2007 年 2 巻 1 号 p. 77-87
In a solid-liquid two-phase flow, the separation of particles or a reduction of particle concentration is often desired. Utilization of ultrasound has been considered as one technique for the separation of particles. Particles are known to aggregate due to the radiation pressure of ultrasound. However, the effect of ultrasound including cavitation on particle behavior is not well understood. Thus, we horizontally irradiated water with aluminum particles having a density of 2720 kg/m3 and diameters of 50 to 150 μm or smaller aluminum powder (flakes) in a rectangular vessel by ultrasound at frequencies of 23 kHz or 100 kHz. In this way, a standing wave was formed. The following results were obtained. For ultrasound of 100 kHz, the aluminum powder aggregates in vertical bands. When acoustic cavitation existed at the frequency of 23 kHz, we noticed that the aluminum particles aggregate as clumps near antinodes of the sound pressure profile because of the flow induced by acoustic cavitation. When the particles are continuously provided in a relatively high concentration, particle clumps form and remain. Then, the particle clumps become larger and suddenly fall faster than the surrounding small particles. Such phenomena repeat themselves periodically. At relatively low concentrations, particle clumps do not become large and remain stationary at the same positions.
