EP2613614A1 - Device for generating a plasma stream - Google Patents

Abstract

The method involves forming long electric arc type electric shock from a minimum electric voltage to generate an electric arc (12) between a central electrode (5) and an end area using an auto-modulation rule of a power supply source (4). Frequency of modulation is determined by the electric voltage of ignition of the electric shock, where the central electrode is coaxially arranged in a channel (2) and the power supply source. An independent claim is also included for a device for generation and hydrodynamic stabilization of plasma flow.

Description

Technical area

The present invention relates to a device for generating a plasma stream having a low temperature and a relatively high power.

State of the art

In the field of surface treatment, it is known to use a plasma stream so as, in particular, to weld surfaces or to cut surfaces. Such applications of a plasma flow have in particular been described in the patent US 3,515,839 . However, in this state of the art, the plasma flow created has a very high temperature. This plasma flow is therefore not suitable for the treatment of heat sensitive surfaces such as plastic for example. It is also known to use a plasma stream to treat plastic surfaces so as to increase their wettability. Such an application has notably been described in the article " Surface Treatment of Plastics by Plasmajet ", published in the Journal of Adhesion Society of Japan, Volume 6, No. 4, August 2, 1968 . In this document, the plasma flow is generated by applying a voltage between a cathode formed of a thoriated tungsten bar and an anode forming the body of the plasma nozzle. In addition, an argon gas flow circulates in the free space separating the anode and the cathode so as to develop the electric arc formed between these two electrodes to an outlet opening of the nozzle. However, in this document, the average temperature of the plasma jet is about 5500 ° K which is still too high for the surface treatment applications contemplated by the present invention.

Disclosure of the invention

The present invention therefore aims at providing a device enabling generate a plasma stream whose temperature is low while having a relatively high power.

For this purpose, according to the invention, there is provided a device for generating a plasma flow comprising an electrically conductive box of tubular form forming a central channel through which a swirling gas passes, a central electrode disposed coaxially in said channel and a electric power source for applying an electrical voltage V between the electrode and the housing, characterized in that the average diameter of the channel formed by the housing decreases progressively from an area substantially at the free end of the housing electrode to an end zone of said housing, said end zone being configured such that the minimum electrical voltage Vcmin (0) to be applied to develop an electric arc between said electrode and said end zone is strictly greater at said voltage V.

Other possible configurations of the device of the present invention are also defined in claims 2 to 14.

Thus configured, the device according to the invention makes it possible to limit the development of an electric arc inside a conductive casing to an end zone positioned just before the opening of the casing intended to deliver the flow of plasma over the piece to be treated. Indeed, the end zone is configured so as to develop an electric arc with the central electrode only from a certain minimum voltage. Therefore, by applying a voltage lower than said minimum voltage, the electric arc develops inside the central channel of the housing to approach or even reach said end zone, then retracts abruptly in the direction of the central electrode. Subsequently, it resumes its development inside the channel towards said end zone until it retracts again. This succession of development and retraction of the electric arc ultimately generates a relatively powerful plasma flow but whose temperature is low enough to allow its use in many surface treatment applications.

Brief description of the drawings

• la Figure 1 représente une vue schématique, latérale et en coupe d'un dispositif générateur d'un flux de plasma selon l'invention;
• la Figure 2a représente une vue schématique, latérale et en coupe d'une première variante d'une zone d'extrémité utilisable dans le dispositif représenté à la Figure 1;
• la Figure 2b représente une vue de face de la zone d'extrémité représentée à la Figure 2a;
• la Figure 3a représente une vue schématique, latérale et en coupe d'une deuxième variante d'une zone d'extrémité utilisable dans le dispositif représenté à la Figure 1;
• la Figure 3b représente une vue de dessus de la zone d'extrémité représentée à la Figure 3a;
• la Figure 3c représente une vue de dessus de la zone d'extrémité représentée à la Figure 3a, dans sa position d'utilisation;
• la Figure 4 représente une vue schématique, latérale et en coupe d'une troisième variante d'une zone d'extrémité utilisable dans le dispositif représenté à la Figure 1;

