Numerical study on the smoke extraction efficiency and the improvement through a smoke reservoir in the naturally ventilated tunnel with vertical shaft

doi:10.1016/j.tust.2020.103505

Tunnelling and Underground Space Technology

Volume 103, September 2020, 103505

https://doi.org/10.1016/j.tust.2020.103505 Get rights and content

Highlights

•
Large Eddy Simulations are carried out to study the smoke extraction efficiency.
•
Fr* number to represent the proportion of smoke in exhausted flow is proposed.
•
Smoke reservoir is designed to improve efficiency as a new structure of shaft.

Abstract

Plug-holing of the smoke layer is an unfavorable phenomenon for the smoke extraction efficiency in the naturally ventilated tunnel with vertical shaft. In this work, theoretical analysis and large eddy simulation (LES) are carried out to investigate how to prevent the occurrence of the plug-holing phenomenon and improve smoke extraction efficiency. The results show that the occurrence of plug-holing is influenced by the height, cross-sectional area and total pressure loss coefficient of the vertical shaft, as well as the thickness of the smoke layer. A modified dimensionless number Fr* is proposed to represent the proportion of smoke in the exhausted mass flow. Fr* varies as the −1/4 power of the cross-sectional area of the shaft, −1/2 power of the height of the shaft, and increases linearly with the thickness of the smoke layer. The critical value of Fr* for predicting whether the plug-holing phenomenon occurs is determined to be 0.34 by combining the results of the previous small-scale experiment. In addition, a smoke reservoir is designed under the shaft to increase the thickness of the smoke layer, which is proven to be effective in preventing the occurrence of the plug-holing phenomenon and improving the smoke extraction efficiency of the vertical shaft. The findings of this paper contribute to the understandings of smoke extraction for naturally ventilated tunnel with vertical shaft, and the results are also useful to the fire protection engineering in tunnels.

Introduction

Urban road tunnels have developed rapidly in recent years as an effective way to alleviate traffic congestion (Tang et al., 2018, Xie et al., 2018). Meanwhile, their fire safety problems are receiving more and more attention (Hu et al., 2010, Tang et al., 2017). The characteristics of fire plume and smoke flow in tunnel fire are significant different from those in compartment fire. (Zhang et al., 2014, Zhang et al., 2017, Tao et al., 2019, Zhang et al., 2020). Statistics have shown that about 75–85% of the victims in tunnel fires were killed by the toxic compounds of smoke (Alarie, 2002, Yang et al., 2011), so it is very important to prevent smoke and toxic gases from spreading by a proper ventilation system. There are mainly two kinds of ventilation systems in road tunnels, i.e. the naturally ventilated system and the mechanically ventilated system (Harish and Venkatasubbaiah, 2014, Costantino et al., 2014). The naturally ventilated system with vertical shaft has been widely adopted in more and more urban road tunnels, since it does not require extra power and takes up less space (Fan et al., 2013, Tanaka et al., 2017). However, when the smoke is exhausted by the vertical shaft, the ambient cold air will be mixed into the exhausted mass flow, which might result in the occurrence of the plug-holing phenomenon and a significant reduction in smoke extraction efficiency. (Mei et al., 2017, Zhang et al., 2018).

In the early research, Hinkley, 1970, Hinkley, 1986) proposed a dimensionless number Fr to determine the occurrence of plug-holing in the naturally ventilated system, which can be expressed by: $Fr = \frac{u_{v} A}{{(g Δ T / T_{0})}^{1 / 2} d^{5 / 2}}$ where $u_{v}$ is the flow velocity at the exhaust vent (m/s), A is the area of the exhaust vent (m²), d is the thickness of the smoke layer (m), $Δ T$ is the average temperature rise of the smoke layer (K), $T_{0}$ is the ambient temperature (K), and g is the gravity acceleration (m/s²).

It is indicated by former studies that the dimensionless number Fr has good applicability for describing the plug-holing phenomenon in ventilation systems. For instance, Lougheed and Hadjisophocleous (2001) found that the dimensionless number Fr proposed by Hinkley, 1970, Hinkley, 1986) would also be appropriate for describing the plug-holing phenomenon in the mechanical ventilation system. Li et al. (2013) investigated the validity of the dimensionless number Fr for predicting whether the plug-holing occurs in mechanically ventilated tunnels, and the critical value of Fr was recommended to be 2.1.

Ji et al. (2012) have studied the plug-holing phenomenon in naturally ventilated tunnels with vertical shaft by a 1/6th scale model, and proposed a dimensionless number $R i^{'}$ as the criterion to determine the occurrence of the plug-holing phenomenon for the tunnel ventilation systems, which is: $R i^{'} = \frac{F_{v}}{F_{h}} = \frac{Δ ρ g H A}{ρ_{s} v^{2} d w}$ where $Δ ρ$ is the density difference between smoke under the vertical shaft and the ambient air without smoke exhaust (kg/m³), H is the height of the vertical shaft (m), w is the width of the vertical shaft (m), $ρ_{s}$ is the density of smoke in the tunnel without smoke exhaust (kg/m³), and v is the velocity of smoke under the vent without smoke exhaust (m/s). However, it can be seen that the variables in Eq. (2) are somewhat different from those in Eq. (1).

