Abstract
Providing efficient ventilation in road tunnels is essential to prevent severe air pollution exposure for both drivers and pedestrians in such enclosed spaces with heavy vehicle emissions. Longitudinal ventilation methods like commercial jet fans have been widely applied and confirmed to be effective for introducing external fresh air into road tunnels that are shorter than 3 km. However, operating tunnel jet fans is energy consuming. Therefore, for small-scale (~100 m–1 km) road tunnels, mechanical ventilation methods might be highly energetically expensive and unaffordable. Many studies have found that the use of wind catchers could improve buildings’ natural ventilation, but their effect on improving natural ventilation in small-scale road tunnels has, hitherto, rarely been studied. This paper, therefore, aims to quantify the influence of style and arrangement of one-sided flat-roof wind catchers on ventilation performance in a road tunnel. The concept of intake fraction (IF) is applied for ventilation and pollutant exposure assessment in the overall tunnel and for pedestrian regions. Computational fluid dynamics (CFD) methodology with a standard k-epsilon turbulence model is used to perform a three-dimensional (3D) turbulent flow simulation, and CFD results have been validated by wind-tunnel experiments for building cross ventilation. Results show that the introduction of wind catchers would significantly enhance wind speed at pedestrian level, but a negative velocity reduction effect and a near-catcher recirculation zone can also be found. A special downstream vortex extending along the downstream tunnel is found, helping remove the accumulated pollutants away from the low-level pedestrian sides. Both wind catcher style and arrangement would significantly influence the ventilation performance in the tunnel. Compared to long-catcher designs, short-catchers would be more effective for providing fresh air to pedestrian sides due to a weaker upstream velocity reduction effect and smaller near-catcher recirculation zone. In long-catcher cases, IF increases to 1.13 ppm when the wind catcher is positioned 240 m away from the tunnel entrance, which is almost twice that in short-catcher cases. For the effects of catcher arrangements, single, short-catcher, span-wise, shifting would not help dilute pollutants effectively. Generally, a design involving a double short-catcher in a parallel arrangement is the most recommended, with the smallest IF, i.e., 61% of that in the tunnel without wind catchers (0.36 ppm).
Highlights
With the development of urban transportation networks, urban road tunnels have gained popularity and can be commonly found in many large cities
The results showed that the performance of the commercial jet fans varies dramatically with the number, pitch angle, and position of jet fans, whilst the best pitch angle varied along with traffic conditions: When there is no traffic, the best pitch angle is 6 degrees and, during traffic jams, the pitch angle should be maintained between 2 and 4 degrees
The road tunnel environment could be regarded as a special kind of indoor environment but on a much larger scale than an indoor room, it approximately fits the scale of urban street canyons
Summary
With the development of urban transportation networks, urban road tunnels have gained popularity and can be commonly found in many large cities. These tunnels provide an efficient transportation process for vehicles and offer a much safer public traveling experience for pedestrians. They are core connections between different transportation systems like metro stations and bus stops, coupled with the provision of shade and commercial opportunities. According to the city air quality report for the Barbican, London, UK [1], air pollution is quite severe in vehicle-pedestrian tunnels, with the concentration of pollutants like carbon monoxide (CO) and nitrogen dioxide (NO2 ) dramatically exceeding the EU mean annual target. The highest monthly mean NO2 concentration in the Beech Street tunnel in the
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