CN101788786B - Thermal analysis control method of electric arc furnace for preparing magnesium oxide crystals - Google Patents

Abstract

The invention relates to a thermal analysis control method of electric arc furnace for preparing magnesium oxide crystals and belongs to the technical field of simulation control in crystal preparation industry. The method includes the following steps: (1) a finite element model is built for an electric arc furnace; (2) physical parameters are set; (3) harmonic electric field obtains ohmic heat power distribution during solving; (4) the equivalent heat flow of arc is solved; (5) the equivalent heat flux power distribution of ohmic heat and arc can be used as the heating load, and the ambient temperature and convection coefficient are set for calculating the temperature field inside the furnace; (6) According to the calculation results, the control strategy is optimized based on process in the prior art. The invention has the beneficial effects that the invention provides a mathematical analysis tool for the current smelting process of magnesium oxide on analyzing the internal temperature field and molten pool shape, makes more accurate and reliable control strategy for the smelting process, adopts three-dimensional image to demonstrate the change of the temperature gradient field, accordingly is helpful to enhancing the understanding of the process from raw material to crystal growth.

Description

A kind of thermal analysis control method for preparing the magnesia crystal electric arc furnaces

Technical field

The invention belongs to the technical field of simulation control of crystal preparation industry, relate to a kind of thermal analysis system and corresponding Industry Control method of three-phase Submerged-arc Furnace, specially refer to a kind of thermoanalytical Finite Element Method of electric arc furnaces for preparing magnesia crystal.

Background technology

Magnesia crystal now has been applied to many high-tech areas, as the film growth substrate of high-temperature superconductive device, substrate base, plasma display diaphragm, high temperature high-precision optical material and the high-temperature crucibles material etc. of semiconductor material.

The mainstream technology of producing magnesia crystal at present is the three-phase ac electric arc furnace fusion method.This method mainly forms by the high-temperature electric arc heating raw and melts body, stops the back melt in heating process and obtains crystal through natural cooling.The major defect of existing electric arc furnaces technology is, in whole smelting process, distribute and all the unknowns of molten bath form for the vital electric arc temperature field in furnace of control crystal growth, cause the fusion of magnesium oxide breeze, and the poor controllability of crystallization process, seriously restricted the raising of crystal yield and quality.

Summary of the invention

The technical problem to be solved in the present invention provides the heating analogue system of a kind of three-dimensional based on the finite element electric arc furnaces, variation to electric arc furnaces temperature inside field is described according to the thermal model of this system, for the temperature required field of the crystal growth of control provides a cover more effective process.

Technical scheme of the present invention is by to the electric arc body of heater, electric arc, raw material, factors such as electric parameter and other various process are taken all factors into consideration the finite element modeling of finishing electric arc furnaces, again according to the hot analysis result of certain operation stage, the electric arc furnaces injecting power is adjusted accordingly, realized the balance of three electrode injecting powers and stablizing of temperature field.

Concrete technical scheme of the present invention and implementation step have following content.At existing magnesium oxide electric arc furnaces type of furnace size, electrode size, and the difference of relative position is spatially set up corresponding finite element model and is divided grid automatically, the boundary condition that this model is set according to the environment and the electrical operation parameter at scene carries out the analysis of transient state or stable state according to the difference in electric arc furnace smelting technological process stage of living in to electric arc furnaces temperature inside field then again.At the smelting initial stage, the control in starting the arc stage is played crucial effects to the formation in molten bath, for can the quantitative test injecting power and the form in molten bath between mutual relationship, when being set, boundary condition needs artificial electric current and the heat flux set, and humorous electric field obtains distribute power that the equivalent heat flux stack of ohm thermal power and electric arc obtains as adding thermal force when finding the solution, and allowing both heating power values and the general power that equals in the actual process flow process inject, the general power injection is to obtain in real time at the scene according to the data of gathering by computing machine.Carry out transient analysis in this stage, just obtained the temperature field that changes in should the stage electric arc furnaces, the form in molten bath then obtains according to the zone that temperature in the stove is higher than fusing point.According to same principle, in the mid-term and the ending stage of smelting, the steady-state analysis when this model both had been fit to carry out electric arc furnaces and is in thermal equilibrium, also being adapted at certain local smelting stage carries out transient analysis.The result and the actual production technological process that obtain are compared, dope various technological parameters, comprise current amplitude, size that power injects and the optimum of time are provided with scope.

