Study on Tunnel Air Curtain Insulation System for Cold Areas

With the growing number of tunnels in high-latitude and high-altitude areas, freezing damage is all the more obvious. In traditional insulation layer method, insulation layer is laid in the tunnel to prevent invasion of cold air by taking advantage of its low thermal conductivity. However, due to its narrow temperature adaptation range and high construction and maintenance costs, this method cannot completely solve the problem of tunnel freezing damage. To solve this problem, this paper proposes tunnel air curtain insulation system which can generate electricity using solar energy. Based on parameters such as indoor air temperature and wind velocity in the thermal insulation room, on the one hand, it intelligently controls running sets of air curtain, jet ow angle, and velocity, prevents invasion of the external cold temperature into the tunnel, so that the tunnel maintains an interior temperature above 0°C. On the other hand, it intelligently diagnoses air curtain machine to provide system fault warning, so that the system keeps safe and reliable operation. ­e results show that air curtain technology can eectively solve the problem of tunnel freezing damage in cold areas; multiple air curtains in series connection provide better barrier eciency. From the measured data and calculation results of Lafashan tunnel, it can be seen that an air curtain is installed at the tunnel portal with jet speed of 22.7m/s and temperature of 35.24°C, which can meet the tunnel without freezing damage when the external temperature is −10°C, and the external wind speed is 2m/s, providing a new method for tunnel insulation technology in cold area.


