Use of a water mist for smoke confinement and radiation shielding in case of fire during tunnel construction

The ability of a water curtain to stop smoke and radiation from a fire is investigated in the particular configuration of a tunnel during construction work. Experiments are conducted in reduced scale (1/22nd) in a blind tunnel and simulations are performed in small and real scale configurations. The small-scale experimental model consists of a 6.20 m long polycarbonate circular tunnel with a diameter of 40 cm. It is connected to a 1.20 m cubic space, which represents the wellbore of the tunnel boring machine. The scenario considers a fire occurring in the tunnel and the activation of a water mist downstream from the fire, close to the wellbore, in order to stop smoke and radiation emitted by the fire. The spraying system consists of 5 injection nozzles, regularly distributed on the ceiling of the tunnel. The numerical simulations are carried out using Fire Dynamics Simulator. The results from both small and large scale studies show that the water curtain induces a strong mixing of air and smoke due to drag effect, but it does not stop the smoke flow. Similarly, despite the down drag effect due to the curtain, fresh air continuously feeds the fire, crossing the mist area and flowing in the lowest part of the tunnel as a counter-current flow. Upstream from the water curtain the smoke is filling all the tunnel due to a strong de-stratification, partially diluted with air in the lower counter-current flow. There may be a remaining thermal stratification, but contour plots of soot mass fraction show that soot particles are found even in the lowest part of the tunnel in a layer of smoke cooled by the spray and diluted with air. Radiation from the fire is attenuated as expected due to radiation and droplet interactions, with a radiation attenuation up to 90 % for the large scale water flow-rate investigated in the present case (15 m3/h).