Other advantages and features of the present invention will be better understood on reading a particular embodiment of the invention and with reference to the drawings in which:

• the Figure 1 represents a schematic side view in section of a device generating a plasma flow according to the invention;
• the Figure 2a represents a schematic, lateral and sectional view of a first variant of an end zone that can be used in the device represented in FIG. Figure 1 ;
• the Figure 2b represents a front view of the end zone shown in FIG. Figure 2a ;
• the Figure 3a represents a schematic, lateral and sectional view of a second variant of an end zone that can be used in the device represented in FIG. Figure 1 ;
• the Figure 3b represents a view from above of the end zone shown in FIG. Figure 3a ;
• the Figure 3c represents a view from above of the end zone shown in FIG. Figure 3a in its position of use;
• the Figure 4 represents a schematic, lateral and sectional view of a third variant of an end zone that can be used in the device represented in FIG. Figure 1 ;

Detailed description of an embodiment of the invention

The device 10, represented on the Figure 1 , has an electrically conductive box 1 of tubular form, connected to the ground, having an internal cavity joining its two ends, said cavity constituting an elongate central channel 2 within which circulates a swirling gas 3. The gas 3, for example air, is introduced into the central channel 2 from an opening 4 formed in the side wall of the housing 1. The gas 3 is caused to swirl by means of a swirling device (not shown) so that the gas 3 flows inside the channel 2 forming a vortex substantially helically around the longitudinal axis of the channel 2, coinciding with the longitudinal axis of the housing 1. At one end of the housing 1 is mounted an insulating support 6 on which is fixed a central electrode 5 rod-shaped, which penetrates coaxially in the central channel 2. An electric high voltage source 7, which can supply a DC voltage, an AC voltage or a pulsed voltage, is connected to the electrode 5 and the earth. In addition, a device 8 for measuring and regulating the current and the electrical voltage connected between the voltage source 7 and the electrode 5 makes it possible to control the real voltage applied between the electrode 5 and the housing 1. in the configuration shown, the housing 1, formed of a metal and itself connected to earth, serves as a counter electrode so that an electric discharge between the electrode 5 and the housing 1 can be caused. This electric discharge initially occurs in an ignition zone 9, which is in the free space surrounding the electrode 5 and defined by the inner wall of the housing 1. The ignition zone 9 will generally be positioned near the the free end of the electrode 5 and downstream of the opening 4 so as to allow the gas 5 to move along the axis of the housing 1 the micro-electric arcs 11 formed at each discharge. As a result, the micro-arcs 11 lengthen over time along the entire length of the channel 2 and, due to a vortex stabilization of the gas flow towards the axis of the housing 1, form a wire arc The end zone 13 may be similar for example to an end channel oriented along the longitudinal axis of the housing end opening on an open end through which the flow of plasma. It may also have a more complex form, as we will see more fully later with reference to Figures 2 to 4 . Once the arc 12 has formed, the micro-arcs 11 are formed between this arc 12 and the internal walls of the housing 1.

The basic structure of the device 10 as described above, however, does not allow the generation of a low temperature plasma stream. Indeed, in this basic structure, the electric arc 12 is stabilized quickly. The plasma flux is thus generated continuously as long as a voltage V is maintained between the electrode 5 and the housing 1. This operating mode induces the formation of a powerful plasma flux and particularly hot. In addition, in this configuration, the risk is high that the electric arc 12 is formed directly between the electrode 5 and the object to be treated if the latter is metallic. To remedy this, the Applicant has had the idea to limit the development of the electric arc 12, in particular by causing its retraction as soon as it reaches a limit zone inside the housing 1. It turns out that, to maintain sufficient power to the plasma flow, it is advantageous to coincide this boundary zone with the end zone 13 mentioned above.

At this stage, two solutions can be envisaged to cause a retraction of the electric arc 12.

