Studies On Smoke Movement & Control In Shafts And Stairwell In High-rise Buildings

Many high-rise buildings had been constructed in trecent years with rapid development of cities.High-rise buildings makes people's life more convenient, satisfies the architectural artistic and meets the function requirements.However,it also brings about many new fire safety problems for us.High-rise building fire has attached more and more research attention due to the fire disasters occurred in recent years.A stairwell connecting different floors of a building used for occupant transportation maybe becomes a path for smoke spread in case of fire,which would endanger the occupants' lives.Smoke control in stairwell is very important for saving lives in case of high-rise fire.However,in order to provide appropriate fire safety,the dynamics of smoke spreading in shafts and stairwells should be well understood first.Interaction of three factors would make the smoke movement induced by fire in shafts and stairwells much more complex than in normal compartment.The three factors are fire induced buoyancy,the confinement of vertical long-narrow configuration of shafts and stairwells and the flow resistance of the stairwell.In this thesis,the above issues will be focused on.A model was developed for predicting the transient plume rise in the vertical direction in a shaft and its steady upward movement in high-rise building fires as Boussinesq approximation was not suitable when the smoke temperature was high under larger fires.Heat transfer from hot gases to side walls and the density variations due to temperature rise were considered,The traditional models by former researchers usually take considerations of the above both issues for simplification.The temperature rise curves in the growing period roughly fit exponential increase with time,and their non-dimensional curves to be linear increase tendency.The transient front of smoke plume in shaft can be rationally simplified to be an exponential form versus time,and their non-dimensional expressions to be linear functions.An exponential decay model with an equation to predict the vertical temperature distribution in steady stage in shafts was also found by theoretical consideration.The above theoretical models and curves were all further verified by 1/8 scaled experiments.Full-scale fire experiments were conducted in a 27 m tall stairwell,along with CFD numerical simulation,to study the smoke filling and movement induced by fire in stairwell.Dynamic and thermal physics characteristics,including smoke temperature field,upward traveling velocity of smoke,pressure distribution and the flow pattern,were measured.Vortices were found both in the upper and lower zone of each floor and the convection between heat and cool gases was enhanced in the vortices.This was attributed to the greater turbulences in these regions.The size of hot gas vortices increases with higher temperature difference between the upper and lower zones.The smoke stratification found by Qin in lower stairwells was also verified by the experiments and simulations in higher stairwells.These experiments systematically provided important full scale data for high-rise building fire research.Experiments were conducted in a 1/3 scaled 12-level stairwell configuration with the top floor opening to outside to study the smoke movement in stairwell and the effect of stack effect to the adjacent compartment combustion.With the development of fire,more and more smoke and heat was transferred to the stairwell by fire,which promote the inner temperature in stairwell and accelerate the stack effect and the stack effect would further accelerate the traveling speed of smoke.The temperature was found exponentially decay with height in the stairwell in steady stage.The burning rate was also accelerated by the stack effect in the stairwell,because air convection was enhanced by the stack effect and strong air supply was only through the compartment door to outside.An air flow induced by stack effect also made the flame nearly inclined to horizontal.The make-up air separated the flame into two branches.Finally,the function of the stairwelll pressurization system was numerically studied by considering various factors,such as the position of fire and pressurization fan,the stairwell door crack,and ventilation conditions.Some considerations were taken for the optimization of the stairwelll pressurization design in aid of human evacuation.These had provided technical support for the improvement of the design for pressurized smoke control system in high-rise buildings.