A study of internal fire whirl generated in a vertical shaft

Abstract

With appropriate setting of a vertical shaft, internal fire whirl (IFW) can be formed, which is different from a normal fire burnt in free space. The flame height of an internal fire whirl can reach several times that of a normal fire as well as with a faster burning rate. This thesis examines and discusses the criteria for the formation of an IFW inside a vertical shaft. Correlation between the flame height and the single corner gap width has been deduced through analytical study of experimental data from an experiment on IFW. In a vertical shaft with a single corner gap, the gap width is found to be a key factor in generating the fire whirl. The swirling motion of the IFW is also studied in detail in this thesis by using a high speed camera. The formation of an IFW at different stages is analyzed in detail and vortex tubes are found moving around the horizontal pool surface. Further correlation of the flame height and the corner gap width is obtained based on the experimental data and analytical study on vertical distribution of the radial velocity. The arrangement of different roof opening settings of a vertical shaft is also studied in this thesis. Background-oriented Schlieren (BOS) technique is applied to study the hot air flow pattern from the vertical shaft. It is found that compared to a vertical shaft model with the roof closed, the hot smoke layer temperature is lower for the roof half-closed as well as the neutral plane height is higher. Swirling motion of IFW is also studied based on experimental results and solution of the angular velocity equation for circulatory fluid motion. Attenuation of swirling motion of the fire whirl was then studied by including the variation in vertical velocity and viscous dissipation using the basic equations of vorticity. The relative effects of baroclinic force and buoyancy were found to depend on height and on the heat release rate from the fire pool. The height and the heat release rate were found to be dependent factors on baroclinic force and buoyancy. The regions of the fire whirl are observed from bottom to top based on the baroclinic force and buoyancy ratio. Vertical variation of Froude number as well as the rotating velocity and attenuation of vorticity from bottom to top of the fire whirl were studied by analytical study. Furthermore, numerical simulations with computational fluid dynamics on medium-scale IFW using a fully-coupled large eddy simulation incorporating subgrid scale turbulence and a fire source with heat release rates compiled from experimental results were carried out. It is found that the numerical simulation results agree well with the experimental results for flame surface, temperature and flame length. IFW flame region and intermittent region were longer than those of an ordinary pool fire. The modified centerline temperature empirical formula was derived. Variations of vertical and tangential velocity in axial and radial directions are shown. The vortex core radius was found to be determined by the fuel bed size. The study in this thesis is focused on the relationship between the IFW and the vertical gap width in a vertical shaft to provide some ideas for appropriate fire safety management in tall buildings in the future. The use of highspeed camera also provides a new insight into examining the details of formation of IFW at different stages.