Global flow behaviors of multiple shafts in shallow-buried urban road tunnels subjected to fire load

Abstract

Shallow-buried urban road tunnels with shafts (URTS) have reduced traffic congestion in large cities. During fire scenarios, the backflows occur at those shafts far away from the fire source inhibiting smoke exhaust, but its rules have been unknown. A 400 m (length) × 12 m (width) × 5.5 m (height) physical model with 6–7 shafts over the ceiling is established using fire dynamic simulator software. The simulations are carried out after validation by both a small-scale experiment and a full-scale experiment. A total of 16 cases with 4 heat release rates (HRRs) and 4 spacing of fire source from the nearest unit shaft #1-1 (s_f-us1), are designed. Results indicate that the smoke spreading length is nearly independent of HRR but increases with s_f-us1. Ceiling smoke temperatures follow the power exponential laws, and the attenuation coefficients decrease with the increase of HRR and s_f-us1. The farther away from the fire, the more likely the occurrence of shaft backflow. A good power exponential rule of the shaft negative mass flow rate is fitted out, and values of decay coefficient b₂ range from 0.56 to 1.0. Based on dimensional analysis, a power exponential rule of the shaft dimensionless net mass flow rate is fitted for the exhaust shafts and a linear rule for the backflow shafts. The shaft neutral plane heights range from 1.4 m to 3.6 m for the exhaust shafts and 3.2 m to 5.4 m for the backflow shafts. Also, a linear rule is fitted. This study establishes the smoke backflow theory in URTS during fire scenarios and contributes to the tunnel fire protection engineering.

Keywords

Tunnel / Shaft / Fire / Smoke / Backflow / Neutral plane

References

[1] Anghel, I., Octavian, L., Codescu, S., Pesic, D. J., & Constantin, P. (2017). Experimental investigation on the characteristics of fires in a reduced 1/10 scale road tunnel using different combustible liquid pools, Acta Tech. Napocensis-Series. Applied Mathematics and Mechanics (English Edition), 60.

[2] Babrauskas, V . (1983). Estimating large pool fire burning rates. Fire Technology, 19(4), 251-261.

[3] Bibin, C., Devan, P. K., Senthil Kumar, S., Gopinath, S., Sheeja, R., Kanthavelkumaran, N., Gobinath, S., & Ashok, K. G. (2021). Thermal performance of lithium-Ion battery pack using forced air circulation system. Materials Today: Proceedings, 46, 3670-3676.

[4] Cheng, H. H., Liu, C., Chen, J. F., Wu, L., Zhao, Y. T., & Zhong, M. H. (2022). Full-scale experimental study on fire under natural ventilation in the T-shaped and curved tunnel groups. Tunnelling and Underground Space Technology, 123, 104442.

[5] Chow, W. K., & Gao, Y. (2011). Buoyancy and inertial force on oscillations of thermal-induced convective flow across a vent. Building and Environment, 46(2), 315-323.

[6] Chow, W. K., Gao, Y., Zhao, J. H., Dang, J. F., & Chow, N. C. L. (2016). A study on tilted tunnel fire under natural ventilation. Fire Safety Journal, 81, 44-57.

[7] Cong, H. Y., Bi, M. S., Ren, J. J., Li, B., Bi, Y. B., Li, Y. C., Jiang, H. P., & Gao, W. (2020). Experimental studies on the smoke extraction performance by natural ventilation with a board-coupled shaft in a deep buried tunnel. Tunnelling and Underground Space Technology, 106, 103613.

[8] Cong, H. Y., Wang, X. S., Zhu, P., Jiang, T. H., & Shi, X. J. (2017). Improvement in smoke extraction efficiency by natural ventilation through a board-coupled shaft during tunnel fires. Applied Thermal Engineering, 118, 127-137.

[9] Ding, Y. M., Mei, J. X., Li, X., Lu, K. H., & Li, C. H. (2024). A dimensionless model for smoke backflow length prediction with mobile ventilation in tunnel fire. Thermal Science and Engineering Progress, 56, 103026.

