A method for predicting the ablation position of a DC non-transferred arc in a plasma torch

Abstract

The present invention discloses a method for predicting the ablation position of a DC non-transferred arc in a plasma torch, comprising the steps of: drawing a three-dimensional unstructured grid according to the geometric structure of the plasma torch, and locally encrypting the grid anode and cathode walls; describing a magnetohydrodynamic model coupled with a standard k-epsilon turbulence model according to working condition parameters and based on Ansys-Fluent software through a UDF user-defined function; setting the boundary conditions of the inlet, outlet, anode, cathode, workpiece surface and other walls according to actual working conditions; configuring the calculation parameters for steady-state calculation and solution by giving the initial value of the flow field, setting the temperature of the flow field between the anode and cathode regions to be not less than 8000K; combining the physical model and comparing the attenuation degree of each residual value of the steady-state calculation result with the difference in the mass flow rate of the inlet and outlet to determine the convergence condition; data processing and analysis. Compared with the traditional method of setting a large-scale cooling circulation channel to ensure the heat exchange performance of the torch, the present invention effectively reduces energy consumption, prolongs the working life, and improves the processing performance and energy utilization efficiency.

Description

Technical Field

[0001] The invention relates to the technical field of plasma torch processing, and in particular to a method for predicting the ablation position of a direct current non-transferred arc in a plasma torch. Background Art

[0002] As modern optics gradually increases the requirements for the surface quality of optical materials, plasma processing has gradually replaced traditional machining methods that are time-consuming and difficult to process hard materials. As a method that can produce atmospheric pressure plasma jets, DC plasma torches have been widely used. However, this type of plasma generator is very likely to cause ablation inside the plasma torch, which seriously reduces the working life of the DC plasma torch.

[0003] At present, the DC non-transferred arc plasma torch mainly improves the overall heat exchange performance in the torch by setting circulating cooling water near the anode. This setting cannot predict the part that is prone to ablation. Since the location of ablation cannot be determined, the plasma torch cavity needs to be arranged with a large range of cooling circulation channels due to its large axial length. However, the arc excitation range that directly causes ablation in the plasma torch is limited, which is significantly smaller than the axial length of the cavity. The use of such a globally arranged cooling system causes excess energy loss. In addition, in order to reduce the ablation phenomenon, the higher cooling power causes the temperature of some cavities outside the arc excitation range to be too low, which seriously affects the properties of the plasma jet and the processing effect. At the same time, the globally arranged heat dissipation system also increases energy consumption and affects the processing performance. Therefore, in order to extend the working life of the DC plasma torch and improve the energy utilization efficiency, it is necessary to provide a method that can predict the ablation position of the DC non-transferred arc in the plasma torch. Summary of the invention

[0004] It is an object of the present invention to provide a method of predicting the location of ablation of a DC non-transferred arc in a plasma torch and to provide at least the advantages hereinafter described.

[0005] The technical solution of the present invention is as follows:

[0006] A method for predicting the ablation position of a DC non-transferred arc in a plasma torch comprises the following steps:

[0007] a) drawing a three-dimensional unstructured grid according to the geometric structure of the plasma torch, and locally encrypting the anode and cathode wall surfaces of the three-dimensional unstructured grid;

[0008] b) Establishing a coupled MHD magnetohydrodynamic physical model according to the operating parameters;

[0009] c) setting boundary conditions according to actual working conditions, wherein the boundary conditions include the boundary conditions of the inlet, outlet, anode, cathode, workpiece surface and other wall surfaces;

[0010] d) Configure the calculation parameters, use the hybrid initialization method provided by Ansys-Fluent software to give the initial value of the flow field, and use the SIMPLEC pressure-velocity coupling algorithm of the second-order upwind format to perform steady-state calculation and solution on the physical model;

[0011] e) determining the convergence condition based on the physical model in step b) and the characteristics of the steady-state calculation in step d), wherein the determination includes comparing the attenuation degree of each residual value and the difference between the inlet and outlet mass flow rates;

[0012] f) performing data processing and analysis based on the converged calculation results in step e), including predicting the radial current distribution and temperature distribution of the anode wall;

[0013] Preferably, in step a), a first layer of grids with a height no greater than 2e-2 mm and a grid expansion rate of 1.05 is drawn in combination with the application scenario to ensure sufficient calculation accuracy and high calculation speed.

