**1. Introduction**

In recent years, the extra-long railway tunnel has made travel easier, but also introduces new challenges with regards to safety management [1,2]. When fire breaks out inside a tunnel, poison gas and heat accumulate within the long and narrow space in a short period of time, posing a threat to life [3,4]. Over past decades, tunnel ventilation [5–7] and smoke control [8,9] has been investigated by many researchers, which has proved effective for eliminating back-layering and protecting personnel evacuation. Providing a certain flow velocity from one side of the fire source [10,11], or exhausting smoke through the shaft and flue [12,13], is expected to weaken the fire risk; however, there are still many challenges with long-distance ventilation, including smoke exhaust, evacuation, and emergency rescue. For long-distance ventilation, airflow produced by fans will encounter great resistance within the tunnel and it is hard to form a stable air flow, which may easily cause a disaster, due to the obstructed ventilation.

Tunnel ventilation systems, which provide fresh air to tunnels [14–16], are classified as transverse, semi-transverse and longitudinal. Of these, a longitudinal ventilation system equipped with jet fans is the most widely used, owing to lower costs and easier implementation. Using this ventilation mode, wind flow runs for dozens of kilometers against the wall friction; therefore, a large number of fans are required to provide enough air flow to withstand the huge ventilation loss. Unfortunately, due to the limitation of space within the tunnel, jet fans can only be arranged at both ends of the tunnel, which easily forms a local ultra-high wind pressure, threatening both human and traffic safety. In addition, such an arrangement will also present further train and personnel traffic safety issues, such as excessive ventilation pressure at the end of the tunnel.

**Citation:** You, W.; Kong, J. Feasibility Analysis of Cross Passage Ventilation and Smoke Control in Extra-Long Submarine Tunnel. *Fire* **2022**, *5*, 102. https://doi.org/10.3390/fire5040102

Academic Editors: Chuangang Fan and Dahai Qi

Received: 12 June 2022 Accepted: 15 July 2022 Published: 18 July 2022

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**Copyright:** © 2022 by the authors. Licensee MDPI, Basel, Switzerland. This article is an open access article distributed under the terms and conditions of the Creative Commons Attribution (CC BY) license (https:// creativecommons.org/licenses/by/ 4.0/). *fire*

Feng et al. [17] studied the smoke control of a subway tunnel fire, and its influencing factors, by use of CFD numerical simulation and full-scale cold-smoke experiment. It was found that the ventilation velocity through the cross passage increased with the shorter distance between train and cross passage. As the fire location was closer to the cross passage, a lower critical velocity was needed for smoke control. Hou et al. [18] carried out research on the critical velocity in a tunnel cross passage through theoretical analysis, full-scale experiment, and numerical simulation. Their results show that when the train blockage is considered in the fire tunnel, the ventilation speed in the cross passage exceeds critical velocity. In the research of Li et al. [19], critical velocity in the cross passage was related to the fireproof door height, fire load and ventilation velocity. They also proposed the dimensionless prediction model of critical velocity in the cross passage, based on their experimental results. Guo et al. [20] used the ventilation network calculation to study wind pressure and measure the tunnel section velocity, exploring tunnel ventilation energy saving technology. Optimal energy saving was proven to reach 43%, which could be applied to practical projects.

Overall, previous studies mainly focused on the critical velocity of fire smoke control in accident tunnels or cross passages, but few studies focused on the design parameters of ventilation systems using cross passages for smoke control, which is worth investigating.

In this paper, a new combination ventilation mode is designed for extra-long tunnels, which adopts longitudinal ventilation of the passenger tunnel, together with auxiliary ventilation in cross passage driven by pressurization of the service tunnel. The twin-tube complementary ventilation system is commonly used in tunnels, because of the advantage of significant reductions in total air volume, and total ventilation power consumption of the two tunnels [21–23]. We will attempt, therefore, to explore the cross passage, which is used as an important part of the ventilation and exhaust system, providing airflow in the case of an accident within extra-long tunnels.
