U nderstanding the fluid dynamics associated with macro scale ultrasonic separators

Abstract

Acoustic standing wave fields are widely used in MEMS applications to separate micron sized particles from fluids. However, the use and understanding of macro scale ultrasonic separators are still limited and challenging. These systems rely on acoustic radiation forces for trapping and clumping of dispersed phase particles. The clumps of particles then continuously separate out due to enhanced gravity or buoyancy in a flowing system. Typical flow Reynolds numbers are less than 50, particle concentrations up to 20%, ultrasonic standing wave fields at frequencies of 2 MHz, and acoustic pressure amplitudes of about 1 MPa. At such small Reynolds numbers, the flow is dominated by shear forces and the drag on clumps of particles is significantly lower than Stokes drag on a single particle. The fluid dynamics associated with these systems is extremely complex due to the coupling between the fluid flow field, suspended particles, and acoustic radiation forces. This work discusses the key physics involved and explains our current understanding of operation of macro scale acoustic separators. The status of CFD efforts to predict the flow fields and particle clumping in such systems is presented and compared to experimental results.