[0014] Preferably, the three-dimensional unstructured grid in step a) includes a plasma torch cavity and a workpiece processing range area.

[0015] Preferably, the workpiece processing range area includes a columnar area, the diameter of which is more than 60 times the diameter of the plasma torch cavity.

[0016] Preferably, the step b) is specifically to compile a self-developed UDF user-defined function based on the standard k-epsilon turbulence model of Ansys-Fluent software to describe the magnetohydrodynamic steady-state model, and modify the source term of the cathode boundary in the UDF user-defined function according to the geometric parameters and working current of the test model.

[0017]

[0018] Among them, I 工作 is the working current of the test model, r 阴极截面 is the radius of the maximum radial section of the cathode head, r is the radius of the radial section of the cathode head, and j is the source term of the cathode boundary.

[0019] Preferably, the operating parameters in step b) include working gas inlet parameters, physical properties of anode and cathode materials, and cathode operating current.

[0020] Preferably, the boundary condition of the inlet in step c) is a velocity inlet including the incoming flow direction and the incoming flow velocity, the boundary condition of the outlet is a pressure outlet, the boundary condition of the anode is a no-slip heat exchange wall, the boundary condition of the cathode is a no-slip isothermal wall, the boundary condition of the workpiece surface is a no-slip isothermal wall, and the boundary condition of the remaining walls is an isothermal wall.

[0021] Preferably, the thermal conductivity of the no-slip heat exchange wall is set to 1e5 W / m 2 , the free temperature is 500K, the surface potential is 0V, the no-slip isothermal wall temperature of the cathode is set to 3500K, the defined current value is taken from the source term of the cathode boundary in the UDF user-defined function, the no-slip isothermal wall temperature of the workpiece surface is set to 300K, the surface potential is 0V, and the isothermal wall temperature is set to 300K.

[0022] Preferably, in step d), the temperature of the flow field between the cathode and anode regions is set to be no less than 8000K.

[0023] Preferably, the convergence condition in step e) is that the residual value decreases by 4 orders of magnitude and the inlet and outlet mass flow difference is less than 1e-6 kg / s.

[0024] Beneficial Effects

[0025] The present invention provides a method for predicting the ablation position of a DC non-transferred arc in a plasma torch. The position inside a DC non-transferred arc plasma torch where ablation is likely to occur is predicted by combining the arc temperature distribution with the anode wall temperature distribution and the radial current distribution in the cavity. The model has flexible input conditions to expand the working conditions that can be predicted and the size and structure range of the model. It effectively solves the technical problems of high energy consumption, low processing performance and short working life caused by arranging a large range of cooling circulation channels to ensure heat exchange performance in the prior art, and effectively improves energy utilization efficiency. BRIEF DESCRIPTION OF THE DRAWINGS

[0026] Figure 1 A flow chart of an embodiment of a method for predicting ablation position of a DC non-transferred arc in a plasma torch provided by the present invention;

[0027] Figure 2 is the radial current density distribution diagram of the anode wall;

[0028] Figure 3 is the temperature distribution diagram of the anode wall;

[0029] Figure 4 is the temperature distribution diagram in the xz plane;

[0030] Figure 5 It is the temperature distribution diagram in xy plane. DETAILED DESCRIPTION

[0031] The present invention is further described in detail below in conjunction with the accompanying drawings so that those skilled in the art can implement the invention with reference to the description.

[0032] It should be understood that terms such as “having”, “including” and “comprising” used herein do not specify the existence or addition of one or more other elements or combinations thereof.

[0033] like Figure 1 As shown, the present invention provides a method for predicting the ablation position of a DC non-transferred arc in a plasma torch. The magnetohydrodynamic model coupled with the standard k-epsilon turbulence model is described by a user-defined function based on Ansys-Fluent software, and the momentum source term in the magnetohydrodynamic model in three dimensions and the energy source term considering radiation dissipation, Joule heat, and electron transport enthalpy are described. The assumption of local thermodynamic equilibrium but fixed conductivity within 0.1 mm near the anode is adopted to make the numerical simulation results closer to the experimental data. Without changing the geometric structure size of the model, the development and change of the arc in the plasma torch can be simulated by giving four basic parameters such as the incoming gas composition, mass flow rate (or incoming flow velocity), cathode working current size, and basic heat dissipation capacity of the anode, and finally the data of the overall temperature, velocity, and radial current distribution of the arc during the working time are obtained. The data are combined with the temperature distribution data obtained on the anode wall to determine the position in the torch that is prone to ablation. Specifically, the following steps are included:

[0034] S1: Three-dimensional grid drawing. Draw a three-dimensional unstructured grid according to the geometric structure of the plasma torch. Locally encrypt the anode and cathode walls of the three-dimensional unstructured grid. To ensure sufficient calculation accuracy and high calculation speed, the first layer of grid height should not exceed 2e-2mm and the grid expansion rate should be 1.05 to ensure the above requirements in combination with the application scenario.

