Study On Thermal Storage Effect On Soil Around Subway Tunnel Under Variable Heat Flux Boundary Condition

As the most efficient urban transportation system,the subway has developed rapidly in recent years.However,although the heat generated during the train running process is partially discharged from the tunnel through the action of piston wind,the remaining heat is transmitted through the surrounding rock and accumulated in the surrounding soil,which may cause the thermal environment in the tunnel to deteriorate and even result in overheating as the operation time of the line increases.In particular,the increase of intersecting and parallel lines will exacerbate the possibility of this phenomenon.In the interval tunnel,the transfer of heat is affected by the heat dissipation law of the train,the piston wind field,the air temperature in the tunnel,and the physical parameters of the surrounding rock and surrounding soil.Therefore,to reveal the accumulation and transmission of waste heat in the subway tunnel surrounding rock resulting in cumulative temperature rise,this paper conducted the following main research work:(1)Firstly,theoretical formulas were used to calculate the heat production of a single train traveling in a 1120 m tunnel section.Then,IDA Tunnel simulation was utilized to study the distribution of the train’s heat dissipation power along the direction of travel in the tunnel,as well as the distribution of tunnel air temperature and surrounding rock heat flux density under conditions with and without temperature difference between the tunnel and outside environment.The proportion of heat absorbed by the surrounding rock to the total heat production of the train was calculated to be approximately 31.26%.(2)The dynamic piston wind field in the interval tunnel under periodic train operation conditions was simulated,and the influence of five factors,namely train length,tunnel length,train speed,tunnel blockage ratio,and train departure time interval,on the piston wind speed and ventilation frequency were explored.The correctness of the simulation results was verified.The grey correlation analysis method was used to analyze the strength of the influence of each factor on the ventilation frequency.The results showed that the strength of the influence of the tunnel blockage ratio,train speed,tunnel length,train length,and train departure time interval on the ventilation frequency decreased in turn.A fitting equation for the maximum piston wind speed and the ventilation frequency of the interval tunnel with respect to the five factors was proposed by multiple regression analysis.The predicted values of the fitting equation were in good agreement with the simulation calculation results and previous research results,indicating the reliability of the fitting equation.The fitting equation can be used to predict the variation law of the piston wind speed in most metro or railway tunnels at home and abroad under different train operation rules.(3)A ventilation network model for a 5-station,6-section subway line with a length of 10.6 km and a platform screen door system was established based on a train.The model included different train departure time intervals.The dynamic changes in the air temperature,wall temperature,and piston airflow in the tunnel were analyzed under different outdoor temperatures,tunnel depths,and piston shaft areas.The results showed that the outdoor air temperature was the main factor affecting the temperature field in the interval tunnel,and the air(wall)temperature was greatly affected by the train departure time interval and tunnel depth.Increasing the piston shaft area could effectively enhance the ventilation and air exchange effect in the tunnel.The distribution law of heat flux density in the interval tunnel and the fitting relationship between heat flux density and time were proposed,providing boundary conditions for studying heat diffusion in surrounding rocks and soils.(4)Using CFD software,the dynamic temperature field changes in the surrounding rock and soil under the action of heat flow in single-line tunnels,double-line tunnels with different spacing,and parallel transfer tunnels in the left,right,and upper and lower directions were simulated.The results show that the temperature of the surrounding rock and soil basically reaches stability after 30 years.Due to the superposition effect of heat,the smaller the distance between the two tunnels,the higher the temperature of the surrounding rock and soil,and it is higher than the temperature of the surrounding rock and soil in a single tunnel.Therefore,when planning subway lines,the spacing between the two tunnels should be arranged reasonably to avoid a rapid increase in temperature.In addition,relying solely on piston ventilation to remove tunnel residual heat without using mechanical ventilation will cause the tunnel to be in a state of overheating.This must be given sufficient attention to take reasonable mechanical ventilation measures and strategies in advance.