A Simulation Method for Combustible Material Pyrolysis under Electric Arc Action Based on Multiphysics Coupling

Abstract

This invention discloses a method for simulating the pyrolysis of combustibles under electric arc based on multiphysics coupling, belonging to the field of multiphysics coupling simulation technology. The invention involves extracting geometric configurations from the electric arc, electrodes, and porous combustibles, and performing two-dimensional axisymmetric modeling on these geometric configurations to form a computational domain mesh structure. This computational domain mesh structure includes an air domain, an electrode domain, and a porous combustibles domain. By constructing a multiphysics coupling model of electromagnetic-heat transfer-flow-chemical reaction, this invention achieves a unified description and coupled solution of the pyrolysis process of combustibles under electric arc, characterizing the mutual influence relationships between electric arc energy, heat conduction and convection, gas flow, and pyrolysis reaction kinetics. This avoids the shortcomings of traditional single or weakly coupled models in predicting flow rates, temperatures, and reaction rates, thus improving the physical consistency of the simulation results.

Description

Technical Field

[0001] This invention belongs to the field of multiphysics coupling simulation technology, and in particular relates to a method for simulating the pyrolysis of combustibles under the action of an electric arc based on multiphysics coupling. Background Technology

[0002] Electric arc discharge is one of the main causes of building fires, industrial fires, and forest fires. Its high temperature can directly cause the pyrolysis of combustibles and induce ignition or smoldering combustion. Current basic and applied research relies heavily on experimental methods to analyze the arc ignition process. However, due to the short discharge time and high discharge energy, traditional measuring instruments are unable to effectively obtain key data such as the temperature field and flow field of combustible pyrolysis under the action of electric arc, resulting in limitations in our understanding of the pyrolysis mechanism of combustibles under the action of electric arc.

[0003] Existing models for the pyrolysis of combustibles under electric arc often focus on specific physical processes, lacking a holistic description of the multi-physics coupling effects during the pyrolysis process. This leads to significant discrepancies between simulation results and experimental phenomena. For example, pyrolysis models that ignore the electromagnetic field and only consider the combustible domain fail to fully reflect the impact of combustible pyrolysis on plasma motion and energy transfer; or models that only simulate arc behavior based on magnetohydrodynamics cannot simulate the influence of the arc on the combustible domain. Therefore, there is an urgent need to establish a multi-physics simulation method that can effectively couple electromagnetic, heat transfer, flow, and reaction processes to more realistically reveal and predict the mechanisms and processes of combustible pyrolysis under electric arc.

[0004] To address this issue, we propose a method for simulating the pyrolysis of combustibles under electric arc action based on multi-physics coupling. Summary of the Invention

[0005] To address the shortcomings of existing technologies, this invention provides a simulation method for the pyrolysis of combustibles under the action of an electric arc based on multi-physics coupling. The technical problem to be solved by this invention is: how to simulate and predict the complete process of electromagnetic, heat transfer, flow and reaction in the pyrolysis of combustibles under the action of an electric arc by establishing a coupled multi-physics numerical model, and to obtain the evolution law of key parameters such as voltage, temperature, flow rate and reaction rate.

[0006] To solve the above-mentioned technical problems, the present invention is achieved through the following technical solution.

[0007] This invention relates to a method for simulating the pyrolysis of combustibles under the action of an electric arc based on multi-physics field coupling, comprising: S1. Extract the geometric configuration from the electric arc, electrode and combustible material, and perform two-dimensional axisymmetric modeling on the geometric configuration to form a computational domain mesh structure, wherein the computational domain mesh structure includes an air domain, an electrode domain and a porous combustible material domain.

[0008] S2. Based on the computational domain grid structure, assign initial values ​​and boundary conditions to each physical field.

[0009] S3. For the computational domain grid structure, the intermediate electromagnetic field data is generated by solving the coupled control equations based on Maxwell's equations. In the solution process, the influence of material conductivity changes and electromotive force caused by temperature field and flow field on the electromagnetic field is considered.

[0010] S4. For the computational domain grid structure, the heat transfer control equation is used to solve for intermediate temperature field data. In the solution process, the effects of Joule heat, enthalpy transfer, chemical reaction heat and net radiation loss caused by electromagnetic field and chemical reaction are considered.

[0011] S5. For the computational domain grid structure, the Navier-Stokes equations are used to solve for intermediate flow field data. In the process of solving, the effects of electromagnetic field, temperature field, Lorentz force caused by chemical reaction, air pressure change and pyrolysis gas product mass source on the flow field are considered.

[0012] S6. An apparent reaction kinetic model based on the Arrhenius formula is used to solve for the intermediate chemical reaction rate through the temperature field.

