Effect of Liquid Droplets Injection on Powder Spheroidization Process
Using a Low Power DC-RF Hybrid Plasma Flow System
Juyong Jang1, Hidemasa Takana2, Sangkyu Park3 and Hideya Nishiyama2
1
Graduate School of Engineering, Tohoku University, Sendai, Japan
2
3
Institute of Fluid Science, Tohoku University, Sendai, Japan
Department of Mechanical and Automotive Engineering, Woosuk University, Wanju, Chunbuk, South Korea
Abstract: Water droplets are injected into the downstream of plasma flow in
order to enhance plasma enthalpy to improve spheroidization process efficiency
with low power DC-RF hybrid plasma flow system. Small amount of injected
water droplets in the tail of RF plasma flow are vaporized and dissociated in a
low power DC-RF hybrid plasma flow system, then hydrogen and oxygen
molecules are produced. When water droplets are injected radially by the gasliquid atomizer, DC plasma jet expands and elongates, additionally, RF plasma
flow enlarges and changes visible red in downstream of RF plasma with the
emission of the hydrogen Balmer α line at the wavelength of 656 nm. In this
study, the effect of injected water droplets in the downstream of plasma flow on
plasma flow behaviors and spectroscopic characteristics is experimentally
investigated in a low electric power DC-RF hybrid plasma flow system.
Keywords: liquid droplet, spheroidization process, DC-RF hybrid plasma flow
system, evaporation
1. Introduction
Spherical particles provide more homogenous
and stable particle transportation by enhancing the
powder fluidity. This allows fine control of powder
feeding rate without clogging problems. Especially,
spherical powders are preferred in thermal plasma
spray processes for dense coating formation and thin
film fabrications depending on accuracy of powder
feeding rate during spray processing. Therefore,
powder spheroidization process is essential for high
spray processing performance [1,2].
In the previous studies, a DC-RF hybrid plasma
flow system has been optimized for in-flight alumina
powder synthesis [1,2]. However, limitation of low
electric input power in this system resulted in low
process efficiency. To overcome the limitation of
input electrical power, Ar/He mixture gas was used
to increase in plasma enthalpy for high performance
spheroidization process. Furthermore, sinusoidal
central gas of pure argon without helium mixture
was injected to introduce active mixing effect of
DC-RF plasma flow. As a result, in-flight alumina
powder process efficiency has improved in spite of
the limitation of low electric power [3].
Recently, plasma process with liquid injection
has been paid special attention because of
advantages
to
improve
thermofluid
flow
characteristics of plasma flow for ideal in-flight
powder processes. Water droplets provide
environmentally friendly combustion products such
as molecular hydrogen (H2) and oxygen (O2), all
these elementary species are mixed with the plasma
flow [4] and enhance thermofluid flow
characteristics of plasma flow. Water-stabilized
plasma jet, for instance, has been researching, which
produces high concentration of hydrogen to generate
plasma jet with extremely high enthalpy and jet
velocity with water droplets injection [5].
In this study, small amount of water droplets is
injected into the downstream of thermal plasma in
the DC-RF hybrid plasma flow system to avoid
consuming amount of thermal energy of plasma for
phase transition and deflecting plasma flow. The
injected water droplets are heated and vaporized in
the tail of RF plasma flow then dissociated to
hydrogen and oxygen molecules. Therefore, the
effect of water droplets injected below the DC-RF
hybrid torch on the spectroscopic characteristics of
the plasma flow is experimentally investigated in
order to show the fundamental data for high
performance spheroidization processes using the low
electric power DC-RF hybrid plasma flow system.
2. Experimental setup
Figure 1 shows a schematic illustration of the
DC-RF hybrid plasma flow system. The origin point
denoted by “O” is located at the exit of the DC
plasma torch on the central axis. The operating
pressure is 120 torr (16 kPa). The DC power of 1.2
kW and RF power of 6.6 kW (4 MHz) are supplied
to this system. The total heat loss to cooling water
was roughly 2.3 kW. The working gas is argon. The
flow rates of central gas and swirling sheath gas are
5 and 20 Sl/min, respectively. The central gas flow
rate is given as a sum of the DC plasma jet forming
gas flow rate of 4.2 Sl/min and the carrier gas flow
rate of 0.8 Sl/min, respectively.
