Layered structure around an extended gliding discharge column in a methane-nitrogen mixture at high pressure

Abstract

The current work aims at investigating the detailed spatial structure of the thin plasma column of a gliding arc (GA) discharge extended in N2-CH4 gas mixtures, using visualization techniques. The GA discharge was operated at up to 5 atm in a high-pressure vessel with extensive optical access. The results show that the emission intensity from the plasma column increased tenfold with the addition of 0.1% CH4 in nitrogen, compared to that in pure N2. Furthermore, an additional layer located around the GA discharge column is detected. Imaging through spectral filters and spectral analysis of the emitted signal indicate that the emissions of this outer layer are mostly from the CN A-X and CH A-X transitions. This outer layer can propagate and extinguish dynamically, similar to the flame front in combustion. Besides, the separation of this outer layer to the plasma core decreases with pressure. The layered structure and its dynamical behaviors can be explained by a plasma-sustained radical propagation mechanism. The high-power plasma column can produce a high-temperature zone with rich atomic species, surrounded by the relatively cold N2-CH4 mixture. At the mixing layer between the high-temperature zone and the N2-CH4 mixture, some highly exothermic reactions occur to produce excited CN and CH species, which emit their specific spectra. As the high-temperature zone expands with time, the outer layer propagates outward. However, with the propagation continuing, the radical species involved in the outer layer formation are rapidly consumed, and thus, this layer disappears when it propagates too far away from the plasma column.

Full text

Appl. Phys. Lett. 114, 194102 (2019); https://doi.org/10.1063/1.5097908 114, 194102
© 2019 Author(s).
Layered structure around an extended
gliding discharge column in a methane-
nitrogen mixture at high pressure
Cite as: Appl. Phys. Lett. 114, 194102 (2019); https://doi.org/10.1063/1.5097908
Submitted: 29 March 2019 • Accepted: 29 April 2019 • Published Online: 15 May 2019
Chengdong Kong, Jinlong Gao, Zhongshan Li, et al.
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Layered structure around an extended gliding
discharge column in a methane-nitrogen
mixture at high pressure
Cite as: Appl. Phys. Lett. 114, 194102 (2019); doi: 10.1063/1.5097908
Submitted: 29 March 2019 .Accepted: 29 April 2019 .
Published Online: 15 May 2019
Chengdong Kong,
a)
Jinlong Gao, Zhongshan Li, Marcus Ald
en, and Andreas Ehn
AFFILIATIONS
Division of Combustion Physics, Lund University, P.O. Box 118, S-221 00 Lund, Sweden
a)
Author to whom correspondence should be addressed: kongcd1987@gmail.com
ABSTRACT
The current work aims at investigating the detailed spatial structure of the thin plasma column of a gliding arc (GA) discharge extended in
N
2
-CH
4
gas mixtures, using visualization techniques. The GA discharge was operated at up to 5 atm in a high-pressure vessel with extensive
optical access. The results show that the emission intensity from the plasma column increased tenfold with the addition of 0.1% CH
4
in nitro-
gen, compared to that in pure N
2
. Furthermore, an additional layer located around the GA discharge column is detected. Imaging through
spectral filters and spectral analysis of the emitted signal indicate that the emissions of this outer layer are mostly from the CN A-X and CH
A-X transitions. This outer layer can propagate and extinguish dynamically, similar to the flame front in combustion. Besides, the separation
of this outer layer to the plasma core decreases with pressure. The layered structure and its dynamical behaviors can be explained by a
plasma-sustained radical propagation mechanism. The high-power plasma column can produce a high-temperature zone with rich atomic
species, surrounded by the relatively cold N
2
-CH
4
mixture. At the mixing layer between the high-temperature zone and the N
2
-CH
4
mixture,
some highly exothermic reactions occur to produce excited CN and CH species, which emit their specific spectra. As the high-temperature
zone expands with time, the outer layer propagates outward. However, with the propagation continuing, the radical species involved in the
outer layer formation are rapidly consumed, and thus, this layer disappears when it propagates too far away from the plasma column.
