BACKGROUND TO THE INVENTION

THIS invention relates to a plasma torch, and more particularly but not exclusively to an improved spinner for use in a plasma torch, and to a plasma torch incorporating such a spinner.

In this specification, the term “spinner” is used to describe a vortex inducing device or swirl ring which is found in a plasma torch, and which is used to induce or control the swirling action or vortex flow pattern of working gas through an anode channel of a plasma torch. The terms “spinner” and “swirl ring” are regarded as synonymous.

Plasma is one of the four fundamental states of matter, and can broadly be defined as an ionized gas. It mainly consists of a mixture of free electrons and positively charged ions. Ionization is the process where a molecule gains a positive or negative charge by either losing or gaining electrons to form ions. Ionization in turn yields a molecule with an electrical charge - either positive or negative. In order to be classified as plasma, the negative and positive charges should balance each other, and plasma should therefore be electrically neutral. The properties of plasma are very similar to that of gases as it has no specific shape or constant volume.

Plasma technology is commonly used in many industries, including in the manufacturing, automotive, microelectronics, packaging and medical device industries. It is also foreseen for plasma technology to play an increasingly important role in the destruction of waste in a plasma gasification process, due to the high energy density of plasma. A disadvantage associated with plasma gasification is that it consumes a significant amount of electricity. In some applications, such as plasma gasification where plasma is used to convert organic matter into a syngas, the syngas can in part be used to power the plasma torch, but it is nevertheless important for the efficiency of the plasma torch to be as high as possible, irrespective of the application.

Plasmas can occur naturally or can be man-made, with natural plasmas contributing to about 99% of the optical universe. There are numerous methods that can be used to induce a plasma, of which exposing a gas to a high temperature or a high electrical potential are two of the most commonly used techniques in industry. Man-made plasmas consist of a wide range of temperatures, densities and pressures. The temperature range of man-made plasmas can range from as low as room temperature to as high as the average interior star temperature (ranging from a few thousand Kelvin to 50, 000 Kelvin). Some of the methods that can be used to induce man-made plasma include: Inductively Coupled Plasma (ICP), Capacitive Coupled Plasma (CCP), DC or AC Electric Arc Plasma, Micro- Wave Plasma (MWP) or Toroidal Plasma. The present invention is primarily concerned with electric arc generated plasmas. Electric arc generated plasmas can in turn be subdivided into two types, namely transferred arc plasmas and non-transferred arc plasmas, and this invention is primarily concerned with non-transferred arc plasmas.

The difference between the two arc plasmas lies in the electrode design of the plasma torch. In the case of the transferred arc plasma, the electrode is the processing material, and the arc is accordingly transferred from the plasma torch to the processing material. In the case of a non-transferred arc plasma, the arc is generated inside the plasma torch, and then creates a tail flame which in turn processes the material. In a non-transferred arc plasma torch, the arc is generated between a cathode and an anode that are both located inside the plasma torch.

A typical plasma torch used in non-transferred electric arc plasma generation includes a pointed cathode located in a carrier gas inlet zone of the torch, and an annular anode located in an anode channel section of the torch. When energized, an arc forms between the cathode and the anode, and the arc protrudes through the anode channel. The working gas that is forced through the anode channel (and thus the arc) is converted into a high temperature, high velocity plasma jet.

One typical configuration of a plasma torch 10 as is known in the art is shown in Figure 1. The most important components of the torch is the cathode 12 and the anode 13. In the configuration shown, the working gas enters the torch 10 through a tangential gas inlet 18, which leads to an annular receiving volume 19. The annular receiving volume 19 is in flow communication with a spinner or swirl ring 20, and the working gas is forced through radially offset passages 24 extending through the swirl ring. The tangential introduction of the gas into the torch, and the use of the spinner 20 results in the formation of a vortex gas column that forms in an arc chamber 14 of the plasma torch 10 and then moves down an anode Channell 6, which is defined by a hollow bore of the elongate, annular electrode13.

The body of the torch consists of various cooling channels (17, 18) for use in cooling both the cathode 12 and the anode 13. The cathode 12 cooling channel 18 includes and inlet 18.1 , a flow passage (from 18.2 to 18.3) in contact with an inner surface of the cathode 12 and a coolant outlet 18.4. The anode 13 cooling channel 17 includes an inlet 17.1 , a flow passage (from 17.2 to 17.3) in contact with an outer surface of the anode 13, and a coolant outlet 17.4.

The cathode and anode assemblies are insulated from each other by a non-conductive material 30 and are connected to a main power supply. In use an arc (not shown) is formed between the cathode 12 and the anode 13, and the working gas is ionized when travelling through the arch chamber 14 and down the anode channel 16, in so doing forming a plasma jet when the plasma gas exits the anode channel. It should be noted that the arc formed between the cathode and the anode is not stiff and straight, but randomly moves inside the anode channel before attaching at the bottom end. This results in a loss of energy, and hence a loss of efficiency.

