Discharge characteristics of kilohertz AC gliding arc in a vortex air flow
Tian-Liang Zhao, Xiao-Song Li, Jing-Lin Liu, Jin-Bao Liu, Ai-Min Zhu
Laboratory of Plasma Physical Chemistry, Dalian University of Technology, Dalian 116024, China
Abstract: The kilohertz AC gliding arc discharge in a vortex air flow at atmospheric
pressure is investigated via the electrical measurement, high-speed imaging and optical
emission spectra in this paper. The electron density is estimated according to the current
density formula and decreases with increasing the flow rate. The vibrational and rotational
temperatures are determined by fitting to overlapped spectra of N2(C-B, Δν=1) with
OH(A-X, Δν=0) and N2(C-B, Δν=-2,-3) with N2+(B-X, Δν=0) and the rotational temperature
of N2(C-B) is a better estimate of gas temperature compared with that of OH(A-X).
Keywords: Gliding arc discharge, Optical emission spectra, Vibrational and rotational
temperatures.
1. Introduction
inlets. The arc, pushed by the swirling gas, rotates stably
The gliding arc discharge, as a unique non-thermal
along the inner surface of the outer electrode. The
plasma, has a relatively high plasma density and energy
discharge voltage is measured using a high voltage probe
efficiency and has been extensively applied to pollution
(Tektronix P6015A, 1000:1), and the discharge current is
control [1], enhancement of ignition and combustion [2],
determined according to the voltage drop across a
fuel conversion [3], surface modification of materials [4],
50
and so on. The DC [5, 6] and AC [7, 8] gliding arc
waveforms are monitored by an oscilloscope (Tektronix
discharges have been studied by several groups. In order
TDS 2024B).
resistor. Both discharge voltage and current
to better understand the characteristics of the gliding arc
discharge, the kilohertz AC gliding arc discharge in a
vortex air flow at atmospheric pressure is investigated via
the electrical measurement, high-speed imaging and
optical emission spectra in this paper.
2. Experimental
The experimental setup consisting of a power supply, a
plasma reactor, and electrical and optical measurement
units, is schematically shown in Fig.1. The stainless steel
inner rod electrode is fixed on the machinable ceramics
and connected to high voltage which is supplied by an AC
power with a frequency of 30 kHz. The outer electrode is
grounded. A stainless steel spiral wire is attached to the
inner electrode so that the arc can be initiated at the
narrowest gap (about 2 mm). The working gas is
introduced to the bottom of the reactor via two tangential
Fig.1 Schematic view of the experimental setup.
The high-speed imaging is performed using an
intensified CCD (ICCD) camera (Andor iStar DH734
-18F-03). The ICCD is synchronized with a delay
generator DG 645 (Stanford Research Systems) for
temporally resolved measurements. The temporally
resolved optical emission spectral measurements are
where je is current density, e is the elementary electron
carried out using a high resolution spectrograph (Andor
charge,
Shamrock SR-750), and 32 phase regions in one
field [6, 9]. With increasing the flow rate to 4.0 L/min, the
discharge period are obtained by programming the gate
delay time at 1 μs increment.
electron density decreases by about a half during one
3. Results and discussion
3.2. High-speed imaging by the ICCD camera
e
is the electron mobility and E is the electric
discharge period as shown in Fig.3.
Fig.4 shows the images taken by the ICCD camera at
3.1. Electrical measurement
flow rates of 2.0 L/min and 5.0 L/min, respectively. For
Fig.4 (a) and (c), the ICCD is internally triggered and
operated with Fire-only mode. For Fig.4 (b) and (d), the
ICCD is externally triggered and operated with an
integrate-on-chip mode and the gate width is set as 30 μs .
The arc rotates stably at a flow rate of 2.0 L/min as shown
in Fig.4 (a) and (b). As is shown in Fig.4 (c) and (d), the
arc glides spirally along the inner surface of the outer
electrode with increasing the flow rate to 5.0 l/min. For a
further higher flow rate, the arc is firstly ignited at the
narrowest gap and quickly elongated while rotates spirally.