V
C2019 Author(s). All article content, except where otherwise noted, is licensed under a Creative Commons Attribution (CC BY) license (http://
creativecommons.org/licenses/by/4.0/).https://doi.org/10.1063/1.5097908
Gliding arc (GA) discharge is an easy-handling and low-cost
scheme to produce nonthermal high-power plasma in a large volume
at high pressure (1atm),
1–3
and thus, it has numerous potential
applications in combustion assistance,
4
surface treatment,
5
fuel
reforming,
6
and chemical synthesis.
7
There have been plenty of
reports on the physical and chemical characteristics of GA dis-
charge.
8–11
However, for any specific application, the gas mixtures of
the GA discharge can be unique and so are the discharge characteris-
tics. GA discharges in air and argon have been extensively investi-
gated by experiments
9,12–15
and modeling,
16–20
while GA discharge in
the methane (CH
4
)-nitrogen (N
2
) mixture is rarely reported. There
were some references about dielectric barrier discharge (DBD),
21
radio frequency,
22
low-pressure glow,
23
and microwave
24
discharges
in N
2
-CH
4
mixtures to study the chemical kinetics in the discharge
and afterglow processes. As for the GA discharge, only one group
used the rotating gliding arc plasma to activate methane for hydrogen
production in a N
2
-CH
4
mixture.
6,25
They presented much
information about the emission spectra and the main produced spe-
cies in the afterglow and also proposed detailed kinetic models.
However, the spatially resolved morphologies of the GA discharge
column were not demonstrated. Therefore, knowledge of the GA dis-
charge plasma column in the N
2
-CH
4
mixture is still missing.
In this investigation, we report a finding of a layered structure
around the GA discharge column in well-controlled nitrogen-methane
gas mixtures. The plasma column of GA discharge is anchored on
electrodes, which are installed in a high-pressure chamber and visual-
ized by spatiotemporally resolved techniques [including intensified
charged-coupled device (ICCD) camera, imaging spectrometer, and
high-speed camera]. By increasing the gas pressure up to 5atm, the
high-pressure GA discharge is achieved and the pressure effect on dis-
charge characteristics is explored.
Figure 1 shows a schematic of the experimental apparatus used
in this work. A pair of diverging electrodes to support the gliding arc
discharge is installed on a Teflon plate inside a high-pressure chamber,
Appl. Phys. Lett. 114, 194102 (2019); doi: 10.1063/1.5097908 114, 194102-1
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Applied Physics Letters ARTICLE scitation.org/journal/apl

which was built to provide an elevated pressure environment from
atmospheric pressure to 35 atm. The details about this high-pressure
vessel can be found in Ref. 26. One of the electrodes is connected to an
AC power supply (Generator 9030 E, SOFTAL Electronic GmbH)
through an insulated connector on the wall of the chamber, whereas
the other electrode is connected to the chamber wall, which is
grounded. Three broadband antireflection coated sapphire windows
provide optical access for the discharge inside the chamber. Various
techniques including a digital camera (D7100, Nikon) equipped with a
microNikon lens (200mm, f/4), an intensified charged-coupled device
(ICCD, PIMAX II, Princeton Instruments) mounted with a UV Nikon
lens (105 mm, f/4.5) and different filters [i.e., 420 nm and 430 nm
interferencefilters(IFs)withafullwidthathalfmaximum(FWHM)
of 10 nm from Edmund Optics and a OG 550 long pass filter], a spec-
trometer (SP 2300i, Princeton Instruments), and a high-speed camera
(Phantom 7.2, Vision Research) were employed to characterize the
GA discharge in the N
2
-CH
4
mixture. In the experiment, the gas mix-
ture is ejected into the chamber through a small hole (3 mm) between
theelectrodes.TheflowratesofN
2
and CH
4
are set to 10 standard liter
per minute (SLPM) and 0.01 SLPM, respectively, and fixed even
though the pressure changes. A current monitor (Pearson Electronics)
and a voltage probe (Tektronix P6015A) are used to measure the
waveforms of the current and the voltage simultaneously. The inset of
Fig. 1 also shows a typical current-voltage waveform of the GA dis-
charge in one discharge cycle and the current/voltage in several alter-
nating periods. The frequency of the alternating current power supply
is 35 kHz. The rated power of the high-voltage supply is manually set
to 600W, while the current and the voltage change automatically
depending on the discharge conditions. The plasma initializes at the
shortest gap between electrodes with current spikes. Later, the plasma
column elongates driven by the jet flow. The current amplitude
decreases slightly, while the voltage amplitude increases as the plasma
column elongates. When the voltage reaches the breakdown threshold,
re-ignition occurs at the shortest gap to start a new discharge cycle.