A first solution is to first determine the actual voltage Vcmax from which an electric arc is likely to form between the electrode 5 and the end zone 13 of the housing 1. By controlling the actual voltage Vr by means of the device 8, it is possible to determine when Vr reaches the value Vcmax. The device 8 is then able to send an interruption signal to the voltage source 7 so as to produce an electrical micro-cut which causes a retraction of the arc 12 to the ignition zone 9. following, the restoration and maintenance of the voltage V again produces the expansion of the arc 12 to the end zone 13 and, therefore, its retraction again. By proceeding in this way, an unbalanced plasma stream is generated that is characterized by a relatively low temperature, especially between 30 ° C and 300 ° C.

A second solution consists in configuring the device for generating the plasma flow such that an automatic retraction of the electric arc 12 occurs when it reaches or approaches the end zone 13. This result can notably be obtained by using the particular structure of the housing 1 shown in the Figure 1 . In this structure, the housing 1 has a channel 2 whose section, or the average diameter, decreases progressively from the ignition zone 9 to the end zone 13. This progressive decrease may notably consist in segmenting the inner wall of the housing 1 in a series of successive tubular sections S1, S2, S3 and S4 of decreasing diameter and of identical length. However, it has been found that this gradual decrease in the diameter of the channel 2 causes a concomitant increase the breakdown voltage of said sections S1, S2, S3 and S4, that is to say the minimum electrical voltage to be applied to develop an electric arc between the electrode 5 and said tubular sections S1, S2, S3 and S4 . Therefore, considering that the tubular section S4 corresponds to the end zone 13 and that the breakdown voltage associated with this section S4 is Vcmin (0), it suffices to apply between the electrode 7 and the housing 1 a voltage V less than Vcmin (0) to find that the electric arc 12 will retract as soon as it reaches the end zone 13. In order to maintain a relatively high power plasma flow, it can also be advantageous to allow an uninterrupted development of the electric arc 12 to the section S3 located just before the end zone 13. To do this, simply choose the voltage V so that V is greater than or equal to Vcmin (-1), Vcmin (-1) corresponding to the breakdown voltage of section S3.

With reference to Figures 2a and 2b , there is shown a possible variant of the end zone that can be used in the device represented in FIG. Figure 1 .

In this variant, the end zone 13 defines an end channel oriented along the longitudinal axis of the casing 1, said end channel opening on an open end 14 of conical shape through which the plasma flow exits. In this way, it is found that the micro-arcs 11 leave the end channel 13 following the conical surface of said end 14. This uniform distribution of the micro-arcs 11 on the surface of the cone ultimately generates a flow of plasma more wide and less intense which allows to further reduce its temperature and allows the device 10 to be used on a wider range of surfaces. In a preferred embodiment of the invention, it will be advantageous to configure the open end 14 so that its conical shape partially defines a hyperboloid of revolution and that the ratio between the outside diameter of the cone and the diameter of the inner wall of the cone. housing 1 at the end channel 14 is between 2 and 20.

With reference to Figures 3a to 3c , there is shown a second possible variant of the end zone that can be used in the device represented in FIG. Figure 1 .

In this variant, the end zone 13 defines an end channel oriented along the longitudinal axis of the housing 1, said end channel opening on a channel 15 open at its two ends 16 and forming an angle α with the longitudinal axis of the housing 1, the angle α being less than or equal to 90 °. In the configuration shown, this angle α is substantially equal to 90 °. In this way, the plasma flow F leaves the housing 1 by two openings 16 formed on its side walls and in a direction transverse to the longitudinal axis of the housing 1. This configuration makes it easier to apply the plasma flow F to inside tubes or, more generally, inside hollow objects. Moreover, as represented on Figures 3b and 3c it is also conceivable to use the device 10 to treat wires 17, or any other filiform object such as tubes or cables, capable of being introduced inside the transverse channel 15. Thus, by passing through the channel 15, the wire 17 is in contact with the plasma flow F leaving the end channel 13. To further improve the distribution of the plasma flow F along the outer wall of the wire 17, it will be advantageous to shift the This axis increases the propensity of the plasma flow F to swirl inside the transverse channel 15.

