Numerical study on the impact of gradient on semi-transverse smoke control system in tunnel

• The impact of gradient on semi-transverse smoke control system in tunnel is studied.
• With a gradient of 1%, the flow pattern in tunnel changes greatly.
• With gradients of 3–5%, the ascending tunnel is impassable for evacuation quickly.
• The smoke extraction rate in tunnel with slopes should be increased.

The impact of gradient in tunnels with longitudinal smoke control system has been studied extensively in the last two decades. However, the impact of gradient in tunnels with semi-transverse smoke control has long been neglected. In this paper, numerical simulations have been conducted to investigate the impact of gradient on the performance of semi-transverse smoke control system in a 500 m long and rectangular tunnel. Firstly, tentative simulations are conducted in the horizontal tunnel until the adopted smoke control system can maintain the smoke layer well above the driveway for 30 min and this scenario is then adopted as a benchmark. Secondly, the gradient of the tunnel is increased from 0% to 1% and the simulation shows that the buoyancy drives more smoke moving in the ascending direction, which gradually changes the flow pattern in the tunnel. Further increase the gradient to 3% and 5%, the smoke movement pattern changes more rapidly and makes the ascending section impassable for evacuation. To provide tenable conditions, the smoke extraction rates in tunnel with a gradient of 1–5% should be increased by 5–20% compared those in the horizontal tunnel.