Fine numerical simulation method for direct-current deicing of overhead ground wire

Abstract

The invention relates to a fine numerical simulation method for direct-current ice melting of an overhead ground wire, which considers gap convective heat transfer on the basis of an existing ice melting model, adds a numerical simulation process of ice layer phase change, and provides a more accurate model basis for selecting ice melting current and ice melting time in subsequent simulation research. The method comprises the following steps: S1, investigating and collecting related information, and selecting a direct current ice melting research object; s2, establishing an overhead ground wire geometric model, and endowing material attributes according to physical properties and a sensible heat capacity method; s3, considering gap convective heat transfer, and determining a model initial condition and a control equation of a boundary condition; research objects and initial conditions are changed, and fine numerical simulation is carried out on overhead ground wire direct current ice melting under different ground wire models, wind speeds, icing thicknesses and environment temperatures.

Description

Technical Field

[0001] The present invention relates to a refined numerical simulation method for DC de-icing of overhead ground wires. Background Art

[0002] Ground wire icing refers to the phenomenon that supercooled water in the air condenses on the overhead ground wire, covering the ground wire with ice on a large area. Ground wire icing can cause the gap between the ground wire and the conductor to become smaller, leading to short circuits. In severe icing conditions, the line may dance, fittings may fall off, or in the worst case, the wire may break, the tower may collapse, or ice flash tripping may occur, affecting the safe and stable operation of the power system. When de-icing the line by DC, if the de-icing current is too large, due to the existence of contact resistance, the connections of fittings will heat up severely and even burn out and fuse. If the de-icing current is too small, the de-icing time will be too long or de-icing may not be possible. Therefore, when de-icing the overhead ground wire by DC, it is particularly important to select appropriate de-icing current and de-icing time. The accurate simulation and analysis of the de-icing dynamic process have important reference significance for the selection of de-icing current and the formulation of de-icing strategies. By constructing a numerical simulation model of the de-icing dynamic process of the overhead ground wire, the physical process of de-icing is refinedly simulated to obtain its variation law, and then technical support is provided for the de-icing of the overhead ground wire, which has important theoretical research significance and engineering application value.

[0003] Defects and deficiencies of the prior art:

[0004] Over the years, scholars have proposed de-icing dynamic models, elliptical de-icing models, etc. for the simulation calculation of the thermal de-icing method. However, the existing thermal de-icing research mainly focuses on transmission conductors. The suspension position of the ground wire is higher, the wire diameter is smaller, and it does not carry working current. Compared with the conductor, it is more prone to icing and has lower design strength, making it more likely to suffer from ice damage accidents. Since the de-icing process involves both heat transfer and ice layer phase change, the current research rarely simulates the ice layer phase change process and does not consider the convective heat transfer of the air gap generated during de-icing, which will affect the accuracy of the de-icing dynamic simulation model, and then lead to the calculation results of de-icing current and de-icing time not matching the actual situation. Therefore, considering the convective heat transfer of the gap, a finite element analysis model for the DC de-icing dynamic process of the overhead ground wire is established, the phase change problem of the ice layer is solved by the apparent heat capacity method, and the influence relationship between the half-width dT of the phase change interval and the paste zone constant A m on the ice layer phase change simulation is given. Summary of the Invention

[0005] The purpose of the present invention is to provide a refined numerical simulation method for DC de-icing of overhead ground wires. This method considers the convective heat transfer of the gap on the basis of the existing de-icing model and adds the numerical simulation process of ice layer phase change, providing a more accurate model basis for the selection of de-icing current and de-icing time in subsequent simulation studies.

[0006] To solve the above problems, the technical solution of the present invention is as follows:

[0007] A refined numerical simulation method for DC de-icing of overhead ground wires, comprising the following steps:

[0008] S1: Conduct research and collect relevant information, and select the research object for DC de-icing;

[0009] S2: Establish a geometric model of the overhead ground wire, and endow material properties according to physical properties and the apparent heat capacity method;

[0010] S3: Consider convective heat transfer in the gap, determine the initial conditions of the model, and the control equations for the boundary conditions;

[0011] S4: Discretize the initial mesh, set up the mesh rezoning conditions, and solve the internal heat flux density of the model;

[0012] S5: Export the calculation results, and analyze the influence relationship between the half-width dT of the phase change interval and the paste zone constant A m on the ice layer phase change simulation;

[0013] S6: After extracting the calculation results, according to the above steps S1-S5, change the research object and initial conditions, and conduct refined numerical simulations on the DC de-icing of overhead ground wires with different ground wire models, wind speeds, ice coating thicknesses, and ambient temperatures.