[0013] S7. Iterative calculations are performed using a transient fully coupled method until the convergence condition is met, resulting in a multi-physics field coupled simulation of pyrolysis of combustibles under the action of an electric arc.

[0014] The present invention is further configured such that, in the geometric configuration simulation experiment arrangement, one side electrode of the geometric configuration is a cathode and the other side electrode is an anode, and there is a gap between the cathode and the anode.

[0015] The present invention is further configured such that the coupling control equation includes an electromotive force term caused by the plasma velocity.

[0016] The present invention is further configured such that the coupling control equation is solved by specifying different electromagnetic property parameters for the air domain, the electrode domain and the porous combustible material domain, respectively. The conductivity of the air domain is set as a function related to temperature, the conductivity of the electrode domain is set as a function related to temperature, and the conductivity of the porous combustible material domain is set as a fixed value or a function related to temperature.

[0017] The present invention is further configured such that the heat transfer control equation is a multi-region coupled energy conservation equation, and the heat transfer control equation includes the fluid heat transfer equation of the air domain, the solid heat transfer equation of the electrode domain, and the porous material heat transfer equation of the combustible material domain.

[0018] The present invention is further configured such that the heat source term of the heat transfer control equation includes a Joule heating term, an enthalpy transfer term, a net radiation loss term for the air-domain plasma region, and a chemical reaction heat source term calculated from the intermediate chemical reaction field data, wherein the Joule heating term is calculated from the current density and electric field strength in the intermediate electromagnetic field data.

[0019] The present invention is further configured such that the intermediate flow field data is generated by calculating the Navier-Stokes equations for compressible laminar flow in the air domain and by calculating the fluid flow equations in the porous combustible material domain.

[0020] The present invention is further configured such that the parameters of the apparent reaction kinetic model are obtained based on thermogravimetric analysis, and the intermediate chemical reaction rate is solved based on the Arrhenius equation.

[0021] The present invention is further configured such that the solution process of the apparent reaction kinetic model is based on the intermediate temperature distribution in the intermediate temperature field data.

[0022] The present invention is further configured such that the convergence condition is that the changes in the intermediate data of the multiphysics field in adjacent iteration steps satisfy a preset threshold.

[0023] The beneficial effects of this invention are as follows: This invention constructs a multi-physics field coupling model of electromagnetic-heat transfer-flow-chemical reaction, which realizes a unified description and coupled solution of the pyrolysis process of combustibles under the action of electric arc. It characterizes the mutual influence between electric arc energy, heat conduction and convection, gas flow and pyrolysis reaction kinetics, avoiding the shortcomings of traditional single or weakly coupled models in the calculation of flow rate, temperature and reaction rate, and improving the physical consistency of simulation results. Attached Figure Description

[0024] To more clearly illustrate the technical solutions of the embodiments of the present invention, the accompanying drawings used in the description of the embodiments will be briefly introduced below.

[0025] Figure 1 This is a flowchart of the geometric modeling and mesh generation process of the present invention.

[0026] Figure 2 The flowchart shows the process of solving the electromagnetic field.

[0027] Figure 3 The flowchart shows the process of solving the coupled flow field. Detailed Implementation

[0028] The technical solutions of the present invention will be described below with reference to the accompanying drawings. The described embodiments are only some embodiments of the present invention, and not all embodiments.

[0029] Example 1 Please see Figures 1-3 This invention is a method for simulating the pyrolysis of combustibles under the action of an electric arc based on multi-physics field coupling, including... S1. Extract the geometric configuration from the electric arc, electrodes, and combustible material. Perform two-dimensional axisymmetric modeling on the geometric configuration to form a computational domain mesh structure, which includes an air domain, an electrode domain, and a porous combustible material domain. The geometric configuration is used to simulate the experimental setup. The upper electrode of the geometric configuration is the cathode with a tip angle of 40°, and the lower electrode is the anode. The gap between the cathode and anode is 10 mm.

[0030] S2. Based on the computational domain grid structure, initial values ​​and boundary conditions are assigned to each physical field. The initial values ​​for the governing equations of magnetic field, electric field, heat transfer, flow, and reaction are magnetic vector potential A = 0 Wb·m. -1 Electric potential V=0V, temperature T=300K, flow velocity u=0m·s -1 The normalized mass of the combustible material is Y=1. Boundary conditions include axisymmetric, current-driven, grounded, wall-driven, and open boundary conditions.

