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A new method for producing nonspherical
cavitation bubble using flexible electrodes
Bai Lixin, Xu Wei-lin, Deng Jingjun, Li Chao, Xu Delong
13th - 16th August 2012
Singapore
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Motivation and Background
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Motivation and Background
Reyleigh(1917), Plesset(1971), Mørch(1979), Leighton(2000), Versluis(2000), and Ohl(1995)
In the field of cavitation research the assumption of a spherical shape for the entire bubble lifetime helped to evaluate and
interpret many experimental results.
Kornfeld & Suvorov(1944), Naude & Ellis(1961), Lauterborn(1975), Tomita(1986), Vogel(1989),
Philipp(1998), Lindau(2003), Hara(1984) and Kezios(1986), Ceccio & Brennen(1991)
But in fact, nonspherical bubbles occur commonly in cavitation phenomenon. Surface instabilities induced by rigid boundaries,
neighboring bubbles, flow disturbances, can cause the bubble shape to deviate from the spherical.
Tomita(2000), Vogel(1994), Ohl(1998)
Besides the nonspherical cavitation bubble mentioned above, the laser-induced or spark-induced cavitation bubble is
nonspherical in the very earlier stage of formation. When the bubble collapses, disturbances stored in the flow field are
amplified and can severely influence the bubble shape at minimum volume and might make it become nonspherical.
Kang Yuan Lim(2010)
To investigate nonspherical cavitation bubbles, methods for producing nonspherical bubbles are needed. Kang Yuan Lim put
forward a technique of generating arbitrarily shaped nonspherical laser-induced cavitation bubbles inside a liquid gap. But the
bubble surface is not smooth and boundary influences the bubble dynamics in the final stage of bubble collapse.
So we propose a new method, in this paper, as supplementary, for producing nonspherical isolated
cavitation bubble using flexible electrodes in a quiescent liquid without the disturbance of boundary and
flow field.
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Experimental Setup
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High voltage pulse is produced using
a needle-plate discharge device.
Water resistor R1
Voltage changer T2
Switch
K
The frames are
illuminated with a cold
light resource (150 W)
and fibre bundle.
Silicon stack
D
The initial high voltage pulse between the
electrodes (10kV) formed plasma allowing
the fast discharge of a previously charged
capacitor with a capacitance of 0.25μF.
Needle plate
Capacity C
Voltage
regulator
T1
R2
Electric
resistance
Power
supply
R3
Hloder
GND
GND
Slider
Cold light
source
Optical
system
Flexible electrodes
Light
fibers
The movements of
cavitation bubbles are
recorded with a CMOS
high-speed camera
equipped with a long
distance microscope
High-speed
camera
Bubble chamber
Flexible tungsten electrodes are used in the
experiment. The electrode is helix-shaped. The
distance between two electrodes can be
adjusted arbitrarily.
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The electrodes are placed in a glass
chamber (170 × 50 × 90 mm3) filled with
deionized water.
Results
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Grid electrodes - Spark discharge – Spherical bubble
Cavitation bubble
Stiff electrodes
2 mm
1
2
3
4
5
6
The growth and collapse of spherical cavitation bubble induced by spark. (Frame rate 5000 fps. Exposure time 20μs.)
High voltage
9000 V
A spherical cavitation bubble can
be induced by spark discharge
with grid electrodes
First of all, charge a capacitor
with a high voltage of 9000v
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Discharge breakdown
Deionization
Then spark breakdown in water, and the initial high
voltage between the electrodes formed plasma allowing
the fast discharge of a previously charged capacitor.
Shortly after this discharge, formed a volume of
superheated water vapor that gave rise to a bubble.
Finally, deionization.
Flexible electrodes - Arc discharge – Nonspherical bubble
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Flexible electrodes - Arc discharge – Nonspherical bubble
Attract each other
High voltage
Temperature jumps
Touching the electrodes
At the beginning of
discharge, the two
flexible electrodes
attract each other.
Separate from
each other
Cavitation bubbles formed
At the instant of two electrodes touching
each other, electric current density at
contacts is very high. The temperature at
the contracts jumps because of contact
resistance. A cavitation bubbles is formed
by rapid evaporation of the surrounding
fluid.
Plasma produced
Deionization
Nonspherical cavitation bubble
Then the elastic force of electrodes cause
the two electrodes separate from each
other. Plasma is produced by the high
electric-field intensity in the narrow gap of
electrodes. The cavitation bubble becomes
highly nonspherical with the separation of
electrodes. The electrodes gap resumes the
insulating state after the pulse discharge. A
nonspherical bubble is formed.
The flexibility of electrode is the precondition to generate nonspherical cavitation bubble (two electrodes
touch each other and then separate from each other) and is important for the shape of nonspherical bubble.
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The growth and collapse of a nonspherical cavitation bubble. The air bubble placed
on the electrode is an indicator. (Frame rate 3000 fps. Exposure time 23μs.)
The discharge voltage and discharge time of electrodes can be
controlled by adjusting the gap of the needle-plate discharge
device and variable resistance (R2). The gas content in the
nonspherical bubble (hydrogen, oxygen, water vapor) can be
changed in this way. The collapse velocity of nonspherical bubble
in the right figure is slower than that in left figure. The minimum
volume during the collapse in the right figure is larger than that in
left figure. Gas bubbles left after the bubble collapse. The noncondensable gas in the bubble will significantly slow down the
velocity of bubble wall. The relation between the time scale of
bubble motion and the time scale of mass and thermal diffusion
can be changed by adjusting the parameter of high-voltage pulse.
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The influence of high-voltage pulse on the production of nonspherical
cavitation bubble. (Frame rate 3000 fps. Exposure time 23μs.)
A small structure on the tip of electrodes can
influence the production of nonspherical
cavitation bubble (as shown in this figure ).
At the early stage of growth the cavitation
bubble is approximate spherical. The bubble
grows bigger in the direction of point end
with the separation of electrodes and the
growth of bubble. The shape of electrode tip
can influence arc and the shape of
nonspherical bubble.
The influence of electrode tip on the production of nonspherical cavitation bubble.
(Frame rate 3000 fps. Exposure time 23μs.)
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Discussion and Conclusion
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A
B
The repeatability of nonspherical cavitation bubbles. A and B are in the same experiment with time interval of 0.3s
(Frame rate 3000 fps. Exposure time 23μs.)
Generally speaking, arc induced nonspherical bubble is hard to accurately control. It is impossible to
have two identical nonspherical bubbles, especially the surface character in its maximum volume
and the shape in its minimum volume, because the high-voltage pulse and the motion of flexible
electrodes are all dynamic process, the coupling of the two dynamic process is very complicated. But
rough similar nonspherical bubbles can be produced by adjusting the parameter of high-voltage
pulse, the gap of electrodes, flexibility of electrode and small structure on the tip of electrode (as
shown in this Figure ).
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A new method for producing an isolated nonspherical cavitation bubble in
infinite liquid using flexible electrodes is developed. The motion of electric
arc-induced nonspherical cavitation bubble is followed by using high-speed
photography. This method may be useful in the investigation of bubble
dynamics. Further experimental researches on the production and control
of nonspherical cavitation bubble are needed. I will be happy if anyone use
this method to produce nonspherical bubbles.
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Thank you
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