Studying the Synergetic Effect of Point-Extraction and Longitudinal Ventilation on the Maximum Smoke Temperature and Back-Layering Length in Tunnel Fires

This study investigates the impact of combining longitudinal and point-extraction ventilated systems on temperature distribution and back-layering length in tunnel fires. Numerical simulations are conducted using a fire dynamic simulator (FDS), and reduced-scale tunnel fire experiments with a scale of 1/10 are introduced to provide supplementary data. Results indicate that the longitudinal velocity is more critical than other factors in reducing the highest temperature and casualties. Lowering the temperature below the tunnel ceiling is not caused by increasing the ceiling extraction velocity. Additionally, the study reveals that the fire source-ceiling distance and their relative positions play a crucial role in temperature distribution and plug-holing phenomenon in the tunnel. By using the Taguchi method, it is determined that a fire at a height of 0.125 m has a maximum ceiling temperature of 1.8, 1.3, and 1.1 times when the fire source happens on the floor with longitudinal velocities of 0.133, 0.265, and 0.53 m/s, respectively. The extraction point has diverse effects provided that the longitudinal ventilation velocity is set by critical velocity. The study’s objective is to provide tunnel engineering managers with a correlation to predict the highest temperature, which is a vital parameter for emergency evacuation. In conclusion, this study highlights the importance of considering longitudinal and point-extraction ventilated systems and their relative speeds in reducing the severity of tunnel fires.