Theoretical predictions and field measurements for potential natural ventilation in urban vehicular tunnels with roof openings

Naturally ventilated urban vehicular tunnels with multiple roof openings have emerged in China; thus, the ventilation strategy needs to be studied and validated. The safety standards for CO concentration depend on people's exposure time: 125 mg/m3 for 5 min, 100 mg/m3 for 15 min, and 35-44 mg/m3 for 1 h. Airflow and contaminant equations were established and solved based on one-dimensional and steady state assumptions. Three naturally ventilated and two mechanically ventilated urban vehicular tunnels were investigated from 11/2013 to 1/2014 using TSI7575-X and KIMO-VT200 for continuous and single-point measurements during congested periods. The survey reveals that piston winds existed in each tunnel but the effect in mechanically ventilated tunnels was more apparent compared to that in naturally ventilated tunnels. Furthermore, all temperature as well as CO and CO2 concentrations increased from inlets to outlets. The theory model was validated by comparing the analytical air velocity and CO concentration of the XIANMEN Tunnel to that of the field measurement. Further theoretical analyses indicate that under a constant traffic flow of 1700 veh/h.lane, the air velocities depend largely on the vehicle speed vt and the opening area ratio Rf; moreover, the maximum CO concentrations increase with a decrease in vt and an increase in tunnel length L but are minimally affected by Rf. At 20 km/h, drivers are exposed for different times based on the tunnel length, and they are always safe against CO exposure in tunnels up to 3000 m.