[0014] The beneficial effects of the present invention are as follows:

[0015] (1) The present invention provides a refined numerical simulation method for DC de-icing of overhead ground wires. This method considers convective heat transfer in the gap on the basis of the existing de-icing model, and adds a numerical simulation process of ice layer phase change, making up for the deficiencies in convective heat transfer and phase change dynamic process simulation that occur after the formation of the air gap in the existing de-icing model.

[0016] (2) The method of the present invention utilizes the advantages of convenience and low cost of numerical simulation to construct a refined numerical model for DC de-icing of overhead ground wires, and analyzes the influence relationship between the half-width dT of the phase change interval and the paste zone constant A m on the phase change simulation, providing a model basis for more accurate selection of de-icing current and de-icing time in the future.

[0017] (3) The present invention conducts modeling through the finite element method, avoiding the cumbersome complexity of engineering tests, and can conduct refined numerical simulations on the DC de-icing of overhead ground wires with different ground wire models, wind speeds, ice coating thicknesses, and ambient temperatures according to the research object and initial conditions. Description of the Drawings

[0018] The following further describes the present invention with reference to the drawings:

[0019] Figure 1 is the overall flow chart of the method of the present invention;

[0020] Figure 2 Schematic diagram of the OPGW geometric model in the embodiment of the present invention;

[0021] Figure 3 Schematic diagram of the grid division of the model calculation domain in the embodiment of the present invention;

[0022] Figure 4 Flow chart of the DC ice melting simulation numerical calculation in the embodiment of the present invention;

[0023] Figure 5 Schematic diagram of the temperature distribution and phase change distribution during the ice melting process of the present invention;

[0024] Figure 6 Temperature curve and phase change curve diagram during the ice melting process under different half widths of the phase change interval of the present invention;

[0025] Figure 7 Temperature curve and phase change curve diagram during the ice melting process under different paste zone constants of the present invention. Detailed implementation manners

[0026] Next, the technical solutions in the embodiments of the present invention will be clearly and completely described in conjunction with the accompanying drawings in the embodiments of the present invention. Obviously, the described embodiments are only a part of the embodiments of the present invention, rather than all the embodiments. Based on the embodiments of the present invention, all other embodiments obtained by those of ordinary skill in the art without creative efforts shall fall within the protection scope of the present invention.

[0027] As Figure 1 shown, a refined numerical simulation method for DC ice melting of an overhead ground wire includes the following steps:

[0028] S1: Investigate and collect relevant information, and select the research object of DC ice melting;

[0029] S2: Establish the geometric model of the overhead ground wire, further simplify the geometry with reasonable assumptions, and assign material properties according to physical properties and the apparent heat capacity method;

[0030] S3: Consider convective heat transfer in the gap, determine the initial conditions of the model, and the control equations of the boundary conditions;

[0031] S4: Discretize the initial grid, set the grid rezoning conditions, and solve the internal heat flux density of the model;

[0032] S5: Export the calculation results, and analyze the influence relationship between the half width dT of the phase change interval and the paste zone constant A m on the ice layer phase change simulation;

[0033] S6: After extracting the calculation results, according to the steps S1 - S5 above, change the research object and initial conditions, and conduct refined numerical simulations on the DC de - icing of overhead ground wires under different ground wire models, wind speeds, ice - coating thicknesses, and environmental temperatures.

[0034] Further, in step S1, conduct research and collect relevant information, select the research object of DC de - icing of overhead ground wires. The research and collection of relevant information include icing meteorological conditions, ground wire parameters, and physical properties of other involved materials.

[0035] Further, in step S2, establish a geometric model of the overhead ground wire. The methods of assigning material properties according to physical properties and the apparent heat capacity method include:

[0036] S2.1: According to the research object determined in the steps described in S1, establish a geometric model based on the parameters obtained through research.

