Visualization of arc and plasma flow patterns for advanced material ...

Visualization of arc and plasma flow patterns for advanced material processing

O. P. Solonenko, H. Nishiyama, A. V. Smirnov, H. Takana & J. Jang

Journal of Visualization ISSN 1343-8875 J Vis DOI 10.1007/s12650-014-0221-6

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Author's personal copy J Vis DOI 10.1007/s12650-014-0221-6

REVI EW PAPE R

O. P. Solonenko • H. Nishiyama • A. V. Smirnov H. Takana • J. Jang

•

Visualization of arc and plasma flow patterns for advanced material processing

Received: 17 March 2014 / Revised: 23 June 2014 / Accepted: 4 July 2014 Ó The Visualization Society of Japan 2014

Abstract Results are presented for physical experiments that illustrate the possibilities and efficiency of visualization for studying the effect of operating conditions (backward-facing stepped forming nozzle, exit diameter of anode, mass flow, and composition of working gas) on plasma flows at low Reynolds numbers for advanced coating and powder processing. In particular, the shadow method, based on adaptive visualization transparency, is used for imaging electric arc and plasma jet flow patterns for different operating conditions. Because of visualization, the optimal geometrical characteristics of the backward-facing stepped forming nozzle, mass flow rate of the working gas, and its composition were found. These provide: (1) the absence of micro-shunting of the arc inside the backward-facing stepped nozzle for a transfer arc and twin arcs; and (2) compared to transient and turbulent jets, a higher density for the heat flux from a quasi-laminar flow to the surface of a flat substrate and the powder material to be treated, for nontransfer arc DC (direct current) torches and DC–RF (direct current and radio frequency) hybrid plasma flow system. Keywords Visualization Transfer arc torch Twin torch Nontransfer DC torch DC–RF hybrid plasma flow system Plasma jet Reynolds number Optimization

1 Introduction Free flow of thermal plasma (open electric arcs, including heteropolar current jets interacting in the open atmosphere, and plasma jets with multicomponent gases) flowing at low Reynolds numbers (Re ¼ 4Gf0 =pDn lf0 , where Gf0 is the mass flow rate of the working gas, Dn is the inner diameter of the outlet electrode, and lf0 is the mean-mass dynamic viscosity of the working gas at the exit cross-section of the outlet electrode) are of much interest for various technological applications such as production of microspheres and hollow powders (Solonenko et al. 2007, 2011; Jang et al. 2011; Takana et al. 2011), plasma spraying of coatings and their subsequent post-treatment by highly concentrated energy fluxes (Solonenko 1995; Solonenko et al. 2000), and surfacing (Yamamoto et al. 2005; Solonenko et al. 2005), including a reduction in the level of oxides when welding metals (Hamatani et al. 2012). Plasma torches with a transfer arc are effective tools for surface treatment (cutting, welding, surfacing, surface modification, etc.) (Choi and Gauvin 1982; Hsu and Pfender 1984; Gauvin 1989; Tanaka et al. 2003; Bini et al. 2007), because the density of the heat flux in the arc spot may become 108–109 W/m2 or more. Depending on the functionality, the requirements for such plasma torches are specified in different ways. For O. P. Solonenko (&) A. V. Smirnov Khristianovich Institute of Theoretical and Applied Mechanics SB RAS, Novosibirsk, Russia E-mail: [email protected] H. Nishiyama H. Takana J. Jang Institute of Fluid Science, Tohoku University, Sendai 980-8577, Japan

Author's personal copy O. P. Solonenko et al.

