**4. Conclusions**

The distribution characteristics of temperature below a curved tunnel ceiling induced by fire is studied through a small-scale tunnel model. The maximum temperature and longitudinal attenuation of temperature are analyzed. The following are some of the conclusions that may be drawn:

(1) The tunnel turning radius has limited influence on the maximum temperature and the longitudinal attenuation of the temperature below the ceiling in a strong plume impingement scenario. The temperature distribution profiles induced by a strong plume in a curved tunnel have similar tendencies to those observed in a straight tunnel (Figures 4 and 8).

(2) The maximum temperature below the ceiling under strong plume impingement cannot be well-predicted by previously proposed equations based on weak plume assumptions. This is because the thermocouple is in direct contact with the flame and the measured temperature is related to the axial temperature of the fire plume at the ceiling height (Figures 5 and 6).

(3) Improved correlation for predicting the temperature attenuation along the tunnel are proposed based on Heskestad and Hamada's model, which is capable of predicting the temperature decay along the tunnel under a strong plume-driven ceiling jet (Figure 10).

**Author Contributions:** H.T.: Writing—original draft and funding acquisition. Z.X.: project administration, resources, and supervision. D.Z.: data curation, investigation, validation, and visualization. All authors have read and agreed to the published version of the manuscript.

**Funding:** This work was supported by the National Key R&D Program of China (Grant No. 2017YFB1201204), the Hunan Provincial Innovation Foundation for Postgraduates (Grant No. CX20190131), and the Fundamental Research Funds for the Central Universities of Central South University (Grant No. 2022zzts0746).

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

**Informed Consent Statement:** Not applicable.

**Acknowledgments:** We acknowledge the High Performance Computing Center of Central South University for its support.

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