Shrouding of Thermal Plasma Jets Generated by Gas-Water Torch

Transcription

1 WDS'05 Proceedings of Contributed Papers, Part II, , ISBN MATFYZPRESS Shrouding of Thermal Plasma Jets Generated by Gas-Water Torch T. Kavka, M. Hrabovsky, O. Chumak, and V. Kopecky Institute of Plasma Physics, ASCR, Za Slovankou 3, Prague, Czech Republic Abstract. The ambient air is entrained intensively into atmospheric pressure thermal plasma jets. A system for shielding plasma jet has been used to improve jet characteristics and reduce oxygen content. Different gases have been applied as a shroud gas argon, acetylene, methane, mixtures of acetylene-nitrogen and hydrogen-nitrogen. All shroud gases used in experiments affected plasma jet making the profiles of the jet characteristics flattened with higher temperatures and velocities and reduced amount of oxygen. Introduction Plasma spraying of metals is usually accompanied by oxidation of sprayed material, which affects resulting coating. The plasma spraying is a two stage process. In the first stage particles are introduced into the plasma jet, where they are heated, melted and accelerated toward the substrate, where the second stage, i.e. deposition, cooling down and solidification takes place. Processes occurring in both stages have an influence on resulting coating properties and one of the serious problems is oxidation mentioned above. Although the second stage, during which the deposited solidified material interacts with ambient atmosphere consisted mostly of the air, is much longer than the first stage, the in-flight oxidation in the high temperature environment plays critical role in the resulting oxide content in the sprayed coatings. In-flight oxidation begins due to gas-solid interaction during heating of particles in the plasma jet containing oxygen from air entrained into the jet. Hence, reduction of the oxygen content in the plasma jet is a question of great importance. There are several methods how to avoid or at least reduce oxygen content in plasma jets. The simplest and the most economical method is shroud nozzle attached to the exit of the plasma torch. The shroud nozzle can provide both a solid shield and a gas shroud. The solid shield acts as a barrier preventing the air entrainment into the jet. In a gas shroud the solid shield is replaced by gas formed around the jet preventing mixing of the plasma with air [Gawne et al., 2002; Thomson et al., 2001, Kang et al., 1999]. Usually gases contained in plasma forming medium (e.g. Ar, N 2 ) are used as a shroud gas and they are supplied close to the exit nozzle parallel to the main flow. In the present paper the effect of the reactive gases as acetylene, methane and a mixture of oxygen with hydrogen is studied together with the effect of nonreactive argon shroud gas. Reactive gases combust intensively when react with oxygen in the plasma jet and are significantly more effective for decreasing the oxide content in plasma sprayed powders. Previous study of metallic coatings sprayed with different shrouding gases has shown that inert shrouding is ineffective in decreasing the oxide content and the oxide layers are thicker in comparison with the shrouding by reactive gases [Volenik et al., 1999]. The aim of the present paper is a study of the shroud effect on the properties and composition of the plasma jet. Both the shroud gas and the solid shield were used in experiments. The effect of shroud gas nature and flow rate on properties of the plasma jet is discussed. Experimental setup Plasma jet was generated by hybrid argon-water plasma torch [Brezina et al., 2001]. The schematic diagram of the torch with the shielding system is shown in Figure 1. The plasma jet generated by this torch is formed by a mixture of argon with water vapor. Argon is supplied along the cathode in the upstream part of the torch, while water vapor entrances and mixes up with argon plasma in the downstream water stabilized part. A long arc column results in high arc voltage and thus, high arc power gives rise to high temperatures of the plasma. The important feature of the torch is the external rotating anode, which provides reduction of the strong erosion of the anode surface in the atmosphere containing oxygen. The anode makes it difficult to provide good protection of the plasma jet close to the exit nozzle in the air atmosphere. That is why in the present experiments the shielding system was positioned at the distance 25 mm from the exit nozzle downstream the anode. 337