Because the imbalance that some on-the-spot accidentalia can cause power to inject, model can be analyzed according to this unscheduled event, dope the trend of the local overheating of electric arc furnaces internal temperature field, and remedial measures is further analyzed, to provide advisory opinion.For example: in order to reduce injecting power, prevent local overheating and change three-phase power imbalance that model can analyze and reduce reasonable interval single-phase or the polyphase current amplitude; For to increase injecting power, improve the electric arc furnaces overall operation efficiency and change three-phase power imbalance, model then can analyze the reasonable interval of the single-phase or polyphase current amplitude that needs increase.

Effect of the present invention and benefit are to be magnesian smelting process of present stage, the tool of mathematical analysis of a kind of electric arc furnaces internal temperature field and molten bath form is provided, variation tendency according to the electric arc furnace temperature that dopes, formulate more accurate, control strategy more targetedly, the fluctuation of furnace wall temperature is controlled in 10%; Model helps deepening the understanding of the process from the raw materials melt to the crystal growth by the variation of 3-D view demonstration temperature gradient field.

Description of drawings

Fig. 1 is the finite element method software of the inventive method carries out transient analysis according to the real-time change of the electric arc furnaces working of a furnace a process flow diagram.

Fig. 2 is the process flow diagram that carry out steady-state analysis of finite element method software when electric arc furnaces stable operation of the inventive method.

Embodiment

Be described in detail specific embodiments of the invention below in conjunction with technical scheme and accompanying drawing.

The difference of the operation stage of smelting according to magnesia crystal, transient state and steady-state analysis control are two kinds of typical control methods.The smelting initial stage is adopted transient analysis control, as shown in Figure 1; Smelt mid-term or later stage and adopt steady-state analysis control, as shown in Figure 2.Concrete implementation step is as follows:

Step (1), the three-dimensional finite element model of setting up electric arc furnaces according to the size and the relative position of electric arc furnaces, electrode and breeze;

Step (2) then is provided with the temperature variant parameter of various physical characteristicss of raw material, comprises conductivity, thermal conductivity, magnetic permeability, specific heat and density;

Step (3), ohm thermal power that humorous electric field is calculated when finding the solution distributes, and expression formula is:

$Q=\frac{1}{2}\mathrm{Re}\left({\left\{J\right\}}^T{\left\{E\right\}}^{*}\right)$

Re represents real part, Q representation unit volume Joule heat power, and { J} represents the current density vector, and { E} represents electric field vector, and { E}* represents the { conjugate complex number of E};

Step (4), find the solution the equivalent heat flux of electric arc, the equivalent heat flux of electric arc is made up of convection heat transfer' heat-transfer by convection flow, Thomson effect heat flux, the total condensation heat flow of electronics and radiant heat transfer flow four partial stacks, adopts cylindrical-coordinate system, and the expression formula of convection heat transfer' heat-transfer by convection flow is:

$Q_C=\frac{0.915}{{\mathrm{\σ}}_w}{\left(\frac{{\mathrm{\ρ}}_b{\mathrm{\μ}}_b}{{\mathrm{\ρ}}_w{\mathrm{\μ}}_w}\right)}^{0.43}{\left({\mathrm{\ρ}}_w{\mathrm{\μ}}_w\frac{v_{r,b}}{r}\right)}^{0.5}{C}_p(T_b-T_w)$

C _pThe specific heat of expression plasma, subscript w represents weld pool surface, σ _w, ρ _w, μ _w, T _wBe respectively turbulent flow Prandtl number, density, dynamic viscosity coefficient and the temperature at weld pool surface place; Subscript b represents the boundary layer of arc-plasma, v _{R, b}, ρ _b, μ _b, T _bRadially flow velocity, density, dynamic viscosity coefficient and the temperature of representing plasma boundary layer place respectively,

The expression formula of Thomson effect heat flux is:

$Q_E=\frac{{5J}_a}{2e}k({\mathrm{\αT}}_b-T_w)$

J _aThe current density at expression anode place, α is expressed as the ratio of electron temperature and plasma temperature, and e is an electron charge,

The expression formula of the total condensation heat flow of electronics is:

Q _A＝J _a(V _a+q _a)

V _aBe anode drop, q _aBe work function, V _aAnd q _aSum is got 8V,

The radiant heat transfer flow

$Q_{R,\mathrm{ij}}=v_j\frac{S_R}{4\mathrm{\π}}\frac{\cos \mathrm{\ψ}}{r_{i,j}^2}{\mathrm{dv}}_j$

S _RThe radiation loss of representation unit volume, r _{I, j}Expression connects the vector of j control volume in i unit of anode surface and the zoning, and ψ represents vector r _{I, j}And the angle between the anode surface;

Step (5) loads boundary condition, and ohm thermal power distributed load to the molten bath, is loaded into electrode below in the model with the equivalent heat flux of electric arc, and the environment temperature the furnace wall outside is set to 25 and spends, and convection coefficient is made as 12.5W/ (m ²℃), by instantaneous or steady-state thermal analysis, calculate the temperature field of electric arc furnaces;

Step (6) exceeds the limit that the furnace wall can bear if analysis result is found near the temperature certain electrode, then improves electrode on original technology basis, reduces voltage, reduces electric current; Do not form the needed temperature conditions in molten bath if reach, then stabilized electrodes on original technology basis improves voltage, increases electric current.