[0035] S2: Establish a physical model of coupled MHD magnetohydrodynamics. Use the standard k-epsilon turbulence model provided by Ansys-Fluent software and compile the self-developed UDF user-defined function to establish the physical model. It is necessary to modify the source term of the cathode boundary in the UDF user-defined function according to the geometric parameters and working current of the test model. The equation for calculating the cathode boundary source term of the UDF user-defined function is:

[0036] Among them, I 工作 is the working current of the test model, r 阴极截面 is the radius of the maximum radial cross section of the cathode head, r is the radius of the radial cross section of the cathode head, and j is the source term of the cathode boundary.

[0037] S3: Set boundary conditions. Set the boundary conditions of the inlet, outlet, anode, cathode, workpiece surface and other walls according to the actual working conditions. The inlet is set as the velocity inlet, including the incoming flow direction and the incoming flow velocity. The outlet is set as the pressure outlet to meet the normal pressure processing requirements. The anode is set as the non-slip heat exchange wall. The default thermal conductivity is 1e5 W / m 2 The free temperature is 500K and the surface potential is 0V. The cathode is set to a no-slip isothermal wall with a default temperature of 3500K. The current value is determined by the cathode boundary source term of the UDF user-defined function. The workpiece surface is set to a no-slip isothermal wall with a default temperature of 300K and a surface potential of 0V. The remaining walls are set to an isothermal wall with a default temperature of 300K.

[0038] S4: Configure the calculation parameters and use Flunet's built-in hybrid initialization method to give the initial value of the flow field. At the same time, in order to ensure that the arc can be stably excited, the temperature of the flow field between the cathode and anode is set to no less than 8000K. The SIMPLEC pressure-velocity coupling algorithm of the second-order upwind scheme is used to perform steady-state calculations and solve the physical model.

[0039] S5: Determine the convergence condition. Combine the characteristics of this physical model and compare the steady-state calculation results, that is, by comparing the attenuation degree of each residual value and the difference in inlet and outlet mass flow, it is generally considered to be converged when the residual value drops by 4 orders of magnitude and the difference in inlet and outlet mass flow is less than 1e-6kg / s. At this time, the physical phenomenon of the test object reaches stability.

[0040] S6: Data processing and analysis, including prediction based on the radial current distribution and temperature distribution on the anode wall.

[0041] In one embodiment of the method for predicting the ablation position of a DC non-transferred arc in a plasma torch provided by the present invention, as Figure 2 As shown,

[0042] Considering that the arc formed between the cathode and cathode in the cavity has an extremely high current density, it will generate extremely high Joule heat, which is the main reason for the anode wall ablation. In addition, since the "arc foot" is the main location where the arc is connected on the anode surface, the location where the "arc foot" is more likely to form on the anode surface can be predicted by comparing the distribution of radial current density along the anode wall. Therefore, the processing of the calculation results after convergence will focus on analyzing the radial current distribution on the anode wall and combining it with its temperature distribution for prediction. Figure 2 The radial current density is higher in the range of 25 to 30 mm on the horizontal axis, and the current flowing into the anode wall in this range is the highest, which is the main location where the arc "foot" is generated.

[0043] In one embodiment of the method for predicting the ablation position of a DC non-transferred arc in a plasma torch provided by the present invention, as Figure 3As shown, the higher temperature range is Figure 2 The range with higher radial current density overlaps. Considering that the material of the anode is mainly copper, whose melting point is generally less than 1350K, the position where the anode is prone to ablation under this working condition can be delineated. Figure 4 is the temperature distribution diagram in the xz plane, Figure 5 It is the temperature distribution diagram in xy plane.

