Abstract
We have set up a Computational Fluid Dynamics (CFD) modeling, and performed a user evacuation model, for evaluating the risk level in one-way road tunnel tube when used for bi-directional traffic in particular circumstances. The simulations were carried out by considering both peak-hour traffic volumes during the day and off-peak hours overnight. The investigated one-way tube is ventilated by natural ventilation only, and has a length of less than 1000 m. With reference to the worst environmental conditions, which are downstream of the fire due to the direction of natural ventilation, the consequences on escaping users, caused by different types of burning vehicles located in various longitudinal positions along the tube, are shown. The results prove the positive effects on environmental conditions (in terms of temperature, visibility distance, CO and CO2 concentration) along the user evacuation path when the tube is used for bi-directional traffic at night rather than daytime. Only for the case of 100 MW fire and in the proximity of the exit portal, the last escaping user might be affected by a visibility distance and CO concentration exceeding the threshold values. In this special case, countermeasures for reducing smoke concentration or emergency services at the portals should be provided. However, the quantitative risk analysis, based on a probabilistic approach, showed that the F-N curve of the tube when used for bi-directional traffic with reference to the night always lies below that of the daytime, and the reduction in the risk level is between 80 and 100% for the night traffic compared to daytime one. It is to be focused on the fact that our modeling may represent a reference in investigating the effects of hourly traffic volumes on the risk level in tunnels and may help decisionmakers in understanding when to temporarily close a tube for maintenance, repair, or rehabilitation activities and use the adjacent tube for bi-directional traffic.
Highlights
Caliendo et al [18], by applying the Fire Dynamics Simulator (FDS)+Evac codes for a unidirectional road tunnel tube equipped with natural ventilation only and implementing the peak-hour traffic volume (VHP) during the day, evaluated the effects on escaping users upstream of the fire due to different types of burning vehicles located at different longitudinal positions along the one-way tube
Since the closure of the adjacent tube during the day might create very high levels of congestion in the investigated tube due to its use for bi-directional traffic, we assumed in the risk analysis that the hourly volume in each of the aforementioned six hours were equal to the bi-directional capacity of the tube (i.e., 3200 vehicles/hour for the two lanes)
One can see that: (i) the smoke layering length is not symmetrical in the two directions downstream and upstream of the fire because the natural ventilation pushes the smoke towards portal B, which explains the worst environmental conditions downstream of the fire; (ii) by increasing the Heat Release Rates (HRRs) the smoke layering length increases; (iii) the smoke layering length is at night shorter than at day for HRR of 8, 30, and 50 MW, while it is the same for a HRR of 100 MW
Summary
Publisher’s Note: MDPI stays neutral with regard to jurisdictional claims in published maps and institutional affiliations. Due to the ventilation direction (mechanical or natural) within the originally unidirectional tube, the hot airflow is generally pushed towards people who are in the zone downstream of the fire, and that having left their own motor vehicles in the queue to run away from the burning vehicle in the direction of a safe place (emergency exits or portal) It is to be stressed, in the case of unidirectional traffic tube, that users downstream of the fire can, instead, escape with their own vehicles towards the exit portal without any significant risk. The mentioned topic of discussion in the risk analysis of tunnels (i.e., the use of oneway tube for bi-directional traffic during both daily peak-hours and overnight in the event of a fire) indicates a lack of knowledge that is needed to be filled, which is the scope of the present paper.
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