[10] Fan, C. G., Li, X. Y., Mu, Y., Guo, F. Y., & Ji, J. (2017). Smoke movement characteristics under stack effect in a mine laneway fire. Applied Thermal Engineering, 110, 70-79.

[11] Gao, Z. H., Li, L. J., Sun, C. P., Zhong, W., & Yan, C. B. (2022). Effect of longitudinal slope on the smoke propagation and ceiling temperature characterization in sloping tunnel fires under natural ventilation. Tunnelling and Underground Space Technology, 123, 104396.

[12] Gao, Z. H., Li, L. J., Zhong, W., & Liu, X. (2021). Characterization and prediction of ceiling temperature propagation of thermal plume in confined environment of common services tunnel. Tunnelling and Underground Space Technology, 110, 103714.

[13] Gong, L., Jiang, L., Li, S. Y., Shen, N., Zhang, Y. C., & Sun, J. H. (2016). Theoretical and experimental study on longitudinal smoke temperature distribution in tunnel fires. International Journal of Thermal Sciences, 102, 319-328.

[14] Guo, Q. H., Li, Y. Z., Ingason, H., Yan, Z. G., & Zhu, H. H. (2021). Theoretical and numerical study on mass flow rates of smoke exhausted from short vertical shafts in naturally ventilated urban road tunnel fires. Tunnelling and Underground Space Technology, 111, 103782.

[15] Guo, Q. H., Zhu, H. H., Zhang, Y. X., Shen, Y., Zhang, Y. P., & Yan, Z. G. (2020). Smoke flow in full-scale urban road tunnel fires with large cross-sectional vertical shafts. Tunnelling and Underground Space Technology, 104, 103536.

[16] He, K., Cheng, X., Li, Y. Z., Ingason, H., Shi, Z., Yang, H., & Zhang, H. (2021). Experimental study on flame characteristics of double fires in a naturally ventilated tunnel: Flame merging, flame tilt angle and flame height. Tunnelling and Underground Space Technology, 114, 103912.

[17] Hu, L. H., Huo, R., Li, Y. Z., Wang, H. B., & Chow, W. K. (2005). Full-scale burning tests on studying smoke temperature and velocity along a corridor. Tunnelling and Underground Space Technology, 20(3), 223-229.

[18] Hu, L. H., Tang, F., Yang, D., Liu, S., & Huo, R. (2010). Longitudinal distributions of CO concentration and difference with temperature field in a tunnel fire smoke flow. International Journal of Heat and Mass Transfer, 53(13-14), 2844-2855.

[19] Ingason, H. (1995). Design fires in tunnels. In Proceedings of Asia flame 95: The International Conference on Fire Science and Engineering (77-86). September, 1995, Hangzhou, China.

[20] Ji, J., Gao, Z. H., Fan, C. G., & Sun, J. H. (2013a). Large Eddy simulation of stack effect on natural smoke exhausting effect in urban road tunnel fires. International Journal of Heat and Mass Transfer, 66, 531-542.

[21] Ji, J., Gao, Z. H., Fan, C. G., Zhong, W., & Sun, J. H. (2012). A study of the effect of plug-holing and boundary layer separation on natural ventilation with vertical shaft in urban road tunnel fires. International Journal of Heat and Mass Transfer, 55(21-22), 6032-6041.

[22] Ji, J., Han, J. Y., Fan, C. G., Gao, Z. H., & Sun, J. H. (2013b). Influence of cross-sectional area and aspect ratio of shaft on natural ventilation in urban road tunnel. International Journal of Heat and Mass Transfer, 67, 420-431.

[23] Jiao, A., Zeng, W., Fan, C., Bu, R., Chen, H., Chen, Y., Xie, B., & Li, Y. (2025). Experimental study on multiple fire hazards both inside and outside a naturally ventilated tunnel: Burning rate and flame characteristics of unequal fires. Tunnelling and Underground Space Technology, 158, 106442.