[0031] S3. For the computational domain mesh structure, intermediate electromagnetic field data is generated by solving the coupled control equations based on Maxwell's equations. During the solution process, the effects of material conductivity variations and electromotive force caused by temperature and flow fields on the electromagnetic field are considered. The coupled control equations are for the air domain, electrode domain, and porous combustible material domain, with different electromagnetic property parameters specified for each domain. The conductivity of the air domain is set as a temperature-dependent function, the conductivity of the electrode domain is set as a temperature-dependent function, and the conductivity of the porous combustible material domain is set as a fixed value or a temperature-dependent function.

[0032] S4. For the computational domain mesh structure, the heat transfer control equations are solved to generate intermediate temperature field data. During the solution process, the effects of Joule heating, enthalpy transfer, and chemical reaction heat on heat transfer, as well as net radiation loss, caused by electromagnetic fields and chemical reactions, are considered. The heat transfer control equations are multi-domain coupled energy conservation equations, including fluid heat transfer equations for the air domain, solid heat transfer equations for the electrode domain, and porous material heat transfer equations for the combustible material domain. The heat source terms in the heat transfer control equations include Joule heating, enthalpy transfer, and net radiation loss terms for the air domain plasma region, as well as chemical reaction heat source terms calculated from the intermediate chemical reaction field data. The Joule heating term is calculated from the current density and electric field strength in the intermediate electromagnetic field data.

[0033] S5. For the computational domain grid structure, the Navier-Stokes equations are used to solve for intermediate flow field data. During the solution process, the effects of electromagnetic fields, temperature fields, Lorentz forces caused by chemical reactions, air pressure changes, and the mass source of pyrolysis gas products on the flow field are considered. The intermediate flow field data is generated by performing calculations based on the compressible laminar Navier-Stokes equations in the air domain and on the fluid flow equations in the porous combustible material domain.

[0034] S6. An apparent reaction kinetic model based on the Arrhenius formula is used to solve for the intermediate chemical reaction rate through the temperature field. The parameters of the apparent reaction kinetic model are obtained based on thermogravimetric analysis, and the intermediate chemical reaction rate is solved using the Arrhenius formula.

[0035] Where Y is the mass fraction of the combustible condensate, dimensionless; t is time, in seconds; and A is the pre-exponential factor, in seconds. -1 For example, the value is 1×10 11 s -1 n is the reaction order, dimensionless, for example, a value of 2; E is the activation energy, in kJ·mol⁻¹. -1 For example, a value of 120 kJ·mol -1 R is the gas constant, which is 8.314 J·mol⁻¹. -1 ·K -1 T represents temperature, in Kelvin (K). The solution process for the apparent reaction kinetic model is based on the temperature distribution in the intermediate heat transfer data.

[0036] S7. Iterative calculations are performed using a transient fully coupled method until the convergence condition is met, resulting in a multiphysics coupled simulation of combustible pyrolysis under electric arc. The convergence condition is that the changes in intermediate multiphysics data in adjacent iteration steps meet a preset threshold.

[0037] Initialization and Input: Set the initial time step, for example, take... Read the intermediate magnetic vector potential obtained in the previous step. Intermediate potential intermediate temperature Intermediate flow velocity and the mass fraction of condensed phase in the chemical reaction field In the initial iteration k=0, the combustible mass fraction is Y(0)=1.0, at which point the mass source term... =0, reaction heat source term =0. The mass source term resulting from the generation of the pyrolysis gaseous products is considered. With reaction heat source term The equations are incorporated into the flow and heat transfer equations and iterated repeatedly until the preset convergence criterion is met.

[0038] Update the reaction source term from the temperature field: Within the combustible material domain, the condensed phase reaction rate dY / dt is obtained according to apparent kinetics, and the mass source term and reaction heat source term are calculated: Quality source items are denoted as The unit is The mass source term of the pyrolysis gas products was calculated in the combustible domain, and its maximum value varies with time along the r-axis of the cylindrical coordinate system.

[0039] Reaction heat source term notes The unit is Based on experiments or literature, obtain the pyrolysis reaction heat, such as ΔH = J kg -1 Then the heat source term for each point of the combustible material is: .

[0040] Mass source terms are fed back into the flow field coupling solution: As a source term in the continuity equation, it is incorporated into the porous combustible material domain, and viscous drag, inertial drag, and mass source-related drag are considered in the porous momentum equation.

[0041] The reaction heat source term is included in the heat transfer control equation: Reaction heat source term Substituting the heat source terms such as Joule heat, enthalpy transfer, and net radiation loss into the energy conservation equation and resolving the temperature field, we obtain... .

[0042] To prevent divergence and relaxation, under-relaxation is applied to the backfill source item:

[0043]

[0044] If convergence is not achieved after approximately 20 iterations, the calculation is interrupted to analyze the rationality of the preset parameter values, etc.

