Studies On Smoke Characteristics Of Fire Under Water Mist In Large Cross-Section Underwater Road Tunnel

With the increasing development of economic and technology, there is a large number of underwater road tunnels being constructed and coming in to service with supper length and large section. Though they have brought convenience to everyday life, the following fire hazards could not be ignored. Due to many advantages.such as clean, high fire extinguishing and so on, water mist system, is utilized gradually in underwater tunnels, which aims to fulfill the demand of fire-suppression and prevention in tunnel. However, the water mist fire suppression and temperature distribution with water mist applying in tunnel are not well known. Based on previous researches, the experimental bench with water mist fire extinguishing-concentration exhaust-longitudinal ventilation under synergy was designed and established in the paper. This paper used diesel pool fire as fire source to study the temperature distribution in tunnel, water mist fire suppression and fire enhancement,the smoke characteristics under water mist and ventilation,by means of small scale model and numerical simulation.The main research achievements were as follows:(1)According to the difference of height, vertical temperature distribution could be divided into linear attenuation stage and stable stage in near fire regions and far fire regions; while near the tunnel exit, vertical temperature was presenting change of down-up-down a three-stage process by the impact of pressure and temperature difference between the inside and outside. The longitudinal smoke temperature distribution along the ceiling followed the natural exponential decay laws from the ceiling above the source to the left and right side under non-water mist condition or non-controlling fire water mist condition; and the temperature distribution along the ceiling appeared the piecewise function decay laws under controlling fire or suppressing fire water mist condition in tunnel.(2) With increasing pressure in the nozzle, the fire combustion time was shortened as a whole, only when nozzle pressure reached the critical pressure achieved short-term extinguishing fire. The extinguishing time decreased firstly and then rised as preburning time increased when the preburning time was less then the critical preburning time; and when the extinguishing time was over the preburning time, it rised conspicuously as preburning time increased and then the short-term extinguishing fire failed. Besides, by means of numerical simulation was the cooling effect of water mist in different nozzle characteristic parameters showed that the first level size water mist> the second level size water mist> the third level size water mist; the solid cone pattern>the hollow cone pattern; the spray cone angle of 30°> the spray cone angle of 60°> the spray cone angle of 90°>the spray cone angle of 120°.(3)The phenomenon of fire enhancement would occur if only when the pressure in the nozzle is above the the critical pressure. The interaction of fire enhancement can be divided into three levels, that is instantaneous weak degree enhancement, discontinuous medium degree enhancement and the continuous strong degree enhancement.Rising as the preburning time, the phenomenon of fire enhancement would escalate and the fire risk would ratchet up. At the same time, the scope and endurance of expansion flame and explosive flame would increase.(4) Under the condition of water mist-air supply cooperative action,the development of fire plume was limited greatly, due to the vertical and horizontal interaction of water mist and ventilation.It affords good results on smoke controlling and reducing temperature on the condition of water mist-air supply cooperative action.