In the experiment, a glare from the GA discharge is detected
when 0.1 vol. % CH
4
is mixed with nitrogen. Owing to this 0.1% CH
4
addition, the emission intensity is tenfold stronger than that from the
discharge in pure N
2
gas. Because of this intense emission, the expo-
sure time and the f-number of the digital camera are manually
adjusted to avoid overexposure. Here, the exposure time was set to
1/8000 s and the f-number was 22. Figure 2 illustrates the images of
the plasma column at 1–5 atm captured using the digital Nikon cam-
era. A special layer around the purple GA column is identified in those
images. This outer layer moves closer to the central discharge column
as the pressure is increased. It should also be noted that this layer is
not smoothly surrounding the plasma column. There seem some
wrinkles or gaps in the layer, and so the color is inhomogeneous along
the discharge column.
In order to gain more insights into this outer layer, the ICCD
camera mounted with different filters was utilized to visualize the GA
discharge channels. Figure 3 shows images captured with two band-
pass interference filters (IF 420 nm, IF 430 nm) and a longpass filter
(OG 550), together with the emission spectra in the relevant wave-
length bands. By using the 420 nm interference filter, the strong emis-
sion from the plasma column is acquired without the apparent outer
FIG. 1. Schematic of the experimental setup, together with a typical current-voltage
waveform.
FIG. 2. Images of the GA column at 1–5 atm captured using the digital Nikon
camera.
FIG. 3. Emission spectra and images captured with different filters. (a) Emission
spectra of N
2
-CH
4
and pure N
2
discharges in a wavelength range of 410–440 nm.
(b) Emission spectra of N
2
-CH
4
discharge in a wavelength range of 580–680 nm.
(c) Image of N
2
-CH
4
discharge captured with a 420 nm interference filter; the ICCD
gate is 15 ls. (d) Image of N
2
-CH
4
discharge captured with a 430 nm interference
filter; the ICCD gate is 30 ls. (e) Image of N
2
-CH
4
discharge captured with the OG
550 filter; the ICCD gate is 15 ls.
Applied Physics Letters ARTICLE scitation.org/journal/apl
Appl. Phys. Lett. 114, 194102 (2019); doi: 10.1063/1.5097908 114, 194102-2
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layer. However, the outer layer is distinctly detected by using the
430nm interference filter and the OG 550 filter. The emission inten-
sity of the outer layer detected with IF 420nm is at least twenty times
stronger than that of the plasma column detected with IF 430 nm.
Meanwhile, the measured spectra indicate that in the wavelength range
of 410 nm to 440 nm, only the emission from CN(B
2
P
þ
) and N
atoms is clearly identified for the N
2
-CH
4
discharge. Therefore, it can
be confirmed that the emission signal acquired with IF 420 nm is
mainly from the CN (B-X) transition, while the weak emission signal
of the outer layer acquired with IF 430 nm is difficult to determine.
The emission wavelength of atomic N is covered by the 430nm inter-
ference filter, but the atomic N should be confined in the discharge
channel, where the energetic electrons can dissociate nitrogen mole-
cules and excite the N atoms. A more possible explanation is that the
weak layered emission signal comes from the CH (A-X) transition,
which has been detected when the methane concentration in nitrogen
is higher.