instance, cutting metals and alloys with the transfer arc requires continuous evacuation of material from the resulting molten pool. This could be provided only with a sufficiently high dynamic pressure in the impinging plasma flow. On the contrary, for welding, surfacing, or surface modification, one should guarantee a low level of dynamic pressure at the arc spot, providing a high level of heat flux density to prevent destruction of a fused surface layer. Moreover, Yamamoto et al. (2005) demonstrated that the scanning amplitude of the surface by the anode spot of an electric arc was increased when an alternating external magnetic field was applied (Fig. 1). It is necessary to reduce the velocity of a coaxial plasma flow, while keeping a stable arc, i.e., to reduce the Reynolds number. However, the decrease in the velocity of the plasma flow in the nozzle of a cathode assembly, at rather high currents, leads to intensive micro-shunting of the electric arc to the output edge of the nozzle, which causes a high level of erosion. Another example of a plasma torch with a transfer arc is a twin torch (Zheenbaev and Engelsht 1983; Williams et al. 1995; Kittaka et al. 2005; Ando et al. 2009; Iwao et al. 2009; Tang et al. 2010), which now has a wide range of practical applications, in particular, in spectral analysis, as a source of spectral excitation of various materials; in technologies for silicon solar cell production; protective coating spraying; and powder materials processing. This torch also has a problem: it reduces the working gas flow rate (Reynolds number) into the cathode and anode current jets, while simultaneously preventing the micro-shunting of an arc to the outlet edges of the cathode and anode assemblies. It is necessary to laminarize the plasma flows (Fauchais and Vardelle 2000) in the cathode and anode current jets, simplifying the input of dispersed material, which allows a significant increase in the efficiency of powder processing. Finally, a similar problem occurs in plasma jets flowing from nontransfer single-arc torches, which are used for surface treatment, fusion of coatings, refractory powder processing, etc. Outflow of plasma jets can be realized in quasi-laminar, transient, or turbulent modes. Outflowing jets in quasi-laminar and transient regimes are characterized by small expansion angles. As a result, considerable elongation of the highenthalpy zone in the flow (Fig. 2c) and a rather low level of acoustic noise make these jets very promising candidates for use in the techniques of plasma spraying, powder material processing, and surface layer treatment, including coating post-treatment (Solonenko 1995). They are also attractive for stabilizing the plasma flow and increasing the powder processing efficiency in hybrid direct current–radio frequency (DC– RF) plasma flow systems (Jang et al. 2011; Takana et al. 2011). At the same time, plasma torches with self-installing arc length and intensive aerodynamic twisting of the working gas, generating turbulent plasma flows, became widely used in plasma spraying and powder material processing. These plasma torches have the following disadvantages (Solonenko 1995): 1. The low-frequency pulsations of the arc voltage, i.e., the thermal power of the outflowing plasma jet, due to arc length variation and movement of the anode spot attachment, conditioned by magnetic gas dynamic interaction of a radial arc arm with a cold wall boundary layer (Zhukov et al. 1985; Duan and Heberlein 2002; Fauchais 2004). This results in considerable inhomogeneity in the powder particle treatment (Fauchais 2004; Zhukov and Solonenko 1990; Outcalt et al. 2006; Leblanc and Moreau 2000; Goutier et al. 2008).

Fig. 1 Transfer arcs: without external magnetic field (a), oscillating arc with swirling gas flow (b), and oscillating arc without swirling gas flow (c). The plasma gas flow rate was of 15 Sl/min and the arc current was 130 A. In the cases (b) and (c), the wave form of the applied external magnetic field is rectangular, its amplitude, Bo, was 3 9 10-3 T and the oscillating frequency was 60 Hz

Author's personal copy Visualization of arc and plasma flow patterns

Fig. 2 Comparison of quasi-laminar and turbulent plasma jets: a argon jet generated by the commercial Praxair spraying torch 2086A, b argon plasma jet generated by an in-house turbulent torch, and c argon–nitrogen jet generated by a cascade torch developed by Hamatani et al. 2012

