Influence of longitudinal wind on natural ventilation with vertical shaft in a road tunnel fire

doi:10.1016/j.ijheatmasstransfer.2012.10.063

International Journal of Heat and Mass Transfer

Volume 57, Issue 2, February 2013, Pages 671-678

https://doi.org/10.1016/j.ijheatmasstransfer.2012.10.063 Get rights and content

Abstract

The influence of longitudinal wind on natural ventilation with vertical shaft in a road tunnel fire was investigated numerically by Large Eddy Simulation. The smoke flow characteristics of a road tunnel fire under the combined function of longitudinal wind and stack effect of shaft were analyzed. Results show that the stack effect, plug-holing and boundary layer separation are the dominating factors on the natural ventilation performance. Plug-holing occurs at small longitudinal velocity and causes the reduction of exhaust effect. At high longitudinal velocity, the driving force of smoke exhausting is weak and obvious boundary layer separation occurs, resulting in a poor exhaust capacity. There is a critical value of longitudinal wind velocity, in which a better smoke exhausting effect can be obtained.

Introduction

Nowadays, more and more road tunnels are under construction in cities all over the world. However, due to the special structure of tunnels, smoke and toxic gases (e.g. CO) induced by fires will be accumulated in the tunnel, and are among the most fatal hazards to the evacuees. One important reason can be attributed to that smoke prohibits the safe evacuation of occupants and the firefighters from extinguishing the fire, whose influence is comparable with that of fire protection system failure [1]. Another reason is that soot from the flame and smoke layer formed in the tunnel significantly strengthens the radiative heat flux [2], [3], which may reignite surrounding combustibles and cause damage to evacuees in the tunnel.

There are mainly two kinds of ventilation systems in road tunnels, i.e. the natural ventilation and the mechanical ventilation. The mechanical ventilation requires a large space of excavation for the installation of ventilation equipments and ends up with a costly maintenance and electrical energy consumption whereas the natural ventilation using vertical shafts can avoid the air fans that will reduce the tunnel section height and does not consume any power in the operation process. The natural ventilation is particularly applicable to those short tunnels with a light traffic flow [4].

In tunnel fires research, the critical velocity and backlayering length due to mechanical ventilation systems [5], [6], [7], [8], the maximum temperature beneath the ceiling [9], [10], [11], [12] and the heat release rates [13], [14] have been widely studied. Relatively few studies have been carried out on the smoke movement under the action of natural ventilation with vertical shafts.

When a road tunnel fire occurs, the fire plume rises up until it hits the ceiling. Then the ceiling jet spreads radially until it reaches the side walls. The next stage is the transition of the ceiling jet from radially spreading to horizontally spreading which is regarded as a one-dimensional smoke flow. The smoke spreading process then reaches the next stage in which a fully developed one-dimensional smoke flow forms in the tunnel [5], [11]. When the hot smoke reaches the shaft bottom in the stage of flowing horizontally, the stack effect takes place, and then the smoke will be exhausted from the shaft. The more smoke is exhausted by shafts, the lower temperature and smoke spread velocity will be achieved in tunnels, which benefits the evacuation of crowd. Shafts not only contribute to the discharge of high temperature smoke and weaken its impact on the lining structures and equipments, but also facilitate the airflow exchange between inside and outside of tunnels during daily operation to improve interior air quality.

Several researchers have conducted preliminary studies on the fire-induced smoke flow in road tunnels under natural ventilation with shafts. Wang et al. [15] conducted full-scale experiments on fires in tunnels with roof openings and tested the effect of natural ventilation. Bi et al. [16] studied the effect of natural ventilation with shafts in an road tunnel by STAR-CD software and obtained the maximum spreading distance of smoke in the tunnel. Yoon et al. [17] investigated the pressure of natural ventilation in the shaft of a road tunnel and found that the natural ventilation pressure induced by the shaft had a significant impact on the efficiency of the ventilation system.

In our former work [18], a set of burning experiments were conducted to investigate the effect of shaft height on natural ventilation in road tunnel fires. When shaft height is relatively small, the boundary layer separation is significant and vortexes form in the upstream region inside the shaft, restricting the smoke from being exhausted. With the increase of shaft height, the boundary layer separation becomes inconspicuous and the plug-holing occurs, leading to some 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 exists 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.

However, above studies did not perform detailed analysis on influence of longitudinal wind on natural smoke exhausting with vertical shaft in road tunnel fires. In fact, the longitudinal wind with a certain speed will always exist in a road tunnel. In this paper, a numerical study was conducted to investigate the effect of longitudinal wind on natural smoke exhausting in a road tunnel fire. The detailed characteristics of plug-holing and boundary layer separation were analyzed in detail.

Section snippets

Fire Dynamics Simulator

The rapid development of computer provides efficient tools to fire safety risk assessment such as Computational Fluid Dynamics (CFD) and in particular Large Eddy Simulation (LES) codes for modeling fires. The software package, Fire Dynamics Simulator (FDS) [19], a LES code coupling with a post-processing visualization tool, Smokeview, developed by National Institute of Standards and Technology (NIST), USA, could now be regarded as a practical tool for simulating fire-induced environment. The

Smoke layer height and temperature in tunnel

The height and average temperature of the smoke layer in the tunnel are shown in Fig. 4, Fig. 5, respectively. The smoke layer height is obtained by the calculation method proposed by Janssens et al., more details of which can be found in FDS user’s guide [19]. Depending on the interface height, the average temperature is calculated by averaging the values of gauging points within the smoke layer in the stable period. It can be seen that both the height and average temperature of smoke layer

Conclusion

Large Eddy Simulation was conducted to investigate the influence of longitudinal wind on natural ventilation with vertical shaft in a road tunnel fire. The smoke flow characteristics of a road tunnel fire under the combined function of longitudinal wind and stack effect of shaft were analyzed.

Simulation results show that the stack effect, plug-holing and boundary layer separation are the dominating factors which affect the exhaust ability of vertical shaft. Various longitudinal wind velocities

Acknowledgments

This work was supported by National Natural Science Foundation of China (NSFC) under Grant No. 50904055 and the key technologies research and development program of Henan province under Grant No. 102102210379.

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Influence of longitudinal wind on natural ventilation with vertical shaft in a road tunnel fire

Abstract

Introduction

Section snippets

Fire Dynamics Simulator

Smoke layer height and temperature in tunnel

Conclusion

Acknowledgments

Fire Saf. J.

Tunn. Undergr. Space Technol.

J. Wind Eng. Ind. Aerodyn.

Fire Saf. J.

Fire Saf. J.

Tunn. Undergr. Space Technol.

Fire Saf. J.

Int. J. Heat Mass Transfer

Fire Saf. J.

Fire Saf. J.

Fire Saf. J.

Int. J. Heat Mass Transfer

Fire Saf. J.

Int. J. Heat Mass Transfer

Tunn. Undergr. Space Technol.

Fire Saf. J.

J. Hazard. Mater.