[0045] Convergence criterion: After each cycle of calculating the reaction source term, coupling the flow field, coupling the heat transfer, and updating the reaction, the relative changes in the solutions of adjacent iterations are checked to see if they meet a preset threshold, such as: Temperature field:

[0046] Velocity field:

[0047] Condensed phase mass fraction:

[0048] Select from the examples .

[0049] Example 2 Please see Figure 2 Based on Example 1, electromagnetic field coupling control equations were constructed and solved step by step based on Maxwell's equations. By assigning different electromagnetic parameters to air, electrodes and low-conductivity combustibles, intermediate electromagnetic field distribution data required for the pyrolysis process of combustibles under electric arc were obtained.

[0050] 1. Governing equations The electromagnetic field is solved using a coupled system of equations illustrated in the diagram. The unknowns are the electric potential V, the magnetic vector potential A, and the derived E, J, and H. The system of equations is written as follows: Charge conservation equation: ·J=0.

[0051] Ampère's circuital law: ×H=J.

[0052] Magnetic vector potential: ×A=B.

[0053] Electric field strength: .

[0054] Ohm's Law: J = σ(E + u × B).

[0055] Magnetic flux density:

[0056] Electric displacement vector:

[0057] Where, the initial value of u is u=0ms -1 The equations are obtained from the Navier-Stokes equations, and subsequent iterations are calculated from the flow field.

[0058] 2. Assigning material parameters Material parameter values ​​are assigned separately according to the principles of different regions and different physical properties: Air domain: Relative permeability: .

[0059] Relative permittivity: .

[0060] Electrical conductivity: a temperature-dependent function Piecewise interpolation is convenient for implementation. Example data:

[0061]

[0062]

[0063]

[0064] Electrode domain: The density of copper is taken as 8960. The relative permeability is approximately Relative permittivity .

[0065] Conductivity corrected for temperature, example:

[0066]

[0067]

[0068] Combustible area: Relative permeability: .

[0069] Relative permittivity: .

[0070] Electrical conductivity: A very low fixed value is used to reflect the insulation characteristics. Example: .

[0071] For combustibles with good insulation properties, this setting inhibits current from penetrating into the interior of the combustible and allows the formation of a physically consistent potential gradient at the interface, which can better reproduce the electromagnetic field changes of combustibles during the pyrolysis process under the action of an electric arc.

[0072] 3. Boundary conditions and operating data Geometric conditions: The upper electrode is the cathode with a tip angle of 40° and the gap between the cathode and anode is 9.70 mm.

[0073] Electric field boundary: The top of the upper electrode, with the current density set based on the actual current conditions. The bottom of the lower electrode is grounded, where the potential V=0V.

[0074] Magnetic field boundary: The magnetic field uses n×A=0 as the boundary condition.

[0075] Axis boundary: axisymmetric condition.

[0076] Example 3 Please see Figure 3 Based on Examples 1 and 2, continuity equations and Navier-Stokes equations were established in the air domain and the porous combustible material domain, respectively. Lorentz force and mass source term of pyrolysis gas products were introduced and coupled to obtain intermediate flow field data such as velocity field and pressure field under the action of electric arc.

[0077] 1. Air domain Continuity equation:

[0078] Momentum equation:

[0079] Wherein, the viscous stress tensor is taken as:

[0080] Lorentz force term:

[0081] 2. Porous combustible domain 2.1 Porosity, Equivalent Particle Size, and Drag Coefficient Combustible materials are treated as porous media, with a porosity of [value missing]. =0.65, equivalent particle diameter is taken as .

[0082] Based on the Ergun isopermeability model, the permeability and inertial drag coefficient are calculated:

[0083]

[0084]

[0085]

[0086] 2.2 Continuity Equation in Porous Domains

[0087] in, For the mass source term of the pyrolysis gas products: the first round of coupling calculation first takes =0, calculate the pyrolysis rate and then incorporate it into the flow field. A commonly used example of backfill range in engineering implementation: Due to the extremely high energy of the electric arc, it is effective in the active pyrolysis zone of combustible materials. Reachable .

[0088] 2.3 Momentum Equation for Porous Domains .

[0089] The viscous stress tensor of the porous combustible domain is taken as:

[0090] Lorentz force press F L =J×B calculation. When the conductivity of the combustible material is extremely low, this term is usually very small. It is retained near the interface to ensure that the equation form is consistent.

