【作者】 师文喜；

【导师】 孙金华； 纪杰；

【作者基本信息】 中国科学技术大学 ， 安全科学与工程， 2014， 博士

【摘要】 随着城市化进展的加快和土地资源的日益稀缺,我国高层建筑的数量和高度得到了快速增长,再加上近年来房价的飞速上涨和住房需求量的不断攀升,进一步刺激各房地产企业加快投资和建设高楼大厦的速度,各地(超)高层建筑如雨后春笋般不断拔地而起,具有数量多、高度高、结构复杂、功能多样化的特点,并且许多建筑已经成为城市的标志性建筑,同时,高层建筑也是对施工技术和安全保障的严峻考验,尤其是高层建筑火灾安全问题备受人们关注。近年来发生了许多大型高层建筑火灾事故,并有愈演愈烈之势。高层建筑由于楼层多、结构复杂、体积大,且烟气蔓延途径多,其火灾危险性更大,尤其是烟囱效应和外界风带来后果尤其严重,因此,研究高层建筑烟气运动规律和火灾发展特性具有重要意义。湍流混合运动和烟囱效应是楼梯间内烟气运动的两种主要机制,两种运动机制分别在火灾发展不同阶段起主要作用。在第一个稳定阶段,楼梯间内烟气主要依靠湍流混合作用向上缓慢的运动；在第二个稳定阶段,楼梯间内烟气主要依靠烟囱效应向上快速的运动。两个稳定阶段的持续时间都会受到火源功率和通风口位置的影响。楼梯间内烟气运动机制影响其竖向温度分布。与湍流混合运动作用阶段相比,在烟囱效应作用阶段,楼梯间烟气竖向温度衰减较慢,温度衰减系数β值较小。随着楼梯间通风口位置升高和火源功率增大,β值越小,封闭楼梯间的烟气温度衰减最快,β值最大。着火层位于高层建筑中间楼层时,着火层上方和下方通风口状态对楼梯间内竖向温度分布有较大的影响。在烟囱效应作用下,甲醇和正庚烷池火向楼梯间方向倾斜,本文引入理查森数Ri分析烟囱效应对火焰倾角的影响。对于甲醇池火,火焰倾角θ与理查德森数Ri1呈线性递增关系,而对于正庚烷池火,在烟囱效应作用下各工况火焰倾角θ相差不大,其平均值为69.7°,楼梯间通风口位置对正庚烷的火焰倾角θ影响较小。对于火焰长度和火焰高度,实验结果表明在烟囱效应作用下正庚烷池火的无量纲火焰长度(Lf/D)和火焰高度(Hf/D)与无量纲火源热释放速率(Q*)的1/4次方呈正比。对于火焰温度,在烟囱效应产生的风速的冷却作用下,不仅倾斜火焰连续区的温升(△T)低于开放空间的火焰连续区温升,AT大约776℃,而且火焰连续区和间歇区的范围也有所增大,对于火焰连续区,L/Q2/5从无风情况下0-0.08增加到0-0.12,而对于火焰间歇区,L/Q2/5从无风情况下0.08-0.2增加到0.12-0.25。烟囱效应产生的风速和火源的燃烧速率相互作用、相互影响,研究结果表明风速V与火源功率Q的1/3次方呈线性递增关系。楼梯间通风口位置和状态对火源的燃烧速率、着火房间和前室的温度场分布以及楼梯间内辐射和总热流值有一定的影响。通过设计和建造1/6尺寸外界风楼梯间实验台,研究结果表明在不同外界风向和外界风速作用下,楼梯间内烟气运动和相连房间的火灾特性具有四种明显不同的模式。模式Ⅰ：随着燃烧时间增加,楼梯间内各层烟气温度和总热流也逐渐增加,同时房间内火焰逐渐向楼梯间方向倾斜,最终温度、热量值、火焰倾斜程度都达到最大。烟囱效应是楼梯间内烟气运动的主要驱动力。模式Ⅱ：在燃烧初期,楼梯间内各层温度和总热流值都非常小,燃烧一段时间后,楼梯间内温度和总热流突然地迅速升高。随着燃烧速率的增大,烟囱效应逐渐克服外界风成为主要的驱动力。模式Ⅲ：当外界风速比较大时,在整个燃烧过程中,楼梯间内各层烟气温度和总热流值都非常小,房间内火焰一直向室外方向倾斜。火源产生的热烟气始终不能克服外界风的阻力进入楼梯间内部。模式Ⅳ：在燃烧期间启动风机,楼梯间内各楼层的烟气温度和总热流呈现下降的趋势,房间内火焰逐渐向室外方向倾斜,随着外界风速增大,烟气温度和热流下降越快。房间内火焰的倾斜方向主要与烟囱效应作用力Fstack和外界风作用力Fwind有关,通过理论分析引入修正的弗洛德数Fr来判断和分析房间内火焰倾斜方向,最终计算出火焰方向转变的临界弗洛德数Fr值为0.814。当Fr小于O.814时,火焰向室外方向倾斜,否则,火焰向楼梯间方向倾斜。更多还原

【Abstract】 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.更多还原