An experimental study on smoke back-layering and critical velocity in tunnel fires with canyon cross wind

Abstract This paper investigated experimentally the thermal smoke back-layering and critical velocity of the highway tunnel in alpine gorge area with canyon cross wind. A series of small-scale tests were performed using a pool fire as the fire source. The downstream tunnel portal was under the action of canyon cross wind, and the longitudinal ventilation device was set on the upstream tunnel portal. The smoke movement phenomena were studied by changing the velocities of the canyon cross wind and longitudinal ventilation, and the longitudinal position of the fire source in the tunnel. The experimental results indicated that the smoke back-layering length decreases with the increasing longitudinal ventilation velocity, and both the critical velocity and smoke back-layering length increase with the increasing canyon cross wind velocity. This is because the airflow velocity towards upstream in the tunnel increases with the canyon cross wind velocity. In contrast, the downstream smoke spread length of the fire decreases with the increasing canyon cross wind velocity, and increases with the increasing longitudinal ventilation velocity. An equation for the critical velocity was developed by taking the canyon cross wind into account.