2 The system for shielding consisted of a ring for shroud gas distribution and supply, and a solid wall. The ring was 78 mm in diameter and 10 mm in width of the front part (Fig. 2). It was made of stainless steel and was not cooled. Several holes with diameter of 1 mm were made in the ring to distribute shroud gas around the plasma jet. The shroud gas flow was directed parallel to the jet axis. The solid wall was made from a ceramic tube 100 mm in length, which was placed downstream the ring. The tube was removable and was not water cooled. The ceramic wall protected surrounding of the plasma jet from air entrance altering surrounding atmosphere. The shielding system was centered with the respect to the exit nozzle. First, the feeding of the shroud gas was made through one port entering to the distributing ring at its top (Fig. 2 a)). But this arrangement turned out to be unsatisfactory as it could not provide proper distribution of the shroud gas around the jet. As a result the flow rate of the shroud gas at the bottom part of the ring was higher than at the top part. To avoid improper distribution the ring was divided inside into three sections by partitions and shroud gas was supplied from three different sides (Fig. 2 b)). Figure 1. Hybrid argon-water torch with the system for shielding Figure 2. The ring for gas shroud distribution: a) one gas entrance; b) three gas entrances The choice of the shroud gases was dictated not only by necessity to reduce oxygen content entrained from the surrounding, but also by plasma gas contained oxygen from the products of dissociation and ionization of water. Several gases were applied. First, effect of nonreactive argon was studied. Further, argon was replaced with reactive acetylene, methane, and safety mixture of hydrogen with nitrogen (5% of H 2 ). The problem with gas distribution appeared when pure acetylene was used as shroud gas. At the temperatures about 400 C the pyrolysis of acetylene starts. As the ring was not cooled its temperature rose fast during acetylene combustion. The carbon precipitated inside the ring blocking the holes. To avoid this effect acetylene was mixed with nitrogen. Measurements of the plasma jet characteristics were done with the help of the enthalpy probe system connected to the mass spectrometer. Enthalpy probe allowed to measure plasma jet temperature and velocity while mass spectrometer was used to determine plasma composition. The plasma torch was moved in horizontal and vertical direction allowing a scanning of free jet in axial and radial directions with the step of 5 mm. All present measurements were done under atmospheric pressure conditions. In all experiments argon flow rate was 17.5 slm and arc current was 300 A. Arc voltage under these conditions was about 250 V, which corresponded to arc power of 75 kw. Such a high arc power resulted in high heat fluxes in the plasma jet in spite of strong air entrainment and it was impossible to make measurements in the regions close to the nozzle because of the permissible level of heat flux, which could be sustained by enthalpy probe tip. The enthalpy probe scanned the jet at the distance of 200 mm from the exit nozzle, which corresponded to the 65 mm from the end of the ceramic wall. The freezer was inserted into the gas sample line of the enthalpy probe system to avoid entrance of water vapor into the system, which could damage it. As enthalpy probe evaluates gas characteristics based on the measured gas composition, the recalculation of all measured characteristics should be done due to the fault determination of the gas composition. But present measurements were done at the distance of 200 mm from the exit nozzle. At such a distance plasma jet represents mostly heated ambient gas rather than plasma forming gas because of the strong entrainment. The amount of plasma 338

3 Figure 3. Properties of the plasma jet at the distance 200 mm without shroud gas and with argon and acetylene shrouding (shrouding with the ceramic tube) forming medium drops here to about 5%. Under these condition the error caused by water vapor, which was not taken into account, was negligible and measured values supposed to be equal to real values of the jet characteristics. Results and discussion The plasma jet generated by hybrid argon-water torch is characterized by an intensive interaction with surrounding atmosphere and air entrainment [Kavka et al., 2002]. The effect of the shroud gas was studied for two cases with and without ceramic wall. Figure 3 represents properties of the plasma jet when solid wall was applied, while in the Figures 4 and 5 there are results when ceramic tube was removed. Temperature, velocity and oxygen content in the jet are shown for free jet, when no shroud gas was applied, and for the cases of shrouding by argon and acetylene and its mixture with nitrogen. The oxygen percentage in the free jet shows that the jet consisted mostly of the heated air. The zero point on the x axis corresponds to centerline of the jet, which is usually shifted with respect to plasma torch axis because of interaction of the main plasma flow with an anode jet [Hrabovsky et al., 2004]. The results showed just small effect of the argon shrouding on the properties of the plasma jet. The profiles of the plasma jet characteristics shows that the shroud gas improved free stream region as plasma jet became wider without strong gradients at the jet fringes. Temperature of the jet went down slightly because of the influence of cold argon consuming a part of energy from the jet for its heating. The velocity of the jet remained unchanged. Concentration of oxygen decreased just a little. Further increasing of the argon flow rate up to 120 slm resulted just in minor changes of oxygen content in the jet center accompanied by minor temperature reduction. These results indicate that air entrainment is most intensive in the part of the jet close to the nozzle exit, which was not protected by the gas shielding while in the region, where shroud gas acted, entrainment rates already slowed down. 339

