Effects of Pressure on the Characteristics of the
Dual-Jet DC Arc Plasmas
Gui-Qing Wu1, Nan Ge1, An Yang2, He-Ping Li1,* and Cheng-Yu Bao1
1
Department of Engineering Physics, Tsinghua University, Beijing 100084, P. R. China
2
Biomedical Engineering Institute, Southern Medical University, Guangzhou 510515, P. R. China
(*Corresponding author, H.-P. Li: [email protected])
Abstract: The influences of the operation pressure on the arc voltages and
the projective areas of the two-dimensional (2D) high-temperature plasma
regions are studied experimentally. The experimental measurements show
that the operation pressure has significant influences on the shapes and areas
of the 2D high-temperature regions of the dual-jet dc arc plasmas, especially
under small plasma working gas flow rates.
Keywords: Arc, Dual-jet plasma, Pressure
1. Introduction
In recent years, with the increasing requirements for
the establishment of the healthy and low-carbon environment, thermal plasma hazardous waste treatment would be a prominent technology due to their
unique features with high energy densities and the
opportunities to generate valuable by-products [1, 2].
In previous publications, different configurations of
the thermal plasma generators, such as single/multiple traditional non-transferred direct-current
(dc) arc plasma torches [3-5], dc arc plasma torches
processes, the non-equilibrium features near the cold
solid walls (e.g., the electrodes or the work-pieces),
and the arc instabilities since both the anode and
cathode regions can be observed directly during the
discharge processes compared with those using the
non-transferred arc plasma torch. Secondly, it is possible to produce the large volume, high temperature
arcjet region for the treatment of different kinds of
materials (e.g., the conducting and/or non-conducting
materials) since no current passing through the treated materials.
with hollow electrodes [6, 7], dual-jet dc arc plasma
With plasma hazardous waste treatment as the re-
generators [8-10], were employed for the thermal
search background, most of the present studies con-
plasma hazardous waste treatment. Among the pre-
cerning the dual-jet dc arc plasmas focus on the fol-
ceding plasma sources, the dual-jet dc arc plasma
lowing aspects: (i) how to expand the volume of the
source shows their unique features and attracts much
high temperature plasma region; (ii) how to reduce
attention of the researchers. Firstly, it is very con-
the power consumption or improve the thermal effi-
venient to study the plasma-electrode interaction
ciency; (iii) how to reduce the electrode erosion, the
harmful gas emissions, and so on. Previous theoretical and experimental studies have shown that different operation parameters, such as the arc current (I),
the distance between the electrode tips (L) and declination angle () between electrodes, the chemical
composition of the plasma-forming gas and its flow
rate injected from the cathode (Qc) and anode (Qa)
sides, etc., could influence the voltage-current characteristics, the heat transfer and flow patterns inside
the dual-jet dc arc plasma generators [9].
chain coding technique.
Since the luminous flux of the thermal plasmas is too
intensive to be captured without attenuation by an
ordinary camera, the transmittance of the visible light
is reduced to 3.125% through the neutral density filters. The parameters of the camera are as follows: the
aperture number F=3.9, exposure time Tex=1/50 s,
focal length f=25 mm and ISO speed=80. In addition,
the operation parameters are also fixed at I=60 A,
L=30.0 mm and =140°.
In actual applications, the operation pressure (p) inside the reactor chamber may be maintained at less
than 1.0 atm to prevent the leakage of the gas and to
avoid the possible air pollution. And simultaneously,
the gas discharge characteristics may be different
under different operation pressures. Thus, it is necessary to study the influences of the operation pressures on the voltage-current characteristics, the temperature and flow fields of the dual-jet dc arc plasmas. In this paper, the effects of the operation pressures on the arc voltages and the shapes of the high
temperature arcjet regions are studied experimentally.
2. Experimental setup
In this study, the experimental setup is shown in Fig.
1, which is similar to that presented in Ref. [9]. In
order to quantitatively in situ describe the 2D grayscale contour distributions of the arcjet plasmas during the discharge processes, the method presented in
Ref. [9] is employed as a visual and qualitative tool
to evaluate the relative shape changes of the high
temperature regions for the arcjet plasmas under different operation conditions. And then, the variations
of their corresponding projective areas can be given
quantitatively by using the eight-direction Freeman
Figure 1. Schematic diagram of the experimental setup.
