COMSOL 6.4 - Inductively Coupled Plasma (ICP) Torch

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 Equilibrium Discharges, Out-of-Plane Currents interface (available in 2D and 2D axisymmetric) to simulate the plasma generated in an inductively coupled plasma torch.

Figure 1 displays the geometry of the to-be-modeled inductive plasma torch.

Figure 1: Geometry of an inductively coupled plasma torch. The torch is composed of three concentric quartz tubes in which gas are injected from the bottom and exit from the top the torch. In this model, a fixed power of 11 kW is transferred to the plasma by a three-turn coil operating at 3 MHz.

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

Model Definition

This model is based on the work presented in Ref. 1 and uses the following assumptions:

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The plasma torch is modeled by a fully axisymmetric configuration.

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The current inside the plasma is dominated by the induced current which is out-of-plane, that is, in the azimuth direction.

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The coil consists of parallel current carrying rings with a circle cross section, 6 mm in diameter. This implies neglecting the axial component of the coil current.

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Steady state, laminar pure argon plasma flow at atmospheric pressure.

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Optically thin plasma under local thermodynamic equilibrium (LTE) conditions.

Figure 2 shows the geometry of the model.

Figure 2: Schematic of the ICP torch. Flow enters from the base (v1, v2, and v3) and leaves out the top. The dimensions of the different part of the model are given in the Modeling Instructions section.

In this model excitation is provided to a three turns coil at 3 MHz. The gas flowing in the sheath tube (plasma confinement tube) is then ionized by Joule heating.

The model is solved using a frequency-stationary study in combination with a single turn coil feature which set a fixed power to the system (11 kW). By fixing the power, the current and electric potential can vary in the coil as the plasma electric conductivity builds up.

In this model the three different gas stream velocities (v1 for the carrier tube, v2 for the central tube, and v3 for the sheath tube) are composed of pure argon. The temperature-dependent physical properties of argon are loaded from the material library under Equilibrium Discharge. Note that the temperature range of the physical properties span from 500 K to 25,000 K. Note also that a minimum electric conductivity of 1 S/m is used for numerical stability reasons.

If the initial temperature is too low chances are that the solution found corresponds to a flat profile of the minimum electric conductivity (the default is 1 S/m). This is obviously a solution without interest and in fact it is the easiest solution to obtain. To avoid this start at a higher temperature closer to the experimental value as it is done in the present example. Always make sure to plot the conductivity to see if it is set to the minimum electric conductivity.

Results and Discussion

Figure 3 and Figure 4 show the plasma temperature distribution and velocity magnitude, respectively, of the argon plasma. The temperature peaks near the coils at 10,000 K. The plasma conductivity increases with the temperature and it has a maximum in the regions of maximum temperature as shown in Figure 5 where the electric conductivity of the plasma is plotted. Figure 6 displays the magnetic flux norm. Note that the electric conductivity of the plasma screens the magnetic flux as a consequence of the skin effect.