| Cold regions are widely distributed in China,the problem of freezing damage of tunnels built in cold regions is a serious puzzle for the engineering community.Affected by severe cold environment,the phenomenon of tunnel freezing damage in northern China is very serious,once the problem of freezing damage occurs,it will affect the normal operation of the tunnel and endanger the safety of driving,and even seriously affect the healthy service of the tunnel and reduce its service life.Considering the various engineering hazards caused by the freezing damage of tunnels in cold regions,this thesis intends to carry out two aspects of research.One is to establish a theoretical model to describe the heat transfer process of the surrounding rock-lining structure at the tunnel entrance in cold regions,and derive an analytical solution to the temperature field;The second is to propose a method of installing a air curtain at the tunnel entrance to isolate external cold air and prevent frost damage,and carry out systematic numerical simulation research around this,analyzing and comparing the impact of various parameters.The main contents and conclusions of this article are as follows:(1)Through summarizing and sorting out the current research status of prevention and control measures,theoretical research,technical research,and numerical simulation analysis for freezing damage of tunnels in cold regions at home and abroad.It is pointed out that the existing display formulas for the analytical solution of the temperature field of the surrounding rock and lining at the tunnel entrance in cold regions under the condition that the air temperature in the tunnel varies with time are lacking,and should be studied;at the same time.It is pointed out that the important reason for freezing damage at the tunnel entrance in cold regions is the inflow of external cold air,and it is proposed to set up a air curtain at the tunnel entrance to block the external cold air in order to achieve the purpose of preventing and controlling freezing damage in cold regions.(2)A heat transfer model for a multi-layer structure of surrounding rock-lining-insulation layer under non-uniform initial temperature conditions was established;the temperature response under the action of an instantaneous heat source is obtained using the separation of variables method,and the analytical solution of the temperature field distribution of a multi-layer structure is obtained using the Green function method.It is pointed out that the minimum insulation thickness of a tunnel without freezing damage during its service life can be regarded as the zero point of the historical minimum temperature function outside the tunnel lining;using the analytical solution of the temperature field and the dichotomy method to search for this zero point,accurate and automatic calculation of the minimum insulation layer thickness can be achieved.The results of calculation examples show that the cooling of tunnel surrounding rock in cold regions is not only affected by the cold air in the tunnel,but also by the initial temperature gradient of the surrounding rock;after excavation of the tunnel,the original thermal stability was broken,and the initial geothermal gradient caused additional cooling of the surrounding rock.(3)A air curtain jet velocity attenuation test system was established in the laboratory,and experimental tests were carried out on the velocity attenuation in the axial direction of the air curtain.The evolution characteristics of the axial wind velocity attenuation with distance were obtained;due to the large length-width ratio of the air curtain outlet section and the high outlet velocity,the jet can be approximated as a planar turbulent jet,using the theory of planar turbulent jets,the attenuation of jets with distance is calculated and predicted,and the theoretical prediction results agree well with the measured results.This indicates that the outlet jet of the air curtain machine can be approximated as a planar turbulent jet.(4)A numerical simulation study was conducted to investigate the process of cold air curtain insulation with a single side arrangement in a hole space(a scaled tunnel model).The simulation results of the air curtain jet angle outward 3 ° show that: the shape of the air curtain formed at this angle is relatively ideal,and the average temperature in the enclosed space at one hour is increased by 4 ℃ to 5 ℃ compared to that without the air curtain,which plays a certain role in cold insulation;the air curtain jet transfers cold energy into the hole space by absorbing hot air in the hole space and bouncing into the hole space by striking the opposite wall surface;as the thickness of the air curtain increases or the speed of the air curtain increases,the heat transfer of both of the above methods increases,leading to a weakening of the cooling insulation effect of the air curtain.The cold insulation results of the air curtain jet angle outward of 6 ° and 9 ° indicate that when the air curtain jet velocity and jet width do not match,vortices tend to appear on both sides of the jet,which consume some of the jet energy,causing the jet to dissipate or lack power and not fully reach the opposite wall,resulting in poor cold insulation effects of the air curtain.(5)Based on the Qilianshan Tunnel,a numerical simulation study was conducted to investigate the effect of arranging cold air curtains on both sides of the tunnel portal to block cold air flow into the tunnel under the pressure difference.The outlet of the calculation model portal is provided with a reflux device to simulate the inflow of air into the calculation area at the depth of the tunnel.This calculation can characterize the air curtain effect of a long tunnel in a relatively short time.The air curtain plays a significant role in cold insulation and thermal insulation by diverting air from the depths of the tunnel to flow towards the portal,with a maximum temperature increase of about 20 ℃ to 30 ℃ compared to that without the air curtain.Through numerical simulation examples,the influence of parameters such as air curtain outlet velocity,air curtain outlet angle,and air curtain setting position on the blocking effect of cold air inflow outside the tunnel caused by wind pressure and thermal pressure is further studied.(6)Taking the Qilianshan Tunnel as the background,a numerical simulation study was carried out to block the flow of certain external wind speeds into the tunnel by arranging air curtains on both sides of the tunnel portal.A symmetrical boundary is set at the entrance and exit of the calculation model to simulate the situation where air curtains are installed at both ends of a full-length tunnel to isolate cold.The numerical simulation results show that when using cold air curtain,the average temperature in the tunnel under the optimal parameter combination is 5.86 ℃ higher than that without air curtain.When using a hot air curtain(to heat the air outside the tunnel),the average temperature inside the tunnel increases with the increase of the temperature of the hot air curtain,which has a better effect.However,after the collision of the hot jets on both sides,a large portion of the air flows directly into the outer space of the tunnel,resulting in a waste of energy.A combination of cold air curtain and heat source in the tunnel is proposed for frost damage control,which has a better effect compared to directly using hot air curtain. |
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