Flow of hot gases in vertical shafts with natural and forced ventilation

This paper presents the results from an experimental investigation on the flow of hot gases induced by pressure and buoyancy in vertical shafts. This flow circumstance simulates the propagation of smoke and other combustion products in vertical elevator shafts due to fire in high-rise, multistoried, buildings. The effect of natural and forced ventilation, occurring through shaft openings or vents, on the spread of smoke and hot gases is of particular interest with respect to the development of hazardous conditions in the building. Different configurations are investigated, such as natural and forced ventilation at the top or the bottom of the vertical shaft. The inflow conditions for the hot gases, at a vertical opening near the base of the shaft, are varied and characterized in terms of the inlet Grashof and Reynolds numbers. Extensive temperature measurements are taken within the shaft. These data are used to investigate the steady state thermal field that arises for various scenarios. These results may then form the basis for developing appropriate approaches for smoke mitigation in high-rise building fires. Flow visualization, using smoke and a Schlieren system, is also used to study the flow characteristics in the shaft and near the hot gas inlet. The results obtained indicate that a wall plume arises in several cases and a relatively well-mixed flow in others. A strong wall flow arises at high Grashof numbers and low Reynolds numbers, while increasing Reynolds numbers leads to greater mixing in the shaft. The resulting flow and its effect on the spread of hot gases and temperature decay are investigated for different shaft configurations and inlet conditions. It is found that the best smoke or hot gas removal and lowest shaft temperatures occur with natural ventilation at the top and forced ventilation up from the shaft bottom. It is also shown that forced downward flow at the top can be used to arrest smoke spread, as well as to minimize the effects of the fire.