**5. Conclusions**

In this paper, a ventilation mode with service tunnel and cross passages for auxiliary air supply and smoke control was studied using FDS simulation. The study proposed a ventilation and smoke exhaust scheme, and solved the problem of longitudinal ventilation in extra-long submarine tunnels. Our main conclusions are as follows:

(1) In the case of a 20 MW fire, the longitudinal ventilation velocity of 2 m/s will be applied in the accident tunnel, and the airflow speed of 1.3 m/s will be supplied at both ends of the service tunnel, together with three cross passages open to provide airflow, which can effectively control the fire smoke and improve the human evacuation.

(2) A ventilation network model is established according to the design parameters of extra-long tunnels. The calculation results show that the longitudinal wind speed at the fire source reaches 4.5 m/s, exceeding the critical velocity for smoke control of 3.5 m/s, which is in accordance with the numerical simulation results. In theory, cross passage pressurized air supply technology is proved feasible.

It should be noted that the distance between cross passages, and the angle between cross passages with main tunnel, will influence ventilation efficiency. Therefore, further experiments and simulations are needed, and parameters should be extended in order to investigate the optimum ventilation scheme. In this paper, design parameters are closely related to the tunnel structure, which are not necessarily applicable to other projects. With regards to ventilation design, the methods and ideas highlighted in this paper are significant; perhaps other tunnels need similar structural models and calculation conditions.

**Author Contributions:** Conceptualization, W.Y. and J.K.; methodology, W.Y.; software, J.K.; validation, W.Y. and J.K.; formal analysis, W.Y.; investigation, W.Y.; resources, W.Y.; data curation, W.Y.; writing—original draft preparation, J.K.; writing—review and editing, J.K.; visualization, W.Y.; supervision, W.Y.; project administration, J.K.; funding acquisition, J.K. All authors have read and agreed to the published version of the manuscript.

**Funding:** This research was funded by Philosophy and Social Science Planning of Zhejiang, grant number 20NDJC199YB; National Natural Science Foundation of Zhejiang, grant number GF22F030254.

**Institutional Review Board Statement:** Not applicable.

**Informed Consent Statement:** Not applicable.

**Data Availability Statement:** Not applicable.

**Conflicts of Interest:** The authors declare no conflict of interest.
