22nd International Symposium on Plasma Chemistry
July 5-10, 2015; Antwerp, Belgium
Atmospheric discharge using an interface of magnetic fluid
S. Uehara and H. Nishiyama
Institute of Fluid Science, Tohoku University, Sendai, Miyagi, Japan
Abstract: A novel device of corona discharge using magnetic fluid for air cleaning system
is investigated in this paper. High-frequency AC voltage is applied between magnetic fluid
spike formed under the magnetic field and flat-plate electrode. Characteristic of
atmospheric discharge is investigated by measuring a discharge current. Efficiency of the
device is evaluated by measuring the ozone concentration.
Keywords: atmospheric discharge, magnetic fluid spike, pollutant degradation
1. Introduction
Corona discharge is widely utilized for air purification
device [1, 2]. A high chemical activity of radicals which
are generated by non-thermal plasma is efficient for
decompose a pollutant such as SO x and NO x in the air.
Furthermore, polluted micro particle in the vicinity of
electrode is charged by the plasma. Then, charged
particle is adsorbed on the surface of electrode by
Coulomb force.
Corona discharge is initiated when the charged object
has a sharp point to generate a locally high electrical
potential. Hence, needle-like shape electrode is required
for the device.
The problem of corona discharge in the device is that
the surface of electrodes become damaged because of
oxidation by radicals such as ozone generated by plasma.
It should be avoided to sustain a stable discharge.
In this paper, a novel device of atmospheric discharge
using magnetic fluid spike for air purification system is
investigated to solve these problems.
Magnetic fluid can be formed spike-like structures
naturally under the magnetic fields (Fig. 1). Therefore, it
is utilized as a needle electrode for initiate a discharge in
atmosphere. Furthermore, due to the fluidity of an
electrode of a magnetic-fluid spike, oxidation problem of
electrode surface can be overcome by renewing the
surface with the fluidity.
In the experiment, high-frequency AC voltage is
applied between the electrodes in order to initiate the
atmospheric discharge. The purification performance is
evaluated by measuring a concentration of generated
ozone.
2. Experimental methods
Magnetic fluid is a suspension of nano-sized iron
particle in a carrier liquid. Under a gravity and magnetic
field, the forces which subject to magnetic fluid and
surface tension are counterbalance with forming a spikelike shape [3]. In this research, water base magnetic fluid
was used to obtain the fundamental data of atmospheric
discharge characteristics.
Fig. 2 shows the schematic of atmospheric discharge
using interface of magnetic fluid. Upper stainless
electrode is set above the magnetic fluid spikes. The
surface of the upper electrode is covered by dielectric tape
in order to prevent discharge from transition to arc
discharge.
Fig. 2. Schematic of atmospheric discharge using
magnetic fluid interface.
Fig. 1. Image of magnetic fluid under the magnetic field
of neodymium magnet which set under the magnetic
fluid.
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The distance between upper and bottom electrode is
variable by controlling the micro positioning stage. In this
study, it is fixed as 5 mm. The distance between the spike
tip and upper electrode is changed depending on the size
of a spike.
Magnetic fluid is dropped directly on the bottom
electrode. Neodymium magnet is set 15 mm below the
electrode and acrylic stage in order to form the magnetic
fluid spike. AC voltage applied from a function generator
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(WF1973, NF, inc.) is amplified by high voltage amplifier
(Model 20/20c, Trek, Inc.).
Discharge current is measured using a current probe
(TCP 312, Tektronix, Inc.). High voltage probe (P6015A,
Tektronix, Inc.) is used to measure voltages which applied
between electrodes. Digital oscilloscope (DSO-X3034A,
Agilent Technologies, Inc.) is used to obtain the electric
data of discharge characteristics.
3. Results and discussions
Fig. 3 shows the typical wave form of applied voltage
and discharge current. Spike-like current caused by the
discharge is measured when positive voltage is applied to
bottom electrode.
Fig. 4. Image of atmospheric discharge from tip of the
spikes of magnetic fluid.
Fig. 5. Distance between the surface of upper electrode
and a tip of magnetic-fluid spike. Filled symbols indicate
the case of visible discharge is observed.
Fig. 3. Waveforms of applied voltage of 5.8 V pp and
2000 Hz (red line, left vertical axis) and discharge current
(blue line, right vertical axis)
Fig. 4 shows the image of atmospheric discharge from a
tip of spikes when the 5.8 V pp and 2000 Hz AC voltage is
applied between the electrodes. Red-purple colored
discharge is observed from the tip of each spikes of
magnetic fluid. Focusing on the spike located on the right
side, around the tip of the spike is most bright. The
discharge is progress toward the upper electrode with
partial break down. However, the streamer from center
spike is reached to the upper electrode with total electric
break down. The reason of these differences is caused by
the non-uniformity of the distance between upper
electrode and tip of magnetic-fluid spike.
The permanent magnet set under a magnetic fluid is
cuboid shape. Therefore, magnetic field is not uniform
hence the shape of spikes is not uniform. It should be
noted that the tip of a spike of magnetic fluid is sharpen
when the discharge is initiate.
Fig. 5 shows the variation of the distance between
upper electrode and tip of magnetic-fluid spikes for
discharge. Three spikes are observed in Fig. 5. The
distance decreases as applied voltage increase. This result
indicates that charged magnetic fluid is stretched toward
the upper electrode by increasing electric field. The
distances decreased drastically before a discharge is
initiated.
2
Fig. 6 shows the concentration of ozone generated
during the discharge. The concentration increases with
the increase of the frequency. Maximum concentration
was 46 ppm. It is found that the device has a possibility
to decompose organic pollutants.
However, it is
necessary to evaluate the efficiency of ozone generation
with considering the input energy.
Fig. 6. Concentration of ozone generated in the condition
of 5.8 kV pp and 2000 Hz.
4. Conclusions
A novel device of atmospheric discharge using
magnetic fluid for air purification system is investigated.
The characteristic of purification performance is
evaluated by measuring the ozone concentrations.
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5. Acknowledgements
This work was supported by JSPS KAKENHI Grant
Number 26630045.
6. References
[1] K. Urashima, et al. IEEE Trans. Dielectr. Electr.
Insul., 7, 602 (2000)
[2] R. Hackam, et al. IEEE Trans. Dielectr. Electr.
Insul., 7, 654 (2000)
[3] Sudo, et al. Energ. Convers. Manage., 43, 289-297
(2002)
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