Ceiling jet characteristics in tunnel fires

In the past decades, research on tunnel fire has mainly focused on design fires and smoke control in longitudinally ventilated tunnels. There is a clear lack of research on detailed ceiling jet characteristics in tunnel fires. This investigation, conducted by Ying Zhen Li and Haukur Ingason of SP Fire Research, Sweden provides valuable information about the flame length and the possible fire spread, which indicate the hazards of any given tunnel fire, and are the key parameters in the design of a tunnel fire safety system. 

 A total of 43 tests were carried out in two model tunnels with a scaling ratio of 1:10. The parameters tested include heat release rate, ventilation velocity, fire source height and tunnel geometry. The model tunnels are 12.5 m long and 0.6 m high. The tunnel widths are 1 m and 0.6 m.

In each test, either the ventilation velocity was fixed with a varying heat release rate, or the heat release rate was fixed with a varying velocity. The heat release rate, HRR, varied between 16 kW and 632 kW, corresponding to full scale, HRR of 5 MW and 200 MW respectively. Both natural ventilation and forced ventilation conditions were tested. Fire spread to fuel targets close to the floor was also tested.

Flame length
Under low ventilation, i.e. when the dimensionless velocity is less than 0.3, there exists both upstream  flame and downstream flame, and the upstream flame length decreases linearly with the increasing velocity. Under high ventilation, i.e. when the dimensionless velocity is greater than 0.3, only downstream flame exists. Regardless of ventilation velocity, the downstream flame length increases linearly with the heat release rate, and decreases with tunnel width and effective tunnel height. The total flame length, i.e. the sum of downstream and upstream flame lengths can be as long as twice the downstream flame lengths. Correlations for downstream flame lengths, upstream flame lengths, and total flame lengths are proposed.

Ceiling jet velocity
Theoretical model of ceiling jet velocity in tunnels under different ventilation conditions is proposed and validated using test data. Under natural ventilation, the ceiling jet velocity increases with heat release rate and decreases with effective tunnel height. Under forced ventilation, the ceiling jet velocity increases with the ventilation velocity and the ceiling jet temperature.

Ceiling jet flow rate
The mass flow rate of the fire plume increases with heat release rate and effective tunnel height, under  natural ventilation. Under high ventilation, the smoke mass flow rate increases linearly with ventilation velocity, independent of heat release rate. Therefore the smoke flow rate is not a constant for a given fire size as documented in some regulations.

Distribution of gas temperature
For large tunnel fires, there exist virtual origins. Between the virtual origins and the fire source, the gas temperatures decrease very slowly. This is due to the large amount of heat released within the intensive ceiling combustion region. Correlations for both the ceiling gas temperatures and the virtual origins under low and high ventilation are proposed.

Ceiling jet radiation
For tunnel surfaces in the upper smoke layer, that are directly exposed to smoky gases and/or flames, the incident heat flux in the upper smoke layer can be simply correlated with the smoke temperature. For the lower layer, the view factor must be accounted for, together with the ceiling jet temperature and the emissivity of the smoke volume.

Fire spread
Fire spread to targets on the floor level or at a certain height above floor occurred when the radiation heat flux is greater than approximately 20 kW/m2. The net heat flux on the fuel surface at the ignition is found to be a positive value.

More information can be found in SP Report 2015:23. The correlations proposed in the work can be applied for performance-based tunnel fire safety design.