The stabilization of the plasma arc initiated in the plasma torch is important, as a more stable plasma arc will result in a more efficient plasma torch. One aspect of plasma arc stabilization is to ensure that the arc is focused in the center of the anode channel. This can be achieved in two conventional ways namely wall stabilisation and vortex stabilisation. The former refers to stabilisation that is induced by lowering the electrical conductivity whilst simultaneously cooling the outer wall. The result is not only the stabilisation of the plasma jet but also minimisation of heat loss. The latter involves generating a swirling or vortex gas flow pattern, which is achieved by the tangentially injection of the working gas into the plasma torch, which is augmented by the spinner. The vortex or swirling gas flow pattern serves as insulation between the wall of the torch and the arc, and also creates a low-pressure zone in the center of the anode channel which serves to urge the plasma arc into the low pressure zone, and also to align the plasma arc flow with the central axis of the anode channel. Increased stabilization results in decreased heat loss, and therefore increased efficiency.

As mentioned, the vortex flow is initiated using a spinner or swirl ring 20, and a configuration known in the art is shown in Figure 2. The spinner essentially comprises an annular disc, with a plurality of angularly offset flow passages 24 provided therethrough.

The inventor previously proposed increasing the plasma torch efficiency by replacing a conventional spinner with radially offset but straight flow paths (as seen in Figure 2), to a spinner having arcuate or curved flow paths. The flow path of the improved spinner had flow paths having a curvature radius of 96mm. Experiments shown that the curved flow paths had a positive impact on the plasma torch efficiency, although no optimal solution was found. The result of this experiment was disclosed in a paper entitled “Plasma Torch Optimisation by Additive Manufacturing of Components", IJ van der Walt, PL Crouse, WB du Preez, ISPC 23 Conference in Montreal, Canada from 30 Jul to 4 Aug 2017, the content of which is incorporated herein by reference. The experiment confirmed that improved vortex formation and stability (in this case using curved flow passages) improved the efficiency of the plasma torch. It is therefore clear that further improvement in vortex formation and stability can potentially further increase the efficiency of a plasma torch, but the prior art does not provide any further guidance as to who this can be achieved.

It is accordingly an object of the invention to provide an improved plasma torch with increased efficiency.

It is also an object of the invention to provide a spinner for use in a plasma torch, and which has the effect of improving the efficiency of the plasma torch. SUMMARY OF THE INVENTION

According to the invention there is provided a non-transferred arc plasma torch including: a cathode; an anode spaced apart from the cathode, wherein an arc chamber is defined by a space between the cathode and the anode; a gas inlet chamber through which a working gas is introduced into the plasma torch; and a spinner located between the gas inlet chamber and the arc chamber; the spinner including a body which is annular when viewed in plan, the annular body having a central axis extending therethrough and which is in use substantially co-axial with a longitudinal axis of the plasma torch; the body having an operatively outer face which faces the gas inlet chamber and an operatively inner face which faces the arc chamber, with a plurality of flow passages extending between the operatively outer face and the operatively inner face; characterized in that the operatively inner face of the body has a first edge and a second edge, with a distance between the first edge of the of the operatively inner face and the central axis of the body is larger than a distance between a second edge of the of the operatively inner face and the central axis of the body.

There is provided for the first edge to be an edge that is in use closer to the cathode, and for the second edge to be an edge that is in use closer to the anode. There is also provided for the operatively inner face to be slanted, and to form an acute inner face offset angle relative to the central axis of the body of the spinner.

There is provided for the inner face offset angle to be between 70° and 45°, preferably between 65° and 50°, and more preferably between 60° and 55°.

There is also provided for each flow passage to have an acute flow passage offset angle relative to the central axis of the annular body.

There is provided for the flow passage offset angle to be between 87.5° and 70°, preferably between 85° and 75°, and more preferably between 82.5° and 80°.

A further feature of the invention provides for the flow passages to be curved, and for the flow passages to have a curvature radius.

There is provided for the spinner body to have an outer diameter, and for the ratio of the spinner body outer diameter to the flow passage curvature radius to be larger than 1 .5, and preferably larger than 2.

According to the invention there is provided a spinner for use in a nontransferred arc plasma torch, the spinner including: a body which is annular when viewed in plan, the annular body having a central axis extending therethrough and which is in use substantially co-axial with a longitudinal axis of the plasma torch; the body having an operatively outer face which faces the gas inlet chamber and an operatively inner face which faces the arc chamber, with a plurality of flow passages extending between the operatively outer face and the operatively inner face; characterized in that the operatively inner face of the body has a first edge and a second edge, with a distance between a first edge of the of the operatively inner face and the central axis of the body is larger than a distance between a second edge of the of the operatively inner face and the central axis of the body.

