Ultrasonic horn transducers are frequently used in applications of acoustic cavitation in liquids, for instance, for cell disruption or sonochemical reactions. They are operated typically in the frequency range up to about 50 kHz and have tip diameters from some mm to several cm. It has been observed that if the horn tip is sufficiently small and driven at high amplitude, cavitation is very strong, and the tip can be covered entirely by the gas/vapor phase for longer time intervals. A peculiar dynamics of the attached cavity can emerge with expansion and collapse at a self-generated frequency in the subharmonic range, i.e., below the acoustic driving frequency. Here, we present a systematic study of the cavitation dynamics in water at a 20 kHz horn tip of 3 mm diameter. The system was investigated by high-speed imaging with simultaneous recording of the acoustic emissions. Measurements were performed under variation of acoustic power, air saturation, viscosity, surface tension, and temperature of the liquid. Our findings show that the liquid properties play no significant role in the dynamics of the attached cavitation at the small ultrasonic horn. Also the variation of the experimental geometry, within a certain range, did not change the dynamics. We believe that the main two reasons for the peculiar dynamics of cavitation on a small ultrasonic horn are the higher energy density on a small tip and the inability of the big tip to wash away the gaseous bubbles. Calculation of the somewhat adapted Strouhal number revealed that, similar to the hydrodynamic cavitation, values which are relatively low characterize slow cavitation structure dynamics. In cases where the cavitation follows the driving frequency this value lies much higher probably at Str ) 20. In the spirit to distinguish the observed phenomenon with other cavitation dynamics at ultrasonic transducer surfaces, we suggest to term the observed phenomenon of attached cavities partly covering the full horn tip as acoustic supercavitation. This reflects the conjecture that not the sound field in terms of acoustic (negative) pressure in the liquid is responsible for nucleation, but the motion of the transducer surface.
COBISS.SI-ID: 13368347
Experiments within the cavitation erosion incubation period were performed on simple and modified two-dimensional hydrofoils with circular leading edges. A pit-counting method, based on computer-aided image processing, was used for direct measurement of the cavitation erosion by evaluating the damage of the surface. Cavitation phenomenon above hydrofoils at different flow conditions (pressure, water gas content, flow velocity) was observed. A clear relation between characteristics of cavitation structures and cavitation damage was established. A study of influence of gas content in water and flow velocity on the cavitation erosion aggressiveness was performed. There we found a clear influence which shows a drop in aggressiveness of cavitation erosion as the gas content of water is increased. Also a power law was confirmed for velocity influence on cavitation erosive aggressiveness. Due to the extreme length of experiments, many studies tend to perform tests only within the incubation period and the mass loss rate is then predicted by extrapolation. A rotating disc test rig that generates a very aggressive cavitation and pure copper specimens, as erosion sensors, were used to investigate the correlation between the damage within the incubation period and mass loss rate. Like in the case of a single hydrofoil we also observed dependency of the cavitation erosive aggressiveness on the size and dynamics of cavitation structures. Results presented in these studies will serve as a basis for achieving the final goal of the ongoing work-to develop a method that will enable accurate prediction of cavitation erosion with minimal experimental effort or even solely by using computational fluid dynamics.
COBISS.SI-ID: 13369115
An experimental investigation has been made to detect cavitation in a pump-storage hydropower plant prototype suffering from leading edge cavitation in pump mode. Vibrations and acoustic emission on the housing of the turbine bearing and pressure fluctuations in the draft tube were measured and the corresponding signals were recorded and analyzed. The analysis was based on the analysis of high-frequency content of measured variables. The pump-storage hydropower plant prototype has been operated at various input loads and Thoma numbers. Several estimators of cavitation were evaluated according to a coefficient of determination between the Thoma number and cavitation estimators. The best results were achieved with a compound discharge coefficient cavitation estimator that is based on the discharge coefficient and several rms estimators: vibrations, acoustic emission, and pressure fluctuations. The compound discharge estimator was set as a product of the rms estimator and the squared discharge coefficient. Cavitation estimators were evaluated in several intervals of frequencies; the best frequency interval for the vibration sensor on the turbine cover was from 24 to 26 kHz, for the vibration sensor on the guide vane 36-40 kHz, for the acoustic emission sensor on the turbine cover 140-145 kHz, and for the pressure fluctuation sensor in the draft tube wall 130-150 kHz.
COBISS.SI-ID: 13375771