25
Here, the CH (A) emission is too weak compared to that
from CN (B) and thus not detected by the spectrometer. In the wave-
length range of 570– 680 nm, the CN (A
2
Q
I
X
2
P
þ
) red emission is
identified by comparison with the reported spectra.
27
Since the outer
layer is detectable when the OG 550 filter is used, we can confirm the
existence of CN (A) in the outer layer.
The dynamical behavior of this special outer layer around the GA
discharge channel is acquired using a high-speed camera. Figure 4 dis-
plays snapshots of the moving discharge column at 1 atm. The outer
layer closely surrounds the central plasma column throughout its
movement. It also propagates further out from the discharge channel,
as marked in Fig. 4. This rapid propagation may be due to the local
thermal expansion and convection of the hot gas surrounding the glid-
ing arc. However, when the outer layer moves too far away from the
plasma column, it breaks up and becomes difficult to identify (see Fig.
4). Therefore, the outer layer is not smooth that in some segments of
the plasma channel, it is distinguished with a sharp boundary, while in
others, it is broken up and blurred. From the view point of dynamical
behavior, this outer layer is similar to the flame front in combustion,
which can propagate and extinguish. When the plasma column is re-
ignited at the shortest gap between electrodes, the emission from the
shortcut plasma column decays fast at a time scale of 0.4 ms, but the
outer layer decays slower with a typical decay time of 1 ms. The slower
vanishing of the outer layer infers that it is not directly controlled by
the electric field.
This outer layer behaves similar to the flame front, and thus, a
plasma-sustained radical propagation mechanism is proposed to
explain the detected phenomena, in analogy with the flame front prop-
agation mechanism. As illustrated in Fig. 5, the plasma column is
located in the center, absorbing the externally input electrical power,
and serves as a heat and radical source, while the surrounding is the
cold N
2
-CH
4
mixture. With the heat and radical species spreading out
from the plasma column, the surrounding N
2
-CH
4
mixture is heated
up as well as mixes and reacts with the radicals from plasma. At the
mixing layer, chemical reactions (see R1–R5 in Fig. 5)
21,22,25,28
includ-
ing atomic N and excited N
2
could take place to form the excited
CH,CN,etc.,andemittheirspecificspectra.Here,thekeypointisthe
formation and transport of energetic species to support the chemical
reactions of forming excited CN and CH species. In order to verify the
formation of atomic species in the plasma column, thermodynamic
analysis based on the assumption of chemical equilibrium is first per-
formed. Since the temperature profile around the plasma column is
crucial for the thermodynamic analysis, but it is difficult to measure, a
simplified heating model is employed to simulate the temperature pro-
files around the plasma column. The governing equation of heat trans-
ferisgivenby
qcp
@Tg
@trkrTg¼Qht;(1)
where qis the gas density; c
p
is the heat capacity at constant gas pres-
sure; T
g
is the gas temperature; kis the thermal conductivity; Q
ht
denotes the heating power density. The heat capacity and the thermal
conductivity as a function of temperature are obtained from the
FIG. 4. Snapshots of the moving GA column and its outer layer.
FIG. 5. A schematic of the mechanism of the outer layer formation around the
plasma column.
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Appl. Phys. Lett. 114, 194102 (2019); doi: 10.1063/1.5097908 114, 194102-3
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database of the CEA (Chemical Equilibrium with Applications) pro-
gram from NASA.
29
Q
ht
is estimated according to the measured cur-
rent and voltage as well as the length of the plasma column. In this
work, the mean current reaches 290mA and the input power density
is around 4 kW per meter of the plasma column. With a presumed
radius of 0.5mm, the input power density in the plasma column is as
high as 2.5 10
9
W/m
3
. Assuming that all input energy converts into
heat, the peak temperature is estimated to be around 5000 K in the
plasma column, and the temperature drops with the increased radial
distance from the plasma core. It should be noteworthy that the GA
discharge is nonthermal, and so the temperature cannot be well
defined. In the N
2
discharge, as we know, the input electrical energy is
first transferred to the population of the vibrational states of nitrogen
and later converts to heat through the V-T transition. It infers that the
vibrational temperature should be higher than the translational tem-
perature.