2. Absence of axial symmetry of the velocity and temperature fields in the outflowing plasma jet, resulting in nonuniform heating and acceleration of particles in jet cross-sections; the latter causes the absence of axial symmetry in the particles’ parameter distributions within a spraying spot. Discussions regarding other disadvantages of conventional DC plasma torches can be found elsewhere (Fauchais 2004; Zhukov and Solonenko 1990; Leblanc and Moreau 2000; Outcalt et al. 2006; Goutier et al. 2008). One of the methods for overcoming these problems is the generation of high-enthalpy, quasi-laminar, lengthy flow without aerodynamic swirling at the outlet of the plasma torch; therefore, the admixture of cold gas in a dust-laden plasma jet and its turbulization is significantly reduced. Previous studies (Pan et al. 2002; Cheng et al. 2006; Pan et al. 2006; Wang et al. 2007) have discussed long argon outflowing plasma jets from nozzles with rather small internal diameters, *5 mm. At the same time, further development of the techniques for plasma spraying, synthesis, and processing of powder materials as well as surface treatment require the design of plasma torches working with various plasma gases and generating lengthy, quasi-laminar jets with significantly larger transverse diameters, *20 mm (Solonenko et al. 2011; Hamatani et al. 2012). However, as shown by Pfender (1994) and Lemanov et al. (2013), when the laminar–turbulent transition is used in free isothermal outflowing jets with a wide range of Reynolds numbers, the location of the transition to the turbulent mode is displaced with respect to the nozzle with the increase in its internal diameter. During the last decades a great attention has been paid to the non-transferred arc torches, with the averaged arc length in non-changeable, but longer than that of the self-installing length of arc (Zhukov et al. 1985; Bernecki et al. 1988; Zhukov 1994; Solonenko et al. 2000, 2001; Hawley et al. 2010; Chen et al. 2014). The increase in the average arc length is achieved by neutral interelectrode insert placed between the anode and the cathode. This type of torch will be referred below as cascade torch. Below is a brief review of the research and development, carried out by the authors of the present paper, to increase the length and stability of the outflowing plasma from various types of DC torches at power and low Reynolds numbers. As is shown, this can be achieved by using the backward-facing step forming nozzles, as well as increasing the inner diameter of the anode and/or mixing the main working gas with an additional gas with more enthalpy (for instance, nitrogen or helium). An effective method for studying and optimizing free plasma flows is visualization, as proved by both isothermal and nonisothermal subsonic jet flows (Kozlov et al. 2013a, b). The objective of the present work is to visualize and explain the different kinds of plasma flow patterns to increase the process efficiency for advanced material processing by increasing the length and stability of the plasma flow. Transfer and non-transfer arcs with different torch geometries are explained for advanced coating and powder processes in Sect. 2. The DC–RF hybrid plasma flow systems, combining a DC plasma jet with RF-ICP (radio frequency inductively coupled) plasma flow is discussed for powder treatment in Sect. 3.


2 Transfer and nontransfer arcs with different torch geometries As was shown for the first time by Solonenko (1995) and Kuz’min et al. (1995), an efficient method for increasing the stability of outflowing free plasma jets with small Reynolds numbers is to use forming nozzles with an internal geometry that has the form of a backward-facing step (Fig. 3). Investigation were carried out (Solonenko 1995; Kuz’min et al. 1995) on the quasi-laminar nitrogen outflowing plasma jets with low Reynolds numbers from a cascade plasma torch, with a nominal power of 50 kW. Keeping other parameters constant, jet flows for three geometries of output electrodes (anodes) were studied: cylinder, diffuser, and backward-facing step. The maximum length of the quasi-laminar jet and its stable outflow at Dn,i = 8 mm occurred for a backward-facing step with ratio L/Dr B 5. The qualitative sketch of the flow pattern in the output area of the nozzle is presented in Fig. 4c. In this regard, we will qualitatively analyze the effect of characteristics of the backward-facing stepped nozzle on the flow within it. For a rather long forming nozzle, L/Dr [ 5, the dynamic boundary layer of the jet reattaches to the wall inside the step, therefore, an internal closed vortex is formed (Fig. 4a). Behind the reattachment point, there is a flow separation leading to additional turbulization of the flow in the step. As L/Dr gradually decreases, the reattachment point of the flow boundary layer to the wall moves towards the output section of the stepped nozzle. At a certain value of L/Dr, gas from the surroundings begins to flow into the stepped nozzle, forming the flow qualitatively presented in Fig. 4b; therefore, as can be seen, the internal closed vortex is still present. The flow pattern in the backward-facing stepped nozzle undergoes a fundamental change at L/ Dr \ 5 (Fig. 4c). In this case, a considerable mass of gas flows into the internal area of the stepped nozzle from the surroundings, leading to the destruction of the internal closed vortex and providing additional stabilization to the outflowing jet. In our opinion, this is caused by the suppression of transversal pulsations of velocity, increase in the spatial scale of its longitudinal pulsations, and reduction of the turbulence level in the mixture layer at the jet’s outflow into the surroundings. By measurements of the radial temperature distributions in cross-sections of the plasma jets, while keeping other parameters constant, it was shown that