In addition, several studies were conducted to investigate how to prevent the occurrence of the plug-holing phenomenon and improve the smoke extraction efficiency. Ji et al. (2013) numerically studied the effects of the vertical shaft’s cross-sectional aspect ratio on the performance of natural ventilation in urban road tunnels, and found that the plug-holing phenomenon occurred when the cross-sectional aspect ratio increased or decreased to a certain value. Cong et al., 2017, Cong et al., 2018) replaced the traditional vertical shaft with the board-coupled shaft to prevent cold air from entering the vertical shaft and increase the proportion of smoke in the exhausted mass flow. However, the use of board-coupled shaft may reduce the extraction efficiency for large fires, which needs to be further studied. Zhou et al. (2019) added solid screens below vertical shafts to prevent the occurrence of the plug-holing phenomenon and found that the solid screen with an acute angle could result in a better smoke exhausting effect than that with an obtuse angle.

Physically, the occurrence of plug-holing phenomenon is closely related to the proportion of smoke in the exhausted mass flow. However, it has not been quantitatively correlated. In this study, theoretical analysis and numerical simulations are conducted to investigate the essential influence factors of the proportion of smoke in the exhausted mass flow and the plug-holing phenomenon in naturally ventilated tunnels with vertical shaft. After understanding the essential parameters in affecting the plug-holing phenomenon, the smoke reservoir (as shown in Fig. 1) is also discussed as a potential way to improve the smoke extraction efficiency.

Section snippets

Theoretical analysis

When the content of smoke in the exhausted mass flow decreases to the critical value, the occurrence of the plug-holing phenomenon can be observed. In fact, in spite of different expressions, the Fr number and the $R i^{'}$ number might have a similar physical connotation in determining the occurrence of the plug-holing phenomenon, and are closely related to the proportion of smoke in the exhausted mass flow.

Thus, theoretical analysis is conducted to find out the essential variables that relate to

Numerical modeling

Fire Dynamics Simulator (FDS) is a computational fluid dynamics software using large eddy simulation (LES) model to process smoke flow calculations (Mcgrattan et al., 2016). The filter function is used in LES to divide the eddy into large eddy and small eddy. The large eddy is solved directly by numerical calculation, and the small eddy is solved by the Sub-Grid Models (SGM). The equations used to maintain conservation in LES are as follows:

for the conservation of mass: $\frac{\partial ρ}{\partial t} + \nabla \cdot ρ u = 0$

for the

Verification of the influencing variables

According to the results of the theoretical analysis, the proportion of smoke in the exhausted mass flow is related to the thickness of the smoke layer (d), the height of the vertical shaft (H), the cross-sectional area of the vertical shaft (A) and the total pressure loss at the vertical shaft ( $ξ_{total}$ ). In this section, series of numerical scenarios are designed to verity the variables in influencing the proportion of smoke in the exhausted mass flow.

Table 2 shows the results obtained from

Conclusion

In this paper, theoretical analysis and numerical simulation are adopted to investigate the influencing factors of the plug-holing phenomenon and the proportion of smoke in the exhausted mass flow. The effect of the smoke reservoir on improving the smoke extraction efficiency of the naturally ventilated system with vertical shaft is also studied. The main conclusions are:

(1)
As described in Eq. (22), the proportion of smoke in the exhausted mass flow ( ${\dot{m}}_{s} / {\dot{m}}_{es}$ ) can be well predicted by the modified

CRediT authorship contribution statement

Zhisheng Xu: Resources, Project administration, Funding acquisition. Weikun Xu: Conceptualization, Formal analysis, Investigation, Methodology, Writing - original draft. Lu He: Formal analysis, Validation. En Xie: Investigation, Data curation. Tianxiong Wang: Formal analysis, Visualization. Haowen Tao: Writing - review & editing, Funding acquisition.

Declaration of Competing Interest

The authors declared that there is no conflict of interest.

Acknowledgements

This work was supported by National key R&D program of China (Grant No. 2017YFB1201204), Ministry of Science and Technology of the People’s Republic of China, Hunan Provincial Innovation foundation for postgraduate (Grant No. CX20190131), Provincial Education Department of Hunan; Fundamental Research Funds for the Central Universities of Central South University (Grant No. 2019zzts075), Ministry of Education of the People’s Republic of China.

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Numerical study on the smoke extraction efficiency and the improvement through a smoke reservoir in the naturally ventilated tunnel with vertical shaft

Highlights

Abstract

Introduction

Section snippets

Theoretical analysis

Numerical modeling

Verification of the influencing variables

Conclusion

CRediT authorship contribution statement

Declaration of Competing Interest

Acknowledgements

Energy Procedia

Appl. Therm. Eng.

Appl. Therm. Eng.

Appl. Therm. Eng.

Int. J. Heat Mass Transf.

Energy

Fire Saf. J.

Tunn. Undergr. Space Technol.

Int. J. Heat Mass Transf.

Int. J. Heat Mass Transf.

Tunn. Undergr. Space Technol.

Tunn. Undergr. Space Technol.

Int. J. Heat Mass Transf.

Atmos. Environ.

Saf. Sci.

Fire Saf. J.

Procedia Eng.

Appl. Therm. Eng.

J. Wind Eng. Industrial.

Appl. Therm. Eng.

Fire Saf. J.

Int. J. Therm. Sci.

Sustain. Cities Soc.