30
As a result, the effective temperature for chemical reactions
is larger than the translational temperature due to the nonthermal
properties. Actually, according to the emission spectra of CN (B-X) in
the plasma column, the fitted vibrational temperature reaches 7000K,
higher than the peak temperature based on the thermal equilibrium.
With the assumption of chemical equilibrium, the chemical species
compositions with respect to temperature can be calculated using the
CEA program. This indicates that in the center of the plasma column,
the temperature (e.g., 5000K) is extremely high so that gas species are
partially decomposed into atoms (e.g., N, C, and H). Outside the
plasma column, with the temperature decrease, the concentrations of
those active atomic species naturally drop to form more stable species
according to the chemical equilibrium calculation. However, there
always needs some time to reach equilibrium, and at higher tempera-
ture, the lifetime of atomic species is longer. Hence, atomic species
(mainly N and H) can transport out of the plasma column. The trans-
port distance is basically determined by the local temperature and spe-
cies. When enough energetic radical species can reach the mixing
layer, highly exothermic chemical reactions occur to form the excited
species. Since the energetic radical species are sustained by the plasma
column and can be quickly consumed in the relatively cold gas, they
cannot be further away from the plasma column. This is the underly-
ing reason for the disappearance of the sharp layer when the layer
propagates too far away from the plasma column, as demonstrated in
Fig. 4.
The pressure effect on the layered structure can be explained by
the fact that the gas density and heat capacity per volume increase
with pressure. Thus, with a constant input power density, the hot vol-
ume around the plasma column naturally decreases with pressure.
Furthermore, the atomic species can be consumed faster at higher
pressure. Therefore, the outer layer can only exist closer to the plasma
column at higher pressure.
In conclusion, the structure of the GA discharge column in the
N
2
-CH
4
mixture has been visualized using different detection schemes.
A tenfold increment of emission intensity with the addition of 0.1%
CH
4
in nitrogen is found. More importantly, a special layered struc-
ture around the GA discharge column has been detected. Imaging
through spectral filters and spectral analysis indicate that this detected
outer layer is basically due to the existence of CN (A) and CH (A).
The separation of this outer layer to the plasma core decreases with
pressure. Besides, it can propagate and extinguish, behaving similar to
the flame front except that the flame is self-sustained by the fuel-
oxidant reactions, while the outer layer is sustained by the plasma col-
umn. This outer layer is detected owing to the chemical reactions of
energetic radical species from plasma and surrounding hydrocarbon
compounds to form excited CH and CN species.
This work was financially supported by the Swedish Energy
Agency, the Swedish Research Council, the Knut and Alice
Wallenberg Foundation, and the European Research Council.
REFERENCES
1
A. Fridman, A. Chirokov, and A. Gutsol, J. Phys. D: Appl. Phys. 38, R1 (2005).
2
A. Fridman, S. Nester, L. A. Kennedy, A. Saveliev, and O. Mutaf-Yardimci,
Prog. Energy Combust. Sci. 25, 211 (1999).
3
J. J. Zhu, J. L. Gao, Z. S. Li, A. Ehn, M. Ald
en, A. Larsson, and Y. Kusano,
Appl. Phys. Lett. 105, 234102 (2014).
4
J. L. Gao, C. D. Kong, J. J. Zhu, A. Ehn, T. Hurtig, Y. Tang, S. Chen, M. Ald
en,
and Z. S. Li, Proc. Combust. Inst. 37(4), 5629 (2019).
5
Y. Kusano, J. J. Zhu, A. Ehn, Z. S. Li, M. Ald
en, M. Salewski, F. Leipold, A.
Bardenshtein, and N. Krebs, Surf. Eng. 31(4), 282 (2015).