There is provided for the first edge to be an edge that is in use closer to a cathode of the plasma torch, and for the second edge to be an edge that is in use closer to an anode of the plasma torch.

There is also provided for the operatively inner face to be slanted, and to form an acute inner face offset angle relative to the central axis of the body of the spinner.

There is provided for the inner face offset angle to be between 70° and 45°, preferably between 65° and 50°, and more preferably between 60° and 55°.

There is also provided for each flow passage to have an acute flow passage offset angle relative to the central axis of the annular body.

There is provided for the flow passage offset angle to be between 87.5° and 70°, preferably between 85° and 75°, and more preferably between 82.5° and 80°.

A further feature of the invention provides for the flow passages to be curved, and for the flow passages to have a curvature radius.

There is provided for the spinner body to have an outer diameter, and for the ratio of the spinner body outer diameter to the flow passage curvature radius to be larger than 1 .5, and preferably larger than 2.

According to a further aspect of the invention there is provided a spinner for use in a plasma torch, the spinner including: a body which is annular when viewed in plan, the annular body having a central axis extending therethrough, wherein the central axis is in use substantially co-axial with a longitudinal axis of the plasma torch; the body having an operatively outer face and an operatively inner face, with a plurality of flow passages extending between the operatively outer face and the operatively inner face; wherein the flow passages form an acute angle relative to the central axis of the annular body.

According to a further aspect of the invention there is provided a method of generating a plasma jet, the method comprising the steps of:

- providing a plasma torch as described above;

- introducing a working gas into the plasma torch; and

- effecting an electric arc between electrodes of the plasma torch in order for a plasma arc to be initiated from the working gas.

BRIEF DESCRIPTION OF THE DRAWINGS

The invention is described by way of a non-limiting examples and by way of experimental results, and with reference to the accompanying drawings in which:

Figure 1 is a cross-sectional side view of a plasma torch as is known in the art;

Figure 2 is a cross-sectional perspective view of a spinner or swirl ring as is known in the art, and which is in particular used in the plasma torch of Figure 1 ;

Figure 3 is a cross-sectional plan view of a new spinner in accordance with one embodiment and one aspect of this invention; Figure 4 is a top plan view of another new spinner in accordance with another aspect of this invention;

Figure 5 is a cross-section side view of the spinner of Figure 4, taken through a centerline (X-X) of the spinner of Figure 4;

Figure 6 is a cross-section side view of the spinner of Figure 4, taken through a plane (Y-Y) that is aligned with one of the flow passages extending through the spinner;

Figure 7 is a perspective view of the spinner of Figure 4;

Figure 8 is a cross-sectional side view of a plasma torch incorporating the new spinner of Figure 4;

Figure 9 is an enlarged view of an arc chamber section of the plasma torch of Figure 8;

Figure 10 is a power - current graph for the different spinners at a nitrogen gas flow rates of 1 .55 g/s;

Figure 1 1 is a power - current graph for the different spinners at a nitrogen gas flow rates of 2.07g/s;

Figure 12 is a power - current graph for the different spinners at a nitrogen gas flow rates of 2.59g/s;

Figure 13 is a power - current graph for the different spinners at a nitrogen gas flow rates of 3.1 Og/s;

Figure 14 is a heat loss - current graph for the different spinners at a nitrogen gas flow rate of 1 .55 g/s; Figure 15 is a heat loss - current graph for the different spinners at a nitrogen gas flow rate of 2.07 g/s;

Figure 16 is a heat loss - current graph for the different spinners at a nitrogen gas flow rate of 2.59 g/s;

Figure 17 is a heat loss - current graph for the different spinners at a nitrogen gas flow rate of 3.10 g/s; and

Figure 18 is a bar chart showing the efficiency of the different spinners tested.

DETAILED DESCRIPTION OF INVENTION

Before the invention is explained in detail, it is to be understood that the invention is not limited in its application to the details of construction and the arrangement of components set forth in the following description or illustrated in the following drawings. The invention is capable of other embodiments and of being practiced or of being carried out in various ways. Also, it is to be understood that the phraseology and terminology used herein is for the purpose of description and should not be regarded as limiting. The use of "including," "comprising," or "having" and variations thereof herein is meant to encompass the items listed thereafter and equivalents thereof as well as additional items. It is noted that, as used in this specification and the appended claims, the singular forms "a," "an," and "the," and any singular use of any word, include plural referents unless expressly and unequivocally limited to one referent. As used herein, the term “include” and its grammatical variants are intended to be non-limiting, such that recitation of items in a list is not to the exclusion of other like items that can be substituted or added to the listed items. Referring to the drawings, in which like numerals indicate like features, a non-limiting and simplified example of a plasma torch incorporating the spinner(s) in accordance with the invention is generally indicated by reference numeral 10 in Figure 8.

