A numerical study on the effect of the shaft group arrangement on the natural ventilation performance in tunnel fires

Highlights

• The layout of natural ventilation shaft was numerically simulated.

• The spatial location and number of shaft groups are different.

• Temperature distribution, velocity field and total smoke emission were studied.

• If the number of shafts is >2, the shaft groups are better placed on the sideline.

• The Richardson number of 1.4 is still applies to this article.

Abstract

A series of numerical simulations about tunnel fires are carried out to study the performance of different arrangements of natural ventilation shafts in a 500 m tunnel. Two to four identical shafts are arranged in a group at the centerline and the sideline of the tunnel ceiling. The characteristics of the temperature distribution, velocity field and the total amount of smoke exhausted from each shaft group are investigated for each arrangement. The results have shown that when the fire heat release rate is 20 MW, boundary layer separation occurs in all the shafts in the series of simulations. At the same fire heat release rate, when the number of shafts, n, is ≤ 2, the smoke exhaust performance of the shaft groups arranged on the centerline of the tunnel ceiling is better than that on the sideline of the tunnel ceiling while n is >2, the shaft groups arranged on the sideline have a better overall smoke exhaust performance. The Richardson number of 1.4 proposed by Ji et al. is still applicable to determine the flow phenomena between boundary layer separation and plug-holing.

Introduction

With the development of modern cities, more and more road tunnels have been built to alleviate the pressure on urban traffic (Le et al., 2018, Ying and Li, 2017). However, tunnel fires are still one of the frequent tunnel accidents (Chen et al., 2016, Yi et al., 2013). Toxic smoke from fire spreads rapidly in a long and narrow tunnel spaces (Tsai et al., 2014), which can seriously hinder the safe evacuation of the stranded people in the tunnel and the rescue operations of firefighters (Qiang et al., 2017). Therefore, it is essential to effectively control the smoke in tunnel fires and remove it in an appropriate time.

Compared with a mechanical exhaust system, natural ventilation has merits of economic efficiency and functional stability such that it has gradually gained its popularity in urban road tunnels (Cong et al., 2018). For example, the Chengdu Metro Shallow Buried Interval Tunnel (China) (Fan, 2015), the Nanjing Xi’an men Tunnel (China) (Fan, 2015), and the Chiba section of the Tokyo Outer Ring Road (Japan) (Ura et al., 2014) have adopted natural ventilation. The increase of recent research on the performance of natural ventilation in tunnel fires also substantiates the elevated interests in natural ventilation (Cong et al., 2017, Yuan et al., 2013). Ji conducted a series of combustion tests to study the effect of shaft height on natural smoke exhaust from urban road tunnel fires. The results showed that there is a critical height for the shaft to achieve the best smoke exhausting effect, and a parameter, R_i, was proposed to classify the performance of the exhaust; plug-holing occurs in the shaft when R_i is greater than 1.4, otherwise the boundary layer separation occurs (Ji et al., 2012). Ji also analyzed the influence of cross-sectional area and the aspect ratio of exhaust shafts on natural ventilation performance using a computer modeling tool. Several smaller shafts showed better ventilation performance than a larger shaft having the same cross-sectional opening area (Ji et al., 2013). Based on the research of Ji, Zhang uses numerical simulation method to study the plug-holing phenomenon under the different spacing between the shaft and the fire source (Shaogang et al., 2018). The results show that the critical height of the shaft where the plug-holing phenomenon occurs is mainly related to the thickness of the smoke layer, the area of the shaft, the width of the tunnel and the length of the side of the shaft, which is less affected by the heat release rate of the fire source (Shaogang et al., 2018). By conducting fire field tests in the tunnel, Wang established a theoretical model for the maximum temperature of smoke under the ceiling, and proposed a prediction model for smoke counter-current length in such tunnel fire(Wang et al., 2016a, Wang et al., 2016b, Yan et al., 2016, Yan et al., 2015). Takeuchi conducted a series of fire experiments using the reduced-size model tunnels to investigate the effects of the aspect ratio of the model on natural ventilation systems, they confirmed that Ri of 1.4 is valid to differentiate boundary layer separation and plug-holing (Takeuchi et al., 2018).

In summary, most of these studies are about the influence of the height, size, shape and location of a single shaft on the natural ventilation performance. However, the effect of shafts arrangement on natural ventilation performance have rarely been addressed. And in actual engineering, there are cases where shafts are arranged in groups: Wuhan Donghu Tunnel in China shown in Fig. 1(a) having two shafts in a group along the centerline of the tunnel ceiling and Nanjing Xi'an men Tunnel shown in Fig. 1(b) having four shafts in a group on the sideline of the tunnel ceiling. The effects of the number of shafts in a group and their installed location on the ventilation performance is currently unknown and our understanding on the physical phenomena for the potential performance difference is also limited. Therefore, it is necessary to carry out relevant research. A series of numerical simulations are carry out to investigate the effects of different number and placement of shafts on natural ventilation performance when the total area of the shaft group is the same. In the current study, Two to four identical shafts are arranged in a group at the centerline and the sideline of the tunnel ceiling. The characteristics of the temperature distribution, velocity vector field, and the total amount of smoke exhausted from each shaft group are investigated.