[0037] S2.2: Since the actual ice - coating morphology and de - icing working conditions of overhead ground wires are very complex, for the convenience of DC de - icing modeling and calculation, while taking into account its accuracy, the following assumptions and simplifications are made:

[0038] ① The ice - coating cross - section is circular: Due to the small diameter of the ground wire and its relatively small torsional stiffness, when ice accumulates on the windward side, it is more likely to twist under the action of the ice layer gravity, making the ice - coating cross - section shape close to circular.

[0039] ② Neglect the axial heat transfer of the ground wire: When the ground wire de - ices synchronously or the de - icing section of the line is relatively long, the axial heat transfer of the ground wire can be neglected, and a two - dimensional field is used to analyze and solve the temperature distribution of the ground wire.

[0040] ③ Neglect the de - icing shear force: The process of ground - wire de - icing is the process of continuously expanding the air gap. When the ice - layer thickness on the upper surface of the ground wire is zero, the ice layer falls off from the ground - wire surface.

[0041] ④ Neglect the influence of the strand gap of the overhead ground wire on heat transfer: The diameter of the overhead ground wire is relatively small, and the strand gap is also small, which has a small impact on the heat - transfer process of de - icing. To improve the quality of the meshing and the convergence of the solution, geometric models of the corresponding aluminum and steel parts are established respectively with the equivalent aluminum and steel cross - sectional areas as a whole.

[0042] S2.3: Assign corresponding material properties according to physical properties. The material properties of the ice layer are processed using the apparent heat capacity method. The specific method is as follows:

[0043] In the physical process of ice - layer melting, there are three types of regions in the phase - change distribution of the ice layer: solid phase, mushy zone, and liquid phase. The mushy zone is a semi - solid region that exists as the interface between the melting zone and the unmelted zone of the phase - change material. According to the actual temperature T of the solution region and the melting point T of the ice layer m, the relative magnitude of the phase change interval width ΔT. Using the apparent heat capacity method to solve the phase change process of the ice layer during the DC de-icing of the overhead ground wire, the three types of regions can be characterized by the liquid fraction of the piecewise function Characterized by:

[0044]

[0045] In the formula: T m is the melting point, K; ΔT is the phase change interval width, K.

[0046] The material properties in the solid phase, mushy zone, and liquid phase during the phase change process of the ice layer can be expressed as follows:

[0047] A: Equivalent heat capacity C e (T)

[0048]

[0049] In the formula: C fs and C fg are the specific heat capacities of the ice layer and air respectively, J / (kg·K); L f is the latent heat of phase change, and its contribution to the equivalent specific heat capacity is reflected by the Gaussian function D(T) centered on T m as follows:

[0050]

[0051] B: Equivalent heat transfer coefficient k e (T)

[0052]

[0053] In the formula: k fs and k fg are the heat transfer coefficients of the ice layer and air respectively, W / (m 2 ·K).

[0054] C: Equivalent density ρ e (T)

[0055]

[0056] In the formula: ρ fs and ρ fg are the densities of the ice layer and air respectively, kg / m 3 .

[0057] ρ f (T) is related to the solid fraction φ fs . The solid fraction of the phase change material is 1 in the solid phase, 0 in the liquid phase, and linearly related to the temperature distribution in the mushy zone, expressed as:

[0058]

[0059] In the formula: φ fs is the solid phase fraction, and φ fg is the liquid phase fraction.

[0060] Furthermore, in step S3, considering the convective heat transfer in the gap, the initial conditions of the model and the governing equations of the boundary conditions are determined. The specific method is as follows:

[0061] During the DC ice melting process, part of the Joule heat generated by the current flowing through the ground wire is used to heat the ground wire and the ice layer and provide heat for the latent heat of phase change; the other part is transferred to the outer surface of the ice layer in the form of heat conduction through the ground wire and the ice layer, and finally lost to the external environment through convective and radiative heat exchange. Therefore, the energy equation during the ice melting process can be expressed as:

[0062] I 2 Rt = ∫h(T i - T a )dl·Δt + ∫∫ρ θ C θ dSdT + ρ ice L f ΔS (7)

[0063] In the formula: I is the ice melting current, A; R is the DC resistance of the ground wire, Ω; h is the heat transfer coefficient between the outer surface of the ice layer and the external environment, W / (m 2 ·K); T i , T a are the temperatures of the outer surface of the ice layer and the external environment, respectively, K; ρ θ is the density of the area microelement in the solution region, kg / m 3 ; C θ is the specific heat capacity of the area microelement in the solution region, J / (kg·K); ρ ice is the density of the ice layer, kg / m 3 ; L f is the phase change enthalpy of ice, J / m 3 ; ΔS is the area of the ice layer cross-section melted within Δt, m 2 .