Liquid atomization is used by a two-phase
atomizer (YS-03, Yaezaki Inc., Japan). The atomizer
was installed at 95 mm in radius and 195 mm in
downstream of DC plasma jet nozzle in order to
avoid high heat load from the core of plasma flow as
shown in Fig. 1. The internal nozzle section diameter
for water supply and the external nozzle section
diameter for atomizing gases are 0.3 and 1.2 mm,
respectively. The pure water and the atomizing gas
of argon were used. The small amount of water flow
rate (Qw) was 1 – 15 Sml/min. Atomizing gas flow
rate (Qg) for water atomization was 4 – 8 Sl/min.
The average injected water droplets size of 30-50
Figure 1. Schematic illustration of DC-RF hybrid plasma flow
system.
µm was measured by the PTV system. Weber
number as the ratio of the force exerted by the gas
on the liquid to the surface tension force [4] was
estimated 5.5 - 22.0 under the experimental
conditions.
Optical emission spectra were measured by an
optical emission spectroscopy (OES) with a
wavelength range of 330 – 1100 nm (Otsuka Inc,
Japan).
3. Experimental results and discussion
Figures 2 (a) – (c) show the effect of injecting
gas and water droplets injection on DC-RF hybrid
plasma luminance. The photographs were taken by a
charge coupled device camera (Nikon Inc., Japan)
with the shutter speed of 0.5 ms. Comparing Fig. 2
(a) with Fig. 2 (b), injecting Qg = 8 Sl/min without
water droplets injection, DC plasma jet expands and
elongates, RF plasma flow shrinks in downstream by
entrainment of cold gas. Comparing Fig. 2 (c) with
Fig. 2 (b), when water droplets with Qw = 1 Sml/min
at z = 190 mm and Qg = 8 Sl/min are injected, DC
plasma jet similarly expands and elongates,
additionally, on the other hand, RF plasma flow
under the RF coils expands and changes visible red
in downstream mainly due to the light emission from
hydrogen Balmer α line.
(b)
(a)
(c)
Figure 2. Photographs of DC-RF plasma flow (a) without and
(b) with injecting gas, and (c) with water droplets injection at z =
190 mm.
Emission intensity (A.U.)
0.8
0.6
0.4
Qat = 8 Sl/min
z = 70 mm
Ar at 811 nm
w/o Qg and Qw
Qw = 1 Sml/min
Qw = 5 Sml/min
Qg = 8 Sl/min w/o Qw
Qw = 10 Sml/min
Qw = 15 Sml/min
without
water
droplets
0.2
with water droplets
0.0
-30
-20
-10
0
10
20
30
w/o Qg and Qw
0.6
Qg = 8 Sl/min
Ar
with Qw = 15 Sml/min
0.4
ArAr
H
0.2
H
(a)
Ar
Qg = 8 Sl/min w/o Qw
Ar
O
Ar
0.12
Ar
0.0
500
600
700
800
900
Wavelength (nm)
Figure 3. Optical emission spectra with and without the water
droplets injection.
Emission intensity (A.U.)
Emission intensity (A.U.)
Radial distance (mm)
Qat = 8 Sl/min
z = 70 mm
Hat 656 nm
w/o Qg and Qw
0.09
0.06
Qw = 1 Sml/min
Qw = 5 Sml/min
Qg = 8 Sl/min w/o Qw
Qw = 10 Sml/min
Qw = 15 Sml/min
with
water
droplets
0.03
without water droplets
0.00
-30
-20
-10
0
10
20
30
Radial distance (mm)
(b)
Qg = 4 Sl/min
r = 0 mm, z = 70 mm
Qg = 6 Sl/min
1.0
Qg = 8 Sl/min
0.4
0.8
Ar at 811 nm
0.6
0.2
0.4
0.2
H at 656 nm
0
5
10
15
0.0
Emission intensity (A.U.) from H
Emission intensity (A.U.) from Ar
1.2
Qw (Sml/min)
Figure 4. Emission intensity of plasma flow at 811 and 656 nm
as a function of water flow rate for water droplets injection.