On the whole, the construction and configuration of the non-transferred arc plasma torch 10 remains the same as what is known in the art, and shown in Figure 1 . In summary, the plasma torch 10 includes a cathode 12 and an anode 13, parts of which are spaced apart but in relative close proximity to one another in order for a plasma arc (not shown) to be formed between the cathode 12 and the anode 13 when current is applied to the circuit. An arc chamber 14 is defined by the space between the cathode 12 and the anode 13, and in use a working gas is introduced into this arc chamber 14. The working gas is ionized in the arc chamber to form a plasma jet, which is then expelled from the plasma torch through an anode channel 15, the inside of which is in this case formed by the anode 13 being of a tubular configuration, with the elongate bore of the anode thus forming the anode channel. Due to the very nature of the operation of the plasma torch, the electrodes (both the cathode 12 and the anode 13) heats up significantly, and a cathode cooling circuit 18 as well as an anode cooling circuit 17 are provided for the removal of heat from the cathode 12 and the anode 13, as is known in the art.

The working gas is introduced through a tangential gas inlet port 18, which is orientated perpendicularly relative to a longitudinal axis (A-A’) of the plasma torch. The working gas is first introduced into an annular gas inlet chamber 19, upstream of the spinner 20. As mentioned previously, it is important for the working gas to be imparted with a swirling or vortex flow pattern, for the reasons already set out above. For this purpose, a spinner or swirl ring 20 is located in the space between the annular gas inlet chamber 19 and the arc chamber 14. In all the embodiments of the present invention, the spinner 20 is essentially in the form of a body 21 which is annular when viewed in plan (as seen in Figure 4), with the annular body 21 having a central axis (B-B’ - shown in Figures 5 and 6) extending therethrough. The central axis (B-B’) of the spinner 20 is substantially co-axial with the longitudinal axis (A-A’ - shown in Figure 9) of the plasma torch 10. The annular body 21 has an operatively outer face 22 and an operatively inner face 23, with a plurality of flow passages 24 extending between the operatively outer face 22 and the operatively inner face 23. The operatively outer face is an upstream face which is in fluid communication with a source of the working gas used in the plasma torch (via the tangential inlet port 18), and the operatively inner face is a downstream face which is in fluid communication with the arc chamber 14 of the plasma torch 10. The spinner 20 furthermore has a flat base surface 25, as well as a flat top surface 26, with o-ring grooves provided in both surfaces.

In one embodiment of the invention (shown in Figures 4 to 9), the spinner does not have the shape associated with prior art spinners, in that one side of the spinner is of a slanted or tapering configuration, as opposed to a generally square cross-sectional profile associated with prior art spinners, for example the one shown in Figure 2. More particularly, the operatively inner face 23 forms an inner face offset angle 0 (shown in Figure 5) relative to the central axis (B-B’) of the body, whereas in prior art configurations (such as those shown in Figure 1 and 2) the operatively inner face of the spinner is parallel to the central axis. This of course means that the outer face 22 and the inner face 23 of the spinner are also not parallel to one another. The distance between a first edge 23.1 of the inner face 23 and the central axis (B-B’) is larger than a distance between a second edge 23.2 of the inner face 23 and the central axis (B-B’). The first edge 23.1 is the edge on the cathode 12 side of the torch shown in this embodiment of the invention, and the second edge 23.3 is the edge on the anode 13 side of the torch. A flow passage 24 extending through this spinner is not, as is the case in the prior art, perpendicular to the central axis (B-B’) of the spinner, and hence the longitudinal axis (A-A’) of the plasma torch, but is orientated at a gradient. The gradient is directed from the cathode location towards the anode location, and a flow path offset angle a is formed between the flow passage 24 and the central axis (B-B’).

Defined differently, the inner face offset angle 0 result in a corresponding angle p to be defined between the inner face 23 and the base surface 25 of the spinner. Likewise, the flow path offset angle a result in a corresponding angle to be defined between the flow passage 24 and the base surface 25 of the spinner. Because the design and configuration of the base surface 25 and the outer face 22 of the spinner may potentially be changed, the most accurate method to describe the salient aspects of the invention is to first and foremost focus on the inner face offset angle 0 and the flow path offset angle a which relates back to the central axis (B-B’) of the spinner.

It should be noted that, although preferable and practical, the inner surface does not necessarily have to be a flat surface, but could be convex, concave or even stepped, provided that the general configuration is one where the first edge 23.1 is spaced apart further from the central axis than the second edge 23.2. When reference is made to the inner face offset angle, it should be appreciated that the offset angle is defined by a plane between the first edge 23.2 and the second edge 23.1 , even if the inner face itself is not fully aligned with such plane.

In the design shown in Figures 4 to 9, the following dimensions apply:

The rationale behind this new design is to improve the efficiency of the plasma torch by improving vortex stabilization. It is desirable to bring the outlets of the flow passages 24 closer to the arc chamber 14, and also to decrease the compressible gas volume in the arc chamber 14.