Introduction
According to relevant statistics [1][2][3], perennially frozen soil, seasonally frozen soil, and transient frozen soil account for about 50% of the total land area of the world, in which, perennially frozen soil accounts for about 25% of the total land area of the world. Frozen soil is mainly distributed in Russia, western and northern China, outer Mongolia, Japan, northern Europe, Alaska in the northern United States, Canada, and the Antarctic. In Russia and Canada, perennially frozen soil accounts for about 50% of the country's total area. In Alaska in the northern United States, it accounts for about 15% of the country's total area. Frozen soil in the western and northern China accounts for about 75% of the country's total area, mainly distributed in Qinghai-Tibet Plateau, northeast China, and Inner Mongolia, in which, the perennially frozen soil in Qinghai-Tibet Plateau accounts for about 75% of the country's total [4].
With the continuous development of transportation industry, infrastructure construction of railway, and highway tunnels is extending to high-latitude and high-altitude cold areas with severe climatic conditions [5]. According to the statistics of freezing damage in Japan, Norway, and China, in 2017, there were 2,135 tunnels with frozen damage out of 3,819 railway tunnels in Japan, accounting for 56% of the total number of tunnels. Accidents endangering tra c safety due to freezing damage occur frequently in winter [6]. In 2016, nearly all of Norway's railway and highway tunnels su ered from water leakage, ice hanging, and ice slip in varying degrees. In 2014, 5,990 out of 11, 516 railway tunnels su ered from freezing damage, accounting for 52.4% of the total number of tunnels, of which, 2,937 railway tunnels su ered from severe freezing damage, accounting for 25.5% of the total number of tunnels. Construction and operation of a large number of railway and highway tunnels in cold areas indicate that [7,8], low temperature-induced freezing damage is widespread in the existing railway, highway tunnels, which brings serious damage to the tunnel structure, and also results in major security hidden dangers to safe operation of railway and highway, so tunnel freezing damage has become a difficult problem restricting the development of tunnel projects in the world.
At present, there are mainly two types of measures against freezing damage: one is thermal insulation measures, and the other is drainage measures. ermal insulation measures are mainly divided into two categories: one is passive thermal insulation measures, such as insulation layer method adopted in Japan, Ethafoam Plank adopted in the United States, isolation wallboard adopted in France, offwall lining support method adopted in Norway, and insulation layer method and cold-proof door method adopted in China. e other is active thermal insulation measures, such as electric heating method and heating water pipe method adopted in Japan, electric heating method adopted in Russia, heating water pipe method and ground source heat pump method adopted in China, etc. ese thermal insulation measures against freezing damage have greatly reduced occurrence of tunnel freezing damage in cold areas, but there are still some problems. e traditional insulation layer method, as a passive thermal insulation measure, requires a certain range of temperature adaptation. Under the extreme low temperature condition, thermal insulation failure will easily occur. Active thermal insulation measures provide obvious thermal insulation effect, but feature high-energy consumption and high-maintenance cost. Drainage measures adopted in China mainly include thermal insulation ditch, deeply buried ditch in the center and cold-proof drain tunnel. Generally, thermal insulation ditch is suitable for cold area with a mean temperature between −10°C∼−15°C in the coldest month, where, the maximum freezing depth of viscous soil is between 1.0 m-1.5 m and tunnel has or possibly has water in winter. Deeply buried ditch in the center is generally applicable to cold area with a mean temperature between −15°C∼−25°C in the coldest month, where the maximum freezing depth of viscous soil is between 1.5 m∼2.5 m and tunnel has or possibly has water in winter. Cold-proof drain tunnel is generally applicable to cold area with a mean temperature below −25°C in the coldest month, where the freezing depth of viscous soil is above 2.5 m, and tunnel has water in winter. e above measures have greatly alleviated tunnel freezing damage in cold areas, but the widespread phenomenon of "tunnel leakage" still exists, and tunnel freezing damage is becoming all the more serious.
To solve the problem of tunnel freezing damage, this paper proposes air curtain insulation technology. By applying air curtain device to small mine roadway, Guyonnaud et al. [9] studied the relationship between parameters like roadway height, air curtain temperature pressure difference, nozzle width, nozzle angle, jet velocity, and barrier efficiency. e research results are applicable to roadway at a height of 0.2 m∼1.44 m, and no general calculation formula is proposed. Shepelev and Batulin [10,11] studied air curtain jet flow and its flow pattern of cold storage door and provided theoretical calculation formula for air supply volume.
Shepelev [9] applied air curtain device to mine roadway. Due to the mine roadway size far above the cold storage door size, calculation theory of cold storage door is no longer applicable. Spanish scholar Tomas Gil-Lopez [12] installed air curtain on the cold storage door, showing that air curtain could block 80% of airflow heat transfer. Xiao et al. [13] developed a dust separation system based on air curtain technology. e experimental results showed that when the jet velocity of air curtain was set at 6.27 m/s∼8.39 m/s with jet angle set at 10°∼20°, a stable air curtain could be formed and the dust control efficiency was up to 92.6%. Taking Lanzhou West Railway Station as an example, Li et al. [14] conducted field test and analysis on indoor temperature and wind velocity of passenger passage under different start-stop conditions of electric hot air curtain. e results indicated that running hot air curtain could well block cold air penetration at the passenger passage exit. Bian [15] studied the partition effect of parallel air curtain on roadway airflow by means of theoretical calculation, numerical simulation, field effect analysis and test, etc. e research results suggested that parallel double-unit air curtain has better resistance and partition effect than single air curtain, and local resistance variation increases with the increase of angle. With real cold storage as the research object, Nan et al. [16] established a three-dimensional numerical model involving multiple parameters such as air temperature inside cold storage, air curtain design parameters as well as outdoor wind flow field. By studying the relationship between nozzle width, jet velocity, jet angle, and air curtain partition efficiency as well as thermal insulation effect, the research found that wider nozzle width is not necessarily better, and there is optimal jet velocity and jet angle in the calculation model. e above analysis indicates that air curtain technology has been widely used in the fields of mine roadway, cold storage, supermarket, and subway, but has not yet been used in the field of tunnel insulation in cold areas. e main cause of freezing injury in tunnels in cold regions is that the cold air outside the tunnel intrudes into the inside of the tunnel, and the cold air exchanges heat with the air and surrounding rock in the tunnel, so that the temperature inside the tunnel is lower than 0°C; In addition, the phenomenon of "ten tunnels and nine leaks" has existed in the tunnel for a long time. e low temperature causes the tunnel to freeze, resulting in frost damage problems such as lining frost crack and hanging ice [17,18]. Lai et al. [19] set snow guard and cold-proof door at the front of Osakayama Tunnel entrance, and the field test results showed that thermal insulation materials installed on the inner surface of the tunnel lining layer to solve the problem of freezing damage do not produce heat, but only reduce the heat transmission and speed of freezing and thawing, so simply installing thermal insulation materials on the tunnel lining surface cannot prevent the formation of perennially frozen soil, and a reliable way to eliminate freezing damage is to raise air temperature in the tunnel. Snow guard can not only prevent snow accumulation at the entrance and exit of the tunnel but also improve temperature of the air into the tunnel, which can better eliminate freezing damage to the tunnel when used together with thermal insulation door.

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However, in November 2000, the cold-proof door was damaged by a car, so this thermal insulation may endanger driving safety. Zhang et al. [20] based on the ZaDunHe Tunnel in Inner Mongolia, designed a ground source heat pump heating system (GSHP) using ground temperature energy to heat the tunnel portal section. At the same time, they proposed the design and calculation method of the heating section length, heating load, and heat extraction section length of the tunnel heating system using ground temperature energy, which can ensure the occurrence of tunnel freezing injury during the operation of the system; however, GSHP system has high technical content and is difficult to be applied on a large scale. Lai et al. [21] proposed a thermal radiation heating system (EHT) and applied it to DongNanLi Tunnel, the system converts electric energy into heat energy and transmits the heat energy to tunnel lining and tunnel pavement through heating cable for heating, which can effectively prevent tunnel freezing injury. However, the heating cable needs frequent replacement and maintenance under harsh environmental conditions, and the plastic cladding of the heating cable is easy to age after long-term use. For this reason, this paper proposes an air curtain insulation system for tunnels in cold areas, thus providing a new option for solving freezing damage problem of tunnels in cold areas.

Composition of the Tunnel Air Curtain Insulation System
When the tunnel air curtain insulation system is built on the sunny side of the mountain, it is composed of four parts: thermal insulation room, air curtain, solar panel, and PLC intelligent control system. When the tunnel air curtain insulation system is built on the dark side of the mountain, it is composed of three parts: thermal insulation room, air curtain, and PLC intelligent control system. e thermal insulation system generates electricity via solar energy and stores electricity using batteries to guarantee normal operation of the power supply system. When the total solar panel power generation exceeds power consumption of the system, or the tunnel air curtain insulation system does not need to work, the excess power can be used to supplement industrial power consumption, thus effectively saving energy and reducing maintenance cost of the system. Structures of the tunnel air curtain insulation systems on the sunny side and the dark side of the mountain are shown in Figures 1 and 2 respectively. e tunnel air curtain insulation system is installed at the front end of the tunnel portal, and the air curtain machine is installed at the top of the insulation room. e air curtain adopts a customized air curtain machine that can adjust the jet wind speed, jet temperature, and jet angle. e working state of the air curtain machine is divided into normal temperature and heating. e jet wind speed is 10∼24 m/s, the jet temperature is 40-55°C, and the jet angle is 0-90°. e air curtain machine extracts the amount of air in the insulation room and sprays it after heating inside the air curtain machine. It blocks the external cold air and heats it through the hot air curtain wall. e installation diagram of tunnel air curtain is shown in Figure 3.

Governing Equation.
Air curtain barrier efficiency is an important parameter of air curtain insulation system, whose calculation model is shown in Figure 4.  As shown in Figure 4, it is assumed that the synthesized airflow f is composed of the airflow f 1 flowing into the tunnel and the airflow f 2 ejected by the air curtain, then f is (1) e airflow f 1 flowing into the tunnel: e airflow f 2 ejected by the air curtain: As shown in Figure 5, xOy rectangular coordinate system is established. When the synthesized airflow f is vertically downward f H , that is, y � 0, Equation (1) is simplified as follows: Calculated based on equation (2), when the air curtain is closed, the air amount into the tunnel Q 0 is When the air curtain is opened, the synthesized airflow f H of the air curtain is as follows: Air curtain operation fails to block air amount Q m : Air curtain barrier efficiency η is as follows:

Parameter Optimization.
e relationship between the optimal jet angle, jet velocity, and jet thickness of the air curtain and the barrier efficiency is studied. e calculated parameters are shown in Table 1.

Jet angle.
It is set that variation range of the jet angle is 0-90°, the wind velocity outside the tunnel is 1.5 m/s, the air curtain jet velocity is 20 m/s, and the jet thickness is 0.5 m. e above calculation parameters are substituted into the equation (8), and the relationship between the air curtain jet angle and barrier efficiency is calculated as shown in Figure 6. As shown in Figure 6, the barrier efficiency increases first and then decreases with the change of the air curtain jet angle, and its maximum value is 1.242. At this time, the optimal jet angle is 77.6°. When the barrier efficiency is greater than or equal to 1, the airflow outside the tunnel is completely blocked.

Jet velocity.
It is set that the jet angle is 77.6°, the jet thickness is 0.5 m, the wind velocity outside the tunnel is 1.5 m/s∼12 m/s, and the jet velocity is 8 m/s∼20 m/s. e relationship between jet velocity and wind velocity outside the tunnel, barrier efficiency is shown in Table 2.
As shown in Table 2, the barrier efficiency decreases with the increase of wind velocity outside the tunnel, but increases with the increase of jet velocity. erefore, on the premise of not affecting driving safety, jet velocity is recommended to be 20 m/s.

Jet thickness.
It is set that the jet angle is 77.6°, the jet thickness is 0.1 m-0.5 m, the wind velocity outside the tunnel is 1.5 m/s-12 m/s, and the jet velocity is 20 m/s. e relationship between jet thickness and wind velocity outside the tunnel, barrier efficiency is calculated as shown in Table 3.   As can be seen from Table 3, the barrier efficiency decreases with the increase of wind velocity outside the tunnel, but increases with the increase of jet thickness. When the wind velocity outside the tunnel is 1.5 m/s, the jet thickness can be 0.4 m, and the barrier efficiency of the air curtain is 0.994. When the natural wind velocity at the tunnel entrance is beyond 1.5 m/s, jet thickness of the air curtain can be appropriately increased.

Comparative Analysis of Measured Air Curtain Barrier
Efficiency. Wang [19] conducted field application test research on air curtain airflow resistance enhancement in Longshou mine, testing barrier efficiency of the air curtain under different temperatures and different external wind velocities. e test adopted single machine start-up mode, and the test site was on the main slope of Longshou mine. e main slope has a roadway sectional area of 17.9 m 2 and a height of 2.5 m. e model parameters of air curtain used in the test are shown in Table 4.
e measured data are substituted into equations (8), with measured data [22] and theoretical calculation results shown in Table 5.
As can be seen from Table 5, both the measured and theoretically calculated air curtain barrier efficiency decreases with the increase of external wind velocity. In theoretical calculation, the impact of temperature change on airflow pressure difference in the tunnel is not considered, resulting in a big deviation between the measured and calculated value when the external temperature is −10°C.

Analysis on Air Curtain Barrier Efficiency.
Train wind greatly affects temperature field in cold tunnel. According to the measured data [23], when a train with a speed of 200 km/ h passes through Jingzhuling Tunnel, the average wind velocity of the train is 12.5 m/s. Based on calculation in equation (8), the change law of air curtain barrier efficiency is obtained as shown in Table 6.
As shown in Table 6, there are a total of seven air curtains. e first six air curtains have a jet velocity of 20 m/s, the last air curtain has a jet velocity of 14 m/s, and the wind velocity in the tunnel after the barrier is 0.0755 m/s. en, the barrier efficiency of the air curtain is up to 94.15%.

Calculation
Model. Fluent software has high accuracy for the steady-state analysis of tunnel flow field and temperature field under the action of air curtain insulation. In this calculation, Lafashan tunnel in China is selected as the calculation prototype. In order to simplify the calculation model, the following provisions and assumptions are made in the process of using Fluent to calculate the thermal insulation effect of the tunnel: ① the tunnel is equivalent to the prototype section; ② It is assumed that the airflow is   Advances in Civil Engineering laminar, incompressible, and the air pressure does not change with the change of temperature; ③ It is assumed that the solid wall boundary condition is adopted, in which the thermal conductivity, specific heat capacity, density, air convection heat transfer coefficient, and other thermodynamic parameters are constants. Figure 7 shows the Finite element model.

Calculation Parameters.
According to the field measured data and meteorological data of Lafashan tunnel, the external average wind speed of the tunnel in winter is 2 m/s and the external average temperature is −10°C. e velocityinlet boundary condition is velocity-inlet. e boundary condition of the bottom surface of the air domain is wall, while the boundary condition of other surfaces is pressureoutlet. e upper, lower, left, and right boundary conditions of the insulation chamber are wall, and the rear boundary condition is pressure-outlet. e boundary condition of the air curtain nozzle is velocity-inlet, and the jet angle is set to 70°. e jet wind speed and temperature are related to the outside wind speed and temperature. According to the calculation of the inlet boundary conditions of the air domain, the air curtain jet wind speed is set to 22.7 m/s, and the jet temperature is set to 35.24°C.

Calculation
Results. Import the finite element model into ANSYS FLUENT software, set calculation parameters for numerical calculation, and the calculation results of temperature field and flow field are shown in Figures 8 and 9.
It can be seen from Figures 8 and 9 that when the air curtain is opened, the external cold airflow and air curtain air flow form a swirling air flow at the opening of the insulation room, which effectively prevents the external cold airflow from invading the inside of the tunnel. When the external temperature is −10°C, the wind speed at the entrance is 2 m/s, and the jet air temperature of the air curtain is 35.24°C, the negative temperature distribution appears only at the bottom of the tunnel 60 m away from the tunnel  Advances in Civil Engineering entrance. With the increase of the tunnel depth, the ground temperature of the surrounding rock increases. At this time, the temperature of the mixed gas in the tunnel is maintained above 0°C, which effectively prevents the occurrence of tunnel freezing injury.

Conclusions
(1) is paper proposes an air curtain insulation system for tunnels in cold areas and provides a new option for solving freezing damage of tunnels in cold areas by applying air curtain technology to tunnel insulation in cold areas. (2) is paper puts forward the calculation method of tunnel air curtain barrier efficiency, which lays a theoretical foundation for the popularization of tunnel air curtain insulation technology. (3) e air curtain barrier efficiency increases first and then decreases with the change of jet angle. e optimal jet angle is 77.6°. e barrier efficiency increases with the increase of jet velocity. On the premise of not affecting driving safety, jet velocity is recommended to be 20 m/s. When the natural wind velocity at the tunnel entrance is 1.5 m/s, the jet thickness can be 0.4 m, and the barrier efficiency of the air curtain is 0.994 at this time. When the natural wind velocity at the tunnel entrance is above 1.5 m/s, jet thickness of the air curtain can be appropriately increased.
(4) From the measured data and calculation results of Lafashan Tunnel, it can be seen that an air curtain is installed at the tunnel portal with jet speed of 22.7 m/ s and temperature of 35.24°C, which can meet the tunnel without freezing damage when the external temperature is −10°C and the external wind speed is 2 m/s, providing a new method for tunnel insulation technology in cold area.

Data Availability
e data used to support the findings of this study are available from the corresponding author upon request.

Conflicts of Interest
e authors declare that they have no conflicts of interest.