With reference to the Figure 4 , there is shown a third possible variant of the end zone that can be used in the device represented in FIG. Figure 1 .

In this variant, the end zone 13 defines an end channel oriented along the longitudinal axis of the housing 1, said end channel having a plurality of openings 18 opening on a plurality of transverse channels 19 oriented substantially perpendicular to the longitudinal axis of the housing 1 and one of the ends 20 is open. The plasma flow F therefore exits through each of said open ends 20. This "comb" distribution of the plasma flow F thus makes it easier to treat wide surfaces. On the other hand, because the plasma flux exiting the apertures 20 has a variable intensity depending on the position of the apertures 20 in the end channel 13, it may be advantageous to make an additional aperture 21 at the end of the channel. end 13 way partially letting out said plasma stream through said opening 21 and thus uniformize the intensity of the plasma flows leaving the openings 20.

As an indication, various embodiments of the device of the invention are given below.

Example 1

This example uses the device of the invention in its configuration represented on the Figure 1 .

Operating parameters:

Energy source direct current Electrical voltage applied between the electrode and the housing 3 kV Carrier gas Air Carrier gas flow 60 l / min Outside pressure atmospheric Diameter of the central electrode 3 mm Diameter of the central channel at the zone of ignition 4 mm Diameter of section S1 8 mm Diameter of section S2 6 mm Diameter of section S3 4 mm Diameter of section S4 2 mm Length of each section 35 mm

Result:

There is a succession of development-retraction of an electric arc between the central electrode and the section S4 at the frequency of 2 kHz.

Example 2

This example uses the device of the invention in its configuration represented on the Figure 1 .

Operating parameters:

Energy source direct current Electrical voltage applied between the electrode and the housing 2 kV Carrier gas N2 / H2 Carrier gas flow 20 l / min Outside pressure atmospheric Diameter of the central electrode 3 mm Diameter of the central channel at the zone of ignition 4 mm Diameter of section S1 8 mm Diameter of section S2 6 mm Diameter of section S3 4 mm Diameter of section S4 2 mm Length of each section 35 mm

Result:

There is a succession of development-retraction of an electric arc between the central electrode and the section S4 at the frequency of 1.5 kHz.

Example 3

This example uses the device of the invention in its configuration represented on the Figure 2 .

Operating parameters:

Energy source alternating current of 22 kHz frequency Electrical voltage applied between the electrode and the housing 3 kV Carrier gas Air Outside pressure atmospheric Carrier gas flow 50 l / min Diameter of the central electrode 3 mm Diameter of the central channel at the zone of ignition 4 mm Diameter of section S1 8 mm Diameter of section S2 6 mm Diameter of section S3 4 mm Diameter of section S4 3 mm Length of each section 10 mm Diameter of the end channel 3 mm Outer diameter of the cone 35 mm

Result:

There is a succession of development-retraction of an electric arc between the central electrode and the end of the cone at the frequency of 4 kHz.

Example 4

This example uses the device of the invention in its configuration represented on the Figure 3 .

Operating parameters:

Energy source pulsed non-polar current of 40 kHz frequency Electrical voltage applied between the electrode and the housing 6 kV Carrier gas Air Outside pressure atmospheric Carrier gas flow 50 l / min Diameter of the central electrode 3 mm Diameter of the central channel at the zone of ignition 4 mm Diameter of section S1 8 mm Diameter of section S2 6 mm Diameter of section S3 5 mm Diameter of section S4 4 mm Length of each section 15 mm Diameter of the end channel 4 mm Diameter of the transverse canal 4 mm Distance between the longitudinal axis of the housing and the axis of the transverse channel 2 mm

Result:

There is a succession of development-retraction of an electric arc between the central electrode and the section S4 at the frequency of 3 kHz.

Example 5

This example uses the device of the invention in its configuration represented on the Figure 4 .

