Investigation of a multi-frequency ultrasonic acoustic pressure source for acoustic agglomeration.

This paper represents numerical and experimental investigations of an ultrasonic multifrequency piezoelectric acoustic pressure source whose target application is acoustic agglomeration of fine and ultrafine particles. The operation of source is based on three vibration modes at 25.83 kHz, 34.73 kHz and 52.41 kHz. Multi-frequency operation allows to obtain three different patterns of acoustic pressure levels which allows to increase performance of the agglomeration process while particles sizes change over time or process. Moreover, acoustic pressure levels, as well as their patterns, were investigated while the source was driven by rectangular and sawtooth signals. Excitation by nonharmonic signals ensured possibility of obtaining modified patterns which results changes in the acoustic pressure levels gradients and allows to obtain different amplitudes of particles vibrations in the agglomeration chamber. Results of numerical and experimental investigations have shown that the ultrasonic acoustic pressure source under excitation by square and sawtooth signals is able to provide maximum sound pressure in the range from 121.6 dB to 132.2 dB while maximum SPL values generated by harmonic signal were indicated in range from 116.4 dB to 129.3 dB. Finally, experimental investigations of acoustic fields impacting particle decrement in air flow have shown that generation of acoustic field by square and sawtooth-shaped signals is able to provide up to 21.38 % and 27.88 % decrement level of 0.3 µm and 1 µm sized particles.