To ensure only one train operates in each ventilation section within an extra-long tunnel, a ventilation shaft was typically installed to divide the entire tunnel into multiple sections. Given the crucial role of piston wind in the metro tunnel environment and ventilation, a deeper understanding of train-induced unsteady airflow in a metro tunnel with a ventilation shaft is desirable. This study uses the unsteady flow theory of the Bernoulli equation to mathematically model piston wind in metro tunnels both with and without ventilation shafts. The influence of various shaft parameters on piston wind development is systematically analyzed. The results indicate that the shaft significantly impacts the piston wind. The maximum piston wind speed and ventilation rate in tunnels with ventilation shafts surpass those in tunnels without them. Moreover, shaft location and the cross-sectional area notably affect the maximum piston wind speed, ventilation rate, and airflow in the shaft, whereas shaft height has no significant effect. It is found that a ventilation shaft with a larger cross-sectional area positioned in the middle of the tunnel enhances the performance of piston ventilation.

The numerical method was employed to analyze the transient pressure characteristics of a metro train passing through a tunnel in high-altitude regions. The transient pressure evolution inside and outside the train under varying ambient pressures is analyzed and compared. The findings indicate that while ambient pressure minimally impacts the waveform of the exterior transient pressure, it significantly influences the peak value. Specifically, as ambient pressure rises, the maximum transient pressure (P-max) and the peak-to-peak transient pressure (ΔP) on the train’s exterior surface increase linearly, whereas the minimum transient pressure (P-min) decreases linearly. Moreover, this study analyzed pressure changes within the metro train under varying ambient pressures to assess their impact on passengers’ ear comfort. The trend of pressure peak reduction and delay inside the metro train with a certain degree of airtightness remains well aligned for different ambient pressures. In areas of high altitude with low atmospheric pressure, the requirements for the tightness performance of the train are lower.

When a fire occurs in a tunnel during construction, the smoke cannot be discharged in time and continues to settle near the ground, which threatens the safety of personnel. It is essential to understand smoke layer distribution for safe evacuation. To fill the knowledge gap for the spatio-temporal distribution of the smoke layer, a series of fire experiments are carried out in 1/20 reduced-scale tunnel models. Multiple variables are considered, including longitudinal fire location, heat release rate, aspect ratio of the main tunnel, and the inclined shaft length. Two fire scenarios are defined according to the longitudinal fire location in the main tunnel: near the upstream closed end (scenario 1) and near the downstream closed end (scenario 2). The results show that the structural evolution of the smoke layer inside the main tunnel experiences roughly three stages: single-layer smoke flow stage, transition stage, and two-layer smoke flow stage. In different fire scenarios, the reasonable N value is 10, determined by comparing the smoke layer interface height (h_s) predicted by the N-percentage method with the observed results. Moreover, we find that the FDS simulation method has significant deviation in predicting poor stratification situations. Furthermore, the spatio-temporal distributions of h_s in the main tunnel are predicted based on N = 10. The coupled effects of heat release rate and the longitudinal fire location on the h_s values are analyzed. The t_ar value (time of smoke arrival at the respiratory height) is determined, and its spatial variations are predicted. By comparing the t_ar values at position 2^# (near the inclined shaft) in different fire scenarios, we can provide a reference for the evacuation of personnel.

Pressed-in ventilation provides the possibility of implementing fire smoke control in tunnels during construction. In this study, the impact of the velocity at the air duct outlet, the heat release rate (HRR), and the tunnel geometry on the longitudinal temperature decay of the ceiling (ΔT) and smoke’s back-layering length (SBL) is investigated, using a reduced-scale experiment and the Fire Dynamics Simulator (FDS, version 6.7.6). The results indicate that an increase in the velocity at the air duct outlet and a decrease in the HRR lead to a reduction in the value of both ΔT and SBL in the main tunnel. Predictive models for the dimensionless longitudinal temperature decay of the ceiling and the dimensionless SBL are proposed. Near the fire source, the predicted SBL is relatively high due to thermal radiation. The research results provide valuable references for preventing tunnel fires during construction.

Air pollution inside tunnel has attracting more people’s attention in China. The occurrence of traffic accident in the rush hours would cause widespread traffic congestion inside tunnel. This may further lead to accumulation of air pollutants near the accident location. To analyze the accumulative effect, the air velocity and CO concentration under different jet fans layout are predicted by use of the multi-segments model. The results show that the occurrence of the accident does influence the spatial-temporal distributions of air velocity and CO concentration. After the accident, air velocity in the congestion region drops significantly and CO concentration jumps downstream the accident location. For a traffic accident-induced congestion scenario, increasing the jet fans in the upstream area is more effective to dilute the air pollutants inside tunnel.