**4. Conclusions**

Based on the mine ventilation network test platform, the influence of the air-door position in the branch adjacent to the fire source on the characteristics of smoke backlayering was analyzed when the back-layering occurred in the mine ventilation network with a diagonal branch. To simplify the experiment, the influence of the closing degree of the air-doors, which were adjacent to the fire source on the back-layering in the mine

ventilation network with parallel branches was first investigated. The main conclusions are as follows:


**Author Contributions:** Conceptualization, H.W. and Z.X.; methodology, H.W., Z.X. and L.W; validation, C.F. and Y.Z.; formal analysis, H.W. and Z.X.; investigation, L.W., C.F. and Y.Z.; data curation, L.W.; writing—original draft preparation, Z.X.; writing—review and editing, H.W., Z.X. and C.F.; funding acquisition, H.W. All authors have read and agreed to the published version of the manuscript.

**Funding:** This research was funded by the National Natural Science Foundation of China (No. 51874313), the National Key Research and Development Program of China (2018YFC0808100).

**Data Availability Statement:** Not applicable.

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