Keywords

1 Introduction

According to relevant surveys, road traffic accidents caused by snow and ice on roads account for 30% of overall traffic accidents, and the problem of road ice is gradually becoming a major traffic problem in China. Common passive snow removal methods include the removal method and melting method, which cost high, have low snow removal efficiency, use bad effects [1] and pollute the environment [2]. Compared with the traditional snow-melting methods, the active snow-melting pavement itself has the function of melting snow and ice, which is more efficient. To solve the drawbacks of passive snow removal, in recent years, methods including solar energy [3], circulating thermal fluid [4], geothermal energy and heat pumps [5], and electric heating systems [6] have also gradually attracted more and more attention.

This research adopts an electric heating tube to establish road snow melting experimental system, analyze the snow melting effect of the road snow melting system and the heat transfer on the road surface, explore the main factors affecting the road snow melting effect, and carry out indoor simulation experiments of asphalt slabs for control based on numerical simulation, to provide effective guidance for the practical application of electric heating tube snow melting road system.

The electric heating tube usually consists of five parts, which are sealing material, electric heating wire, magnesium oxide powder, metal tube sleeve, and lead rod. The metal tube sleeve mainly plays the role of protection, has the role of heat resistance, and heat conduction. The lead rod is used to conduct electricity and plays the role of connection. The role of magnesium powder is insulation and heat insulation. At the same time, the electric heating wire should be wound into a circle when used to increase the resistance, and extend the service life of the electric heating tube.

Compared with other snow melting methods, electric heating tube gain an efficiency heating and a better mechanical performance with long service life and simple installing combination. Once the current is turned on, heat can be diffused to the surface of the heating tube through the magnesium oxide crystals, making the internal temperature of the heating tube rise and exchange to the outside world, to play the role of heating.

2 Materials and Methodology

2.1 Materials

The electric heating tube snow melting system in the electric heating tube generally adopts a linear layout, snow melting system structure as shown in Fig. 1, the road structure from bottom to top is soil, lime soil, cement stabilized gravel base, and asphalt concrete, and the electric heating tube parallel to the direction of travel buried in the road concrete. This system adopts the intermittent working way: when the power is turned on, the electric heating tube generates heat energy, the temperature is raised, and the heat is transferred to the snow and ice layer through the asphalt layer so that it absorbs heat and warms up to achieve the effect of melting snow and turning ice, and the main parameters of the materials in the model are shown in Table 1.

Fig. 1
A structural diagram of a melting system has an electric heating tube, temperature control center, subgrade, cement, cement-stabilized gravel base, and an asphalt surface layer.

Electric heated snow melting system

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Table 1 Structural design parameters of each layer of road
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The electric heating tube is buried as shown in Fig. 2, D indicates the buried spacing of electric heating tubes, the electric heating tube is parallel to the direction of traffic, buried around the wheel track of the traffic belt, when the power is on, the electric heating tube will transfer heat to the asphalt layer, melting the ice and snow of the traffic belt, which can minimize the cost of the electric heating tube snow melting system.

Fig. 2
A structural diagram of a heating pipe has a rectangular structure with a length marked as pavement width. It has 3 columns inside marked as electric heating tubes and an upward arrow for the direction of travel.

Electric heating pipe layout scheme

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The electric heating snow melting system includes a heating system experiment and control system, as shown in Fig. 1. The system adopts a controlled relay control circuit system, set temperature sensors on the electric heating tube and in the asphalt layer, and set the temperature break control threshold: lower than the rated temperature (low temperature) energized, the electric heating tube continuous power supply, higher than the rated temperature, the circuit automatically shut down, to ensure that the temperature around the electric heating tube is not too high to destroy the material allowable temperature.

2.2 Numerical Simulation

Assumptions of the Model. The electric heaters are laid in the asphalt concrete, and the heaters generate heat when they are energized, and the snow absorbs the heat and melts. To simplify the problem, the following assumptions are made: no consideration is given to the bending of the tube deformation due to vehicle loading at the burial depth. The temperature field of the structure is time-dependent, so the whole snow-melting heating process is a non-stationary process and ignore the heat transfer in the direction of the length of the heating tube. The ice and structural layers are assumed to be solid, and the study area is a homogeneous and isotropic material.

Initial and Boundary Conditions. The initial conditions of the model are

$$ \lambda \left( {\frac{{\partial^{2} t}}{{\partial x^{2} }} + \frac{{\partial^{2} t}}{{\partial y^{2} }}} \right) = \rho c\frac{\partial t}{{\partial \tau }} $$
(1)

The initial condition is \(\tau = 0,t(x,y) = t_{0}\) (t0 is a known constant value). The ice melt system is an intermittent operation system.

2.3 In-Situ Testing Experiment

Considering the large temperature variation and dispersion of outdoor sites, this study proposes a large model test of large slab low-temperature indoor ice melting. By analyzing the warming situation and snow melting effect of a concrete large slab containing an electric heating tube under different input power and ambient temperature, so as get the influence of each factor on the warming effect of the concrete slab surface. The power consumption of the ice melting process and the constant temperature of the slab surface are obtained by the power-on heating and power-off cooling time within the limited temperature interval of the electric heater. The electric heaters (12 mm in diameter and 800 mm in length) were laid at a spacing of 20 cm. The experiments were conducted using a low-temperature controlled icehouse to simulate the external conditions, and the temperature was controlled from −10 to −5 °C. To obtain the real-time data of pavement temperature change it is proposed to lay temperature measurement points on the surface of the asphalt concrete surface, including 15 measurement points for two heating tubes and 14 measurement points for three heating tubes in Figs. 3 and 4. The distance between the measurement points along the tube direction is 20 cm, and 10 cm perpendicular to the tube direction (the measurement points are arranged alternately above and between the tubes), and the measurement points are generally distributed in the middle of the concrete panel (concrete panel length 110 cm, width 80 cm).