Fig. 3 Schematic diagram of the backward-facing step forming nozzle used in the experiments. The following definitions are used: Dn,i is the inlet diameter of the forming nozzle, Dn,e is the outlet diameter of the nozzle, Dr = (Dn,e - Dn,i)/2, L is the length of the forming nozzle step, and M is the total length of the nozzle

Fig. 4 Behavior of plasma flow at different values of L/Dr, characterizing the backward-facing stepped nozzle: a L/Dr [ 5, b L/Dr ^ 5, and c L/Dr \ 5


most enthalpy loss for the jet occurred for the diffuser output electrode. The maximum axial temperature was measured for the stepped electrode. The cylindrical output electrode was in the intermediate position. A similar problem, i.e., stabilization of the plasma flow at a small flow rate of working gas (small Reynolds numbers), arises when designing plasma torches with long transfer arcs, which are used for surface treatment, including submelting of thin surface layers. As shown by Solonenko et al. (2005), application of forming nozzles with the form of a backward-facing step is also a very promising technique. In this study, use of a backward-facing step, with L/Dr * 2.5 and Dn,i = 6 mm, allowed a wide range of operating ˚ ): (1) to provide stable arcing at a small flow rate of argon, 0.1 g/s; (2) to exclude the currents (150–400 A micro-shunting of the arc to an output edge of the cathode assembly of the plasma torch, which provided a long operating lifetime. At the same time, when using the cylindrical nozzle or a nozzle with L/Dr [ 3, it was impossible to prevent this micro-shunting. When designing the twin plasma torch, we used the same principle for stabilization of the current jets. Here, for example, it is necessary to significantly increase the residence time of the particles in the zone of active heat treatment for spheroidization of metal oxide powders with low thermal conductivity. It is impossible to provide at high flow rate of the plasma-forming gas. Besides, the overall length of the cathode and anode jets, providing current transfer during the operation of the twin torch, is approximately 150 mm, which shows increased requirements for open arc burning at low Reynolds numbers. We will briefly explain some results obtained by optimization of plasma torches. 2.1 Transfer arc The transfer arc was ignited at a standoff distance, L = 25 mm, between the nozzle exit section and the water-cooled anode (Fig. 5). Having ignited the transfer arc, one could increase the standoff distance to L = 100 mm. During plasma torch operation, the heat flux in the water-cooled anode and the arc voltage were recorded, to investigate the spectra of the voltage fluctuations. The latter can indirectly characterize arc root stability (arc length). In each series of experiments, with fixed values of Dn,i and Dn,e, the step length L was varied in the following sequence: L/Dr = 6, 5, 4, 3, and 2. It was found that the most stable arcing without micro-shunting of the arc to the exit edge of the nozzle took place at L/Dr * 2.5, with an argon flow rate of G = 0.3 g/s for both inlet diameters, Dn,i = 4 and 6 mm, of the backward-facing stepped forming nozzle. Typical images of the transfer arc operation at these conditions are shown in Fig. 6. A shadow method, based on adaptive visualization transparency (AVT), was used for arc imaging (Shevchenko et al. 2006).

Fig. 5 Schematic diagram of the transfer-arc plasma torch with water-cooled anode


Fig. 6 Three consecutive images of the 100 mm transfer arc operating at: an argon flow rate of 0.3 g/s, I = 200 A, U = 100 V, Dn,i = 6 mm, Dn,e = 11 mm, and L/Dr * 2.5

