Research On Point Smoke Extraction System Of Independent Smoke Exhaust-duct In Case Of Fire For Extra-long Highway Tunnels

In recent years, with the abundant appearance of various extra-long road tunnels in China, the risk of tunnel fire is increasing. Based on fully investigating ventilation system design of extra-long road tunnel at home and abroad, considering the characteristics of the extra-long road tunnel in Zhejiang province, a new design concept is introduced to a tunnel in Zhejiang province. Point smoke extraction system in the tunnel with independent smoke exhaust duct is studied using numerical simulation, theoretical analysis and model experiment. The main results are as follows:1. Smoke exhaust system designs of extra-long road tunnels over the world are analyzed, and different smoke exhaust systems are compared. Three independent smoke exhaust duct systems under longitudinal ventilation have respective characteristics, and the extra-long road tunnel in Zhejiang province has long slope, high traffic volume, complex traffic organization and higher fire frequency than national average. Through comparative analysis of different smoke extraction systems in economic, technical and safety aspects, the design concept that point smoke extraction system with independent smoke exhaust duct in fire conditon and longitudinal ventilation system consisting of shafts and jet fans in normal condition is proposed in mountain tunnel.2. When a fire occures, the smoke extraction systems in tunnel with and without smoke duct are analyzed through three-dimensional numerical simulation, and temperature distributions and fire-induced smoke movements in tunnel with the two smoke exhaust systems above are compared. It can be seen that when fire occurs in tunnel using point smoke extraction system with independent smoke duct, smoke can be controlled in limited area, and the adverse impact of two-way tunnel fires with uni-or bi-direction traffic secondary fires by incidents and chimney effect can be well controlled. In addition, smoke is distributed in upper part of driveway and controlled effectively, and it is safer to rescue and evacuate. At the same time, fire-induced smoke mainly spread in smoke duct under the influence of chimney effect, so passenger and drivers can escape safely, and the maximum smoke temperature below ceiling is decreased. Theoretical basis is provided for extra-long road tunnel using the smoke exhaust sy.stem.3. The characteristics of temperature distribution, smoke spread and induced wind speed under point smoke exhaust mode in tunnel with independent smoke exhaust duct are studied through model experiments. The influence of tunnel slope on high temperature range and smoke spread is explored, and the effectiveness of smoke control under unidirectional and bi-directional exhaust mode are compared. According to the test results, the formula of critical wind speed in slope condition is modified, and the impact of slope change on induced wind speed is revealed. The selection of induced wind speed and wind direction under different tunnel slopes and different ignition source positions was raised. The numerical simulation results are complemented and verified by experiment results and it can provide theoretical basis for ventilation and exhaust design of extra-long tunnels.4. The research on smoke distribution, heat extraction effectiveness and exhaust system setting under point smoke exhaust mode shows that:smoke flow velocity in smoke exhaust duct and at exhaust valves decreases when the distance from the exhaust fan increases; the maximum smoke flow velocity under bi-directional exhaust mode is less than that under unidirectional exhaust mode; the maximum smoke flow velocity is related to total area of exhaust valves, area of each valve and space between different valves; the theoretical model of flow velocity and pressure intensity of independent smoke exhaust duct system is established; different smoke exhaust modes have different distribution characteristics of heat extraction efficiency; in general, the heat extraction efficiency of exhaust fans under bi-directional exhaust mode is higher than that under unidirectional exhaust mode; the main factors that influence the smoke exhaust system are ascertained, according to which proper setting scheme of exhaust system under point smoke exhaust mode in tunnel with independent smoke exhaust duct is proposed and smoke exhaust organization design is specified; it can provide key index parameters for similar engineering designs.