Elsevier

Fire Safety Journal

Volume 116, September 2020, 103207
Fire Safety Journal

Critical shaft height for complete smoke exhaustion during fire at the worst longitudinal fire location in tunnels with natural ventilation

https://doi.org/10.1016/j.firesaf.2020.103207Get rights and content

Highlights

  • A worst longitudinal fire location for smoke control exists.

  • Critical shaft height is independent of the heat release rate.

  • Critical shaft height decreases and then stays stable with increasing shaft length.

  • A theoretical model for critical shaft height was developed and validated.

Abstract

In this study, the smoke control for fire situations in tunnels with naturally ventilated shafts was studied based on a 1/20 reduced-scale tunnel model, and the effects of the longitudinal fire location, shaft settings, and heat release rate (HRR) on the smoke control were discussed. The experimental results show that a worst longitudinal fire location for smoke control exists. Moreover, the critical shaft height for complete smoke exhaustion was studied experimentally and theoretically. According to the experimental results, the critical shaft height is almost independent of the HRR. With increasing shaft length, the critical shaft height decreases first and then remains stable until it finally decreases to zero. Furthermore, it increases with increasing interval between two shafts. In addition, a theoretical model for predicting the critical shaft height was developed and validated. The research results can serve as a useful reference for the design of natural ventilation systems with vertical shafts for the control of tunnel fires.

Introduction

Tunnel fire safety has attracted great attention owing to the catastrophic consequences of fire accidents; for example, the Yanhou tunnel fire caused 40 casualties and injured 12 persons in China in 2014. Statistics have shown that hot and toxic smoke is the main reason why people become injured or die in tunnel fires [1]. Owing to the long and narrow structure of tunnels, the hot and toxic smoke due to the fire can quickly spread along the tunnel. How to exhaust the hot and toxic smoke effectively and maintain the smoke in an acceptable range is a crucial problem in the design of tunnel ventilation systems.

The widely applied longitudinal ventilation method for tunnels has been studied by many researchers [[2], [10], [15], [16], [17], [27]]. It provides a smoke-free environment at upstream of a fire using critical velocity. However, it could destroy the stratification structure of the smoke layer at downstream of a fire [31]. This may endanger passengers of the following train due to short departure interval of the train. Compared with those of the longitudinal ventilation method, natural ventilation has lower investment and operation costs [18] and maintains the stable stratification structure of the smoke layer [22]. Therefore, natural ventilation has drawn increasing attention. For instance, Wang et al. [26] and Tong et al. [24] conducted field tests to investigate the smoke exhaustion performance of natural ventilation with roof openings and vertical shafts during tunnel fires. More specifically, they analyzed the ceiling temperature distribution, maximal temperature, and smoke layer height. Ji et al. [12] studied experimentally two special phenomena: plug-holing and boundary layer separation. They discussed the effect of the shaft height on the plug-holing and boundary layer separation and proposed the Ri number for predicting the occurrence of the two phenomena. Kashef et al. [13] and Yuan et al. [30]investigated experimentally the effects of the shaft interval and dimensions of the vertical shaft on the ceiling temperature distribution, smoke diffusion distance, and smoke exhaust rate of the shaft and developed theoretical prediction models. Moreover, Fan et al. [4] adopted numerical simulations to investigate the effect of the shaft arrangement on the smoke exhaust performance for natural ventilation; they concluded that the total mass flow rate of the smoke exhausted by the shafts increases with the shaft quantity for a fixed total area of shafts. Zhang et al. [32] studied the critical shaft height for the occurrence of plug-holing and proposed a theoretical model based on Ji et al.‘s research study and numerical simulation. Furthermore, He et al. [6] conducted a series of reduced-scale experiments to investigate the critical length of the roof opening for complete smoke exhaustion; the results revealed that the critical length is independent of the heat release rate (HRR). In addition, they developed a theoretical model for its prediction. Takeuchi et al. [21] discussed the effect of the scale ratio of the tunnel and aspect ratio of the tunnel cross-section on the ceiling temperature distribution with two reduced-scale tunnel models. They discovered that the efficiency of exhausting the smoke heat depends on the aspect ratio of the tunnel cross-section. Moreover, they provided a theoretical equation for predicting the ceiling smoke temperature for any scale ratio of the tunnel model and aspect ratio of the tunnel cross-section. Guo et al. [5] analyzed the effects of the HRR, shaft height, and shaft interval on the smoke back-layering length based on a series of model tests; the results showed that the smoke back-layering length decreases with increasing shaft height and increases with increasing shaft interval. In addition, Yao et al. [28]used numerical simulations to investigate the overall smoke control in a road tunnel. The HRR, longitudinal fire location in the tunnel, shaft length, and interval between two shafts were varied in the simulations. According to the results, the total smoke spread length on both sides of the fire source is almost independent of the HRR and longitudinal fire location, and the shaft length plays an important role in the smoke control. In addition, downdraught occurs when the smoke front stabilizes at the bottom of the shaft, which destroys the structure of the smoke layer. He et al. [7] investigated complete smoke exhaustion under natural ventilation with a wide vertical shaft for metro tunnel fires. The experimental results showed that there exists a critical shaft height for complete smoke exhaustion, and a theoretical model was established for its prediction. Furthermore, Wang et al. [25] investigated experimentally the characterization of the ceiling temperature profile and maximal temperature caused by double fires in a natural ventilation tunnel. They revealed that the burner separation distance has a significant effect on the ceiling temperature profile and maximal temperature and proposed a corresponding theoretical model for their prediction.