Fig. 7 Photos of the cathode and forming nozzle after a 10-h lifetime test

In other cases, including the cylindrical nozzle, an intensive erosion of the nozzle exit edge occurred. Additional experiments have shown that the ratio L/Dr * 2.5 was also the most appropriate for argon flow rates of 0.1 and 0.2 g/s. The essential difference between the optimal value of ratios L/Dr * 2.5 and L/Dr * 5 for the typical stabilization of free quasi-laminar jets by the backward-facing step (Solonenko 1995) can be explained as follows. It is considered that an intensive Joule heating release took place within the nozzle, which would increase the thickness of the thermal and dynamic boundary layers inside the stepped channel. Therefore, the relative step length has to be shorter to avoid a reattachment of the dynamic boundary layer of the plasma flow to the wall within the step. After a 10-h lifetime test, no noticeable erosion of the forming nozzle was observed, and the erosion of tungsten cathode was 3 9 10-13 kg/C. Such a value of cathode erosion would provide continuous service (up to a full ablation of a 1.5 9 10-3-m-long tungsten rod of 4 9 10-3 m diameter) for *800 h. Figure 7 shows photos of the cathode and backward-facing stepped nozzle after the 10-h lifetime test. 2.2 Twin arcs The suitability of using this type of plasma torch in practice (Fig. 8) is because of: (1) higher temperature in the current-conductive cathode and anode jets in comparison with the free jet; (2) increased residence time of powder particles, and treatment at temperatures significantly exceeding their melting temperature; (3)


Fig. 8 Diagram of the twin torch coupled with a powder injector (a), general view of plasma flow generated by twin torch (b), and general view of plasma flow with axial powder injection (c)

Fig. 9 Schematic diagram of the cascade torch

more efficient transformation of electric energy into the thermal energy of the plasma flow; and (4) realization of the possibility of distributed feeding of processed powder material, thereby, increasing the loading of the plasma flow with powder. The most stable arcing without micro-shunting of the arcs to their exit electrode edges took place at: Dr = 2.5 mm, i.e., at. L/Dr * 2.4; an argon–nitrogen flow rate of G = 0.5 g/s (GN2 = 0.3 g/s, G Ar = 0.2 g/s); and for the inlet and outlet diameters, Dn,i = 8 mm and Dn,e = 13 mm, for the backwardfacing stepped forming nozzles used for the cathode and anode of the twin torch. Figure 8b, c shows typical views of the open arc, operating at G = 0.5 g/s and I = 250 A. 2.3 Cascade nontransfer arc A major contribution to research and development of cascade plasma torches was made by Zhukov and colleagues (Zhukov et al. 1985; Zhukov 1994). Results of the investigations and developments of cascade torches applied to plasma spraying, surface treatment, and powder processing were published by Solonenko (1995) and Solonenko et al. (2000). Insulated inserts are placed between the cathode and anode in the cascade torch (Fig. 9) to extend and stabilize the arc over a wide range of gas flow rates. The main advantages for the cascade approach are: (1) a significantly lower level of low-frequency power pulsations; (2) very low or no voltage drifting; (3) a relatively low arc current results in a low level of electrode erosion, hence, sprayed coatings and treated powder would not be contaminated by products of erosion (Cu, W, etc.); and (4) the ‘‘cascade’’ approach allows changing plasma gas flow rates (i.e., Reynolds number) over a relatively wide range. Therefore, effective plasma temperature, specific enthalpy, and velocity could be also changed within a wide range, increasing the operational envelope. All of the advantages listed above result in consistent coating and power products quality and deposition efficiency.


Fig. 10 Quasi-laminar argon–nitrogen outflowing jet from a 50-kW cascade torch equipped with optimized backward-facing stepped forming nozzle (a), the same plasma jet with admixture of hydrogen, 3 % (b) and 6 % (c)

