WDS'05 Proceedings of Contributed Papers, Part II, 337–342, 2005.
ISBN 80-86732-59-2 © 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, N2) 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.
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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 H2). 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
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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
Figure 3. Properties of the plasma jet at the distance the jet remained unchanged. Concentration
200 mm without shroud gas and with argon and of oxygen decreased just a little. Further
acetylene shrouding (shrouding with the ceramic increasing of the argon flow rate up to
tube)
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.
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KAVKA ET AL.: SHROUDING OF THERMAL PLASMA JETS
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
Figure 4. Properties of the plasma jet at the distance
reduced process of the plasma jet
z = 200 mm without shroud gas and with CH4, N2/H2
deceleration caused by entrainment. Thus,
and C2H2/N2 shrouding (shrouding without the
plasma jet velocity was increased.
ceramic tube)
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
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KAVKA ET AL.: SHROUDING OF THERMAL PLASMA JETS
combustion one molecule of methane binds 2 molecules of O2, 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 H2.
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 H2/N2 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 H2/N2 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 C2H2/N2
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 C2H2/N2 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
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KAVKA ET AL.: SHROUDING OF THERMAL PLASMA JETS
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 H2/N2 shrouding only replace air from the jet surrounding, while CH4 and C2H2 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.
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