Separation of aerosol fine particles by high-power ultrasound

Abstract

The application of high-intensity acoustic fields to an aerosol can produce the agglomeration of the suspended particles, shifting their size distribution to a large range. Acoustic agglomeration is a process in which acoustic forces cause particles to interact and eventually to collide. The complex mechanism of this process involves orthokinetic and hydrodynamic effects. The orthokinetic effect refers to direct collisions produced among particles of different size and/or density which are differently entrained by the acoustic field. The hydrodynamic effects refer to particle interactions generated through the surrounding fluid by hydrodynamic forces induced by the acoustic field. One important hydrodynamic effect is the acoustic wake effect which is related to the asymmetry of the flow field around the moving particles. Acoustic agglomeration has a potential use in the separation of fine particles (smaller than 2.5 μm) which are emitted from industrial and residential combustion and from vehicle exhaust. In this paper recent theoretical and experimental advances in the knowledge of the basic mechanisms of acoustic agglomeration are presented, together with some results about the improvement of fine particle separation efficiency of conventional filters when the aerosol is previously treated with high-power ultrasound. [The authors acknowledge the support of CICYT-AMB96-A211-C02.]