fire hazards, as well as to determine the zone model with suitable sub-models such as the plume model for different types of chamber. Air flow induced by a point heat source in a natural ventilated compartment was studied. Three models, fully-mixed model, water-filling box model and emptying air-filling box model, on air flow in a chamber reported in the literature were reviewed. With a point heat source located at the center of the compartment, two zone models were developed: Model 1 is for a compartment without any walls and Model 2 is for an enclosed compartment with adiabatic walls; but with two openings. Equations in the developed zone models were solved by symbolic mathematics. The relationship between the neutral plane height and the outlet area was drawn. Results of the two new models are compared with those predicted by CFD, and fairly good agreement was found. Thermally-induced plume in a chamber under forced ventilation was also studied. Plume equations derived under natural ventilation available in the literature were reviewed first. CFD was then applied to study two "models" induced by a heat source in a chamber with forced ventilation. They are the parallel flow model and the jet flow model. Based on the plume expression in a chamber with natural ventilation, two plume equations under forced ventilation were then derived from seven sets of CFD simulations. Those are four cases with different thermal power of the point heat source; and three cases on different air speeds. Again, symbolic mathematics was used to solve some equations concerned. These developed air flow models and derived plume equations have the potential to be used in fire assessment for compartments with mechanical ventilation. As a summary, originality and contribution to fire science and engineering can be summarized into three parts. The first part is on attempting to apply performance-based fire code in China with carpark as an example. The second part is on demonstrating zone models and CFD field model as simulation tools for fire safety assessment. Lastly, new zone models for studying aerodynamics in chambers and plume equation under forced ventilation were developed.|
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