| With the development of China's transport and the upgrade of road grades, the number and length of road tunnel as well as the road traffic density are growing, therefore, it recalls more and more attention to provide a safe, reliable, comfortable operating environment for tunnels. Until now, our investigation and research in this field is far behind abroad, and is badly hindering the tunnel construction.In this paper, fluent software was applied for the longitudinal ventilation systems to get three-dimensional computer simulation and analysis which were widely used in our country nowadays. This paper mainly simulated the air distribution of the tunnel by the establishment of mathematical models on the basic of some structure parameters in Tianjin Expressway tunnel. In order to ensure the reliability of numerical simulation, a tunnel ventilation model was designed based on similar principles, to simulate the jet flow ventilation and analyze the jet characteristics. After verifying the reliability of the numerical simulation, multi-operating modes were simulated, and this paper focused on the study of pressure—adjusting characteristics and performances, the main factors impacting the length of jets diffusion section. According to the different dilution effects of CO and other finite which in the condition of different jet fans composition, the optimal selection and layout of jets ventilation system were analyzed.This paper also studied the critical velocity through establishing the spread of fire smoke model and numerical simulation, determined the critical velocity magnitude in the fire scale of 20MW.
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