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

Longitudinal ventilation systems are commonly installed in new tunnels in large cities of the Far East including Mainland China, Hong Kong and Taiwan. Many tunnels are found in big cities and some of them are inclined at an angle to the horizontal. However, smoke movement in tilted tunnels is not fully understood. In some of the tunnels, the ventilation system was designed based on presumed smoke movement pattern without experimental demonstration. Smoke movement pattern in a tilted tunnel model was studied by using a scaled model. A 1/50 tunnel model of length 2 m with adjustable angle to the horizontal was constructed by transparent acrylic plastics. A small 0.097 kW propanol pool fire was used as the heat source combined with burning pellets generating smoke. A fan placed at the upstream end was used to create longitudinal ventilation. Different ventilation rates were set using a transformer to control or adjust the fan speed. Experiments were performed with the tunnel angle varying up to 30° to the horizontal. Effect of smoke screens was also studied. The 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 at greater angles to the horizontal would give larger amount of smoke flow. Smoke movement pattern for a tilted tunnel with smoke screens was observed to be very different from some design projects. All results will be reported in this paper.