Characterization of HIFU transducers designed for sonochemistry application: Acoustic streaming

Area of Science:

• Acoustics
• Fluid Dynamics
• Chemical Engineering

Background:

• High Intensity Focused Ultrasound (HIFU) sonoreactors are crucial for various chemical processes.
• Cavitation distribution and its quantification via Sonochemiluminescence (SCL) are well-documented.
• Understanding acoustic streaming is vital for optimizing HIFU reactor performance.

Purpose of the Study:

• To investigate acoustic streaming in HIFU sonoreactors using complementary methods.
• To compare mass transfer measurements with optical methods like Particle Image Velocimetry (PIV).
• To analyze the influence of cavitation modes on hydrodynamic behavior.

Main Methods:

• Electrodiffusional method for mass transfer measurements.
• Particle Image Velocimetry (PIV) for optical flow visualization.
• Experiments conducted with and without electrodes to control wave propagation perturbations.

Main Results:

• Maximum acoustic streaming velocity shifts from the focal point towards the transducer, especially at higher powers.
• Both stationary and moving cavitation bubbles significantly impact hydrodynamic behavior.
• Electrochemical measurements corroborate the shift of the active focal zone and low activity in the post-focal region.
• Observed equivalent liquid velocities are an order of magnitude higher than PIV measurements, indicating bubble-induced effects.

Conclusions:

• HIFU cavitation focus shifts towards the transducer, influenced by power and cavitation modes.
• Bubble oscillation and collapse near surfaces are primary drivers of enhanced mass transfer, exceeding pure acoustic streaming.
• Wave distortion at the sonochemical focal point leads to increased absorption and high activity.