[24] Karlsson, B ., & Quintiere, J. G. (2000). Enclosure fire dynamics. CRC Press.

[25] Li, L. J., Ji, J., Fan, C. G., Sun, J. H., Yuan, X. Y., & Shi, W. X. (2014). Experimental investigation on the characteristics of buoyant plume movement in a stairwell with multiple openings. Energy and Buildings, 68, 108-120.

[26] Li, Y. Z., & Ingason, H. (2018). Overview of research on fire safety in underground road and railway tunnels. Tunnelling and Underground Space Technology, 81, 568-589.

[27] Liang, W. K., Wang, Y., Wen, Z. H., Hao, M., Zhou, R., & Jiang, J. C. (2025). Influence of lateral ambient wind speed on fire smoke behavior in tunnel with unpowered ventilation caps shaft. Case Studies in Thermal Engineering, 66, 105776.

[28] Liu, W., Deng, L., Li, H. R., Li, X. C., Shi, C. L., Meng, N., & Tang, F. (2023). Experimental study and machine learning on the maximum temperature beneath tunnel ceiling induced by adjacent tandem fires in longitudinally ventilated tunnel. International Journal of Thermal Sciences, 187, 108169.

[29] Luan, D., Chu, T., Bielawski, J., Fan, C., Wezgrzyn´ski, W., & Huang, X. (2025). Smoke movement and stratification of tunnel fires under coupled effects of rainfall and ventilation. Fire Safety Journal, 152, 104323.

[30] Mao, S. H., & Yang, D. (2016). One-dimensional analysis for optimizing smoke venting in tunnels by combining roof vents and longitudinal ventilation. Applied Thermal Engineering, 108, 1288-1297.

[31] McGrattan, K., Hostikka, S., McDermott, R., Floyd, J., Weinschenk, C., & Overholt, K. (2014). Fire dynamics simulator user’s guide. NIST Special Publication.

[32] Qiu, P. Y., Long, Z., Liu, C., Yang, Y. X., Tian, X. L., & Zhong, M. H. (2021). Full-scale experiment on smoke propagation in metro station tunnel fire under opening the platform screen doors. Tunnelling and Underground Space Technology, 107, 103662.

[33] Shao, Z. Y., Li, Y., Cui, H. H., Yu, B. S., & Cong, H. Y. (2024). Numerical investigation of longitudinal location and heat release rate of fire source on neutral plane height and induced flow in longitudinal sloping tunnel fires under natural ventilation. International Journal of Thermal Sciences, 200, 108981.

[34] Shi, C. L., Li, J., & Xu, X. (2021). Full-scale tests on smoke temperature distribution in long-large subway tunnels with longitudinal mechanical ventilation. Tunnelling and Underground Space Technology, 109, 103784.

[35] Soundararajan, G., Bibin, C., Kumar, A., Devarajan, Y., & Mishra, R. (2025). Synergistic effects of graphene quantum dot additives in waste plastic oil blends: Combustion stability and emission reductions analysis. Results in Engineering, 25, 104130.

[36] Tanaka, F., Majima, S., Kato, M., & Kawabata, N. (2015). Performance validation of a hybrid ventilation strategy comprising longitudinal and point ventilation by a fire experiment using a model-scale tunnel. Fire Safety Journal, 71, 287-298.

[37] Tanaka, F., Yoshida, K., Ueda, K., & Ji, J. (2021). A simple model for predicting the smoke spread length during a fire in a shallow urban road tunnel with roof openings under natural ventilation. Fire Safety Journal, 120, 103106.

[38] Tong, Y., Wang, X., Zhai, J., Niu, X. F., & Liu, L. L. (2014). Theoretical predictions and field measurements for potential natural ventilation in urban vehicular tunnels with roof openings. Building and Environment, 82, 450-458.

