Some investigations on the use of ultrasonics in hydrodynamic cavitation control

Abstract

Ultrasonic Control of Hydrodynamic Cavitation: Experimental and Theoretical Studies

Abstract and Synopsis Introduction This thesis investigates the possibility of using ultrasonics to control hydrodynamic cavitation. Both travelling bubble cavitation and shear layer cavitation are studied through experiments and theoretical models.

Travelling Bubble Cavitation

Generated in a venturi system.

Cavitation nuclei number and size distribution upstream of the venturi were manipulated using ultrasonic waves produced by a cylindrical piezoelectric crystal, termed the Ultrasonic Nuclei Manipulator (UNM).

Acoustic pre-cavitation at the UNM location was examined in both continuous-wave (CW) and pulsed excitation modes.

CW excitation performance depended strongly on dissolved gas concentration (C), while pulsed excitation was effective across the entire range of C.

Pulsing parameters (number of bursts, pulsing period) were varied to understand this behavior.

Numerical and experimental studies of bubble-acoustic field interactions modeled the physics of cavitation control.

Deployment of two crystals in tandem showed improved performance compared to a single crystal.

Benefits: reduced cavitation-induced noise, delayed onset of cavitation, and increased maximum flow rate.

Example: Two-crystal UNM delayed cavitation onset by 45% when C < 0.5 and by 10% when C 1.

Maximum velocity increased from ~15 m/s (UNM off) to ~22 m/s (two-crystal UNM on) at low C.

Shear Layer Cavitation

Studied downstream of a nozzle with sudden expansion.

Cavitation control achieved by modifying bubble pressure via an ultrasonic pressure field imposed on the flow using the Ultrasonic Pressure Modulator (UPM).

Numerical and experimental results indicated that control requires driving frequency higher than the resonant frequency of the nuclei.

Experiments at 690 kHz showed an almost monotonic increase in flow rate with increasing driving power to UPM.

This novel approach reduces cavitation-induced noise and extends the operating regime of flow-measuring devices such as orifices and flow nozzles.

Conclusions Ultrasonics can effectively manipulate cavitation nuclei and delay cavitation onset.

Pulsed excitation is robust across dissolved gas concentrations, while CW excitation is sensitive to gas content.

Tandem crystal deployment enhances cavitation control.

Shear layer cavitation can be controlled by ultrasonic pressure modulation, improving flow measurement accuracy and reducing noise.