Fig. 3
A layout diagram of a surface has two horizontal pipes. It has 3 columns, with points 1 to 5 in column 1, 6 to 10 in column 2, and 11 to 15 in column 3. Pipe 1 has points 4, 9, and 14, and Pipe 2 has 2, 7, and 12.

Schematic diagram of two heating pipe concrete surface temperature measuring points layout

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Fig. 4
A layout diagram of a surface has 3 horizontal pipes. It has 2 columns, with points 1 to 7 in column 1 and 8 to 14 in column 2. Pipe 1 has points 6 and 13, pipe 2 has points 4 and 11, and pipe 3 has points 2 and 9.

Schematic diagram of three heating pipe concrete surface temperature measuring points layout

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3 Results and Discussion

3.1 Analysis of Numerical Simulation

It is concluded that as the heating time increases, the maximum and minimum surface temperatures both increase approximately according to an approximately linear law, due to the existence of heat transfer, thermal convection and thermal radiation, the temperature does not change exactly according to linear, where the average temperature difference tends to decrease with the increase in electrical heating time. The surface center temperature initially rises faster, and with the thermal conduction process, the internal heat storage in the structural layer, the temperature rise rate slows down, and the final curve tends to level off, and the temperature is nearly kept constant.

In this calculation model, when the electric heating tube is buried at a depth of 4 cm, the surface temperature can meet the requirements of snow melting temperature and temperature equilibrium when the initial ambient temperature is set at −10 ℃ and the electric heating tube is at 500 W line power with a burial depth spacing of 20 cm. And the spacing is 20 cm for further calculation. Set the spacing to 20 cm, buried depth of 8 cm, simulation, and simulation results in the buried depth of 4 cm structure for comparison, the results are shown in Fig. 5.

Fig. 5
A graph of temperature versus time has 6 increasing trends for burial depth of 4 and 8 centimeters. It has 3 sets of lines, each for the maximum temperature, minimum temperature, and temperature difference.

Temperature curves of different buried depths

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According to the above theory, the operational effect of the heating electric heating tube road ice system can be improved by reducing the laying depth of the heating electric heating tube in practical application. It is recommended to use an 8 cm burial depth and tube spacing of 20 cm.

3.2 Analysis of In-Situ Testing Experiment

According to the temperature change curves of the measured points of asphalt concrete specimens from the deployment of two electric heating tubes and three electric heating tubes in Figs. 6 and 7, it can be concluded that the heating efficiency of three tubes is higher compared with two tubes. In a short period of time, the temperature of the test points at the edge of the slab does not produce a large change, and the closer the heating tubes are, the temperature of the test points rises more quickly. The heating efficiency of three electric heating tubes is significantly higher than that of two tubes. Therefore, in the actual engineering construction project, the number of buried electric heating tubes should be given priority in the case of project funds.

Fig. 6
A graph of temperature versus time has 6 increasing lines. Node 1 and 4 from (5, negative 5) to (175,15) and (175, negative 5), 5 and 9 from (0, negative 4) to (175,0) and (175,9), 10 and 14 from (0, negative 5) to (175, negative 1) and (175, negative 5). Values are approximate.

Temperature change diagram of buried two electric heating tubes

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Fig. 7
A graph of temperature versus time has 6 increasing lines. Nodes 1, 2, and 3 from (5, negative 3) to (140, negative 1), (140,20), and (140,3). Node 8 from (5, negative 5) to (140, negative 4). Node 9 from (5, negative 2) to (140,10). Node 15 from (5, negative 4) to (140,4). Values are approximate.

Temperature change diagram of buried three electric heating tubes

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4 Conclusion

This study established a solution as electric heating tube road snow melting test platform and a finished in-situ test under complex situation caused by melting during snowing, such as low efficiency, narrow scope of application, and low energy utilization of traditional road snow melting methods. In this study, the numerical simulation calculation was carried out using ABAQUS finite element software. The following conclusions can be drawn:

  1. (1)

    The heat transfer effect of the electric heating tube can effectively raise the temperature of the road and speed up the rate of snow melting on the road.

  2. (2)

    The burial parameters of the electric heating tube have an important influence on the snow melting effect of the electric heating tube snow melting system. Reducing the burial depth and taking proper spacing of electric heating tube burial can improve the temperature of the road and accelerate the snow melting rate. In addition, the number of electric heating tubes also has a certain influence.

  3. (3)

    In this study, the control variable method was used to optimize the burial depth and deployment spacing. Since the influence of external temperature and snow characteristics on the snow, melting performance was not considered further research and experiments are still needed.

  4. (4)

    In this paper, when analyzing the influence of electric heating tube burial parameters, the control variable method is used to optimize the burial depth and deployment spacing, but from the overall consideration, the model is not well considered for snow melting performance, and the influence of external temperature and snow characteristics on snow melting performance is not considered. In addition, the phase change simulation does not consider the corresponding snow melting, which will have a certain influence on the distribution of the temperature field, which is worth further research and experiment.