Effects of shaft inclination angle on the capacity of smoke exhaust under tunnel fire

Vertical shaft is one of the most important approaches for smoke control under tunnel fires. However, the boundary layer separation is a common phenomenon of hampering the smoke exhaust for vertical shafts. A tilted shaft has been proposed to solve problems and improve the capacity of smoke exhaust. In this study, the effect of shaft inclination angle (theta decreases from 90 degrees to 14 degrees) and shaft height on the capacity of smoke exhaust was addressed numerically. A series of scenarios were simulated in a full-scale road tunnel. Numerical results showed that the tilted shaft could eliminate the boundary layer separation. However, small shaft inclination angle could lead to a relatively higher resistance to the smoke and a smaller cross-section area of shaft, which could have an adverse effect on the capacity. Under these two factors, an optimal inclination angle exists in the shaft of around 76 degrees in this study. Based on the smoke flow characteristics and exhaust effect, the inclination angle was roughly divided into three regions. The main influence factor of the inclination angle on the mass flow rate of smoke in each region was examined. For a comprehensive consideration, the low and slightly tilted shaft was applied to tunnel fires, which can improve the capacity of smoke exhaust obviously.