Fire ventilation of tunnels.

A lot of road tunnels are naturally located far away from the big fire brigades, where the fire preparedness measures are not capable of handling severe fires in tunnels.

To be able to handle accidents generally, it is necessary to enter the tunnel. Normally it is not possible to get access against the smoke movement, which means that the fire preparedness measures should be able to attack the fire from both sides, - or the ventilation system must be designed and dimensioned in such a way that the smoke movement can be controlled and directed independent of the position of the fire and the surrounding conditions.

 The current criteria for ventilation of tunnels, especially within tunnels with rather big inclination, could be questioned with respect to necessary basis for smoke control. Therefore, the Directorate of Public Roads asked SINTEF NBL to perform a study to survey the necessary demand for ventilation to control the smoke movement from fires in declined tunnels.

One main task of the project was to come up with simple calculation methods for dimensioning the ventilation system, primarily related to fires in the order of 5 MW, which is a value representative for the thermal load effect from a standard car.

The project consisted of four parts:

• Literature study
• Full scale test in the "Hitra-tunnel"
• CFD modelling
• Teoretical development of simple calculation methods

Conclusions:

•  Literature study

1. Small amount of data and models for declined tunnels exist 
2. New test results indicate that existing empirical models give rather high critical ventilation velocity
3. New test results indicate an upgrading of previously accepted figures for dimensioning thermal actions

•  Tests

1. The results from the tests in the "Hitra-tunnel" show a complicated 3-dimensional flow formation
2. Average flow speed was measured to 3,0 - 3,5 m/s
3. In the "Hitra-tunnel" there should be no problem of directing the smoke movement towards either opening at conditions similar to those during the tests

•  CFD-calculations

1. An average ventilation speed of 3,0 m/s is sufficient for turning the flow related to natural direction in connection with a fire of 4,4 MW
2. Ventilation speeds of 2,5 m/s in tunnels with 3 % inclination and 3,5 m/s in tunnels with 10% inclination are sufficient
3. Due to the calculations an eventual return flow in connection with the low flow speeds will establish a stabilised smoke layer against the roof, with neglegtable temperatures and smoke densities up to a height of 2 m above the ground

•  Calculation models

1. A calculation model based upon simple correlations has been developed, to calculate the buoyancy related to fires in declined tunnels. It is a simplified fire model, but it gives reasonable results for tunnels with inclinations of more than 2 - 4 %. It is not recommended to use the model for fires of sizes like 50 - 100 MW. When designing ventilation systems initial conditions related to external wind and natural flow should be considered.

Contact person:
Kristen Opstad
Telephone: + 47 73 59 10 78
Fax: + 47 73 59 10 44
E-mail: