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In order to cope with the frost damage problems that occur in cold-region tunnels, the engineers have proposed the measure of setting up a heated air curtain at the tunnel entrance. In this paper, a three-dimensional transient heat transfer numerical model was first established to calculate the temperature distribution of the tunnel under the natural wind, which has a good agreement with field test data. The insulation effects of the heated air curtain were demonstrated, and effects of five factors, namely, temperature (Tn) and speed (Sn) of the natural wind, speed (S), temperature (T), and angle (A) of the jet airflow of the air curtain, were discussed. Then, an orthogonal test sensitivity analysis was conducted on the operating parameters (T, S, and A). Finally, effects of the air curtain were predicted by neural networks. The results show that the use of air curtains can effectively shorten the length of the negative temperature section of the tunnel structure. Low temperatures and large speed of the natural wind will weaken the effects of the air curtain. In the two cases, the operating parameters of the air curtain require adjustment according to demand. Increasing S and T can effectively enhance insulation effects. And the optimal A is between − 15° (toward inside the tunnel) and 0°. The sensitivities of the influences of the three operating parameters on the insulation effects are ranked as T (2.142), S (1.832), and A (0.77). The prediction results of the neural network indicate that using Tn, Sn, T, S, and A to predict the length of the negative temperature section (Lv) and temperature of the tunnel vault surface (Tv) is feasible and efficient. This research can provide a reference for the application of air curtains as anti-freeze measures in cold-region tunnels. [ABSTRACT FROM AUTHOR]

High-speed railway tunnels in the cold region, whose numbers are increasing, are widely exposed to the threat of frost damage caused by extremely negative temperatures. The operation of high-speed trains has made this problem even more pronounced. But there are fewer studies focusing on the dynamic changes in temperature during train operation. In this paper, the air thermodynamic behavior of trains passing through a tunnel and its consequences were investigated with the help of the CFD method. In numerical simulations, conditions of different natural airflows, different train speeds, different blockage ratios, as well as two trains intersecting in the tunnel were involved. The following valuable results were obtained. In cold environments, train-induced pressure wave effects will affect the air thermodynamic behavior, but the overall trend in air temperature is mainly influenced by the heat transfer between cold and warm air. The high-speed train will lead to massive cold air being carried into the tunnel and some warm air inside being pushed out. Heat transfer between cold and warm air will cause a significant decrease in tunnel air temperature. This air thermodynamic behavior is particularly prominent in the case of large blockage ratios. When there are same-direction natural airflows, they will be coupled with train-induced airflows, causing a greater decrease in tunnel air temperature. In contrast, opposite-direction natural airflows can reduce the negative effects of the train. Taking the case of opposite-direction natural airflow velocity of 5 m/s as an example, the temperature changes of the three cross-sections are only 16.7 %, 14.8 %, and 42.1 % of those in the case of no natural airflow. Whatever the direction of natural airflows, coupling effects between them and train-induced airflows will be more significant as the natural airflow speed increases. Greater train speed means more cold air entering the tunnel and stronger heat transfer between cold and warm air, causing a more significant temperature decrease. When two trains intersect through the tunnel, airflows caused by them will be coupled. As a result, only a little air enters the tunnel from the entrance and exit, and the heat transfer between air with different temperatures is weak. In this case, the air temperature doesn't decrease but slightly increases as the air is compressed. [ABSTRACT FROM AUTHOR]

Frost heave in cold-region tunnels is caused by complex water-ice phase changes with coupled heat and moisture transfer, which will lead to frost damage and thus threatens the structural stability and traffic safety. To describe the frost heave force, a new model is established for the coupled heat and moisture transfer in the freezing surrounding rock, which consider the major influence factors of the three primary frozen theories for cold-region tunnels simultaneously. An analytical solution for the coupled multifactor is proposed, in which the calculated parameters could be classified into three categories: 1) the frost heave ratios(α fr , α mr , α i); 2) the geometric parameters(R , T mr , T i); and 3) the confinement strength(K r , K fr , K mr , K l , K i). All parameters are studied, in which frost depth is deduced by comprehensively considering the ice content, thermal coefficient and freezing index, and a new method based on a difference value method for the equivalent elasticity coefficient of the lining or ice is also proposed. Furthermore, the method is validated in the Daban Mountain tunnel, and the results have indicated that the proposed analytical solution can accurately reflect frost heave force. • A model for frost heave force was established from the coupled heat and moisture transfer in cold-region tunnels. • Parameters to frost heave force were studied in detail to determine the distribution range or the calculation method. • Frost depth was deduced by comprehensively considering the ice content, thermal coefficient and freezing index. • A method based on a difference value method for the equivalent elasticity coefficient of the lining or ice was proposed. [ABSTRACT FROM AUTHOR]