| Because of the highway tunnel fire occur abruptly,the fire developed quickly,a great deal of poisonous,harmful air and calories were produced when a fire was on. FDS was adopted in this paper for numerical simulation,Mechanism analysis of fire extinguishments with water mist under the mechanical ventilation in highway tunnel.This paper is aimed to study fire suppression effects in highway tunnel by water mist when the pool fire is set at different location,it can provide technical basis for water mist in the construction of long and narrow spaces. FDS (Fire Dynamics Simulator) is based to establish the model of oil fire in highway tunnel,following parameters are discussed to analyze the fire suppression effects,i.e.,the heat release rate of pool fire,the distribution of temperature and CO concentration, the height of smoke and other parameters in the tunnel. By mean of comparison method,this paper analyzed the average temperature over the oil surface,the temperature curve,the volume percentage of CO in the air at 1.5 meters high in the tunnel. It is found that the fire suppression effect by water mist is quite different with the different location of fire. The most effective situation occurs when the fire is located just below the nozzle. Additionally,the effect became worse when the distance between the nozzle and the fire is increased. It's effective to expand the protecting scope of the nozzle by increasing the spray angle,which is helpful to fight against the worse case when the nozzle is far from the fire.This paper is aimed to study fire suppression effects of different heat release rate in highway tunnel by smoke exhaust associated with water mist,and discuss the suppression effects of water mist when turned on the smoke exhaust at different time. Based on Fire Dynamics Simulator,the simulations were carried out when the heat release were 4MW and 2.5MW respectively. And the smoke exhaust was turned on at the moment of 60 seconds,120 seconds and 150 seconds respectively. Following parameters are discussed to analyze the heat release rate of pool fire,the average temperature over oil surface,the volume percentage of CO in the air 1.5 meters high in the tunnel. Results show that when the heat release rate of the fire in the highway tunnel was 4MW,smoke exhaust system was helpful to extinguish the fire. The suppression effect was best when the smoke exhaust and water mist were turned on at the same time. When the heat release rate was 2.5MW,the extinguishment time was the shortest when the smoke exhaust and water mist were turned on at the same time,the suppression effect was better when turned on the smoke exhaust at 60 seconds comparing with the situation when smoke exhaust system was turned on at 150 seconds. The simulation results show that though the smoke was well controlled when the smoke exhaust system was turned on,the suppression effect was different when the heat release rate was different in the collaborate condition of smoke exhaust system associating with water mist system. The start time of smoke exhaust system should be well controlled with consideration of the different Hear Release Rate. FDS is based to simulate and analyze fire suppression effects by water mist when turned on ventilation at different time using the longitudinal ventilation and top smoke exhaust respectively. Focus on discussing fire suppression mechanism the two kinds of mechanical ventilation modes and the smoke flow in tunnel. Simulation results show that the effect of fire suppression by water mist with longitudinal ventilation was better than that of top smoke exhaust,however,the impact to transverse velocity of the water mist droplets was greater. While the vertical velocity was greatly impacted of top smoke exhaust,resulting in the decrease of the penetrability of water mist to plume. The effect of top smoke exhaust was better than the longitudinal ventilation apparently in tunnel.
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