Studies On Fire Behavior And Smoke Movement In Stairwell Of High-rise Buildings Under Different Environmental Conditions

With the development of the national economy and the acceleration of the urbanization process,high-rise buildings have sprung up like mushrooms after a rain.Meanwhile,the increased fire safety problems associated with high-rise buildings have aroused a broad public concern.Statistics have shown that nearly 85%of deaths in fire accidents are caused by the toxic gases produced by fires.The stairwells,as typical vertical passages in high-rise buildings,will become the main channel for hot smoke to spread to other floors and pose a serious threat to the safety of evacuees once entered by hot smoke in the fire accident.Environmental conditions,such as opening states of the stairwell,ambient pressure and external wind,have siginificant impacts on the high-rise building fires.A solid undestanding of the fire behavior and smoke movement in the stairwell under different environmental conditions makes vital practical significance and guiding value for fire prevention and smoke control in stairwells of high-rise buildings.Therefore,this thesis experimentally and numerically studied the fire behavior and smoke movement in the stairwell of high-rise buildings under different environmental conditions by conducting full-scale and model-scale experiments as well as the CFD numerical simulations.Firstly,the full-scale experiments were carried out to study the influence of opening location on the smoke movement and temperature distribution in the stairwell.The experimental results show that the inclined flame induced by the side opening has a wider ignition area and higher threaten to combustibles in the stairwell,compared to the stairwell fire with the opposite openings.The strength of the stack effect in the stairwell initially increases,then decreases with the opening height.According to the temperature attenuation and rising velocity of hot smoke,the stairwell can be divided into an upper and a lower region.In the lower region,both the stack effect and the turbulent mixing play important roles in the smoke movement,which leads to a small temperature attenuation and a large rising velocity.On the contrary,in the upper region,the turbulent mixing is dominant,resulting in a large temperature attenuation and a small rising velocity.Based on the theoretical analysis,the equivalent HRR(heat release rate)of hot smoke in the upper region is determined.An integrated correlation is proposed for predicting the rise-time of the smoke plume in the stairwell,and the resistance coefficients of hot smoke flowing in the stairwells and shafts for different scales are compared.Secondly,the model-scale experiments were conducted to study the influence of top opening state on fire behavior of double fire sources in the stairwell.The experimental results show that the lower fire source,whose burning rate is not affected by the top opening state and the pool size of the upper fire source,is an independent variable.The smoke generated by the lower fire source forms a thermal pressure in the stairwell,which inhibits the stack effect induced by the upper fire source.When the top opening is open,a large amount of smoke flows out from the top opening,and a small amount flows into the upper burning lobby from the stairwell.The horizontal inertial force induced by the smoke flow will change the deflection direction of the upper flame,making the flame change from tilting toward the stairwell to burning vertically.As the HRR of the lower fire source increases,the upper flame begins to tilt intermittently toward the right room.When the top opening is closed,the smoke flow patterns in the stairwell have changed,and most of the smoke flows into the upper burning lobby,causing the flame on the upper floor to tilt directly toward the right room.With the increasing HRR of the lower fire source,the inclination of the flame becomes larger.The burning rate of the upper fire source remains stable with the lower fire source when the top opening is open,but increases with the HRR of the lower fire source after the top opening is closed.Through the data analysis,a prediction formula for the smoke temperature at the ceiling of the upper lobby under different states of top opening is put forward.Thirdly,the numerical simulations of full-scale stairwell fire were carried out to investigate the influence of ambient pressure on the fire behavior and smoke movement in the stairwell.The simulation results show that as the ambient pressure drops,the air mass flow rate decreases and the flame length increases because of the weakened stack effect under lower pressures.Through dimensional analysis,a formula to predict the mass flow rate of fresh air into the stairwell under reduced pressures is proposed.The temperature of hot smoke near the fire source increases as the ambient pressure drops.While on the floors far from the fire source,the temperature decreases with the decreasing pressure due to the larger temperature attenuation under lower pressures.On account of the weakened stack effect,the velocity and hot pressure of smoke in the stairwell both declines as the ambient pressure drops.Through theoretical analysis,a prediction formula for the rise-time of the smoke plume under reduced pressures is found,with the consideration of the convective heat transfer between smoke and walls.Fourthly,the model-scale experiments were carried out to study the external wind on the fire behavior and smoke movement in the stairwell.The experimental results show that there is a synergistic relationship between the positive wind and the stack effect,and the positive wind can accelerate the smoke flow in the stairwell.For a small wind velocity,the inhibition effect of the positive wind dominates on the combustion,while for a large wind velocity,the promotion effect plays a significant role.The airflow velocity at the room door increases with the wind velocity,and the relationship between the airflow velocity and the positive wind velocity is established.In addition,a formula for predicting the smoke temperature distribution in the stairwell coupled with the HRR of the fire source and the positive wind velocity is put forward.While the lateral wind and stack effect are in a competition relationship.The lateral wind can slow down the smoke flow in the stairwell and change the deflection direction of the flame in the burning room.According to the critical criterion R for judging the flame deflection,the lateral wind velocity is divided into three zones.The L zone:0 ≤ R<0.41,the lateral wind cannot overcome the stack effect and the flame deflects toward the stairwell;The M zone:0.41 ≤ R<2.32,there is a relative balance between the lateral wind and the stack effect,and the flame remains burning vertically;The H zone:R≥ 2.32,the lateral wind has completely overcome the stack effect and the flame deflects toward the outdoor space.For a small pool size,the burning rate of the fire source first decreases and then increases with the growth of wind velocity,and finally remains relatively stable.While for the large pool size,the burning rate does not show a stable stage,but continues to grow.The attenuation coefficient of the smoke temperature in the stairwell increases with the increasing wind velocity and then remains constant.