Study of smoke back-layering length with different longitudinal fire locations in inclined tunnels under natural ventilation

https://doi.org/10.1016/j.tust.2020.103663Get rights and content

Highlights

  • The fire smoke backflow in inclined tunnel is mainly driven by downstream length.

  • A non-dimensional empirical correlation of back-layering length and downstream length is established.

  • A new prediction formula of back-layering length account for tunnel slopes and downstream length is developed.

Abstract

To investigate the vehicle fire hazard at different locations in inclined tunnels, the effect of longitudinal fire locations on smoke behavior characteristics induced by inclined tunnel fires was proposed by simulation and theoretical analysis. A series of tunnel models with slope of 4% was conducted by Fire Dynamics Simulator (FDS) software for fire simulation, in which three fire scenarios were considered corresponding to the variation of fire location, upstream length and downstream length respectively. The results show that smoke back-layering length drops progressively with increasing of downstream length, and upstream inlet airflow velocity is linearly positively correlated with downstream length, while the upstream length has a limited impact on smoke movement. A new prediction equation of the dimensionless smoke back-layering length account for downstream length and tunnel slope was established, through the addition of height difference between fire source and downstream exit. The established correlation shows that the dimensionless smoke back-layering length is logarithmically related to cubic power of downstream length, and the predicted results agreed well with simulated data as tunnel slope from 3.5% to 7.5%. The results can provide beneficial suggestion in ventilation system design and emergency evacuation for inclined tunnels.

Introduction

In order to alleviate the ground traffic pressure, numerous tunnels have been built in cities which lead to increased risk of occurring tunnel fires. The threat of tunnel fires is mainly due to hot and toxic smoke (Ji et al., 2010, Xu et al., 2017), which have attracted the interests of scientists worldwide to investigate the smoke extraction (Tang et al., 2018a, Fan et al., 2018), critical velocity (Atkinson and Wu, 1996, Wu and Bakar, 2000), smoke back-layering length (Hu et al., 2006, Ingason and Li, 2010, Li et al., 2010). In tunnels with ventilation system achieved using mechanical smoke control system (Tang et al., 2018b), back-layering length is usually defined as the length of the reversed smoke flow upstream of the fire when the ventilation velocity is lower than that of the critical velocity. In fact, in inclined tunnels under natural ventilation, stack effect plays a role of mechanical ventilation to make a certain amount of smoke flow downstream, therefore, back-layering length here refers to the length of smoke spread upstream.

In tunnels without mechanical ventilation, increasing tunnel slope could reduce or even eliminate the back-layering length, which is equivalent to the effect of critical velocity (Chow et al., 2016). This phenomenon indicates that back-layering length in inclined tunnel is significantly affected by slopes.

Ji et al. (2015) proposed the concept of “airflow induced by stack effect”, which represents the longitudinal velocity formed at upstream entrance when fire smoke flow downstream under the effect of thermal buoyancy, in which airflow at upstream end is similar to longitudinal mechanical ventilation at one end of a horizontal tunnel to control smoke spreading. Back-layering length in inclined tunnels is also considered to be related to longitudinal velocity induced by chimney effect under this scenario.

In addition, scholars have conducted studies on influence factors of back-layering length in inclined tunnels. Fan et al. (2017) investigated the smoke movement induced by fire in a mine laneway and revealed that backflow on the left of fire source in horizontal laneway is closely related to the length of inclined laneway. Increasing either length or angle of the inclined laneway will contribute to promoting the stack effect and decreasing the backflow. Wan et al. (2019) presents the numerical studies on smoke flow behavior in inclined tunnels with a vertical shaft. By changing the position of fire source in longitudinal direction, it is found that increasing upstream length will reduce the inlet air velocity, reflecting that fire source location may have a certain effect on chimney effect.