[0091] The foregoing has only described certain exemplary embodiments of the present invention by way of illustration. Undoubtedly, those skilled in the art can modify the described embodiments in various ways without departing from the spirit and scope of the present invention. Therefore, the foregoing drawings and descriptions are illustrative in nature and should not be construed as limiting the scope of protection of the claims of the present invention.

Claims

1. A method for simulating the pyrolysis of combustibles under the action of an electric arc based on multi-physics coupling. Includes, characterized in that: S1. Extract geometric configurations from electric arc, electrodes and combustibles, and perform two-dimensional axisymmetric modeling on the geometric configurations to form a computational domain mesh structure, wherein the computational domain mesh structure includes an air domain, an electrode domain and a porous combustible domain; S2. Based on the computational domain grid structure, assign initial values ​​and boundary conditions to each physical field; S3. For the computational domain grid structure, the intermediate electromagnetic field data is generated by solving the coupled control equations based on Maxwell's equations. In the solution process, the influence of material conductivity changes and electromotive force caused by temperature field and flow field on electromagnetic field is considered. S4. For the computational domain grid structure, the heat transfer control equation is used to solve to form intermediate temperature field data. In the solution process, the effects of Joule heat, enthalpy transfer, chemical reaction heat and net radiation loss caused by electromagnetic field and chemical reaction are considered. S5. For the computational domain grid structure, the Navier-Stokes equations are used to solve for intermediate flow field data. During the solution process, the effects of electromagnetic field, temperature field, Lorentz force caused by chemical reaction, air pressure change and pyrolysis gas product mass source on the flow field are considered. S6. An apparent reaction kinetic model based on the Arrhenius formula is used to solve for the intermediate chemical reaction rate through the temperature field; S7. Iterative calculations are performed using a transient fully coupled method until the convergence condition is met, resulting in a multi-physics field coupled simulation of pyrolysis of combustibles under the action of an electric arc.

2. The method for simulating the pyrolysis of combustibles under the action of an electric arc based on multi-physics coupling as described in claim 1, characterized in that: The geometric configuration is used to simulate the experimental setup, with one side of the geometric configuration serving as the cathode and the other side as the anode, and a gap existing between the cathode and the anode.

3. The method for simulating the pyrolysis of combustibles under the action of an electric arc based on multi-physics coupling as described in claim 1, characterized in that: The coupled control equations include an electromotive force term caused by the plasma velocity.

4. The method for simulating the pyrolysis of combustibles under the action of an electric arc based on multi-physics coupling as described in claim 1, characterized in that: The coupling control equations are for the air domain, the electrode domain, and the porous combustible material domain, and are solved by specifying different electromagnetic property parameters for each domain. The conductivity of the air domain is set as a temperature-dependent function, the conductivity of the electrode domain is set as a temperature-dependent function, and the conductivity of the porous combustible material domain is set as a fixed value or a temperature-dependent function.

5. The method for simulating the pyrolysis of combustibles under the action of an electric arc based on multi-physics coupling according to claim 1, characterized in that: The heat transfer control equation is a multi-region coupled energy conservation equation, which includes the fluid heat transfer equation of the air domain, the solid heat transfer equation of the electrode domain, and the porous material heat transfer equation of the combustible material domain.

6. The method for simulating the pyrolysis of combustibles under the action of an electric arc based on multi-physics coupling as described in claim 1, characterized in that: The heat source term of the heat transfer control equation includes a Joule heating term, an enthalpy transfer term, and a net radiation loss term for the air-domain plasma region, as well as a chemical reaction heat source term calculated from the intermediate chemical reaction field data. The Joule heating term is calculated from the current density and electric field strength in the intermediate electromagnetic field data.

7. The method for simulating the pyrolysis of combustibles under the action of an electric arc based on multi-physics coupling according to claim 1, characterized in that: The intermediate flow field data is generated by performing calculations based on the compressible laminar Navier-Stokes equations in the air domain and on the fluid flow equations in the porous combustible material domain.

8. The method for simulating the pyrolysis of combustibles under the action of an electric arc based on multi-physics coupling according to claim 1, characterized in that: The parameters of the apparent reaction kinetic model are obtained based on thermogravimetric analysis, and the intermediate chemical reaction rate is solved based on the Arrhenius equation.

9. The method for simulating the pyrolysis of combustibles under the action of an electric arc based on multi-physics coupling according to claim 1, characterized in that: The solution process of the apparent reaction kinetic model is based on the intermediate temperature distribution in the intermediate temperature field data.

10. The method for simulating the pyrolysis of combustibles under the action of an electric arc based on multi-physics coupling according to claim 1, characterized in that: The convergence condition is that the changes in the intermediate data of the multiphysics field in adjacent iteration steps meet a preset threshold.

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