Experimental studies on the smoke extraction performance by natural ventilation with a board-coupled shaft in a deep buried tunnel
Introduction
With the acceleration of urbanization, tunnel engineering, as an important hub for connecting cities, has made unprecedented progress. The longer the tunnel, the greater the traffic volume will be, and the greater the probability of fire will happen in the tunnel. Due to the long-narrow structural characteristics of the tunnel, if there is no effective smoke and heat extraction system in the event of a fire, small fires in the tunnel can even lead to catastrophic consequences.
Extensive researches have been carried out, Fire characteristics and smoke propagation characteristics with the mechanical ventilation system in tunnel fires have been widely studied (Kunsch, 2002, Hu et al., 2005, Ingason and Li, 2010, Li and Ingason, 2012, Chow et al., 2016, Oka and Oka, 2016, Yang et al., 2019, Tang et al., 2020). Compared to the mechanical ventilation systems in the tunnel, natural ventilation with vertical shaft has attracted more and more attention due to the advantages of energy-saving, noise reduction, and stable performance in shallow buried tunnels. Specific examples are the Xi’an men road tunnel and the Chiba section of the Tokyo Out Ring Road (Ura et al., 2014). Details of the definition of the shallow buried and deep buried tunnel can be found in the Appendix.
Klote, 1991, Zukoski, 1995 studied the mechanisms of the stack effect and established relevant theories to evaluate the stack effect. Wang et al. (2016) and Tong et al. (2009) conducted full-scale experiments in a road tunnel with roof-opening, the results validated the effectiveness of the shaft in smoke extraction and smoke control. Beyond that, Vauquelin et al. (2002) compared the smoke extraction characteristics of the shaft with different shapes at different locations. Tanaka et al. (2015) studied the smoke extraction performance of a hybrid ventilation strategy comprising longitudinal and point ventilation in a 1/5 scale model tunnel. Kashef et al., 2013, Yuan et al., 2013 conducted experiments in two 1/15 scale model tunnels equipped with shafts and developed theories to evaluate the ceiling temperature distribution and smoke diffusion. Fan et al. (2014) investigated the influence of shaft arrangement on the smoke extraction performance. Ji et al., 2013, Zhang et al., 2018 conducted numerical simulations to study the critical height of the shaft which can avoid the occurrence of the plug-holing phenomenon. Cong et al., 2018, Cong et al., 2019) introduced a board-coupled shaft (BCS), which can eliminate the plug-holing and improve the smoke extraction performance. Although plenty of researches have been conducted, all of these studies focus on the shafts in shallow buried tunnels. For deep buried tunnels, whether natural ventilation with vertical shaft can be applied, how to applied, and how effective if they are applied, have not been studied yet.
The occurrence of the plug-holing phenomenon is the primary factor affecting smoke extraction performance with vertical shafts. From previous research (Klote, 1991), the main driving force in the shaft is the stack effect, which is proportional to the shaft height and the excess temperature in the shaft. Considering that it is impossible to accurately predict the fire scenario before it happens, the only thing controlled by the designer is the height of the shaft. Theoretically speaking, for any specific fire, (a) if the height of the shaft is very large, the higher the shaft, the stronger the stack effect will be. However, the enhanced stack effect force will lead to the occurrence of plug-holing phenomenon, vast cold air beneath the smoke layer will be sucked into the shaft, and then the smoke temperature in the shaft decreases after the cold air enters, which will, in turn, leads to the decrease of the stack effect force and smoke extraction performance in the shaft; (b) if the height of the shaft is very low, although violent plug-holing may not occurs during the extraction process, the stack effect force is so weak that the smoke extraction performance couldn’t be high. This is the dilemma in designing the shaft in tunnels, which greatly inhibits the application of the shaft. Therefore, considering the inefficient smoke extraction performance caused by the occurrence of the plug-holing phenomenon, natural ventilation with vertical shafts is mainly applied in the shallow buried road tunnels, and is generally not used in the deep buried tunnels. But in theory, if the problem of the plug-holing can be effectively solved, the vertical shaft may also be used in the deep buried tunnels.
In our previous researches (Cong et al., 2018, Cong et al., 2019), the board-coupled shaft (BCS) is introduced, and results indicate that the plug-holing phenomenon in a shallow buried tunnel can be effectively eliminated by installing a thin board under the shaft. Although some works have been conducted by BCS in a shallow buried tunnel, there is no answer whether the BCS can be applied to the deep buried tunnels, let along the smoke extraction characteristics and relevant theories when the BCS is applied in the deep buried tunnels. To answer this questions, a series of experiments were conducted in a deep buried tunnel, and the smoke extraction characteristic by BCS in a deep buried tunnel was investigated. Moreover, a correlation function was developed for analyzing the smoke temperature difference and the CO concentration in the shaft, and an empirical correlation was established for predicting the dimensionless volume flow rate under difference HRRs and board locations in deep buried tunnels.
Section snippets
Theory analysis
In another paper by the authors (Cong et al., 2019), the general condition that the first shaft cannot extract the entirety of the smoke was considered. To analyze the characteristics of smoke flow in and around the vertical shaft and quantify the amount of smoke discharged from the shaft, a holistic approach was employed (Yuan et al., 2013). Based on this method, the whole tunnel can be divided into several zones, and simplify the analysis by analyzing the heat and mass transfer on the zone
Heat release rate and smoke layer thickness
The combustion characteristic of the 0# diesel had been carefully studied, and results indicated that the 0# diesel has a long period of stability during the whole combustion (Cong et al., 2018). Due to the dimension difference of the oil pans, the stable period for each oil pan was different. As shown in Fig. 3, the time intervals for the stable combustion period of 8 cm × 8 cm, 10 cm × 10 cm, and 12 cm × 12 cm oil pans were 250–550 s, 200–400 s, and 150–300 s; and all parameters used in this
Determination of coefficient k
As mentioned above, coefficient k is a value depends mainly on the thermodynamic and hydrodynamic characteristics of the tunnel and varies with different tunnels.
Based on previous research (Hu et al., 2005), the temperature distribution along the tunnel ceiling conforms to the law of exponential attenuation, and it can be expressed as:
In order to determine the coefficient k, the dimensionless temperature attenuation along the tunnel ceiling was investigated, as shown in Fig. 9. It
Conclusion
Experiments were conducted in a 1/20 scale model tunnel to explore the effectiveness of BCS in a deep buried tunnel, and the shaft applied in this study is 5 times the height of the tunnel height. The influences of HRRs and board locations on the smoke extraction performance were investigated. Major finds include:
- (1)
The BCS is effective and has stable smoke extraction performance for the deep buried tunnel applied in this study.
- (2)
Opposite to the traditional shaft, the smoke extraction efficiency of
CRediT authorship contribution statement
Haiyong Cong: Conceptualization, Methodology, Investigation, Formal analysis, Writing - original draft. Mingshu Bi: . Jingjie Ren: . Bei Li: . Yubo Bi: . Yanchao Li: . Haipeng Jiang: . Wei Gao: Resources, Writing - review & editing, Supervision, Data curation.
Declaration of Competing Interest
The authors declare that they have no known competing financial interests or personal relationships that could have appearedto influence the work reported in this paper.
Acknowledgments
The authors appreciate the support of grants from the Key National R&D Program (Grant No. HZ2019-KF07), Natural Science Foundation of China (Grant No. 51906030), Fundamental Research Funds for the Central Universities (DUT20JC04).
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