[0064] During the DC ice melting process, the current-carrying area of the ground wire is the aluminum layer and the steel core. In addition, there may also be a stainless steel pipe. Assuming the ice melting current is I, the current distribution in the ground wire is:

[0065]

[0066] In the formula: I al , I st , I’ st are the currents in the aluminum layer, steel core, and stainless steel pipe regions, respectively, A; σ al , σst are the electrical conductivities of aluminum and steel, S / m; S al , S st , S’ st are the cross-sectional areas of the aluminum layer, steel core, and stainless steel tube, m 2 .

[0067] In the inner region of the ice layer, heat conduction mainly occurs and satisfies the differential equation:

[0068]

[0069] where ρ is the material density, kg / m 3 ; c is the specific heat capacity of the material, J / (kg·K); λ is the heat transfer coefficient of the material, W / (m 2 ·K); Q is the internal heat source intensity, W / m 2 , and can be expressed as:

[0070] There is convective heat transfer and radiative heat transfer between the outer surface of the ice layer and the external environment, so the third kind of boundary condition should be satisfied:

[0071]

[0072] where h is the sum of the convective heat transfer coefficient and the radiative heat transfer coefficient. The convective heat transfer coefficient includes the natural convective heat transfer coefficient and the forced convective heat transfer coefficient, and is expressed as:

[0073]

[0074] where: h D , h R , h N , h F represent the convective heat transfer coefficient, radiative heat transfer coefficient, natural convective heat transfer coefficient, and forced convective heat transfer coefficient respectively, W / (m 2 ·K); λ air is the air thermal conductivity, W / (m 2 ·K); D is the geometric diameter after icing, m; Re, Gr, and Pr are the Reynolds number, Prandlt number, and Grashof number respectively:

[0075]

[0076] where: g is the gravitational constant, g = 9.8m / s 2 ; ν is the kinematic viscosity of air, m 2 / s; μ is the dynamic viscosity of air, kg / (m·s); C air is the specific heat capacity of air, J / (kg·K); V a is the external environmental wind speed, m / s; ρ airis the air density, kg / m 3 ; B, b, C, and n are coefficients determined by the Grashof number and the Reynolds number;

[0077] The radiative heat transfer coefficient h R is expressed as:

[0078]

[0079] where: ε is the emissivity of the outer surface of the ice layer, ε = 0.9; σ r is the radiation constant, σ r = 5.67×10 -8 W / (m 2 ·K 4 ).

[0080] The air gap generated by ice layer melting and other fluid solution regions are incompressible flows, which can be expressed by the Navier - Stokes equations:

[0081]

[0082] To describe the convective flow after the phase change of the ice layer and prevent the solid from flowing, the Darcy's law damping term S(T) is added to the momentum equation, which includes the Carman - Kozeny relationship and has a large value when the liquid - phase fraction approaches 0, forcing the solid - phase velocity field to be 0, and eliminating this term when the liquid - phase fraction is in the region of 1, which can be expressed as:

[0083]

[0084] where: A m is the mushy zone constant. For incompressible flows, A m ≥ 1×10 4 kg / (m 3 ·s); in the case of , to avoid division by zero, a constant ε is added, ε = 0.001.

[0085] In the ice layer melting model, there are thermal equilibrium and mass balance at the phase - change interface, from which the Stefan condition that the interface velocity V s should satisfy can be derived:

[0086]

[0087] where, Q s is the jump of the normal heat flux at the interface, W / m 2 ; ρ ice is the density of ice, kg / m 3 ; λ ice is the heat transfer coefficient of ice, W / (m 2 ·K).

