Numerical Investigation on the Influence of Length–Width Ratio of Fire Source on the Smoke Movement and Temperature Distribution in Tunnel Fires

The behaviors hot and toxic smoke in tunnel fires is important to the engineering applications of fire protection systems. The shapes of fire source may affect the characteristics of smoke movement in tunnels. In the current study, a numerical investigation is conducted to explore the effect of fire source shape on the smoke movement and temperature distribution in a full scale road tunnel. The area of fire was fixed with the length–width ratio (n) varying from 1 to 11 and the heat release rate (HRR) of the fire source varying from 5 MW to 15 MW. Results show that with increasing HRR and decreasing n, the impinging flow on the tunnel ceiling transits from buoyancy plume to continuous flame and the smoke becomes more difficultly to be uniform along the longitudinal direction, resulting in further starting position of one-dimensional spread stage (L). Due to the change of entrainment behavior, for a certain HRR, the smoke mass flow rate decreases with decreasing n, which is proportional to the perimeter of the fire source. As n decreases from 11 to 1, the smoke mass flow rate decreases for 23%. Moreover, a correlation of the longitudinal smoke temperature distribution under the tunnel ceiling is developed, with the temperature at the initial position of the one-dimension spread stage as the reference value. This study obtains the variation laws of smoke mass flow and temperature distribution under the effects of fire source shape. It can provide a reference for the smoke control and safe evacuation in straight tunnel without forced flow.