Plasma Pulsed Arc

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Plasma Pulsed Arc

Thermal plasmas have nowadays a large range of industrial applications including cutting, welding, spraying, waste destruction, and surface treatment. Thermal plasmas are assumed to be under partial to complete local thermodynamic equilibrium (LTE) conditions. Under LTE, the plasma can be considered a conductive fluid mixture and therefore, be modeled using the magnetohydrodynamics (MHD) equations. This model shows how to use the Combined Inductively/DC Discharge interface to simulate the plasma generated in a pulsed arc.

This application requires the Plasma Module and AC/DC Module.

Model Definition

This model is based on the work presented in Ref. 1. In Ref. 1 The authors develop a complex model that includes the description of the weld pool under the action of a pulsed arc. In this work it is only simulated the plasma and the transfer of heat and currents in the metals neglecting the weld pool. This model starts by opening a model of a DC arc which solution are going to be used as initial conditions to the time dependent problem.

The applied current consists of a pulse with a frequency of 1 Hz, a peak current of 160 A, a floor current of 80 A, and a duty cycle of 0.5. The current source is set at the cathode and the bottom plate is grounded. In the 5 mm gap between the electrodes an argon plasma arc is created that heats the metal electrodes and surrounding gas. A shielding flow is added along the cathode.

The temperature-dependent physical properties of argon are loaded from the material library under Equilibrium Discharge. The temperature range of the physical properties span from 500 K to 25,000 K. A minimum electrical conductivity of 1 S/m is used for numerical stability reasons. Another important aspect to keep in mind is that the model used is not valid to describe the plasma sheath region since in this regions there is charge separation and deviations from equilibrium. From the practical point of view, having a fine resolution in the plasma-electrode region causes numerical instabilities (and does not bring a better description of the physics). To make the model more stable use a mesh coarse enough so that the plasma sheath is averaged out.

Results and Discussion

Figure 1 and Figure 2 show the fluid velocity and temperature for instants in the current peak and floor of 160 A and 80 A at 5 s. As expected, at the peak value the fluid velocity and temperature are higher. In the results shown the temperature of the subtract still hasn’t reached a steady state that is reached at about 50 s. The final time of computation was set to 5s for practical reasons. The weld pool (not solved here) also has significant changes during the pulse different phases as shown in Ref. 1.

Figure 1: Plot of the velocity magnitude of the fluid at the current peak of 160 A (left) and current floor of 80 A (right).

Figure 2: Plot of the plasma temperature at the current peak of 160 A (left) and current floor of 80 A (right).

1. A. Traidia, Multiphysics Modeling and Numerical Simlation of GTA Weld Pools, Ecole Polytechique PhD Thesis, Paris France, 2011.

Plasma_Module/Equilibrium_Discharges/plasma_pulsed_arc

Modeling Instructions

This model uses an existing model from a dc arc and sets an ac excitation instead. The solution of the dc arc model are used as initial conditions to the time dependent model.

Open the plasma_dc_arc model from the Application Libraries.

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In the toolbar, click

and choose .

Application Libraries

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In the window, click the root node.

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In the window, select in the tree.

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Add a parameter to define the floor of the current pulse.

Global Definitions

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In the window, under click .

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In the window for , locate the section.

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In the table, enter the following settings:


Name	Expression	Value	Description
delta	80[A]	80 A

Add a square wave function to define the pulsed excitation.

Component 1 (comp1)

In the window, expand the node.

Waveform 1 (wv1)

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In the window, expand the node.

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Right-click and choose .

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In the window for , type current in the text field.

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Locate the section. From the list, choose .

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In the text field, type 0.05.

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In the text field, type 1.

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In the text field, type 180.

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In the text field, type delta/2.

Duplicate the existent Normal Current Density feature and set a pulsed current. The Normal Current Density feature are disabled at the study level when needed.

Electric Currents (ec)

Normal Current Density 2

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In the window, expand the node.

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Right-click and choose .

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In the window for , locate the section.

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In the Jn text field, type -(I0+delta/2+current(t[1/s]))/(pi*(1.6[mm])^2).

For the stationary study disable the pulsed excitation. This way, if this study needs to be solved later the correct current feature is used.

Step 1: Stationary

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In the window, expand the node, then click .