The problem of increasing the stability of outflowing plasma jets from a 50-kW cascade plasma torch at low Reynolds number is considered separately. As noted above, this problem was solved by using the backward-facing stepped forming nozzle (Dn,i = 10 mm, Dn,e = 14 mm) at a ratio of L/Dr B 5. Figure 10 shows the results of the shadow diagnostics of the outflowing plasma jets for three operating modes of the cascade torch with an arc current of I = 200 A, using three different basic mixtures as plasmaforming gas: (1) argon–nitrogen with a mass flow of G = 0.68 g/s, GAr = 0.25 g/s, and GN2 = 0.43 g/s (basic mixture); (2) previous mixture with a 3 % mass admixture of hydrogen; and (3) a basic mixture with a 6 % mass admixture of hydrogen. Argon was additionally fed at a flow rate of GAr = 0.07 g/s to the anode inlet for its protection and to provide an equally possible arc attachment. Figure 10a shows a fragment of the argon–nitrogen jet, in which the quasi-laminar region was * 500 mm long. At the same time, Fig. 10b, c shows that the cross-section of the laminar–turbulent transition was displaced toward the outlet section of nozzle with the admixture of hydrogen and the increase in its flow rate. Long quasi-laminar jets may be generated when hydrogen with a relative mass flow no more than 0.005 % is used as one of the components of working gas. Diagnostics were carried out using the shadow device (Shevchenko et al. 2006) with adaptive visualizing transparency for recording phase heterogeneities. The diameter of the working space of the device is 120 mm and the frame exposure is 3 ls. Each of the pictures presented in Fig. 10 was prepared by overlapping four sequential frames. Let us consider the examples, illustrating the technological possibilities of the quasi-laminar nitrogen plasma jets, generated by using the 50-kW cascade torch. First of them (Solonenko et al. 2000) is concerned with fusing the coatings with thickness of 0.3–0.9 mm deposited by a self-fluxing Ni–Cr–B–Si–C powder using the turbulent plasma. The sprayed coatings have a heterogeneous structure characterized by a rather high level of porosity and relatively low values of adhesion and cohesion. Porosity of the sprayed coatings is 12–16 % (see Fig. 11a) when using the turbulent plasma jet. The size of single pores exceeds 100 lm. Measurements of the adhesion strength of the coatings by using the turbulent plasma jet showed that its value does not exceed 40 MPa. An efficient technique for improving the coatings properties is their partial or complete fusion without evaporation of a sprayed material and overheating a substrate, which can be provided by applying the concentrated energy fluxes of a relatively low density (q * 107–109 W/m2). Figures 11 and 12 confirm the possibility of effective solution of this problem using the developed plasma torch.


Fig. 11 Cross-section of the self-fluxing Ni–Cr–B–Si–C coating sprayed on a low-carbon steel substrate by the turbulent plasma jet (a), and coating after fusing by quasi-laminar nitrogen plasma jet ( z = 8) (b), using the same cascade 50-kW torch

Fig. 12 Heat flux density from the nitrogen plasma jet to the flat substrate vs. nitrogen mass flow and Reynolds number (1 z = 6, 2 - z = 8, 3 - z = 10) (a); the photos of plasma jets impinging onto a flat substrate ( z = 8) at different regimes of plasma jet outflow: b quasi-laminar (Re = 580), c turbulent (Re = 820)

For nitrogen plasma, the transitional Reynolds number (Re = 580) was determined by experiments on the basis of measurements of the integral heat fluxes into a flat surface, for different working gas mass flow and distances between the nozzle exit and the surface (Fig. 12a). Figure 12b, c shows the photographs of jets leaving the nozzle of the plasma torch at Re = 580 and 820 (the dimensionless distance z ¼ z=Dn;e from the outlet of the nozzle to the obstacle was equal to 8). As a result of melting of the coatings, almost monolithic structure is formed, coagulation of pores takes place and their size decreases (see Fig. 11b). Absolute porosity decreases to 1–3 %. As a result, the coatings fusion by quasi-laminar plasma jet, the value of the adhesion strength increases by an order and is not less than 400 MPa. The second example shows the possibility of the quasi-laminar jets, outflowing from the cascade plasma torches for producing the hollow microspheres (HOSP powders) of metal oxides. HOSP powders of metals, metal alloys and oxide ceramics are of great interest for powder metallurgy, material science and thermal spraying. HOSP powders can be produced by different methods (Solonenko et al. 2011). Great interest has been recently expressed in studying the characteristics of thermal barrier coatings sprayed from hollow yttria-stabilized zirconia (YSZ) powders (Karoly and Szepvolgyi 2003; Bica 2005; Han et al. 2006; Wu et al. 2007, etc.). Figure 13 shows the SEM images of the initial and plasma processed spray-dried YSZ powders. Particles were injected into the cascade plasma torch exit region and underwent spheroidization in the quasi-laminar jet. Working gas—air ? CH4; electric power—75 kW, four-point powder injection; productivity of process—20 kg/h. Another example illustrates the hollow microspherical a-Al2O3 powder (Fig. 14) synthesized in the aftermath of plasma treatment of aluminum hydroxide (AlOOH) powder. The powder with particles size from 40 to 120 lm, containing from 10 to 15 wt% of chemically bound water, was processed in the quasilaminar air plasma jet outflowing from 50-kW cascade torch.


