| The buoyancy, due to a fire in an enclosure with openings, makes the flow field appear the thermal stratificated fluid or rotating thermal fluid. The thermal stratificated fluid is a kind of fluid due to different densities in this flow field with a heat source. And the rotating thermal fluid is intrinsically a baroclinic fluid, the coupled result of the buoyancy and induced air flow from the opening, which is in perticular to the fire whirl. Internal fire whirls in an enclosure with different opening arrangements were studied in this thesis. Buoyancy and air flow induced by the opening should be considered in studying swirling flame motion induced by a thermal source. Fire whirls will be onsetted with sufficient heat, adequate centrifugal force, and the rapid pressure change in balancing the circular motion. Very few studies were reported on fire whirls of radius from 0.1 m to 1 m. However, fire whirls of this dimension used to occur in buildings. In-depth study should be devoted to that and being the objective of this thesis. Fire whirl induced by a heat source was studied by experiments, theoretical analysis and numerical simulation.Experiments of the fire whirls were conducted in a small 33 cm square vertical shaft of height 200 cm. Different ventilation arrangements were provided by changing the boundary conditions with one or two corner gaps. Openings at the bottom of the shaft, corner gap lengths at a certain gap width and their effects on the swirling flame were studied experimentally. Results indicated that the fire whirl was induced with faster burning rate for the case with a critical corner gap of suitable width. For example, a fire whirl would be onsetted when the corner gap length is just tenth of the shaft height. The swirling flame was observed to have three regions along the flame height.Key flow parameters of the buoyant plume were then studied. The centerline temperature of the axisymmetric plume was studied and found to be similar to the results reported by McCaffrey. This further confirmed that there are three regions in the rotating flame. Consequently, centerline temperature distribution of the rotating flame at different regions is a function of the height and the averaged heat release rate. A mathematical model was developed for internal fire whirl in a small-scaled square vertical shaft. Correlations of the maximum radial and tangential velocity of the fire whirl were further studied. Finally, the experiments of fire whirls were performed in a large-scaled squared vertical shaft with a single corner gap, and numerical simulations were studied. It was focoused on the effects of the different openings at the bottom of the shaft on the fire whirl onsetting. From the experiments, when the three sides at the bottom of the vertial shaft is connected with the ambient, ic can not onset a fire whirl. Whirl, with a certain corner gap width and fuel pool size, the strongest fire whirl occurs with three vents at the bottom of the shaft. Swirling thermal flow field in a large vertical shaft was studied numerically by using Computational Fluid Dynamics with large eddy simulation. Conditions of onsetting the fire whirl were derived. Variations of the temperature, density and pressure along the radial direction were analyzed. Several cases with different openings at the bottom of the shaft were simulated to illustrate the importance of the bottom shearing force in generating the fire whirls. Distribution of the radial and tangential velocity were derived. The swirling center of the fire whirl was shifted and quantitatively analyzed.As a conclusion, internal fire whirl was studied successfully in this thesis by experiments, mathematical model, and numerical simulations. Motion of the fire whirl, the correlating parameters, and basic physical phenomena were studied. Investigation works of this thesis provided theoretical basics on building fire safety which is useful while implementing performance-based design.
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