Smoke movement in high rise building under various ventilations

The research focused on determining the performance of natural and mechanical smoke control systems in a high rise building fire. Generally, it is known that the flow rate for mechanical ventilation is fixed and that of the natural ventilation depends on the fire size. Therefore in the initial stage, the flow rate of a mechanical ventilation system is greater than that of a natural ventilation system. As the fire grows the performance of the two smoke control systems is investigated through the neutral plane, smoke movement pattern and fire released heat as well as the heat transfer and temperature of the structure during the high rise building fires which are all controlled by the flow rate. The data shows the average smoke velocities and the average temperatures during the high rise building fire. The temperatures and velocities were measured using thermocouples and velocity sensors respectively. The thermocouples and velocity sensors were placed in the middle of each floor of the high rise building. Each floor consisted of a room and a stairwell therefore the average temperature or velocity for every floor was obtained by averaging the figures in the room and stairwell of each floor. The height of each floor was 3m high. The average temperatures in the lower floors near the fire origin were higher than those in the upper floors that were far away from the fire origin . There was a general decrease in temperature with corresponding height from the floor of fire origin . The reduction in smoke temperature could be attributed to cooling by the obstructions as well as air mixture during the upward movement process. For the natural ventilation system, the average temperatures in the floor of fire origin decreased with increasing fire heat release rates whereas the average temperatures of the floor of fire origin for mechanical ventilation increased with increasing fire heat release rates. The velocity of smoke could determine the smoke movement pattern and fire released heat. The smoke movement pattern was in a vortex, which is circular as it migrated upward the stairwell and the upper floor rooms. At the tread in each floor of the stairwell, the pattern of circular motion was observed. However, it was observed that the direction of smoke flow under the stairwell tread in the upper floors was changed from vertical to horizontal and then migrated upward. This flow pattern was repeated and the phenomenon was similar to other fire heat release rates. The velocity of smoke was greater in the vortices and many vortices were located in the lower floors as confirmed by the greater turbulence within those regions. It can be said that the smoke velocity is affected by temperature.

本研究旨在探究高层建筑火灾场景下，自然排烟与机械排烟系统的性能表现。现有共识表明，机械通风系统的流量固定不变，而自然通风系统的流量则取决于火灾规模。因此在火灾初始阶段，机械通风系统的流量高于自然通风系统。随着火灾规模扩大，本研究通过中性面、烟气运动形态、火灾放热速率，以及受流量调控的高层建筑火灾过程中的结构传热与温度特征，对两类排烟系统的性能开展分析。本数据集涵盖高层建筑火灾过程中的平均烟气速度与平均温度数据。其中，温度与速度分别通过热电偶（thermocouple）与速度传感器（velocity sensor）测得。热电偶与速度传感器布置于高层建筑的每一层中部。每层均包含一间独立房间与一处楼梯间，因此每层的平均温度或烟气速度通过该层房间与楼梯间的测量值取算术平均得到。每层建筑高度为3米。起火楼层附近的低层区域平均温度高于远离起火点的高层区域，整体而言，温度随距起火楼层的高度升高呈逐步下降趋势。烟气温度的降低可归因于烟气上升过程中障碍物的冷却作用与空气混合效应。对于自然排烟系统，起火楼层的平均温度随火灾放热速率的升高而降低；而机械排烟系统的起火楼层平均温度则随火灾放热速率的升高而升高。烟气速度可影响烟气运动形态与火灾放热速率。烟气在向上蔓延至楼梯间与上层房间的过程中，呈现涡旋状循环运动形态。在每层楼梯间的踏步处，均观测到圆周运动模式。然而研究发现，上层楼梯间踏步下方的烟气流动方向由垂直转为水平，随后继续向上蔓延。该流动模式可重复出现，且在不同火灾放热速率下均观测到类似现象。涡旋区域内的烟气速度更高，且低层区域存在更多涡旋，这一点可通过该区域更强的湍流得到验证。由此可知，烟气速度受温度影响。