Longitudinal ventilation for smoke control in a tilted tunnel by scale modeling

Longitudinal ventilation systems are commonly installed in new tunnels in big cities of the Far East including Mainland China, Hong Kong and Taiwan. Many tunnels are found and some of them are inclined at an angle to the horizontal. However, smoke movement in those tilted tunnels is not fully understood. Some longitudinal ventilation was designed based on presumed smoke movement pattern without experimental demonstration. Smoke movement pattern in a tilted tunnel model was studied by scale modeling technique. A 1/25 tunnel model of length 2 m with adjustable angle to the horizontal was constructed by transparent acrylic plastics. A small pool fire was put in with smoke generated by burning smoke pellets. Longitudinal ventilation was set up by a fan at one end. Different ventilation rates were adjusted by a transformer on controlling input to the fan motor. Experiments were performed with the tunnel angle varying up to 30° to the horizontal. Observed smoke movement patterns indicated that the shape of the buoyant plume inside the tunnel depends on the tilted angle. Smoke would flow along the tunnel floor due to gravity. The bending angle of the plume depends on the tunnel angle. Tunnel inclined with higher angles to the horizontal would give larger amount of smoke flow. All results will be reported in this paper.