[0088] Further, in step S4, the initial grid is dissected, grid reconditioning is set up, and the internal heat flux density of the model is solved. The method includes:

[0089] After the initial grid dissection, except for the boundaries of each domain, the grids in the computational domain are all free triangular grids, the grids around the domain boundaries are quadrilateral boundary layer grids, corner refinement is adopted in the angular regions, and the outer boundary of the overhead ground wire geometric model is a moving grid boundary, which is used to simulate the downward movement of the ice layer during the de-icing process. After solving the internal heat flux density of the model, the heat balance and mass balance existing at the phase change interface can be determined, and then the interface velocity, that is, the phase change rate at the top of the inner surface of the ice layer, can be deduced according to Equation (15), and this is used as the following velocity of the moving grid. Taking Equation (1) as the judgment condition for whether to perform moving grid tracking, when the liquid fraction is less than 1, the grid does not move, that is, no tracking is performed. When the liquid fraction is equal to 1, the grid starts to move and grid tracking is performed.

[0090] As the grid follows the phase change interface to move, the grid elements will be stretched or squeezed, which may lead to non-convergence or inaccurate results. Automatic grid update is also required. With the minimum relative element volume as the grid re-meshing condition, when it is less than the threshold value of 0.5, the grid is re-meshed. Otherwise, the grid will continue to move until it coincides with the outer boundary of the ice layer.

[0091] Further, in step S5, the calculation results are exported, and the influence relationship between the half-width dT of the phase change interval and the mushy zone constant A m on the phase change simulation is as follows:

[0092] After the solution is completed, through result post-processing, the temperature distribution, phase change distribution nephogram of the full dynamic process of DC de-icing of the overhead ground wire, and the temperature curves and phase change curves along the X-axis intercept line and Y-axis intercept line under different half-widths dT of the phase change interval and mushy zone constant A m can be exported.

[0093] Using the above steps, a refined numerical model for DC de-icing of the overhead ground wire is constructed, and the influence relationship between the half-width dT of the phase change interval and the mushy zone constant A m on the phase change simulation is analyzed, providing a basis for more accurate selection of de-icing current and de-icing time in the future.

[0094] After extracting the calculation results, according to the above steps S1 - S5, by changing the research object and initial conditions, refined numerical simulations of DC de-icing of overhead ground wires with different wire models, wind speeds, ice coating thicknesses, and environmental temperatures can be carried out.

[0095] In this embodiment, the optical fiber composite overhead ground wire OPGW-24B1-98[124.2; 51.8] is selected. Based on the assumptions in step S2, a two-dimensional DC de-icing model of it is established, and the corresponding parameters are shown in Tables 1 and 2. The density, specific heat capacity, and heat transfer coefficient of the ice layer are determined according to the apparent heat capacity method in subsection S2.3.

[0096] In addition, the initial de-icing conditions are that the ambient temperature T a = 268.15K, the wind speed V a = 5m / s, the de-icing current I = 500A, and the ice thickness d = 15mm. After calculating according to the above steps, after the calculation is completed, the temperature distribution, phase change distribution nephogram of the full dynamic process of OPGW DC de-icing, and the temperature curves and phase change curves of the cross-section lines along the X-axis and Y-axis under different phase change intervals with half-width dT and paste zone constant A m are obtained, as shown in Figure 5 、 Figure 6 、 Figure 7 .

[0097] Table 1 OPGW technical parameter table

[0098]

[0099] Table 2 OPGW electro-thermal parameter table

[0100]

[0101] Figure 5 (b) and Figure 5 (d), the phase change distribution is characterized by Equation (1), where the dark blue area is 0, indicating that no phase change has occurred, that is, the ice layer, and the dark red area is 1, indicating that the ice layer has undergone a phase change, that is, the air gap. The area between 0 and 1 represents the phase change transition interval of the ice layer, that is, the paste zone. As can be seen from Figure 5 , regardless of whether the gap convection heat transfer is considered, during the de-icing process, the temperature of the OPGW shows a trend of first increasing and then decreasing. In addition, near the lower part of the OPGW, after considering the gap convection, the maximum difference in the air gap width at the same phase change interface position of the ice layer is about 3mm.

[0102] Taking dT = 0.5K as an example, the cross-section distance ranges of the ice layer, paste zone, air gap, and OPGW are indicated, as shown in Figure 6 (a) and 6(b). The paste zone positions of the three groups of phase change intervals with half-width are shown in Figure 6 (c) and Figure 6 .

