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Monday, October 18, 2010

A novel methodology for simulating tunnel fires

A recent journal paper titled "A Novel Multiscale Methodology for Simulating Tunnel Ventilation Flows During Fires" has recently been published in the journal Fire Technology. Its content is presented here. This is a joint research effort between Politecnico di Torino and University of Edinburgh.

PD NOTE: This paper won this year’s Lloyd’s Science of Risk Prize in the Technology Category. The prize is awarded to academics and aims to keep the world’s leading specialist insurance market abreast of the latest academic knowledge and cutting-edge thinking. See press release by Springer.

In the past decade over four hundred people worldwide have died as a result of fires in road, rail and metro tunnels. In Europe alone, fires in tunnels have destroyed over a hundred vehicles, brought vital parts of the road network to a standstill - in some instances for years - and have cost the European economy billions of euros. Disasters like the Mont Blanc tunnel fire (1999) and the three Channel Tunnel fires (2008, 2006 and 1996) show that fire poses a serious threat.

Comprehensive risk assessments for tunnel fires are not easy to conduct. The development of the possible emergency scenarios is dependent on the combined influence of fire detection technologies, ventilation system, tunnel layout, atmospheric conditions at the portals and the presence of vehicles. Nowadays, the analysis of such complex phenomena is performed using numerical computational fluid-dynamics (CFD) tools. But CFD has a significant drawback: its requires very large computational resources (e.g., weeks or months of computing time). This limitation affects the completeness of the risk analyses because they can only be based on a limited number of possible scenarios but do not explore the wide range of possible events.

This recent paper proposes a novel multiscale modelling approach generated by coupling a three dimensional CFD model with a simple one-dimensional model. This allows for a more rational use of the computational resources. The methodology has been applied to a modern tunnel of 7 m diameter section and 1.2 km in length (similar layout to the Dartford Tunnels in London). Different ventilation scenarios are investigated involving fire sizes ranging from 10MW to 100MW.

The multiscale model is proved to be as accurate as the traditional time consuming CFD techniques but provides a reduction of two orders of magnitude in the computational time. This greatly widens the number of scenarios that can be efficiently explored. The much lower computational cost is of great engineering value, especially when conducting comprehensive risk analyses, parametric, sensitivity and redundancy studies, required in the design or assessment of ventilation and fire safety systems.

The multiscale methodology is the latest contribution to the state-of-the-art in computational methods for tunnel flow simulations. The model has been validated against experimental data of cold flow ventilation and shown to be accurate. This work was published in Building and Environment in 2009. It has also been used to provide the tunnel operator with a comprehensive assessment of the ventilation in the Dartford Tunnels, located under the River Thames about 15 miles east of London. This work was published in Tunnelling and Underground Space Technology in 2010 (open access version).

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