Fig. 13 SEM photos of the initial spray-dried YSZ powder (a), and HOSP powder produced by using the cascade plasma torch: b, c particles’ general view and their cross cut, accordingly

Fig. 14 SEM photos of the hollow a-Al2O3 microspheres (general view) (a), and broken single hollow a-Al2O3 microsphere illustrating the shell’s thickness (b)

3 DC–RF hybrid plasma flow with different nozzles and gas mixing A DC–RF hybrid plasma flow system is a combination of a DC plasma jet with an RF-ICP flow. This is expected to be one of the next-generation thermal plasma flow systems, because it has weak backflow, large high-temperature volume, long plasma flow, and high enthalpy (Yoshida et al. 1983; Kim et al. 2005; Seo et al. 2008; Frolov et al. 2011). In the DC–RF hybrid plasma flow system, there are many operating parameters that can affect the process efficiency because of the complex interactions between the RF-ICP discharge, the DC plasma jet, and in-flight particles (Proulx et al. 1987; Kim et al. 2005; McKelliget and ElKaddah 1990). Therefore, the optimization of operating conditions is essential for investigating the correlations among the plasma flow characteristics, in-flight particle behavior, and process particle properties (Kawajiri et al. 2003, 2005; Kawajiri and Nishiyama 2006; Ye et al. 2004, 2007). An experimental study (Nishiyama et al. 2009) of a DC–RF hybrid plasma flow system (Fig. 15), operated at a constant low operating power (PDC = 1.1 kW, PRF = 6.6 kW), was conducted for optimizing the flow conditions for in-flight alumina powder processing and to control the particle size and morphology. Pure argon was used as the main gas in these experiments. The DC–RF hybrid plasma flow system was successfully optimized with a 3-mm inner diameter of the DC torch nozzle. It was found that a pressure of 150 Torr in the DC–RF discharge chamber provided a stable DC–RF argon plasma flow. However, the process efficiency, such as the spheroidization ratio of an alumina powder with an initial mean size of *4 lm, remained low because of nonoptimal operating parameters of the DC plasma torch. An increase in plasma enthalpy is necessary to improve the process efficiency for this system, even at a low input power. In this regard, supplementary investigations were carried out for the optimization of the DC–RF hybrid plasma flow system using the visualization method for further improving the spheroidization process. To understand the DC–RF hybrid plasma flow behavior, sequential images were recorded using a high-speed camera (FASTCAM SA-5, Photron Inc.), providing registration with 1,000 frames per second. These images were used for studying the effect of the nozzle inner diameter of the DC torch and the pressure in the RF discharge chamber, keeping the other parameters constant. Statistical image analysis was conducted on these monochrome images, to estimate average emission intensity and emission fluctuation of the plasma flow,


Fig. 15 Schematic diagram of a hybrid DC–RF plasma flow system

Fig. 16 Sequential images of plasma flow behavior with DC nozzle diameters of 3 (a) and 4 (b) mm

corresponding to the averaged and standard deviation images, respectively. The averaged image, Iij , was P defined as Iij ¼ N1 Nf¼1 ðIij Þf , where Iij is the emission intensity at each image pixel; and N and f are the total number of frames and frame number, respectively. The standard deviation image, I~ij , was defined as qffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffi P I~ij ¼ 1 Nf¼1 ðIij Iij Þ2f . N

3.1 Effect of the nozzle diameter of DC torch on DC–RF hybrid plasma flow The sequential images (Fig. 16) show the DC–RF hybrid plasma flow behavior depending on the DC nozzle diameters of 3 and 4 mm. The plasma flows were generated in pure argon at a pressure of 150 Torr. The length of the DC plasma jet with a 3-mm DC nozzle diameter is shorter than that with 4 mm, despite its increased flow momentum. The RF plasma flow was pulled to the upper region of the RF coils by the DC plasma jet because of the strong recirculation flow around the DC plasma jet. On the other hand, by enlarging the DC nozzle diameter to 4 mm, the DC plasma jet was elongated downstream and the RF plasma flow was replaced below the RF coils.