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In the window for , locate the section.

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Select the check box.

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In the tree, select .

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Electrical conductivity, Magnetic flux, Temperature, Temperature 3D, Velocity

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In the window, under , Ctrl-click to select , , , , and .

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Right-click and choose .

In the window for , type DC arc in the text field.

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In the toolbar, click

to open the window.

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Go to the window.

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Find the subsection. In the tree, select .

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Click in the window toolbar.

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In the toolbar, click

to close the window.

Step 1: Time Dependent

Prepare the time dependent study. Here, a few changes are necessary: disable the DC excitation, change the relative tolerance to 0.005, use previous solutions as initial conditions, use a fully coupled solver, uncheck the initial step option, and uncheck the reuse sparsity patter option.

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In the window for , locate the section.

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Select the check box.

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In the tree, select .

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Locate the section. In the text field, type range(0,0.1,5).

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From the list, choose .

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In the text field, type 0.005.

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In the toolbar, click

.

Current Density Norm (ec), Electric Potential (ec), Electric Potential, Revolved Geometry (ec), Isothermal Contours (ht), Magnetic Flux Density Norm (mf), Magnetic Flux Density Norm, Revolved Geometry (mf), Pressure (spf), Temperature, 3D (ht), Velocity (spf)

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In the window, under , Ctrl-click to select , , , , , , , , and .

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Right-click and choose .

In the window for , type Pulsed arc in the text field.

Temperature pulsed

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In the toolbar, click

and choose .

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In the window for , type Temperature pulsed in the text field.

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Locate the section. From the list, choose .

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Right-click and choose .

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In the window for , locate the section.

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In the text field, type T.

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Locate the section. From the list, choose .

Step 1: Time Dependent

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In the window, under click .

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In the window for , click to expand the section.

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Find the subsection. From the list, choose .

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From the list, choose .

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From the list, choose .

Solver Configurations

In the window, expand the node.

Solution 2 (sol2)

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In the window, expand the node, then click .

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In the window for , click to expand the section.

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Clear the check box.

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Right-click and choose .

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In the window for , click to expand the section.

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From the list, choose .

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In the window, click .

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In the window for , click to expand the section.

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Clear the check box.

Step 1: Time Dependent

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In the window, under click .

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In the window for , click to expand the section.

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Select the check box.

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From the list, choose .

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From the list, choose .

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In the toolbar, click

.

Electric Potential (ec)

Prepare plots to show the velocity and the temperature for the peak and floor values of the current side-by-side.

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In the window, click .

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In the window for , click to expand the section.

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From the list, choose .

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Click to expand the section. Select the check box.

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In the window, expand the node, then click .

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In the window for , locate the section.

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From the list, choose .

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From the list, choose .

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Locate the section. From the list, choose .

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Right-click and choose .

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In the window for , locate the section.

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From the list, choose .

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Click to expand the section. From the list, choose .

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In the toolbar, click

.

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button in the toolbar.

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In the window, click .

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In the window for , locate the section.

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Select the check box.

Temperature pulsed

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In the window, click .

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In the window for , locate the section.

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From the list, choose .

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Locate the section. Select the check box.

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Locate the section. Select the check box.

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In the window, click .

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In the window for , locate the section.

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From the list, choose .

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From the list, choose .

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Locate the section. From the list, choose .

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Right-click and choose .

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In the window for , locate the section.

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From the list, choose .

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Locate the section. From the list, choose .

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In the toolbar, click

.

Temperature pulsed

Plot the temporal evolution of the applied current and the temperature of the bottom plate.

Current vs. Time

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In the toolbar, click

and choose .

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In the window for , type Current vs. Time in the text field.

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Locate the section. From the list, choose .

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Click to expand the section. From the list, choose .

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Right-click and choose .

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Select Point 4 only.

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In the window for , locate the section.

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In the text field, type -ec.ncd2.nJ*(pi*(1.6[mm])^2).

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In the toolbar, click

.

Temperature vs. Time

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In the toolbar, click

and choose .

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In the window for , type Temperature vs. Time in the text field.

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Locate the section. From the list, choose .

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Locate the section. From the list, choose .

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Right-click and choose .

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Select Point 1 only.

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In the window for , locate the section.

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In the text field, type T.

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In the toolbar, click

.