Fig.2 Dishcarge voltage and current and electron density
When the power can’t compensate the heat loss, the arc
(Power: 44 W, Flow rate: 2.0 L/min, Water vapor: 0.63
extinguishes, then the arc reignites itself at the narrowest
vol%).
gap and a new discharge cycle starts.
Fig.3 Electron density under flow rate of 2.0 and 4.0
L/min (Power: 44 W, Water vapor: 0.63 vol%).
Fig.4 Images taken by the ICCD camera (a) and (b) flow
The discharge voltage and current at power 44 W, flow
rate: 2.0 L/min, exposure time: 110 ms and (c) and (d)
rate of 2.0 L/min and water vapor 0.63 vol%, are shown
flow rate: 5.0 L/min, exposure time: 25 ms.
in Fig.2. The discharge exists in a non-equilibrium state
with a relative low current. The electron density ne is
3.3. Diagnostics by optical emission spectra
calculated according to the formula ne
je
e
eE
,
Optical emission spectra measured in a vortex flow
gliding arc at power of 44 W, flow rate of 4.0 L/min and
water vapor of 0.63 vol% is shown in Fig.5. The spectra
emission intensity for OH(A-X) decreases to some extent
mainly consist of the transitions of N2(C-B), N2+(B-X),
but still remains a fair amount when the discharge is off.
+
OH(A-X) and NO(A-X). The spectra of N2 (B-X) and
In order to determine the vibrational and rotational
OH(A-X) overlap with the spectra of N2(C-B), which
temperatures, an automatic fit code to the overlapped
makes the determination of vibrational and rotational
spectra of N2(C-B, Δν=1) with OH(A-X, Δν=0) and of
temperatures complicated.
N2(C-B, Δν=-2,-3) with N2+(B-X, Δν=0) is performed via
the method of least squares. The experimental and
simulated spectral intensities are normalized to their
maxima in the spectral range investigated, and the
vibrational and rotational temperatures, the full width at
half maximum (FWHM) are set as fit parameters during
the fit calculation. The experimental and fitted spectra at
the time delay of 11 μs are in good agreement and shown
in Fig.7.
Fig.5 Optical emission spectra of gliding arc in a votex
flow (Power: 44 W, Flow rate: 4.0 L/min, Water vapor:
0.63 vol%).
Fig.6 Teporally resolved intensitied for OH(A-X), N2(C-B)
and N2+(B-X) (Power: 44 W, Flow rate: 4.0 L/min, Water
vapor: 0.63 vol%).
The temporally resolved band head intensities for
OH(A-X) at 306 nm, N2(C-B) at 316 and 380 nm and
N2+(B-X) at 391 nm are shown in Fig.6. The emission
intensities exhibit two peaks during a cycle of the
discharge voltage period. When the discharge current
decreases to very low or to zero, the emission intensities
for N2(C-B) and N2+(B-X) are close to zero. However, the
Fig.7 Experimental and fitted spectra at time delay of 11
μs (Power: 44 W, Flow rate: 4.0 L/min, Water vapor: 0.63
vol%).
The temporally resolved vibrational and rotational
temperatures are shown in Fig.8. The rotational
with that of OH(A-X).
temperatures are in the range of 2000-2700 K and
vibrational temperatures are in the range of 3800-5000 K.
Acknowledgements
According to the measured temperatures, the discharge in
This work is supported by National Natural Science
the present work is of “warm” properties [5]. The
Foundation of China (51077009, 10835004).
rotational temperature derived from the OH(A-X) is about
1200 K larger than that from N2(C-B) and N2+(B-X).
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Fig.8 Temporally resolved vibrational and rotational
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4. Conclusion
In this paper, the gliding arc discharge in a vortex air
flow is investigated, and the electron density is estimated
to be about 6.5×1013 cm-3 at the peak current with a flow
rate of 2.0 L/min and decreases with increasing the flow
rate. The arc movement is strongly influenced by the gas
flow rate according to the high-speed imaging by the
ICCD camera. According to the fit calculation to
overlapped spectra, the rotational temperatures are in the
range of 2000-2700 K and vibrational temperatures are in
the range of 3800-5000 K. The rotational temperature of
N2(C-B) is a better estimate of gas temperature compared
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