VISUALIZATION OF ACOUSTIC PARTICLE INTERACTION AND AGGLOMERATION - THEORY AND EXPERIMENTS

This paper presents new insights in interaction mechanisms between sma ll particles under the influence of a strong acoustic field. These mec hanisms are associated with acoustic agglomeration, an effect that cau ses relative motion and collisions between fine particles entrained in gaseous media. The agglomeration process has potential use in air pol lution control to enhance the performance of conventional particle fil tering devices in the fine particle size range. In Sec. I of this pape r, a number of existing acoustic agglomeration models are reviewed and implemented into a numerical scheme. A quantitative analysis of the p roposed theories is conducted with parameters representing those of th e experimental Sec. II. Simulations based on the scattering, orthokine tic, and hydrodynamic agglomeration principles generate numerical traj ectories which reflect their importance in the particle interaction pr ocess. Most importantly it is shown that a hydrodynamic effect based o n asymmetric flow fields around the particles (acoustic wake effect) g enerates significant particle attraction in the direction of the acous tic velocity vector. To evaluate these theoretical findings, Sec. II o f this paper presents new experimental observations of microscopic par ticle trajectories in an intense acoustic field. The-experiments are c arried out in a small-scale observation chamber using a CCD camera in conjunction with a high-resolution video system. A homogeneous acousti c velocity field is generated by two rectangular, hat-membrane loudspe akers which comprise two opposing walls in the observation chamber. Gl ass microspheres (diameters 8.1 and 22.1 mu m) and arbitrarily shaped, quartz particles (diameter <50 mu m) are used for the observation of interaction and agglomeration trajectories under the influence of an i ntense acoustic velocity field (1.2-0.53 m/s @400-900 Hz, respectively ). The recorded digitized images show a number of different interactio n phenomena as well as one distinct pattern that resembles the shape o f tuning fork (thus called the tuning fork agglomeration). The latter appears to be the predominant agglomeration mechanism leading to rapid particle approach and multiple, subsequent particle interactions at l arge acoustic velocities. (C) 1996 Acoustical Society of America.