4 Figure 4. Properties of the plasma jet at the distance z = 200 mm without shroud gas and with CH 4, N 2 /H 2 and C 2 H 2 /N 2 shrouding (shrouding without the ceramic tube) Argon shroud gas was replaced by reactive acetylene. The acetylene reacts very intensively with the oxygen already at the temperatures near 300 C. A reaction of acetylene with oxygen results in the formation of carbon oxides. Moreover, the injection of acetylene introduces a further energy source because of combustion and about 19.5 kw of heat will be released. The results showed strong changes of the plasma jet characteristics when acetylene was applied. Oxygen content was reduced when acetylene flow rate was increased and already 20 slm of acetylene allowed reducing oxygen content to values about 5 % at the jet centerline. Temperature and velocity of the jet increased. This effect was caused by a reaction of oxygen with acetylene accompanied by heat evolution. Consequently, the high-temperature highvelocity region was extended significantly. This is expected to provide better particle heating and acceleration with less particles oxidation, and thus improving spraying process efficiency and deposition quality. The shifted profile of the oxygen content was caused by nonuniform distribution of the shroud gas due to the holes, which became cluttered because of acetylene pyrolysis. Mixing of acetylene with the same amount of nitrogen resulted in better distribution of the shroud gas around the jet. As nitrogen is diatomic gas it consumed energy from the plasma flow for the dissociation. Thus, adding of nitrogen caused reduction of temperature. Moreover, higher total shroud gas flow rate resulted in a higher discharge velocity of the shroud gas through the holes. This could affect process of acetylene combustion. On the other hand, higher shroud gas flow rate led to smaller velocity gradients at the jet fringes, which in turn reduced process of the plasma jet deceleration caused by entrainment. Thus, plasma jet velocity was increased. In the Figure 4 there are results of the experiments when ceramic tube was removed and only the effect of the shroud gas was studied. In addition to mixture of acetylene with nitrogen, methane and the mixture of nitrogen with hydrogen were examined. Methane is also combustible, but is more stable and its pyrolysis starts at much higher temperatures, which allowed us to use it without addition of other gases. Like acetylene, methane is also a source of heat as its combustion is accompanied by release of 12 kw of heat for applied flow rate. In the reaction of 340

