The combined effect of a water mist system and longitudinal ventilation on the fire and smoke dynamics in a tunnel

Abstract

The individual and combined effect of a water mist system (WMS) and longitudinal ventilation (LV) on the fire HRR, smoke temperature and back-layering (BL) degree were explored in a 3 m (width)×2.2 m(height)×30 m (length) tunnel model. A series of 39 tests has been conducted in 6 configurations, varying the velocity, water volume flow rate and nozzles arrangement. When the WMS or LV is imposed individually, the reduction effect on the HRR gets stronger with larger water volume flow rates or higher velocities. Yet, under the combined effect, the effect of the LV velocity on the HRR is non-monotonic: the HRR first reduces more, and then rises again with higher LV velocities for given WMS settings. The reduction in temperature monotonically increases with higher LV velocities, in contrast to the HRR behavior. The above phenomenon is explained to be due to the relatively ‘low’ flame height in the present paper, as a consequence of which the interaction between the flame and the water droplets mainly occurs in the lower part of the tunnel, while the interaction between the hot smoke and the water droplets mainly happens in the upper part. The fire HRR behavior mainly relates to the interaction in the lower part, while the temperature depends on both regions. Besides, for a certain BL distance, the required ventilation velocities reduce by 13%–55% with activated WMS, compared to the same conditions without WMS. The effect of covering the fire with a shield is also discussed. For relatively small velocities, the fire HRR, smoke temperature and BL distance are larger with the shield in place.