Pressure pulses from the arc reduces or extinguishes the gas flow through the spinner channels, and thereby destabilises the plasma arc and can even cause the plasma arc to extinguish. This effect can be overcome, or at least reduced, by decreasing the compressible gas volume between the cathode and the anode. However, this cannot be achieved by simply decreasing the inner diameter of a conventional spinner (in order for the spinner to take up more space), as this will mean that the minimum distance between the cathode 12 and the spinner 20 (denoted by ‘t’ in Figure 9) will become smaller than the distance between cathode 12 and the anode 13 (denoted by ‘y’ ⁱⁿ Figure 9), thus resulting in an arc being formed between the cathode and the spinner, in addition to or instead of between the cathode and the anode, which will result in destruction of the spinner 20. The introduction of the slanted inner face 23 allows for the inner end of the lower part of the spinner to be in closer proximity to the arch chamber, without any part of the surface of the inner face 23 actually getting closer to the cathode 12. On the whole, the compressible gas volume is also decreased by the new spinner filling a larger space, but without any part thereof getting too close to the cathode 12.

Vortex stabilization is further aided by the slanted flow passage 24. The slanted flow path enables the outlet to be lower down in the spinner, while also by implication being closer to the arc chamber due to the tapered design of the spinner body. Furthermore, the gradient ensures that the gas exiting the spinner already has a vector component aligned with the longitudinal axis of the torch, which results in the more efficient formation of a vortex as part of the flow is already directed towards the anode and hence the anode channel. The aim is essentially to reduce direction changes by the working gas. If the gas is introduced at a 90-degree angle from the anode channel, the gas has to change direction by 90 degrees in order to enter the anode channel. There is a change of momentum and hence a certain resistance associated with the change in direction. If that angle is reduced, as is the case in the present invention, the resistance is reduced, and less vortex energy is lost and thereby increasing the energy efficiency of the torch.

The results achieved using this new spinner is discussed in more detail when discussing the experimental results below.

Another feature of the invention relates to the configuration of the flow passages through the spinner. The inventor has previously considered the use of arcuate or curved passages, but it was not clear to what extent such flow passages were indeed beneficial, and what the optimal design parameters of the flow passages were.

Figure 3 shows a cross-sectional top plan view of a spinner in accordance with another embodiment of the invention, where a flow path curvature (RF) is shown relative to the spinner diameter (OD) and spinner outer radius (Rs). In a preferred embodiment the ratio between the diameter and the flow path curvature (OD _S / RF) is at least 1 , but preferably larger, for example more 2 or higher. Although the use of curved flow passages is not new per se, the particular range has now been established with much more certainty.

The inventor also foresees that the concept of curved flow passages can successfully be implemented in the spinner shown in Figures 4 to 9, which will further improve the efficiency of the new spinner.

In all cases, the spinners were manufactured by an additive manufacturing process, which enables the introduction of the curved flow passages. It should furthermore be appreciated that the spinner material of construction could be metallic or non-metallic. In fact, the use of a non-metallic material will be beneficial, in that it will reduce the risk of a short circuit between the cathode and spinner.

Experimental Results

In the experimental verification of the inventive concept a non-transferred arc plasma torch was used to generate an arc, and hence a tail flame protruding from the otrch. Electrodes (anode and cathode) are used to generate the arc inside the torch by supplying electricity to the electrodes. The efficiency of the torch is therefore directly related to the energy loss inside the torch.

Vortex stabilisation is used as a method to shield the plasma torch anode wall against heat transfer from the arc to the wall. This is measured by means of thermocouples indicating heat loss to the cooling water. Three thermocouples were used during this experiment: one to measure the temperature of the inlet cooling water stream and two other thermocouples used to measure the anode and cathode outlet cooling water. The vortex stabilisation method makes use of a spinner which swirls a swirling gas (mainly argon or nitrogen) down the anode wall, creating a gas vortex. Additive manufacturing was used to manufacture newly designed titanium spinners where curved gas flow pathways are being introduced into the design of the spinner.

Dimensions of the newly designed Spinners

Eleven different Spinners were designed and tested and then compared to the standard spinner which is conventionally used. The variables that were tested included:

- Number of flow paths (4, 6 or 8);

Diameter of the flow paths (0.5mm, 1 mm or 2mm)

- Cross-sectional shape of the flow paths (circular or oval) Curvature radius (No curvature, 21 mm, 24mm, 48mm)

- Slope of flow paths (No slope, 8.3 degrees).

In addition, a spinner with a new cross-sectional profile (but without exceeding the minimum spacing constraints between the spinner and the electrodes) were also design and tested.

In total eleven new spinners were designed, and the experiment was repeated with a conventional or standard control spinner as well. The spinners that were tested are summarized in table format in table 1 below.

