Study On Fire Behaviors And Smoke Movement In Staircase And Adjacent Compartment Of High-rise Buildings

Over the past decades, high-rise buildings have been constructed widely in many cities due to the acceleration of urbanization process and the scarcity of land resources in China. The fire safety of high-rise buildings has attracted public attention due to frequent fire accidents which leads to a large number of fatalities and property loss. Statistics show that more than80percent of fatalities in fires are caused by toxic gases such as carbon monoxide instead of the burning fire. Many vertical channels existing in the high-rise building lead to the rapid smoke spread. There are two special fire hazards related to high-rise buildings:the obvious stack effect in the vertical channels and the significant influence of external wind. Therefore, it is worth investigating the smoke movement characteristic and the fire behaviors in the high-rise buildings and the effects of these two factors should be focused.In the current study, a set of experiments has been conducted in a1/3scaled high-rise building model with12floors to study smoke movement mechanisms and vertical temperature distribution in the staircase. It is found that the hot smoke movement along vertical staircase has two quasi-steady state stages during the whole burning period. Turbulent mixing strongly affects the smoke movement at the first stage while the stack effect plays a significant role at the second stage. Moreover, both fire size and location of opened window have significant effects on the duration of the two stages. The mechanisms of smoke movement have a significant effect on the vertical temperature distribution in staircase. The temperature attenuation coefficient β at the first stage is larger than that at the second stage. When the fire source is located at the middle floor, it is also found that the doors state below the fire source has a significant influence on the vertical temperature distribution in staircase. The temperature attenuation coefficient β is found to be larger in cases with the door opened due to the fresh air flowing into the opened staircase.The air flow pattern in the room adjacent to a staircase is affected by the stack effect. A large amount of fresh air is sucked into the fire room by the stack effect, which provides sufficient oxygen for combustion, and the flame would be stretched. The influence of stack effect on fire behaviors in the compartment is studied in the1/3scaled building model. The flames of methanol and heptane pool fires incline towards the staircase under the influence of stack effect. The flame tilt angle θ of methanol pool fire increases with increasing Ri-1. While the values of flame tilt angle of heptane pool fires in all cases are closed, and the average value is about69.7°. The location of window opened in the staircase has little influence on the flam tilt angle. It is furthermore found that the dimensionless flame height (Hf/D) and length (Lf/D) of leaned heptane flame under stack effect are proportional to1/4power of the dimensionless heat release rate (Q*). The temperature rise (AT) at the continuous flame zone is lower under the stack effect than that in a free burning case at open space due to the cooling of airflow. The flame of pool fire is stretched and the lengths of continuous and intermittent flame zones are much greater than those obtained at open space. The value of L/Q2/5in the intermittent flame zone ranges from0.12to0.25in the current research whereas it ranges from0.08to0.2at the open space.The fresh air sucked into fire room plays a significant role in the temperature distribution and burning rate of pool fire in the compartment. Under the action of stack effect, the upper hot smoke temperatures in the atrium are higher than these in the fire room due to the tilted flame. Meanwhile, the opened window position in the staircase has an important effect on the temperature distribution in the compartment. The results show that the velocity of airflow induced by stack effect is proportional to1/3power of the heat release rate of fire source. The radiant heat flux and total heat flux at the left sidewalls of staircase are also investigated, and the measured values in an opened staircase are higher than those in a closed staircase, due to the tilted flame.The smoke movement and fire behaviors in high-rise buildings under the external wind have been studied in a1/6scaled building model. Based on the experimental results, the magnitude and direction of external wind have a significant influence on the smoke movement in staircase and the fire behavior in the compartment. Four different scenarios were found in the experiments. In Scenario I, the flame tilt angle, temperature and heat flux in the staircase increase gradually after ignition and maintain a maximum for a while. In this scenario, there is no or a small external wind. The stack effect is the main factor inducing smoke movement, In addition, it is found that the side wind speed has little influence on the smoke movement and fire behavior. In Scenario II, the temperature and heat flux in the staircase are very low at the initial burning stage and then increase sharply to the maximum values. The flame firstly inclines toward the outdoor owing to the effect of external wind however it becomes straight up gradually and eventually inclines toward the staircase. In this scenario, the stack effect can overcome the external wind resistance and induce the smoke movement after a certain period of time. The modified Fr is proposed to address the flame direction transformation and the calculated critical value of Fr is0.814. In Scenario III, under large external winds, there was no noticeable temperature and heat flux rise in the staircase during the whole burning process. The flame inclines toward the outdoor throughout the experiments. In this scenario, the stack effect can not overcome the external wind which flows into the staircase. In Scenario IV, the temperature and heat flux in the staircase decrease and the flame inclines toward the outside after the wind machine is turned on. In this scenario, the external wind can overcome the block of hot smoke and flow into the staircase to cool it down. The mass loss rate increases sharply to the maximum value when the flame is straight up after the wind machine is turned on. The temperature and heat flux decrease rapidly with the increasing external wind speed.