Numerical Research on the Mixture Mechanism of Polluted and Fresh Air at the Staggered Tunnel Portals

Abstract

Abstract:In longitudinal ventilation, circulating air is formed in portals for closely spaced twin tunnels, which causes mixing between the polluted air exhausted from one tunnel and the fresh air flow of another tunnel, thus leading to the rising costs of ventilation system construction and operation. In this study, for the closely spaced tunnel with staggered inlet and outlet, the computational fluid dynamics (CFD) numerical simulation method was adopted to reveal flow characteristics of the circulating air as well as variation rules of the circulating air mixing ratio ϕ c with tunnel structure and operation parameters. Results show that both reducing inlet air velocity and increasing outlet air velocity and lateral distance can reduce the impact of the negative-pressure zone at the tunnel entrance on the jet flow structure at the tunnel exit, thus weakening the circulating air. When the inlet is placed behind or aligned with the outlet (staggered distance ∆l ≤ 0), ϕ c will increase linearly along with the increase of staggered distance; when the inlet is placed before the outlet (∆l > 0), ϕ c will first increase and then decrease with the increase of staggered distance. An expression to predict circulating air mixing ratio was created by sections. The predictions show a good correlation with the measurements and indicate that the front slope gradient of the tunnel portal is also one of the factors affecting the circulating air mixing ratio.

Environmental Impact of Discharge Patterns and Ambient Wind on the Jet Discharge from a Mechanical Upper Vent of a Tunnel

“…Urban traffic tunnels, a primary form of developing and utilizing underground spaces, are essential for optimal land utilization, communication in key traffic nodes, and alleviation of traffic congestion on the ground [1]. Urban traffic tunnels also cause a variety of environmental problems, such as ambient air quality of some areas near the tunnel exceeding standards under the impact of exhaust gas from the tunnel and difficulty in harmonizing exhaust emissions and regional planning [2][3][4][5]. Currently, exhausting waste gas via upper vents on the top of tunnels is an efficient means to deal with the environmental impact of exhaust from urban traffic tunnels on the regional environment and has been more frequently applied in urban tunnels [6], such as the Hongxing Road Tunnel in Chengdu, the Zijingang Tunnel in Hangzhou, and the Taihu Avenue Tunnel in Wuxi, Jiangsu.…”

Study on Mechanism of Series-Flow of Pollutants between Consecutive Tunnels by Numerical Simulation

“…There have been many studies made for the recirculating air between laterally adjacent tunnels [12][13][14][15], but still few for series-flow so far. Xiao et al analyzed the series-flow characteristics of pollutants between two consecutive tunnels with a spacing of 100 m [16], by use of a 1/30 scale model.…”