Operating parameters:

Energy source pulsed non-polar current of 40 kHz frequency Electrical voltage applied between the electrode and the housing 6 kV Carrier gas Air Outside pressure atmospheric Carrier gas flow 60 l / min Diameter of the central electrode 3 mm Diameter of the central channel at the zone of ignition 4 mm Diameter of section S1 8 mm Diameter of section S2 6 mm Diameter of section S3 5 mm Diameter of section S4 5 mm Length of each section 20 mm Diameter of the end channel 5 mm End channel length 150 mm Diameter of cross channels 1 mm Distance between the axes of the transverse channels 6 mm Number of channels 20 Diameter of the additional opening 1.5 mm Thickness of the housing walls 2 mm

Result:

There is a succession of development-retraction of an electric arc between the central electrode and the end channel at the frequency of 1 kHz. This This configuration made it possible to obtain plasma jets having the same density and oriented in a direction perpendicular to the axis of the central channel, which makes it possible to treat large areas.

Claims (14)

Apparatus (10) for generating a plasma flow (F) comprising an electrically conductive box (1) of tubular form forming a central channel (2) traversed by a swirling gas (3), a central electrode (5) arranged coaxially in said channel (2) and an electric power source (7) for applying an electric voltage V between the electrode (5) and the housing (1), characterized in that the average diameter of the channel (2) formed by the housing (1) decreases progressively from a zone (9) located substantially at the free end of the electrode (5) to an end zone (13) of said housing (1), said zone of end (13) being configured such that the minimum electrical voltage Vcmin (0) to be applied to develop an electric arc (12) between said electrode (5) and said end zone (13) is strictly greater than said voltage V . Device (10) according to claim 1, characterized in that the inner wall of the housing (1) defines a series of successive tubular sections (S1, S2, S3, S4) of decreasing diameter, the tubular section (S3) located just before the end zone (13) being configured such that the minimum electrical voltage Vcmin (-1) to be applied to develop an electric arc (12) between said electrode (5) and said tubular section (S3) is less than or equal to at said voltage V. Device (10) according to claim 2, characterized in that said tubular sections (S1, S2, S3, S4) have the same length. Device (10) according to any one of the preceding claims, characterized in that said end zone defines an end channel (13) oriented along the longitudinal axis of the housing (1), said end channel (13) ) opening on an open end (14) through which the plasma flow (F). Device (10) according to claim 4, characterized in that said open end (14) has a conical section. Device (10) according to claim 5, characterized in that said open end (14) partially defines a hyperboloid of revolution. Device (10) according to claim 5 or 6, characterized in that the ratio between the outer diameter of the cone defined by said open end (14) and the diameter of the inner wall of the housing (1) at said area of end (13) is between 2 and 20. Device (10) according to any one of claims 1 to 3, characterized in that said end zone defines an end channel (13) oriented along the longitudinal axis of the housing (1), said end channel (13) opening on a transverse channel (15) open at both ends (16) and forming an angle α with the longitudinal axis of the housing (1), the angle α being less than or equal to 90 °. Device (10) according to claim 8, characterized in that the angle α is substantially equal to 90 °. Device (10) according to any one of claims 8 or 9, characterized in that the axis of the transverse channel (15) is offset relative to the longitudinal axis of the housing (1). Device (10) according to any one of claims 1 to 3, characterized in that said end zone defines an end channel (13) oriented along the longitudinal axis of the housing (1), said end channel (13) having a plurality of openings (18) opening on a plurality of transverse channels (19) oriented substantially perpendicular to the longitudinal axis of the housing (1) and one end (20) is open. Device (10) according to claim 11, characterized in that said end channel (13) opens on an open end (21) by which partially leaves the plasma flow (F). Device (10) according to any one of the preceding claims, characterized in that the electrical voltage generated by the source of electrical energy is chosen from the group consisting of DC voltages, pulsed voltages and AC voltages of any range of voltages. frequency. Device (10) according to any one of the preceding claims, characterized in that the swirling gas (3) is air.

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