Investigation Of The Piston Wind Induced By Multiple Trains And Model Simplification In Ultra-long Tunnel

With the construction of mountain tunnels and cross-ocean tunnels,the number of ultra-long tunnels is increasing.The situation of two trains running in the same direction in tunnel is occured.The piston wind characteristics induced by two trains are more complex than that of a single train moving in a tunnel.The piston wind has important influence on tunnel environment and fire control.In this study,a combination of theoretical model and experimental research is used to study the characteristics of piston wind in an ultra-long tunnel with two running trains at the same time.It provides the basic research for air quality and fire control in tunnels.In addition,the new simplified research methods are proposed to solve the difficulty of model experiment and threedimentional simulation of ultra-long tunnel.The main completed work of this paper is as follows:(?)Based on the Bernoulli equation for unsteady flow,the lumped parameter theoretical model of the relationship between the piston wind induced by multiple moving trains and train length,tunnel length,train speed,blockage ratio,tracking distance,tunnel resistance et al.is established.A multiple dynamic mesh coupling model of CFD is proposed.The flow field of tunnel with multiple moving trains under fire condition and off-fire condition can be simulated.A tunnel experimental platform is built based on the absolute motion.The accuracy of the above models are verified.(?)Take advantage of the lumped parameter theoretical model,the piston wind characteristics induced by two trains in tunnel are studied.The influence of different factors on piston wind is analyzed.The variation law of piston wind induced by two trains and single train is compared.Based on the CFD numerical model,the influence of piston wind on tunnel environment and ventilation energy consumption is studied.The energy saving rate of each city which has subway tunnels is about 13%-32% by making full use of the piston wind.(?)When a motion fire occurs in a tunnel with two running trains,the smoke of the moving fire spreads under the influence of piston wind.Take advantage of the lumped parameter theoretical model,the critical pressure and speed and its influencing factors of preventing the occurrence of smoke counter-current by the longitudinal mechanical ventilation are studied.It provides a basic research for the selection of tunnel fan.The temperature distribution regularity of tunnel motion fire with two trains is studied based on the CFD numerical model.The influence of six different factors is analyzed.(?)On account of the tunnel experimental model designed under the guidance of similarity theory is too long to accommodate the experimental site and the experiment operation is unfavorable,the distortion model is proposed to shorten the length of the tunnel model.The simulations of the flow field in the prototype tunnel and different distortion ratio model tunnels are conducted,and the results show that the piston wind volume errors between the distortion model tunnels and the prototype tunnel are less than 10% when the distortion ratio is not greater than 4.(?)On account of time-consuming and high requirements for computer performance of tunnel three-dimensional dynamic simulation with motion trains,twodimensional simplified model is proposed to replace three-dimensional model.The simulation results of different tunnels by this two models show that the differences of air volume in the tunnels are less than 5%.The simulation time required for the twodimensional model is about 4%?13% of that for three-dimentional model,and the twodimensional model occupies about half of the computer memory of three-dimensional model.