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Research on the Characteristic of Thermal Whirling Flow Driven by Buoyancy in a Vertical Shaft

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Tutor: ZhouYunJi
School: Harbin Engineering University
Course: General and Fundamental Mechanics
Keywords: thermal buoyancy,vertical shaft,fire whirl,large eddy simulation,approximate pro
CLC: TU998.1
Type: PhD thesis
Year:  2013
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Abstract:
For its formidable whirling force and buoyancy, there will be faster burning rate, higherflame temperature and more losses during fire whirls than in other fire scenarios. With thebigger spaces and more complicated vents in some existing buildings especially in the atriumsor stair wells, it is more possible fire whirls occur. Fire whirl in a building is extremelydangerous for human life and properties. Recently, more researchers pay attentions to thehuge danger of building fire whirls. To study the forming conditions, movement and flowfield of fire whirls is very important to keeping fire whirls from occurring and developing inbuildings. The study is very significant to provide public safety.Experimental studies, numerical simulating and theoretical analysis are the mainmethods for studying fire field. Characteristic of fire whirl and its flow field in a vertical shaftwith a said slot was studied. The detailed contents are as following:Firstly, mathematical models of thermal buoyancy whirl flow field were summarized. Amulti-species, low Mach number£¬with viscous and buoyant flow adapting fire field, includingthe conservation equations, mixture fraction combustion model and finite volume radiationmodel, was brought forward. Turbulence was simulated by large eddy simulation (LES) withthe model equations filtered. The vortex equations, vortex dynamics equations, fluiddynamics equations in whirling system and vortex equations of fire field were then derivedbased on the momentum equation.Secondly, an experimental facility was built to carry out experiments on fire whirl in avertical shaft with a sidewall slot. It was found that the fire whirl was fiercest, highest, andburning fastest when the dimensionless sidewall slot was in a critical range. Keeping a criticaldimensionless sidewall slot, experiments on fire whirl was carried out with three differentpool diameters and fuel amount. The vertical and radial temperature distribution, fuel burningrate, heat release rate and flame height of fire whirl were studied thoroughly. Theexperimental empirical expressions for fuel burning rate and flame height put forward byother researchers were then modified according to the fire whirl experiments.Thirdly, the fire whirl experiments in the vertical shaft were simulated by fire fieldpackage FDS using LES method. Distribution of physical flow field in the shaft, air temperature on centerline, radial temperature at different height and flame height of firewhirls driven by three different pool diameters and heat release were studied. Simulationresults were compared with experiment results in order to prove the validity of model. Thenvertical velocity and whirl velocity of fire whirl were predicted. Variation of the ratio ofvertical velocity to whirl velocity was studied further.Fourthly, some classical vortex exact solutions and their characteristic were reviewedand analyzed. The solution form of whirl vortex was revised based on the experimentalobservation. Approximate progression solutions of the attenuation of fire whirl vortex withradius and whirl velocity with height and radius were derived by introducing semi-empiricalcoefficients. The values of semi-empirical coefficients could be work out according to thesimulation results. Then the approximate progressive solutions of fire whirl vortex and whirlvelocity based on experiment and simulation were derived.The results achieved and the conclusions drawn in this study are of great importance andapplicable in fire prevention and fire fighting against infoor fire whirls. Furthermore, thestudy provides guidelines for further study on physical characteristics of fire whirls.
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