A study of the effect of plug-holing and boundary layer separation on natural ventilation with vertical shaft in urban road tunnel fires

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Abstract

A set of burning experiments were conducted to investigate the effect of vertical shaft height on natural ventilation in urban road tunnel fires. Two special phenomena, plug-holing and turbulent boundary-layer separation were observed, both of which will influence the effect of smoke exhaust. When shaft height is relatively small, the boundary layer separation is significant and vortexes form in the upstream region inside the shaft, causing the backflow of gas mixture and preventing the throughflow of smoke. With the increasing of shaft height, the boundary layer separation becomes inconspicuous and the plug-holing occurs, leading to the ambient fresh air beneath smoke layer being exhausted directly, which will strongly decrease the smoke exhaust efficiency. Therefore, it is not the case that the higher the vertical shaft, the better the smoke exhaust effect, there exist a critical shaft height in which the boundary layer separation can be diminished to a large extent and overmuch entrainment of fresh air such as plug-holing can be avoided. In addition, the critical shaft height related to better effect can be determined by the new criterion of Ri(Ricritical=1.4) proposed in this paper.

Introduction

Statistics have shown that smoke is the most fatal factor in fires, and about 85% of victims are killed by the hot and toxic smoke [1]. Therefore, when a fire occurs, it is very important to stop the smoke and toxic gases from spreading by appropriate exhaust systems. As the requirements of environmental protection for urban road tunnels, an emerging natural ventilation system with vertical shaft is gradually adopted in practice rather than the widely used mechanical ventilation system because of the high pollutant density gathered near the outlet of mechanical ventilation systems [2].

Currently, scholars around the world have conducted a few preliminary studies on the natural ventilation system with vertical shaft in urban road tunnel fires, mainly focus on the feasibility and validity of this system. Wang et al. [3], [4] conducted a set of burning experiments in a full-scale tunnel with roof open, tested the effect of natural smoke exhaust and investigated the ceiling jet temperature and the backflow distance. Yoon et al. [5] investigated the natural ventilation pressure of vertical shaft in two road tunnels with results indicated that the ratio of natural ventilation pressure induced by shaft to the mechanical ventilation pressure came to 29.26%, which had greatly improved the smoke exhausting efficiency compared to traditional natural ventilation without vertical shaft, thus verifying the feasibility of natural exhaust system with vertical shaft. Huang et al. [6] numerically studied the effect of the ventilation shaft arrangement and geometry on natural ventilation performance in a subway tunnel with FLUENT. Mao et al. [7] conducted a fire testing to study the temperature distribution of hot smoke under fire circumstance in natural ventilation city tunnel in a 1/10th scale model, the results indicated that natural ventilation shafts can expel a mass of smoke from the tunnel and take off the majority of heat. However, in these former studies, the flow field within shaft has rarely been addressed.

Therefore, a set of burning experiments were conducted to investigate the effect of shaft height on natural ventilation as well as the flow field within shaft in urban road tunnel fires. A simple criterion of Ri′ was put forward to estimate the optimal shaft height under different fire scenarios. Furthermore, the study on this issue may benefit the current design of natural ventilation system with vertical shaft and complement the current codes.

Section snippets

Experiments

The approach of physical scale modeling is well established and has been used in many researches of smoke movement in tunnels and other corridor-like structures [5], [7], [8], [9], [10]. The idea of applying similar model to fire research was first proposed by Thomas [11], after the development and improvement of the later scholars [12], [13], the approach of physical scale modeling has evolved into an effective way to study the phenomenon of fire and smoke. Measurements in this study are made

Stratification characteristics of the smoke

When a fire occurs, the plume rises from the seat at the fire and turns radially outward at the ceiling until it is deflected at the side walls of the tunnel, then a transition from radial to one-dimensional spreading takes place. The complete flow field can be subdivided into five regions: rising plume, turning region near the ceiling, radial spreading under the ceiling, transition from radial to one-dimensional flow and one-dimensional flow under the ceiling parallel to the tunnel axis [17],

Conclusion

This article provides experimental data to characterize the flow field of smoke within vertical shaft of natural ventilation system in urban road tunnels, for a range of fire sizes and shaft heights. Analysis of these results has provided the following conclusions.

With shaft height increasing and stronger stack effect producing, the flow field within vertical shaft has roughly gone through two stages of the boundary layer separation and the plug-holing. In relatively low shaft height, the

Acknowledgement

This work was supported by National Natural Science Foundation of China (NSFC) under Grant No. 50904055.

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