**1. Introduction**

The development of construction technology has been flourishing, and the number of tunnels with large diameter increases continually [1]. However, the studies on fire safety in tunnels with large diameters are still insufficient [2]. Since the heat dissipation is restricted and the hot smoke spreads longitudinally along the tunnel during tunnel fires [3,4], using a reasonable ventilation system to prevent the spread of smoke and harmful gases is crucial [5,6]. The lateral smoke exhaust has been adopted in various tunnels [7]. Nevertheless, the change in the tunnel width [8,9] and the transverse position of the fire source [10,11] influence the heat exhaust coefficient.

According to Chen et al.'s study [12], the maximum temperature rise of the fire and the attenuation coefficient of smoke flow influenced the heat lost by the smoke exhaust. However, the tunnel width influenced the maximum temperature rise and the attenuation factor of smoke close to the fire source. [13]. Additionally, this investigation did not consider the amount of smoke in the expelled mass flow. The amount of smoke in the exhaust mass flow caused by air entrainment at the exhaust vent is affected by the plugholing phenomena. The plug-holing phenomenon would also be experienced by the lateral smoke exhaust, according to Yang et al. [14]. When plug-holing occurs, a great deal of cold air is entrained, which reduces the efficiency of smoke extraction. Jiang et al. [15] calculated the smoke back-layering length in their study. They redefined the confinement velocity based on the smoke temperature distribution below the tunnel ceiling. Zhong et al. [16] proposed the concept of a virtual smoke vent based on the results of different smoke exhaust

**Citation:** Liu, Q.; Xu, Z.; Xu, W.; Tagne, S.M.S.; Tao, H.; Zhao, J.; Ying, H. Study of the Heat Exhaust Coefficient of Lateral Smoke Exhaust in Tunnel Fires: The Effect of Tunnel Width and Transverse Position of the Fire Source. *Fire* **2022**, *5*, 167. https://doi.org/10.3390/fire5050167 4.0/). *fire*

Academic Editors: Dahai Qi, Tiago Miguel Ferreira and Fei Tang

Received: 4 September 2022 Accepted: 10 October 2022 Published: 15 October 2022

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velocities, heat release rates and smoke layer heights in their study of the occurrence of plugholing penetration in the lateral smoke exhaust. A critical Froude number for predicting lateral smoke exhaust plug-holing was established and a corresponding critical Froude number of 0.5 was given for the occurrence of plug-holing. Zhu et al. [17] proposed a modified Froude number for determining the plug-holing phenomenon through a tunnel fire under the action of lateral smoke exhaust. The critical Froude number ranges from approximately 1.5 to 1.75. Our previous study [18] considered the difference between lateral smoke exhaust and ceiling smoke exhaust. We modified the Froude number to determine the plug-holing of lateral smoke exhaust to establish a new modified Froude number. The results show that when the new modified Froude number is greater than 2.5, the lateral smoke exhaust experiences plug-holing.

However, previous research has concentrated on determining plug-holing in the lateral smoke exhaust. There is a lack of research into the effect of different tunnel widths and the lateral position of the fire source on the effectiveness of lateral smoke exhaust. Additionally, the direction of extraction is different for the ceiling smoke exhaust and the lateral smoke exhaust, especially in large-diameter tunnels. The cold air is encased by the smoke layer and exhaust vent working together. Meanwhile, there is a lack of correlation between the tunnel width, the transverse fire location, and the heat exhaust efficiency. Therefore, by large eddy simulation (LES) and theoretical analysis, the change of the influencing factors heat exhaust coefficient in tunnel fires with various tunnel widths and transverse fire locations was studied.