Table 1 : Spinner Configurations Tested

5 The reference to curvature radius (RF) in spinners 1 to 10 is shown in Figure 3.

Spinner 11 is shown in more detail in Figures 4, 5 and 6. Spinner 11 was designed to shorten the distance between the anode and the spinner itself, 0 and also to reduce the volume of the space between the inner periphery of the spinner and the anode channel. This was done by implementing a smaller inner radius at the bottom of the spinner with the top inner radius remaining unchanged, thus resulting in a spinner having an upper surface slanted at a 33° angle (P). In addition, the flow paths in this spinner was 5 slanted relative to a horizontal axis of the spinner, or relative to a base surface of the spinner, in order for the flow paths to slant towards the anode channel. In other word the inlets of the swirling gas and the outlets are not on the same height. The gradient of the flow paths is 8.3 ⁰ (<t>). 0 Apart from the dimensions given in Table 1 , which is mainly dimensions of the gas flow pathways, the set dimensions of the spinners are listed in Table 2. This includes the set inner and outer diameters of the spinners, which is demonstrated in Fig. 5, as well as the height. Dimension Distance [mm]

Inner Spinner Diameter 36

Outer Spinner Diameter 48

Height of Spinner 7.80

Table 2: Set dimensions of all spinners

The flow path curvature radius in relation to the set dimensions of the spinners can be expressed as follows:

Table 3: Relationship between flow path radius and spinner diameter

Experimental setup

The plasma torch configuration used for the spinners were the same as the configuration as is known in the art, as illustrated in Figure 1 , with Figure 6 showing the same assembly, but with spinner 11 installed in the same locations as the conventional arrangements.

The torch was mounted in front of a demarcated background which was then used to assess the size of the tail flame created by the plasma torch.

Water was used to cool the torch as the plasma arc that is created has a very high temperature. The most heat transfer occurs at the electrodes as the electrodes are the points at which the arc that forms the plasma is generated. Therefore, it was necessary to cool down the electrodes as efficiently as possible. Cooling water therefore runs through the torch to the anode and the cathode. The cooling water exits the torch at the anode and cathode and is then cooled and circulated back to the cooling water inlet of the torch.

Three thermocouples were used to indicate the efficiency of the individual spinners. The thermocouples were attached to the inlet of the cooling water, used to cool the torch, as well as the anode and cathode cooling water outlets respectively. The thermocouples serve as an indication of the heat loss to the wall of the anode and cathode. With this information the efficiency of the gas vortex, and therefore the spinner was determined.

Methodology

The experimentation process was started by first disassembling the torch. The gap between the cathode and the anode was then set to 2 mm by means of a vernier. This was an important step as the gap between the cathode and the spinner should not be smaller than the gap between the anode and the cathode. A smaller gap between the cathode and the spinner can cause the arc that is generated to attach itself to the spinner rather than the anode as the arc attaches itself to nearest contact point. This may result in either short circuiting of the torch or melting of the spinner if the experiments are carried out for a long enough time period.

The newly designed additive manufactured spinners were inserted individually into the disassembled torch, where two o-rings were used, at the top and the bottom of the spinner, to keep the spinner in place. The torch was then reassembled with the spinner inserted.

The efficiency of the additive manufactured spinners were tested by carrying out the experiments with variations in (i) the power supply to the torch at 100, 150, 170, 190, 200 and 220 Amperes respectively, as well as (ii) the gas flow rate of the swirling gas at 1.55, 2.07, 2.59 and 3.10 g/s respectively. The gas flow rates were determined by means of a calibration curve that gives different nitrogen flow values for the different flow settings. Effectively the nitrogen gas flow was varied at flow settings of 60, 80, 100 and 120. The equation of the trend line of this calibration curve is given in equation (1). The R ² value for the trend line is 0.9906.

F = 0.3463(FS)

(1) Where;

F = Nitrogen gas flow rate (l/min)

FS = Flow setting for nitrogen gas flow

The plasma torch was first started at 100 Amperes with argon as a swirling gas. Once the plasma arc started and the tail flame exited the torch, the argon swirling gas was gradually replaced with nitrogen and the first experiment was carried out. The first experiment was carried out starting at the lowest current and nitrogen gas flow rate. The current was held constant whilst the flow rate of the gas was varied. Once the highest gas flow rate was reached the current was raised to the next setting. The gas flow rate was then varied again from the lowest gas flow rate to the highest. This procedure was repeated until all the experiments were carried out for each spinner.

The torch was allowed to stabilise after each new operating condition that was introduced to the torch before taking the readings. After a few experiments it was observed that one minute was sufficient for the torch to stabilise. Each experiment was accordingly carried out for one minute before taking the necessary readings. For each different flow and current setting the temperature measurements were observed and registered. The voltage values on the power supply were also reported for each experiment.

For each different operating condition a photo was taken as a visual representation of the efficiency of the tail flame. This served as a secondary indication of the effectiveness of the torch.

