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