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.