Simulation of fires in tunnels under construction

Fires in road tunnels has been an important research area for SINTEF NBL. Below is a short summary of the work from 1997 and -98 within this area.

Numerical Simulation of Fires in Tunnels Under Construction.

In June 1997 an experimental study was performed in a 1:10-scaled model (50 meters long and 0.8 meter diameter) representing a tunnel under construction. The objectives of this study were to examine the smoke and heat exposure at the working face of a tunnel under construction during a fire. The main conclusions from the experimental study were that the conditions with respect to heat, toxic smoke gases and visibility were considerably improved with forced ventilation of the working face.

To extend the findings from the 1:10-scale experiments to full-scale conditions, including variations in both tunnel inclination and length, as well as location of the fire, it was decided to run numerical simulations with the CFD-computer code KAMELEON FireEx in 1998.

The main conclusions from the project can be summarized in short as follows:

• Supplying sufficient forced ventilation at the working face induces an over-pressure which prevents smoke spread to this area, even at tunnel slopes up to 10%.
• If the fire is not located to close to the working face, forced ventilation efficiently prevents hazardous exposure of personnel to toxic fire gases.
• Due to efficient heat loss to the tunnel walls and ceiling, the temperature exposure of the working area will not reach unacceptable values unless the fire is located close to the working face.
• For fires close to (or at) the working face, the spread of smoke gases may overcome the forced ventilation. In this case, the amount of smoke spread to the working face highly depends on the size of the fire, location of the fire with respect to the working face and the ventilation rate.
• Comparison of the numerical calculations with the experimental results shows a good compliance, but the latter seems to give more conservative results for smoke spread to the working face.
• Within the frames of the project, no practically or economically applicable solution has been found, which ensures uninterrupted ventilation of the working face in case of a fire in a tunnel under construction.
• To ensure acceptable personnel safety, safe areas in the form of rooms with sufficient oxygen supply or at least compressed air breathing equipment with long operational time, is recommended.

Both the experimental work in 1997 and the CFD-calculations performed in 1998 were performed on request from the Federation of the Building Industry in Norway and financed by the Confederation of Norwegian Business and Industry.

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