3. Experimental results and discussions
3.1. Design of the electrode configurations
The experimental observations show that the geometrical configurations of the electrodes can influence the erosion rates and the arc stabilities significantly. Different electrode shapes employed in this
study are shown in Fig. 2. The experimental results
show that: (i) from the aspects of the arc stabilities,
the electrode with conical shape as shown in Fig. 2 (a)
is helpful for maintaining a stable discharge; (ii) from
the electrode erosion processes, the electrode with
conical shape will lead to a higher erosion rate of the
anode due to the higher local current density which
results in the very high electron ‘condensation’ heat,
as shown in Fig. 2 (a); (iii) the arc root moves randomly and frequently on the electrode surfaces, es-
pecially on the cathode side, using the hemispherical
region is enlarged by 12.8% due to the decrease of
electrode shape as shown in Fig. 2 (b) leading to the
the operation pressure.
instabilities of the discharges. Thus, based on the
foregoing experiments, an optimal combination of
the 60-conical-shaped cathode and the semi ellipsoidal-shaped anode, as shown in Figs. 2 (a) and (c),
respectively, is adopted in the following experiments
in order to improve the arc stabilities and reduce the
electrode erosions.
From Fig. 3, it can be also seen that with the decrease
of the operation pressures, the current passage channel expands in the radial direction, which leads to the
decrease of the maximum current density. And this
effect results in: (i) the decrease of the maximum gas
temperatures in the core regions of the arcs; (ii) the
changes of the relative intensities of the cathode and
anode jets. Due to the decrease of the current densities, the cathode jet becomes weaker and the plasma
jet, which is a mixing region of the cathode and anode jets, moves towards the cathode side. In addition,
the experimental measurements show that the arcjet
expands downwards with the decrease of the operation pressure, especially for the cases with higher gas
flow rates. But the reasons are not clear at the present
time.
Figure 2. Schematic diagrams and photos of the electrodes.
3.2. Effects of the operation pressures
The typical discharge images and the corresponding
grayscale contour distributions for the cases of p=1.0
and 0.5 atm are shown in Fig. 3 with the argon flow
rates of 8.0 slpm on both cathode and anode sides,
respectively. Figure 3 shows that, with other parameters (e.g., Qa, Qc, L, α, Iarc and the camera parameters)
being unchanged, the high temperature region of the
partially ionized gas expands downwards and outwards with decreasing the operation pressures from
Figure 3. Typical discharge images [(a) and (b)] and the corresponding grayscale contours [(c) and (d)].
3.3. Effects of the working gas flow rates
p=1.0 atm to 0.5 atm. If we use the grayscale contour
with the value of 159 to indicate the plasma-cold gas
interface qualitatively, the areas of the high temperature region are S1=3756 and S2=3330 in pixels, respectively. It can be seen that the high-temperature
Figure 4 shows the variations of the areas (S) of the
2D high-temperature arcjet regions under different
argon flow rates. It can be seen that, with other parameters being unchanged, the areas of the 2D
high-temperature plasma regions are larger for the
case of p=0.5 atm than those at p=1.0 atm; with the
increase of the argon flow rate, the differences between the values of S for the cases of p=0.5 and 1.0
atm become smaller. This phenomenon may be explained as: although the chamber pressure is kept at
p=0.5 atm, the local gas pressure may become higher
with increasing the argon flow rate, leading to the
non-significant changes of the gas discharge characteristics at different chamber pressures.
4. Conclusions
In this paper, the effects of the operation pressure on
the characteristics of the dual-jet dc arc plasmas are
studied. The main conclusions are as follows: (i)
With other parameters being unchanged, the 2D
high-temperature region of the arcjet becomes larger
with decreasing the operation pressure. (ii) With increasing the argon flow rate, the differences of S at
p=0.5 and 1.0 atm become smaller. (iii) The influences of the operation pressure on the arc voltages
are not significant, especially for the larger argon
flow rates.
Acknowledgement This work was supported by the
National Natural Science Foundation of China (No.
11035005).
References
Figure 4. Variations of S (left) and the arc voltage (right) under
different operating conditions.
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
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
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