Feasibility Study On Thermal Insulation Of Tunnel Openings In Cold Area

With the increase of traffic demand in our country,the infrastructure construction such as railway and highway extends to the cold areas with bad natural environment,so it faces severe freezing injury.In China,passive thermal insulation design is the main design of tunnel cold protection,and there is no clear research on active thermal insulation measures.Therefore,this paper takes a tunnel in cold region as the background,through theoretical analysis and numerical simulation,explores the feasibility of air curtain insulation measures in cold region tunnel,and achieves the following research results:（1）Using Ansys Fluent finite element software,a three-dimensional coupling numerical model of the tunnel entrance shed section is established,including the internal and external environment and the air curtain.The characteristics of the air curtain jet flow field are studied.The results show that the installation of the air curtain device at the tunnel entrance can achieve the effect of blocking the flow and temperature difference,and can play the role of heat preservation for the tunnel.For different natural wind speeds,the jet velocity of air curtain has a reasonable setting range.（2）Through the L₂₅（5⁴）orthogonal test on the sensitivity analysis of the air curtain barrier characteristics,and the parameter scanning analysis of the influence of the air curtain jet angle on the air curtain barrier efficiency,it shows that the sensitivity degree of the air curtain barrier characteristics is jet angle（0.665）>natural wind speed（0.324）>jet width（0.273）>jet velocity（0.253）.By changing the jet angle of the air curtain,the blocking efficiency and temperature difference can be improved to a certain extent.When the jet angle of the air curtain deflects to the outside of the tunnel,the effect is higher.Setting the jet angle between 25°and 35°will get the optimal blocking efficiency.From the analysis of the blocking efficiency and the actual installation environment,this paper suggests that the jet angle of the air curtain be set at 30°.（3）Four different installation forms of air curtain,including single outlet,symmetrical outlet,single side double outlet and double side asymmetrical outlet,are studied.The results show that all four installation forms can achieve the effect of heat preservation and cold protection,and the double side symmetrical series connection form of air curtain is better than the single side series connection form of air curtain.When single side air curtain with interval is used in series,the space between two air curtains is recommended to be between 20 m and 30 m;when double side air curtain with interval is used in series,the space between two air curtains is recommended to be between 5 m and 10 m.（4）Ansys Fluent is used to establish a three-dimensional coupled heat transfer model of fluid and solid.The influence factors of natural wind speed,temperature outside the tunnel and temperature inside the tunnel on the radial temperature change characteristics of tunnel surrounding rock and the air temperature field change characteristics inside the tunnel are studied.The results show that the temperature distribution inside and outside the tunnel is not symmetrical,and the air temperature distribution inside and outside the tunnel needs to be considered.When the air curtain is installed at the tunnel entrance in cold regions,the effect of cold proof and heat preservation can be achieved to a certain extent by reducing the cold air flow into the tunnel and increasing the air temperature in the tunnel.