Numerical study on the critical criterion for predicting the plug-holing under lateral mechanical exhaust in tunnel fires: Considering the effect of smoke flow in longitudinal direction

• Published in: Tunnelling and underground space technology Vol. 128; p. 104666
• Main Authors: Wang, Zhiyong, Ji, Jie, Tan, Tiantian, Gao, Zihe, Zhou, Yang, Zhou, Fan
• Format: Journal Article
• Language: English
• Published: Elsevier Ltd 01.10.2022
• Subjects: Lateral smoke exhaust; Plug-holing; Smoke exhaust effect; Tunnel fire; Tunnel fire; Smoke exhaust effect; Lateral smoke exhaust; Plug-holing
• ISSN: 0886-7798; 1878-4364
• DOI: 10.1016/j.tust.2022.104666

•The smoke flow characterization near the lateral vent is analyzed.•The efficiency and effectiveness of lateral mechanical smoke exhaust is evaluated.•A critical criterion is proposed to predict the occurrence of plug-holing. This paper has analyzed the phenomenon of plug-holing under lateral mechanical exhaust in tunnel fires by conducting large eddy simulations (LES) with Fire Dynamics Simulator (FDS). The lateral mechanical smoke exhaust is a method of smoke controlling in tunnel fires with exhaust vents set in the tunnel sidewall. The results show that the phenomenon of plug-holing occurs at the lateral exhaust vent as the exhaust rate increases. The occurrence of plug-holing will lead more fresh air discharged through the vent, thereby reducing the smoke exhaust efficiency. The smoke layer thickness and temperature downstream the vent in the tunnel are analyzed to evaluate the exhaust effectiveness of lateral mechanical exhaust. When the plug-holing does not occur, increasing exhaust rate can effectively reduce the smoke layer thickness. While the plug-holing occurs, the increasing exhaust rate will increase the smoke layer thickness on the contrary. In terms of reducing smoke temperature in the tunnel, increasing exhaust rate has very limited effect in this study. Therefore, it is not the case that the higher the exhaust rate, the better the smoke exhaust efficiency and effectiveness. There is a critical exhaust rate under which the plug-holing can be avoided. Furthermore, considering the horizontal and vertical inertial forces of the smoke beneath the vent, a critical criterion (Ri'critical) for predicting the occurrence of plug-holing of lateral mechanical exhausting is proposed. Based on the results of different exhaust rates, heat release rates and vent sizes, the critical criterion (Ri'critical) that corresponds to the occurrence of plug-holing can be determined as 0.6.