[0103] From Figure 6 (a) and 6(b), it can be seen that within the cross-section distance ranges of the paste zone and the air gap, the temperature difference does not exceed 0.2K, while Figure 2(b) Within the OPGW cross-section distance range, the maximum temperature difference can reach more than 3K. From Figure 6 (c), Figure 6 (d), it can be seen that for the three sets of half-width dT values of the phase change interval, the starting points and widths of the mushy zone are different, and the phase change distribution within the mushy zone is also different.

[0104] The above analysis shows that different half-width dT values of the phase change interval mainly affect the phase change simulation in terms of the position of the phase change interface and the temperature rise of the heat source OPGW in the model. From Figure 6 (c), 6(d), it can be seen that when dT is larger, the phase change transition region, i.e., the mushy zone, is wider, and the overall temperature in the computational domain is also higher. When dT is too small, the sudden change in material properties due to the phase change will be distorted, as shown in Figure 6 (b) where dT = 0.25K.

[0105] Taking A m = 10 5 as an example, the cross-section distance ranges of the ice layer, mushy zone, air gap, and OPGW are indicated, as shown in Figure 7 (a), 7(b). The positions of the mushy zones for the three sets of mushy zone constants are shown in Figure 7 (c) and Figure 7 (d).

[0106] From Figure 7 (a), 3(b), it can be seen that compared with Figure 7 (a), 7(b), within the cross-section distance ranges of the mushy zone and the air gap, the maximum temperature difference increases to about 1.5K, while within the OPGW cross-section distance range, the temperature difference decreases to less than 1K. From Figure 7 (c), Figure 7 (d), it can be seen that compared with Figure 6 (c), 6(d), for the three sets of mushy zone constant A m values, although the phase change distributions within the mushy zones are also different, the starting points of the mushy zones coincide.

[0107] The above analysis shows that different mushy zone constants A m mainly affect the phase change simulation in terms of the temperature distribution in the mushy zone and air gap regions. When A m = 10 4 , 10 5 , the temperature difference of the OPGW is within 0.25K; when A m is too large, due to the existence of the damping term, it is difficult for convective heat transfer to occur in the air gap. At this time, it is closer to the ice melting simulation process without considering gap convection, as shown in Figure 7 (d) where the phase change curve of the Y-axis intercept shows a step trend when A m = 10 6 .

[0108] After the analysis, following the above steps S1 - S5, by changing the research object and initial conditions, refined numerical simulations of DC de - icing for overhead ground wires with different ground wire models, wind speeds, ice - coating thicknesses, and ambient temperatures can be carried out.

[0109] The content described in the embodiments of this specification is only an enumeration of the implementation forms of the inventive concept. The protection scope of the present invention should not be regarded as limited to the specific forms stated in the embodiments. The protection scope of the present invention also extends to equivalent technical means that can be conceived by those skilled in the art based on the inventive concept.

Claims

1. A refined numerical simulation method for DC ice melting of overhead ground wires, characterized by: The following steps are involved: S1: Investigate and collect relevant information, and select DC ice melting research objects; S2: Establish the geometric model of overhead ground wire and assign material attributes according to physical properties and sensible heat capacity method; S3: Consider the gap convection heat transfer, determine the model initial conditions, and the governing equations of the boundary conditions; S4: Divide the initial grid, set the grid redrawing conditions, and solve the internal heat flux density of the model; S5: Derive the calculation results and analyze the phase transition interval half-width dT and the mushy zone constant A m Impact on ice layer phase change simulation; S6: After extracting the calculation results, according to the above steps S1-S5, change the research object and initial conditions, and conduct refined numerical simulation of DC ice melting of overhead ground wires under different ground wire models, wind speeds, ice thicknesses, and ambient temperatures.

2. According to claim 1, a refined numerical simulation method for DC ice melting of overhead ground wires comprises the following steps: In step S1, the relevant information includes icing meteorological conditions, ground line parameters and material physical properties.