Figure 3 shows the optical emission spectra with
and without water droplets injection at z = 70 mm.
Only a small amount of plasma net energy is
consumed to vaporize the injected water droplets.
Taking into account the latent heat of vaporization
for water of 2257 kJ/kg [6], the maximum power of
0.56 kW is required to vaporize the water droplets in
the operating conditions. By injecting two-phase of
Figure 5. Radial distributions of optical emission spectra at (a)
811 and (b) 656 nm.
argon gas and water droplets, while whole argon line
emission intensities relatively decrease, the
important three peaks of excited hydrogen Balmer
series lines at 486 and 656 nm, and an oxygen line at
777 nm are detected. This result shows that the
thermal energy of argon plasma is consumed to
vaporize injected water droplets and then to
dissociate the vapor.
Figure 4 shows emission intensities of plasma
flow at 811 and 656 nm as a function of injecting
water flow rate. With increasing water flow rate,
argon emission intensity gradually decreases, but
hydrogen emission intensity increases.
Figures 5 (a) and (b) show radial distributions of
optical emission spectra at wavelengths of 811 and
656 nm. In Fig. 5 (a), by injecting only argon gas
without water injection, cold argon gas is entrained
2.5
(1) A small amount of the injected water droplets
are vaporized and hydrogen and oxygen
molecules are produced by the thermal plasma
flow.
r = 0 mm, z = 70 mm
Te (eV)
2.0
1.5
Qw = 1 Sml/min
Qw = 5 Sml/min
1.0
Qw = 10 Sml/min
Qw = 15 Sml/min
0.5
4
6
Qg (Sl/min)
8
Figure 6. Effect of injecting gas flow rate on electron
temperature.
to plasma flow at z = 70 mm and argon line emission
distribution decreases through the quenching effect
by cold injecting argon gas. By injecting water
droplets with injecting gas, the water droplets are
actively entrained to plasma flow with the cold gas
and are vaporized by thermal plasma flow. As a
result, the emission intensity rather decreases by
water droplets injection. With increase in water flow
rate, the argon line emission intensity decreases a
little. As clearly shown in Fig. 5 (b), emission
intensity of the excited atomic hydrogen Balmer α
line increases with increasing water flow rate.
Figure 6 shows the injecting effect of gas flow
rate on electron temperature. By assuming plasma
flow satisfies local thermal equilibrium, electron
temperature Te on correlation among only argon
lines is estimated by the Boltzmann plot method [7].
With increasing injecting gas flow rate, electron
temperature increases. In the case of Qg = 8 Sl/min
under Qw = 5 Sml/min, maximum electron
temperature of 2.1 eV is obtained.
4. Conclusions
The effect of water droplets injection on
behaviors and spectroscopic characteristics of the
plasma flow has experimentally been investigated
using a low electric power DC-RF hybrid plasma
flow system. The obtained results in this study are
summarized as follows:
(2) DC plasma jet expands and elongates by water
droplets injection. RF plasma flow below the RF
coils also expands and changes red in
downstream of RF coils.
(3) With increasing water flow rate, argon emission
intensities decrease, while hydrogen emission
intensities increase.
(4) Injecting gas flow rate effects electron
temperature and maximum electron temperature
of 2.1 eV is obtained at Qg = 8 Sl/min under Qw
= 5 Sml/min.
Acknowledgements
The present study was partly supported by the
Global COE program Grant, “World Center of
Education and Research for Trans. Disciplinary
Flow Dynamics” in the Japanese Ministry of
Education, Culture, Sports, Science, and Technology.
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