By the heat analysis of realistic model, the stability of the crystal yield of this electric arc furnaces, quality and technology all is greatly improved before, can effectively solve the problem that the aforementioned techniques scheme need solve.

Claims (1)

1. thermal analysis control method for preparing the magnesia crystal electric arc furnaces, its feature comprises the steps:

Step (1), the three-dimensional finite element model of setting up electric arc furnaces according to the size and the relative position of electric arc furnaces, electrode and breeze;

Step (2) then is provided with the temperature variant parameter of various physical characteristicss of raw material, comprises conductivity, thermal conductivity, magnetic permeability, specific heat and density;

Step (3), ohm thermal power that humorous electric field is calculated when finding the solution distributes, and expression formula is:

$Q=\frac{1}{2}\mathrm{Re}\left({\left\{J\right\}}^T{\left\{E\right\}}^{*}\right)$

Re represents real part, Q representation unit volume Joule heat power, and { J} represents the current density vector, and { E} represents electric field vector, and { E}* represents the { conjugate complex number of E};

Step (4), find the solution the equivalent heat flux of electric arc, the equivalent heat flux of electric arc is made up of convection heat transfer' heat-transfer by convection flow, Thomson effect heat flux, the total condensation heat flow of electronics and radiant heat transfer flow four partial stacks, adopts cylindrical-coordinate system, and the expression formula of convection heat transfer' heat-transfer by convection flow is:

$Q_C=\frac{0.915}{{\mathrm{\σ}}_w}{\left(\frac{{\mathrm{\ρ}}_b{\mathrm{\μ}}_b}{{\mathrm{\ρ}}_w{\mathrm{\μ}}_w}\right)}^{0.43}{\left({\mathrm{\ρ}}_w{\mathrm{\μ}}_w\frac{v_{r,b}}{r}\right)}^{0.5}{C}_p(T_b-T_w)$

C _pThe specific heat of expression plasma, subscript w represents weld pool surface, σ _w, ρ _w, μ _w, T _wBe respectively turbulent flow Prandtl number, density, dynamic viscosity coefficient and the temperature at weld pool surface place; Subscript b represents the boundary layer of arc-plasma, v _{R, b}, ρ _b, μ _b, T _bRadially flow velocity, density, dynamic viscosity coefficient and the temperature of representing plasma boundary layer place respectively,

The expression formula of Thomson effect heat flux is:

$Q_E=\frac{{5J}_a}{2e}k({\mathrm{\αT}}_b-T_w)$

J _aThe current density at expression anode place, α is expressed as the ratio of electron temperature and plasma temperature, and e is an electron charge,

The expression formula of the total condensation heat flow of electronics is:

Q _A＝J _a(V _a+q _a)

V _aBe anode drop, q _aBe work function, V _aAnd q _aSum is got 8V,

The radiant heat transfer flow

$Q_{R,\mathrm{ij}}=\∫v_j\frac{S_R}{4\mathrm{\π}}\frac{\cos \mathrm{\ψ}}{r_{i,j}^2}{\mathrm{dv}}_j$

S _RThe radiation loss of representation unit volume, r _{I, j}Expression connects the vector of j control volume in i unit of anode surface and the zoning, and ψ represents vector r _{I, j}And the angle between the anode surface;

Step (5) loads boundary condition, and ohm thermal power distributed load to the molten bath, is loaded into electrode below in the model with the equivalent heat flux of electric arc, and the environment temperature the furnace wall outside is set to 25 and spends, and convection coefficient is made as 12.5W/ (m ²℃), by instantaneous or steady-state thermal analysis, calculate the temperature field of electric arc furnaces;

Step (6) exceeds the limit that the furnace wall can bear if analysis result is found near the temperature certain electrode, then improves electrode on original technology basis, reduces voltage, reduces electric current; Do not form the needed temperature conditions in molten bath if reach, then stabilized electrodes on original technology basis improves voltage, increases electric current.

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