Actually, vehicle fire accidents in the tunnel may occur at various driving stages, which makes it practical significance to investigate the smoke flow behaviors under different fire locations in inclined tunnels. However, the issue above is rarely involved in previous studies and generation mechanism of fire positions on smoke layer is also lack of attention in slope tunnels. Therefore, this study aims to discuss the influence mechanism of fire locations on backflow, and analyze the correlation between smoke back-layering length with upstream length and downstream length separately. Moreover, a prediction model for smoke back-layering length is developed to account for height difference. The study on this issue may benefit the design of ventilation and evacuation (Kong et al., 2020) system in inclined road tunnels.

Numerical simulation gains extensive applications in tunnel fire research due to the feature of low cost, flexibility, short period and high degree of data recovery in comparison with full scale experiment (Weng et al., 2014, Li et al., 2011, Lee and Ryou, 2006). Fire Dynamics Simulator (FDS) is used as the fire simulation tool, which prediction results has been proved to match well with experimental data in study of backflow length of smoke and temperature distribution (Hu et al., 2008, Weng et al., 2015). In inclined tunnel researches, Liu et al. (2019) conducted model experiment, full-scale experiment and numerical simulation to predict the maximum ceiling temperature and longitudinal temperature. The simulation prediction shows a good agree well with experiment results. Compared with the existing experimental data (Hwang, 1986) obtained from previous studied, the FDS using LES turbulence model is capable of investigating the back-layering length under longitudinal ventilation (Hwang and Edwardss, 2005). Therefore, FDS is a valid tool to model smoke behaviors in inclined tunnels. This paper expects to present multiple simulation scenarios by the variation of fire source location, upstream length and downstream length respectively in inclined tunnel with slope of 4%.

Section snippets

Back-layering length with longitudinal ventilation velocity

Thomas, 1958, Thomas, 1968 presented a theoretical analysis of the smoke back-layering length in a horizontal tunnel fire under longitudinal ventilation, and proposed the smoke back-layering flow length in a dimensionless form as a function:Lb=LbH=f2gHQρTcpV3A

In a circular tunnel, the tunnel cross-sectional area A can be expressed as a quadratic function of tunnel height H, Eq. (1) can be rewritten as:Lb=LbH=fgQρTcpV3H

Based on the research of Thomas, Vantelon et al. (1991) carried out

Model design

The present study employs FDS (Version 6.5.3) codes developed by NIST for simulating smoke movement induced by fires (McGrattan et al., 2014). The model tunnel is based on design of Qiongzhou strait shield tunnel in southern China, with a horseshoe cross-section of 10 m inside diameter and 8.5 m height. The cross-section area is 74 m2 and overall length is 500 m in Fig. 1. The wall boundary is mainly considered as an inert smooth surface, which is based on the full reference to the boundary

Distribution characteristics of smoke layer

Fig. 3 (a) and (b) show the temperature and visibility fields at steady period of smoke movement, in which images have been artificially rotated to horizontal ones. Therein, fire source divides tunnel into upstream and downstream region, and directly determine the region length. In the process of changing downstream length from 50 m to 350 m, upstream length will decrease from 450 m to 150 m. Attributed to stack effect induced by tunnel slope, hot smoke spreads downstream and exhausts through

Conclusions

The current paper used numerical simulation to model the fires in inclined tunnels with different fire source locations, upstream length and downstream length. The effect of changing the position of fire source on back-layering length was specifically focused. An empirical correlation was developed for predicting the back-layering length by taking into account downstream length and tunnel slope. Major findings include:

  • (1)

    Theoretical analyses and numerical simulations indicate that the influence of

CRediT authorship contribution statement

Jie Kong: Conceptualization, Methodology, Formal analysis, Writing - original draft, Funding acquisition. Zhisheng Xu: Resources, Project administration, Supervision. Wenjiao You: Writing - original draft, Writing - review & editing, Funding acquisition. Beilei Wang: Validation, Formal analysis. Yin Liang: Investigation, Data curation. Tao Chen: Formal analysis, Visualization.

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.

Acknowledgements

This work was supported by the Fundamental Research Funds for the Central Universities of Central South University [No. 2018zzts183], and the Key Laboratory of Safety Engineering and Technology Research of Zhejiang Province [No. 201902].

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