6
H. Zhang, W. Z. Wang, X. D. Li, L. Han, M. Yan, Y. J. Zhong, and X. Tu,
Chem. Eng. J. 345, 67 (2018).
7
W. Z. Wang, D. H. Mei, X. Tu, and A. Bogaerts, Chem. Eng. J. 330, 11 (2017).
8
Z. W. Sun, J. J. Zhu, Z. S. Li, M. Ald
en, F. Leipold, M. Salewski, and Y. Kusano,
Opt. Express 21(5), 6028 (2013).
9
J. J. Zhu, Z. W. Sun, Z. S. Li, A. Ehn, M. Ald
en, M. Salewski, F. Leipold, and Y.
Kusano, J. Phys. D: Appl. Phys. 47(29), 295203 (2014).
10
A. El-Zein, M. Talaat, G. El-Aragi, and A. El-Amawy, IEEE Trans. Plasma Sci.
44(7), 1155 (2016).
11
N. C. Roy, M. G. Hafez, and M. R. Talukder, Phys. Plasmas 23(8), 083502
(2016).
12
X. Tu, H. J. Gallon, and J. C. Whitehead, IEEE Trans. Plasma Sci. 39(11SI1),
2900 (2011).
13
N. C. Roy and M. R. Talukder, Phys. Plasmas 25(9), 093502 (2018).
14
Y. D. Korolev, O. B. Frants, V. G. Geyman, N. V. Landl, and V. S. Kasyanov,
IEEE Trans. Plasma Sci. 39(12SI1), 3319 (2011).
15
O. Mutaf-Yardimci, A. V. Saveliev, A. A. Fridman, and L. A. Kennedy, J. Appl.
Phys. 87(4), 1632 (2000).
16
S. Pellerin, F. Richard, J. Chapelle, J. M. Cormier, and K. Musiol, J. Phys. D:
Appl. Phys. 33(19), 2407 (2000).
17
S. Kolev and A. Bogaerts, Plasma Sources Sci. Technol. 24(1), 015025 (2015).
18
G. Trenchev, S. Kolev, and A. Bogaerts, Plasma Sources Sci. Technol. 25(3),
035014 (2016).
19
S. R. Sun, S. Kolev, H. X. Wang, and A. Bogaerts, Plasma Sources Sci. Technol.
26, 055017 (2017).
20
A. F. Gutsol and S. P. Gangoli, IEEE Trans. Plasma Sci. 45, 555 (2017).
21
G. Dilecce, P. F. Ambrico, G. Scarduelli, P. Tosi, and S. De Benedictis, Plasma
Sources Sci. Technol. 18(1), 015010 (2009).
22
J. Pereira, V. Massereau-Guilbaud, I. Geraud-Grenier, and A. Plain, Plasma
Processes Polym. 2(8), 633 (2005).
23
C. D. Pintassilgo, J. Loureiro, G. Cernogora, and M. Touzeau, Plasma Sources
Sci. Technol. 8(3), 463 (1999).
24
M. Kareev, M. Sablier, and T. Fujii, J. Phys. Chem. A 104(31), 7218 (2000).
25
H. Zhang, C. M. Du, A. J. Wu, Z. Bo, J. H. Yan, and X. D. Li, Int. J. Hydrogen
Energy 39(24), 12620 (2014).
26
P. H. Joo, J. L. Gao, Z. S. Li, and M. Ald
en, Rev. Sci. Instrum. 86(3), 035115
(2015).
27
N. Nishiyama, H. Sekiya, M. Tsuji, and Y. Nishimura, Rep. Inst. Adv. Mater.
Study 1, 35–50 (1987).
28
D. W. Setser and B. A. Thrush, Nature 200(490), 864 (1963).
29
S.GordonandB.J.McBride,NASAReferencePublicationReportNo.1311,1996.
30
C. D. Kong, J. L. Gao, J. J. Zhu, A. Ehn, M. Ald
en, and Z. S. Li, Phys. Plasmas
24(9), 093515 (2017).
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