Effect of liquid subcooling on acoustic characteristics during the condensation process of vapor bubbles in a subcooled pool

Sound characteristics of direct contact condensation of vapor bubbles in a subcooled pool were investigated experimentally with a hydrophone and a high-speed video camera. Three different condensation modes were observed, which were referred to as shape oscillation regime, transition regime and capillary wave regime in the paper. Time domain analysis indicated that the acoustic signals were boosted in their maximum amplitude with increase in subcooling, while their standard and average absolute deviations shifted to decrease after reaching a peak value. In addition, two different waveforms were found, possible sources of which were split-up and collapse of bubbles, respectively. From the amplitude spectra obtained by FFT, the first dominant frequency was found at frequency of 150-300 Hz for all condensation regimes, whereas some peaks in high frequency region were observed only for the capillary wave regime. The first dominant frequency was the result of the periodic variation in the vapor bubble volume, and the peaks in high frequency region were due to the high-frequency oscillation of water in pressure caused by sudden bubble collapse. The frequency of first peak was considered to be resulted from the periodic bubble collapse or split-up and thus was close to the bubble collapse frequency obtained from snapshots of bubble condensation. Moreover, according to results of short-time Fourier transform (STFT), the time intervals in which a certain process of bubble condensing occurred could be well known.