Calculations followed to determine the power supplied to the torch as well as the heat loss to the wall of the anode and cathode. The heat loss was then subtracted from the power supplied resulting in the actual power attained by the torch. The actual power was then used in the determination of the efficiency of the spinner.

Calculations

The spinner efficiencies were calculated by first calculating the applied power to the torch using equation (2):

P = IV

(2) Where;

P = Power in kW l= Current in Amps V = Voltage in Volts

The heat loss of the torch was calculated by determining the heat loss to the cooling water used to cool the anode and the cathode. The equation for heat loss is reported in equation (3):

Where;

Q = Heat loss [kW] mass flow rate of water [kg/s]

Cp = Specific heat capacity of water [kJ/kg.(°C)]

T = temperature [°C] The heat capacity value was determined using equation (4). Where A, B, C, D and E are constants. In this equation temperature is in Kelvin. R is the symbol used to indicate the gas constant which has a value of 8.314 J/mol.K.

Values for B, C, D, and E is negligible for water and A has a value of 9.069. Therefore equation (4) reduces to:

Cp = 90697?

(5)

The Cp value obtained is then converted to the correct units yielding a value of - 4.188 kJ/kg. (°C).

As the Cp value for water is not dependent on temperature and therefore constant, the heat loss equation can be reduced to equation (6).

Q = mCp T ₂ - T

(6)

As heat loss occur to the anode and the cathode the heat loss should be determined for the anode as well as the cathode. The total heat loss can then be determined by using equation (7).

Q-total Q- anode Q- cathode

(7) The efficiencies of the spinners were then calculated by means of (8): Where spinner resembles the efficiency of each spinner.

Average efficiencies for the spinners were obtained by taking the various gas flow rates and power supply to the spinners into account for each spinner.

Results

The applied power to the torch for each spinner at the respective gas flow rates were calculated using equation (2) and can be observed in Figures 10, 11 , 12 and 13.

In Fig. 10 it can be observed that with an increase in current all power values also increase. It should be noted that although Spinner 3 is indicated in the legend there is no power values calculated for this spinner as the plasma torch could not start with this spinner. In Fig. 10 it can also be observed that Spinner 11 produces higher applied power throughout all applied current values whilst the standard spinner produces much lower power values compared to the other spinners. All other spinners produce power values more or less in the same range and an almost linear incline can be observed with an increase in the current.

In Fig. 11 it can also be observed that Spinner 11 yields higher power values. The standard spinner yield much lower power values and do not have a steady incline with increased current applied to the torch as observed with all other spinners. Spinner 3 and 10 are absent in the graph although they are indicated in the legend as the desired gas flow rates could not be reached with these spinners. Spinner 11 seems to approach a steady state power value from 190 amperes. All of the spinners indicated in the graph have power values that increase with an increase in current applied to the torch. All spinners except spinner 3, 7, 10, 11 and the standard spinner seem to yield an applied power in more or less the same range between 150 and 190 amperes.

In Fig. 12 a linear increase can be seen by all spinners excluding the standard spinner for 100-170 amperes. All spinners seem to behave more or less the same with an increase in current except for the standard spinner. Once again all power values increase with an increase in current. The standard spinner seem to approach a steady state value of about 14 kW at 200 to 220 amperes. Spinners 3 and 10 could not reach the desired gas flow rate and are therefore not shown on the graph.

In Fig. 13 all the spinners behave more or less the same in the same range. All spinners show an increase in power with an increase in current with the exception of the standard spinner at 200 amperes. The standard spinner yield much lower power values when compared with the other spinners. Spinner 3,7 and 10 are not shown in this graph as the desired gas flow rate could not be reached with these spinners.

An observation that can be made when comparing Fig. 10 to 13 with one another is that the applied power increase with an increase in the nitrogen gas flow rate. It was also observed that in all graphs the standard spinner yielded much lower power than all the other spinners. Lastly it was seen that in all these graphs the power increased with an increase in current. Spinner 11 proved to be particularly efficient for lower flow rates.

The heat loss against current graphs for all spinners at the respective gas flow rates are shown in Fig. 14 to 17. It can be seen in all these figures that all heat loss values increase with an increase in current and with an increase in the nitrogen gas flow rate. In Fig. 14 all spinners exhibit an increase in power with an increase in the applied current to the torch. Spinner 3 is not shown in the graph although it is mentioned in the legend, the reason being that the spinner could not induce a plasma arc inside the torch. Other observations that can be made from this figure are that all spinners behave more or less the same and operate in the same region with the standard spinner almost behaving as an outlier. Spinner 7 and 10 only report a power value for a current of 100 amperes as the desired higher current values could not be achieved with these spinners. Spinner 11 again performed best, in particular at lower currents.

In Fig. 15, Spinner 3 and Spinner 10 are not shown as the desired gas flow rates could not be reached with these two spinners. All other spinner yield power values that are more or less in the same range with the exception of the standard spinner which behaves as an outlier with less heat loss occurring at the same current values. Most of the spinners have a gradual increase in power with an increasing current.

