| At present, road tunnel fire has been highly regarded by researchers in many countries, who are mainly focusing on steady characteristic of fire. In fact, tunnel fire is unsteady and variational because of throttling effect and buoyancy effect. Therefore, study on its dynamic characteristic is significant to lucubrate tunnel fire.Based on mathematic model of one-dimensional compressible and unsteady airflow, tunnel fire is simulated with the method of finite difference. Through numerical simulation and theoretical analysis, variation of temperature field, velocity field, and pressure field with distance and time are investigated. The development of throttling effect and its formation mechanism in different phases of fire are also indicated.Secondly, variation of velocity at the entrance in condition that fire happens in ascent or descent tunnel is simulated. Moreover, the pressure curve that present indicates the action of fire pressure.Then, according to energy equation and simulation, the traditional concept of thermal drag is revised, and a new formula of thermal drag is deduced. Furthermore, a formula for resistance in fire zone is established and justified by numerical simulation.Finally, the factors that influence dynamic characteristic of fire ventilation network are discussed. Necessity of dynamic ventilation control in descent tunnel and the worst state of horizontal or ascent tunnel, in case of fire, are studied. It is suggested that jet shoud be turned on step by step if fire pressure is big in descent tunnel. Due to fire-throttling, the worst state of ventilation in horizontal and ascent tunnel, which obliquity is small, is the moment when heat release rate just become stable.The results enrich the knowledge of tunnel fire's characteristics, and provide important reference to design schemes in the event of tunnel fire.
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