Because pure natural ventilation may be unreliable during a fire, longitudinal ventilation is used to assist smoke exhaustion. For example, Fan et al. [3] investigated numerically the effect of longitudinal wind velocities on the natural ventilation performance; the results showed that smoke bifurcation occurs when the longitudinal wind speed exceeds a certain value, and the shaft located directly above the smoke bifurcation area cannot discharge the smoke effectively. Zhao et al. [33] studied the effect of the longitudinal velocity on natural smoke exhaustion with a reduced-scale tunnel model. They concluded that the total smoke mass flow rate in the shafts increases with increasing ventilation velocity, which is consistent with the results in Ref. [4]. Moreover, the presence of the vertical shaft promotes smoke stratification.

The researchers of the previously presented studies pointed out that the shaft properties (i.e., shaft interval, cross-sectional area of the shaft, and shaft height) have a key influence on natural smoke exhaustion. It should be noted that the fire occurred mostly in the tunnel center between two shafts in these studies. However, fire may occur everywhere in the tunnel. Fire occurring at other longitudinal locations along the tunnel may cause a more dangerous evacuation environment. In addition, when the fire deviates from the center, the control of the smoke inside the tunnel may be more complicated. It is unclear whether the vertical shaft designed in the previous studies provides a safe evacuation environment. The key objective in engineering practice is that the natural ventilation system with vertical shafts must provide a safe evacuation environment for any position in the tunnel in case of fire. Furthermore, the installation costs and shaft quantity should be optimized [18]. To meet these objectives, the dimensions of the shaft must be investigated because related published studies are rare.

In this study, a series of reduced-scale experiments was conducted to investigate the smoke control effects during a fire in a tunnel with naturally ventilated shafts. The effects of the HRR, longitudinal fire location, interval between two shafts, and length and height of the shaft on the smoke control were determined, and the smoke temperature below the ceiling and smoke configurations were recorded. The research study was organized as follows: first, the effect of the longitudinal fire location on the smoke control was investigated to determine the worst longitudinal fire location for smoke control. Secondly, to ensure that the natural ventilation shafts provide a safe evacuation environment during a fire, it is necessary to determine the shaft height when a fire occurs at the worst longitudinal fire location for smoke control. As shown in Fig. 1, with the increasing shaft height, the driven force of smoke exhaustion, namely buoyancy, becomes stronger. The smoke spread distance beyond the shaft will be shorter. It can be seen that there is a critical shaft height for complete smoke exhaustion, as shown in Fig. 1 (b). When the shaft height further increases, the smoke can still be controlled in the zone between the shafts, as shown in Fig. 1 (c). Therefore, critical shaft height was defined as the minimal shaft height for complete smoke exhaustion at the worst longitudinal fire location. It can not only provide a safe evacuation environment under the worst fire scenario but also save installation costs. Finally, a theoretical model was developed to predict the critical shaft height.

Section snippets

Tunnel model

A tunnel model with a scale ratio of 1:20 was constructed to simulate a subway tunnel based on Froude modeling. The scaling of the HRR and smoke temperature between the reduced- and full-scale experiments can be expressed with Eqs. (1), (2), respectively [20]:Qm/Qf=α5/2Tm=Tfwhere f represents the full-scale experiment, m the model experiment, and α the scale ratio.

As shown in Fig. 2, the dimensions of the tunnel model were 14 m (length) × 0.25 m (width) × 0.25 m (height). One side of the tunnel

Effect of longitudinal fire location on ceiling temperature rise

When the fire source is located exactly underneath the shaft (fire location F in Fig. 6), much smoke is directly discharged from the tunnel through the shaft. However, when the fire source occurs at other longitudinal positions between the two shafts in the tunnel (fire locations A, B, C, D, and E in Fig. 6), the smoke control inside the tunnel is more complicated. Fig. 8 (a) illustrates that the ceiling temperature increase in the fire section is high, which affects the security of passengers

Theoretical prediction of critical shaft height

To predict the critical shaft height theoretically, the control volume was determined based on the previously presented analysis (Fig. 17); ms represents the exhaust mass flow rate in the far-fire shaft, m1 the mass flow rate of fresh air flowing into the control volume, m2 the mass flow rate of fresh air exiting the control volume, and m3 the mass flow rate of smoke flowing into the control volume.

Mass conservation for the control volume results in the following expression:m1+m3=m2+ms

Because

Conclusions

A series of reduced-scale experiments and theoretical analysis was conducted to investigate the smoke control of natural ventilation with vertical shafts in a tunnel during fire. The major conclusions are as follows:

  • (1)

    The longitudinal fire location does not have a considerable effect on the ceiling temperature increase in the fire section when the fire occurs in the fire section of the tunnel.

  • (2)

    When the fire source moves gradually away from the tunnel center between two shafts, the height of the

CRediT authorship contribution statement

Peng Zhao: Formal analysis, Methodology, Investigation, Writing - original draft. Tao Chen: Formal analysis, Methodology, Investigation. Zhongyuan Yuan: Conceptualization, Funding acquisition, Supervision, Writing - review & editing. Yuanyi Xie: Resources. Nanyang Yu: Supervision.

Declaration of competing interest

The authors declare that they have no known competing financial interests or personal relationships that could have appeared to influence the work reported in this paper.

Acknowledgments

This work was financially supported by the National Natural Science Foundation of China (NSFC) [grant number 51708454] and the Fundamental Research Funds for the Central Universities [grant number 2682016CX028]. The authors would also like to thank Yiteng Fan for his assistance during the experiments.

References (33)

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