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

Motivation and Background

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) Tomita(2000), Vogel(1994), Ohl(1998) 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. 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.

Experimental Setup

High voltage pulse is produced using a needle-plate discharge device. 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. Flexible tungsten electrodes are used in the experiment. The electrode is helix-shaped. The distance between two electrodes can be adjusted arbitrarily. The electrodes are placed in a glass chamber (170 × 50 × 90 mm 3 ) filled with deionized water. The movements of cavitation bubbles are recorded with a CMOS high-speed camera equipped with a long distance microscope The frames are illuminated with a cold light resource (150 W) and fibre bundle.

Results

Stiff electrodes Cavitation bubble 2 mm Grid electrodes - Spark discharge – Spherical bubble 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 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. The growth and collapse of spherical cavitation bubble induced by spark. (Frame rate 5000 fps. Exposure time 20μs.)

Flexible electrodes - Arc discharge – Nonspherical bubble

At the beginning of discharge, the two flexible electrodes attract each other. 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. 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. Flexible electrodes - Arc discharge – Nonspherical bubble

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 influence of high-voltage pulse on the production of nonspherical cavitation bubble. (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 non- condensable 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.

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.)

Discussion and Conclusion

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.) A B 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 ).

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.

Thank you