[0044] Although the embodiments of the present invention have been disclosed as above, they are not limited to the applications listed in the specification and the implementation modes. They can be fully applied to various fields suitable for the present invention. For those familiar with the art, additional modifications can be easily implemented. Therefore, without departing from the general concept defined by the claims and the scope of equivalents, the present invention is not limited to specific details.

Claims

1. A method for predicting the ablation position of a DC non-transferred arc in a plasma torch, characterized in that: The following steps are involved: a) drawing a three-dimensional unstructured grid according to the geometric structure of the plasma torch, and locally encrypting the anode and cathode wall surfaces of the three-dimensional unstructured grid; b) Establishing a coupled MHD magnetohydrodynamic physical model according to the operating parameters; c) setting boundary conditions according to actual working conditions, wherein the boundary conditions include the boundary conditions of the inlet, outlet, anode, cathode, workpiece surface and other wall surfaces; d) Configure the calculation parameters, use the hybrid initialization method provided by Ansys-Fluent software to give the initial value of the flow field, and use the SIMPLEC pressure-velocity coupling algorithm of the second-order upwind format to perform steady-state calculation and solve the physical model; e) determining the convergence condition based on the physical model in step b) and the characteristics of the steady-state calculation in step d), wherein the determination includes comparing the attenuation degree of each residual value and the difference between the inlet and outlet mass flow rates; f) performing data processing and analysis based on the converged calculation results in step e), including making predictions based on the radial current distribution and temperature distribution of the anode wall.

2. The method for predicting the ablation position of a DC non-transferred arc in a plasma torch according to claim 1, characterized in that: In the step a), a first layer of grids with a height no greater than 2e-2 mm and a grid expansion rate of 1.05 is drawn in combination with the application scenario.

3. The method for predicting the ablation position of a DC non-transferred arc in a plasma torch according to claim 1, characterized in that: The three-dimensional unstructured grid in step a) includes the plasma torch cavity and the workpiece processing range area.

4. The method for predicting the ablation position of a DC non-transferred arc in a plasma torch according to claim 3, characterized in that: The workpiece processing range includes a columnar region, the diameter of which is more than 60 times the diameter of the plasma torch cavity.

5. The method for predicting the ablation position of a DC non-transferred arc in a plasma torch according to claim 1, characterized in that: The step b) specifically includes compiling a self-developed UDF user-defined function based on the standard k-epsilon turbulence model of Ansys-Fluent software to describe the magnetohydrodynamic steady-state model, and modifying the source term of the cathode boundary in the UDF user-defined function according to the geometric parameters and working current of the test model. Among them, I 工作 is the working current of the test model, r 阴极截面 is the radius of the maximum radial section of the cathode head, r is the radius of the radial section of the cathode head, and j is the source term of the cathode boundary.

6. The method for predicting the ablation position of a DC non-transferred arc in a plasma torch according to claim 1, characterized in that: The operating parameters in step b) include working gas inlet parameters, physical parameters of anode and cathode materials, and cathode working current.

7. The method for predicting the ablation position of a DC non-transferred arc in a plasma torch according to claim 1, characterized in that: The boundary condition of the inlet in the step c) is a velocity inlet including the incoming flow direction and the incoming flow velocity, the boundary condition of the outlet is a pressure outlet, the boundary condition of the anode is a no-slip heat exchange wall, the boundary condition of the cathode is a no-slip isothermal wall, the boundary condition of the workpiece surface is a no-slip isothermal wall, and the boundary conditions of the remaining walls are isothermal walls.

8. The method for predicting the ablation position of a DC non-transferred arc in a plasma torch according to claim 7, characterized in that: The thermal conductivity of the no-slip heat exchange wall is set to 1e5W / m 2 , the free temperature is 500K, the surface potential is 0V, the no-slip isothermal wall temperature of the cathode is set to 3500K, the defined current value is taken from the source term of the cathode boundary in the UDF user-defined function, the no-slip isothermal wall temperature of the workpiece surface is set to 300K, the surface potential is 0V, and the isothermal wall temperature is set to 300K.

9. The method for predicting the ablation position of a DC non-transferred arc in a plasma torch according to claim 1, characterized in that: In the step d), the temperature of the flow field between the cathode and anode regions is set to be no less than 8000K.

10. The method for predicting the ablation position of a DC non-transferred arc in a plasma torch according to claim 1, characterized in that: The convergence condition in step e) is that the residual value decreases by 4 orders of magnitude and the inlet and outlet mass flow difference is less than 1e-6 kg / s.

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