[39] Wang, Z., Deng, W. H., Zhou, M., Fang, Z., Guan, Y. X., & Tang, Z. (2023). Evaluation of fire smoke and heat exhaust performance of shafts by natural venting in tunnels. Tunnelling and Underground Space Technology, 131, 104817.

[40] World Road Association . (1999). Fire and smoke control in road tunnels (Report No. 05.05.B). February 2, 2026, https://www.piarc.org/en/order-library/3854-en-Fire%20and%20Smoke%20Control%20in%20Road%20Tunnels.

[41] Yan, T., MingHeng, S., YanFeng, G., & JiaPeng, H. (2009). Full-scale experimental study on smoke flow in natural ventilation road tunnel fires with shafts. Tunnelling and Underground Space Technology, 24(6), 627-633.

[42] Yang, D., Du, T., Peng, S. N., & Li, B. Z. (2013). A model for analysis of convection induced by stack effect in a shaft with warm airflow expelled from adjacent space. Energy and Buildings, 62, 107-115.

[43] Yang, Y. X., Liu, C., Long, Z., Qiu, P. Y., Chen, J. C., & Zhong, M. H. (2020). Full-scale experimental study on fire under vehicle operations in a sloped tunnel. International Journal of Thermal Sciences, 158, 106524.

[44] Yao, Y. Z., Li, Y. Z., Ingason, H., & Cheng, X. D. (2019). Numerical study on overall smoke control using naturally ventilated shafts during fires in a road tunnel. International Journal of Thermal Sciences, 140, 491-504.

[45] Ying, H. L., Xu, Z. S., Yin, Y. L., & Yu, Z. H. (2025). Experimental study of smoke back-layering length under coordinated ventilation in underground interconnected tunnel. Tunnelling and Underground Space Technology, 159, 106464.

[46] Yuan, Y., Lei, J., & Liu, N. (2024). Experimental research on the flow field and flame geometry of free buoyant diffusion flames with low dimensionless heat release rates. Combustion and Flame, 269, 113659.

[47] Zhang, G. L., Peng, T., Tong, Y., Gong, Y. F., Chen, Z., Huang, W. H., & Dai, B. L. (2024). Full-scale experiments on the fire smoke extraction from naturally ventilated shafts for shallow-buried urban road tunnels in hot summer. Tunnelling and Underground Space Technology, 143, 105441.

[48] Zhang, T. H., Wang, G. Y., Hu, H. H., Huang, Y. D., Zhu, K., & Wu, K. (2021). Study on temperature decay characteristics of fire smoke backflow layer in tunnels with wide-shallow cross-section. Tunnelling and Underground Space Technology, 112, 103874.

[49] Zhao, X. Q., Chi, J. W., Luo, H., Zhou, R., Hao, M., & Jiang, J. C. (2024). Effect of unpowered ventilation caps and shaft parameters on fire smoke spread in the natural ventilation tunnel with shafts. Journal of Building Engineering, 87, 109086.

[50] Zhong, M. H., Shi, C. L., He, L., Shi, J. H., Liu, C., & Tian, X. L. (2016). Smoke development in full-scale sloped long and large curved tunnel fires under natural ventilation. Applied Thermal Engineering, 108, 857-865.

[51] Zhou, Y. Q., Tong, Z. P., Tong, Y., Xiong, D. L., Liu, Z. L., Chen, K. Q., Zhang, G. L., & Gong, Y. F. (2025). Full-scale experiments on fire smoke spreading respectively under natural and hybrid ventilation in a real urban road tunnel with shafts. Applied Thermal Engineering, 260, 124865.

[52] Zhu, B., Cong, H. Y., Shao, Z. Y., Hu, H. R., Ye, L. L., Bi, Y. B., & Zeng, Y. P. (2025). Numerical studies on the synergistic effects of smoke extraction and control performance by mechanical ventilation shafts during tunnel fires. Underground Space, 21, 44-64.