Efficiency of Energy Conversion in Underwater Spark Discharges and Associated Bubble Oscillations: Experimental Results

In this paper low-voltage high-energy spark discharges in water and associated bubble oscillations are studied experimentally. Pressure waves emitted during spark discharges and during following bubble oscillations have been recorded and analyzed to determine the radiated acoustic energy and potential energy of the bubbles. These energies are then used to determine the efficiency of energy conversion. For example, it has been found out that only 2% to 8% of electrical energy stored in the capacitor bank is converted into the potential energy of the bubble at its first maximum volume and only about 30% of this potential energy is radiated as acoustic energy in the first bubble pulse. Due to specific features of the low-voltage apparatus used, bubbles of different sizes have been generated and these bubbles were found to oscillate with different intensities. To explain this great diversity in the generated bubbles, the electrical circuit of the apparatus and the early stages of the spark discharge are analyzed. While it is confirmed that the bubble size grows with the energy used for discharge, the relation between the intensity of bubble oscillations and the circuit parameters is more complex. To explain this relation, two hypotheses, partly supported by the data available, are presented. The first hypothesis concerns the rate of electrical energy delivery into the discharge channel, and the second hypothesis concerns the efficiency of electrical energy conversion into heat in the discharge channel.