Fig. 17 Results of statistical image analysis of the sequential photos of plasma flows, corresponding to: averaged (a) and (b), and standard deviation (c) and (d) images. Figures a and c correspond to a nozzle diameter of 3 mm, and b and d to 4 mm

Fig. 18 Results of statistical image analysis of the sequential photos of plasma flows, corresponding to: averaged (a) and standard deviation (b) images. The nozzle diameter was 4 mm

Fig. 19 Instant photographs of DC–RF hybrid plasma flow for: pure argon and 3-mm nozzle diameter (a), and 4 % helium admixture and 4-mm nozzle diameter (b)

In the statistical image analysis, the averaged and standard deviation images (Fig. 17) clearly show the effect of the DC nozzle diameter on the behavior of the DC–RF hybrid plasma flow. In the averaged image, compared to the DC plasma jet with the 3-mm DC nozzle diameter, the DC plasma jet with 4-mm DC nozzle diameter is wider, although the emission intensity and emission area of RF plasma flow are almost the same. The entire emission area and strong emission intensity region of the RF plasma flow in the RF coils are shifted down. In the standard deviation images, compared to the DC plasma jet with 3-mm DC nozzle diameter, the emission fluctuation of the DC plasma jet with 4-mm DC nozzle diameter is slightly reduced, and the RF plasma flow is comparably reduced. This may be caused by the DC plasma jet with


Fig. 20 Effect of helium gas mixing and exit nozzle diameter on alumina spheroidization ratio

3-mm DC nozzle diameter constraining the RF plasma flow to the upper region of RF coils, which increases the emission fluctuation of the RF plasma flow in the RF coils. 3.2 Effect of chamber pressure on DC–RF hybrid plasma flow When a small amount of nitrogen is added into the argon working gas of the DC plasma jet, the emission area of the DC plasma shrinks and its emission intensity becomes weaker with increasing the chamber pressure. This is caused by the thermal energy of the DC plasma jet being consumed by heating and dissociating the injected nitrogen gas. While the emission intensity of the DC plasma jet is relatively weak, that of the RF plasma flow becomes stronger with the increase in chamber pressure (Fig. 18). Therefore, the analysis of emission fluctuation of the RF plasma flow failed in the vicinity of the RF coils. When increasing the chamber pressure, the emission fluctuation of the RF plasma flow increases below the RF coils. By analogy, the emission fluctuation of the RF plasma flow is predicted to also increase in the vicinity of the entire RF coil. In a recent study (Takana et al. 2011), a small amount of helium gas was added to the argon main gas flow to increase the plasma enthalpy (Fig. 19). Furthermore, the effect of a small increase in the DC nozzle diameter on the increase of in-flight particle heating is experimentally investigated in detail. As a result, the in-flight alumina powder spheroidization process in the low-power DC–RF hybrid plasma flow system (Fig. 20) was improved by optimizing the helium gas mixture percentage and adjusting the plasma enthalpy, in-flight powder velocity, temperature for different DC nozzle diameters, and collected powder location. The spheroidization ratio of alumina powder collected at z = 120 mm is at a maximum, 94.5 % (±0.62), with experimental conditions of 4 % helium admixture and 4 mm DC nozzle diameter. 4 Conclusions A brief review of physical experiments, recently conducted by the authors, illustrates the possibilities and efficiency of optical, in particular, shadow visualization methods for studying the influence of operating conditions on the plasma flow formation in transferred arcs and plasma jets at low Reynolds numbers. These operation conditions include the backward-facing stepped forming nozzle, the inner diameter of anode, as well as the mass flow and composition of the working gas. Because of the experiments, the optimum geometry of the backward-facing stepped forming nozzle and optimum mass flow of working gases and their composition have been found to provide: (1) a long service life for the torches owing to the absence of micro-shunting of the arc inside the backward-facing stepped nozzle for the transfer arc and twin torches; and (2) compared to transient and turbulent jets, a higher density heat flux from the quasi-laminar jets to the surfaces of the flat substrate and powder particles to be treated, for nontransfer arc DC torches and DC–RF hybrid plasma flow systems.


Acknowledgments This work was supported in part within the framework of Interdisciplinary Integration Projects No. 2 and 98, of the Siberian Branch of the Russian Academy of Sciences for 2012–2014. Part of the work was also carried out under International Collaborative Research Project J13060, J14012 at the Institute of Fluid Science, Tohoku University, Japan.

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