Effects of Altitude on Fire Smoke Diffusion in Semi-Lateral Smoke Exhaust Highway Tunnels

This paper aims to study the effects of altitude and the size of smoke outlet on fire smoke diffusion and discharge in semi-lateral smoke exhaust highway tunnels. At first, simulations of semi-lateral smoke exhaust highway tunnels were carried out in FDS (Fire Dynamics Simulator), then the distribution laws of temperature, CO concentration, smoke mass flow, and visibility in the tunnel under the conditions of different altitudes and smoke outlet areas were analyzed to figure out the effects of altitude and size of smoke outlet on fire smoke diffusion and discharge in the said tunnels. The results suggest that, in case of the same fire source power, the velocity of smoke diffusion increases with the altitude; for curves of different altitudes, the tunnel roof temperature features are basically the same, that is, the higher the altitude, the higher the temperature at the tunnel roof. When the fire source power is 20 MW, the smoke mass flow at the smoke outlet decreases with the increase of altitude, but the CO concentration grows with it, indicating that the smoke exhaust efficiency is higher in high-altitude areas. When the altitude reaches 4200 m and the fire source power is 20 MW, with the increase of smoke outlet area, the smoke discharge effect of the tunnel shows an upward trend, taking both the smoke discharge effect and economy into consideration; the smoke outlet should take a size of 4 x 3 m.