5 combustion one molecule of methane binds 2 molecules of O 2, whereas molecule of acetylene 2.5. In spite of this, methane is preferable as a shroud gas since it is better to store and to handle. Beside these, a safety mixture of hydrogen with nitrogen was examined in the experiments 5 % of H 2. Figure 5. Effect of the shroud gas flow rate on the properties of the plasma jet at the distance z = 200 mm without the ceramic tube Similarly as in case of the ceramic tube, shroud gas apply resulted in the flattening of the plasma jet characteristics for all shroud gases. Using of the H 2 /N 2 mixture led to increasing of the temperature at the plasma jet fringes, while centerline temperature was unchanged. Temperature of the jet was a little higher when acetylene and methane were used because of the released heat of combustion. Velocity increased for all shroud gases, the centerline velocity increased by almost 30 % from 45 m/s to near 60 m/s. The results show that H 2 /N 2 shrouding only replaces air from the ambient atmosphere and thus reduces available oxygen to be entrained from the surrounding. Hydrogen binds oxygen atoms but its amount is too small to provide proper oxygen elimination. Acetylene and methane not only replace air but also consume significant amount of oxygen in process of combustion. Acetylene shrouding provided the best results of oxygen reduction together with improvement of the plasma jet characteristics. The effect of acetylene flow rate is illustrated in Figure 5. Acetylene flow rate was set to the values of 10 and 20 slm. Acetylene was mixed with the same amount of nitrogen to get 50 % mixture. Oxygen content decreased for higher flow rates of the shroud gas, while velocity increased. The argon content reflects amount of plasma in the plasma jet. The plasma content increased when C 2 H 2 /N 2 shrouding was used, but it was almost independent on shroud gas flow rate. Increasing of the shroud gas flow rate had no effect on the center of the jet and had only negligible effect on the plasma jet fringes. The results confirmed that the entrainment process was slowed down when C 2 H 2 /N 2 mixture was supplied what would be the consequence of two factors. As the shroud gas discharges from the holes it starts to react intensively with air. Reaction of the acetylene with oxygen is accompanied by 341

6 heat release, which results in heating of the plasma jet surrounding. Thus, air bubbles getting into the plasma jet in the process of the entrainment [E. Pfender et al., 1991] and cooling and slowing down the jet have lower density and could travel quicker in the plasma jet in comparison with cold air bubbles. The process of the plasma jet velocity reduction slows down. Moreover, in contrast to stagnant air surrounding, shroud gas moves concurrently with the plasma jet reducing velocity gradients at the jet fringes and slowing down the process of the bubble formation. Conclusions The effect of the shroud gas was studied on the plasma jets generated by hybrid DC torch with water-argon stabilization of arc. The shroud gas was distributed around the jet with the help of the ring with holes. Different reactive and nonreactive gases were used for shrouding pure argon, acetylene, and methane and mixtures of nitrogen with acetylene and hydrogen. The ceramic tube was also tested to improve shielding of the jet as it protected plasma jet surrounded from the air entrance. The study showed that entrainment of the surrounding air into the jet is significant even with shrouding. The highest entrainment rates are at the regions close to the exit nozzle, where high velocity plasma flow enters to the stagnant ambient air. That is why protection of this part of the jet is a question of great importance for further study. All shroud gases used in experiments affected plasma jet making the profiles of the jet characteristics flattened with higher temperatures and velocities and reduced amount of oxygen. Ar and H 2 /N 2 shrouding only replace air from the jet surrounding, while CH 4 and C 2 H 2 shrouding remove oxygen in the process of combustion as well. Moreover, both acetylene and methane are a source of extra heating as a large amount of heat release during combustion, which would provide better particle heating. Acetylene improved plasma jet characteristics most of all used shroud gases but difficulties connected with handle this unstable gas force to replace it with more stable methane which would be the best shielding gas from the tested in present experiments. Acknowledgement. The authors gratefully acknowledge the support of this work by the Grant Agency of the Czech Republic under the project No. 202/05/0669. References Brezina V., Hrabovsky M., Konrad M., Kopecky V., Sember V., Proc. Of 15 th Int.Symp. on Plasma Chemistry (ed. A.Bouchoule et al.), Vol.ΙΙΙ, 9-13 July 2001, Orleans, , Gawne D.T., Zhang T., Liu B., Surface and coating technology 153, , Kang K. D., Hong S. H. - J. Appl. Phys. 85 (9), , Kavka T., Hrabovsky M., Czechoslovak Jour. of Physics 52, Hrabovský M., Chumak O., Kavka T., Kopecký V., Proc. of 12th Workshop on Plasma Technology, September 2004, TU Ilmenau, Germany, 15-22, Pfender E., Fincke J., Spores R., Plasma Chem. Plasma Process. 11 (4), , Thomson I., Pershin V., Mostaghimi J., Chandra S., Plasma Chem. Plasma Process. 21 (1), 65-81, Volenik K., Hanousek F., Chraska P., Ilavsky J., Neufuss K., Materials Science and Engineering A272, ,