Abstract: Using numerical simulation method, the influence of tunnel length on fire smoke mass flow and smoke layer thickness is studied. The tunnel length is set as 100, 200, 300, 400, 500 m respectively. The results show that: under the set conditions, when the smoke spread distance is greater than 200 m, the smoke sedimentation is obvious, and the smoke mass flow and thickness show a trend of first increasing and then decreasing; There is induced airflow under the smoke layer, and the shear effect between them jointly affects the changes of flue gas mass flow and thickness; Within 200 m, the velocity of induced airflow increases with the increase of distance from the fire source, mainly because the smoke layer has a high temperature and strong buoyancy effect, which will suck the cold air in the lower layer, leading to the increase of flue gas mass flow and thickness, and the decrease of induced airflow space below, leading to the increase of velocity; beyond 200 m, the temperature of the smoke layer is greatly reduced, and the buoyancy effect is weakened. The induced air inertia force plays a major role under the shear of the two, thus entraining the upper smoke, leading to the reduction of the smoke mass flow and thickness.

Key words: tunnel length, fire smoke, smoke thickness, mass flow, temperature