3. According to claim 1, a refined numerical simulation method for DC ice melting of overhead ground wires comprises the following steps: exist In step S2, the corresponding material attributes are assigned according to the physical properties, and the material attributes of the ice layer are processed by the apparent heat capacity method, and the processing method includes: in the physical process of melting the ice layer, the phase change distribution of the ice layer has three types of regions: solid phase, mushy region, and liquid phase; The mushy area is calculated based on the actual temperature T of the solution area and the melting point of the ice layer T m , the relative size of the phase change interval width ΔT, and the apparent heat capacity method is used to solve the phase change process of the ice layer during the DC melting of the overhead ground wire. The three types of regions are represented by the piecewise function liquid fraction Characterization: Where: T m is the melting point, K; ΔT is the phase transition interval width, K; The material properties of the solid phase, mushy region, and liquid phase during the phase transition of the ice layer are expressed as follows: A. Equivalent heat capacity C e (T) Where: C fs and C fg are the specific heat capacities of ice and air, J / (kg·K); L f is the latent heat of phase change, and its contribution to the equivalent specific heat capacity is T m It is represented by the Gaussian function D(T) as the center: B. Equivalent heat transfer coefficient k e (T) Where: k fs and k fg are the heat transfer coefficients of ice layer and air, W / (m 2 K); C. Equivalent density ρ e (T) Where: fs and ρ fg are the densities of ice and air, kg / m 3 ; ρ f (T) and solid fraction φ fs The solid phase ratio of the phase change material is 1 in the solid phase and 0 in the liquid phase. The solid phase ratio in the mushy region is linearly related to the temperature distribution in the mushy region, which can be expressed as: Where: φ fs is the solid phase ratio, φ fg is the liquid phase rate.

4. The method for fine-tuning numerical simulation of DC ice melting of overhead ground wires according to claim 1 comprises the following steps: exist In step S3, the method of considering gap convection heat transfer includes: The air gaps and other fluid solution areas generated by the melting of the ice layer are incompressible flows, which are expressed by the Navier-Stokes equations: Where: ρ is the fluid density, kg / m 3 ; u is the fluid velocity vector; p is the fluid pressure; I is the unit matrix; μ is the fluid dynamic viscosity, S(T) is the damping term; Adding the Darcy's law damping term S(T) to the momentum equation includes the Carman-Kozeny relationship. When the liquid fraction approaches 0, it has a larger value, forcing the solid phase velocity field to be 0. When the liquid fraction is 1, the term is eliminated, which is expressed as: Where: A m is the mushy zone constant. For incompressible flow, A m ≥1×10 4 kg / (m 3 ·s); In order to avoid division by zero, a constant ε is added, ε=0.

001.

5. According to claim 1, a refined numerical simulation method for DC ice melting of overhead ground wires comprises the following steps: In step S4, the method for solving the heat flux density inside the model includes: In the ice layer melting model, there is a thermal balance and mass balance at the phase change interface, and the interface velocity V is derived s Stefan conditions that should be met: In the formula, Q s is the sudden change of normal heat flux at the interface, W / m 2 ρ ice is the density of ice, kg / m 3 ; ice is the heat transfer coefficient of ice, W / (m 2 ·K). After the initial meshing, except for the boundaries of each domain, the meshes of the computational domain are all free triangular meshes. The meshes around the domain boundaries are quadrilateral boundary layer meshes, and angle refinement is used in the angle area. The outer boundary of the overhead ground wire geometric model is the dynamic mesh boundary, which is used to simulate the downward movement of the ice layer during the ice melting process. After solving the heat flux density inside the model, the heat balance and mass balance at the phase change interface are determined. The interface velocity, that is, the phase change rate at the top of the inner surface of the ice layer, is calculated according to formula (9) and used as the following velocity of the dynamic mesh. Formula (1) is used as the judgment condition for whether to perform dynamic mesh tracking. When the liquid fraction is less than 1, the mesh does not move, that is, it does not track; when the liquid fraction is equal to 1, the mesh starts to move and mesh tracking is performed. As the grid moves along the phase change interface, the grid cells will be stretched or squeezed, and the grid will be updated with the minimum relative cell volume as the grid redrawing condition. When it is less than the threshold value of 0.5, the grid will be redrawn; otherwise, the grid will continue to move until it coincides with the outer boundary of the ice layer.

6. The method for fine numerical simulation of DC ice melting of overhead ground wire according to claim 1 comprises the following steps: In step S5, the phase transition interval half width dT and the mushy region constant A are analyzed. m Methods that influence the relationship for simulating ice layer phase change include: After the solution is completed, the temperature distribution, phase change distribution cloud diagram, and half-width dT and mushy zone constant A of the full dynamic process of DC ice melting of overhead ground wire can be derived through result post-processing. m Below, the temperature curve and phase change curve along the X-axis section and Y-axis section.

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