In Fig. 16 all the spinners except for the standard spinner behave almost exactly the same with power values in the same range for the different current values. Spinner 3 and 10 are not shown in the graph as the desired gas flow and current values for these spinners could not be achieved. The standard spinner behaves as an outlier with much lower power values form 150-220 amperes. A gradual almost linear increase in the power is observed for all spinners, excluding the standard spinner, with an increase in current.

In Fig. 17 Spinner 3, 7 and 10 are absent although presented in the legend, as these spinners could not achieve the desired gas flow rates. All other spinners with the exception of the standard spinner behave similarly with a relatively close range to one another. The standard spinner behaves as an outlier as much smaller values for the heat loss is seen for this spinner. There is also a sudden decrease in the power applied at the standard spinner around 200 amperes.

The efficiencies of the different spinners were calculated using equation (8) as mentioned above. The average efficiencies for all the different spinners are shown in Fig. 18.

Observations

The curvature radius that proved to be the most efficient is 21 mm, with a highest efficiency at 51.31%. The second most efficient curvature was proven to be the curvature with a radius of 24mm. The efficiencies of the 24mm radius curvature range from 2.91-47.99% (Spinner 4-7). Lastly the curvature radius of 48mm yielded efficiencies ranging from 0.01-44.9 %. The order of the different spinner efficiencies for the different curvature radiuses is therefore: 21 mm > 24mm > 48mm. This shows that a larger curvature radius results in a decrease in the spinner efficiency, and that a curvature radius of below 25mm seems to be an optimal solution. The average efficiency of the standard spinner was found to be 32.65%. This shows that introducing a curvature, in particular a curvature radius below 50mm, to the spinner design will have a positive effect on the plasma torch efficiency as most of the spinners exhibit a higher efficiency than the standard spinner.

Expressed in terms of the spinner geometry, the spinner with a higher outer diameter / curvature ratio performed better than a spinner with a lower ratio. In this case, the spinners with an outer diameter / curvature ratio of 2 or higher performed better than the one with a ratio below 2.

The effect of the gas flow hole shape was tested by comparing two spinners that are exactly the same when considering curvature radius, total area of the spinner holes and number of holes (4 holes), the only difference between these two spinners is that one spinner has an elliptic hole shape whilst the other has the conventional circular shape. This results in a difference in the entry region. Spinner 4 (elliptic gas hole inlet) proved to be less efficient than spinner 5 with efficiencies of 45.18% and 47.99% respectively. Therefore a change in the shape of the hole from circular to elliptic rendered the spinner less efficient.

The number of holes introduced to the spinner designs was 4, 6 and 8. The size of the holes allocated to corresponding number of holes is 2mm, 1 mm and 0.5mm respectively. The spinners with 4 holes (2mm in size) obtained average efficiencies ranging between 32.65-51.31%, with the lowest efficiency being that of the standard spinner at 32.65%. Increasing the number of holes from 4 to 6 and decreasing the size of the holes from 2mm to 1 mm yielded average efficiencies between 44.9-48.36%.

As expected the tail flame becomes larger and longer for spinners with a higher efficiency as more of the energy is converted to the flame rather than the wall of the anode.

Of the spinners tested, the three spinners that are most suitable to replace the standard spinner are spinners 1 , 2 and 11 with efficiencies of 51.36, 48.36, and 49.83 respectively. Although entirely unique in its design, shortening the distance between the anode and the spinner, Spinner 11 showed to be the second most effective spinner overall, even though it did not yet include the curved flow passages of spinners 1 and 2. Spinner 11 provided to be particularly efficient at low flow rates, even without the curved flow passages.

The experiment showed that introducing a curvature has a positive effect on the torch efficiency and the torch can therefore be optimised by implementing a curvature to the flow path of the working gas in the spinner design. It was also shown that shortening the distance between the spinner outlets and the anode, thus reducing the volume of the arc chamber, and adding a gradient to the flow path (i.e. spinner 11) also resulted in a more efficient spinner, even with the flow paths in this spinner being straight. It follows that a combination of a spinner with the configuration of spinner 11 and a curved flow path will result in the optimal spinner configuration.

It will be appreciated that the above is only one embodiment of the invention and that there may be many variations without departing from the spirit and/or the scope of the invention. It is easily understood from the present application that the particular features of the present invention, as generally described and illustrated in the figures, can be arranged and designed according to a wide variety of different configurations. In this way, the description of the present invention and the related figures are not provided to limit the scope of the invention but simply represent selected embodiments.

The skilled person will understand that the technical characteristics of a given embodiment can in fact be combined with characteristics of another embodiment, unless otherwise expressed or it is evident that these characteristics are incompatible. Also, the technical characteristics described in a given embodiment can be isolated from the other characteristics of this embodiment unless otherwise expressed.