Details

Stack effects have an important influence on smoke movement in high-rise buildings. The stack effect in a room with a shaft causes a negative pressure condition in the room. Smoke can be extracted out of the room through the shaft and fresh air will enter the room through inlet openings. The flow through the inlet may be unidirectional or bidirectional. So, the smoke may also flow out of the room from the inlet for some conditions. The design of the inlet is important to prevent smoke from flowing out. The ventilation factor is a basic parameter to study compartment fires. In this paper, a theoretical analysis was developed to obtain a critical ventilation factor to prevent smoke to overflow from the fire room through an inlet. Numerical simulations with different ventilation factors were also conducted to verify the theoretical model. The results showed that a negative pressure was generated at the bottom of the shaft. The magnitude of the negative pressure difference increased with increasing ventilation factor. However, the negative pressure difference increased at a low rate only within a narrow range of ventilation factors. The magnitude of the negative pressure induced at the opening for supplying air decreased with increasing ventilation factor. At some point, the pressure inside the room was even higher than that outside for some ventilation factors. Consequently, smoke moved out of the fire room through the air-supply opening under a larger ventilation factor. This result illustrated that there is a critical value for the ventilation factor. When the ventilation factor is less than this critical value, there is no smoke flow out of the room. The critical value of ventilation factor was calculated using the theoretical model and verified by numerical simulations.