Mamoru Kuwabara

Effect of Process Parameters on Ultrasonic Separation of Dispersed Particles in Liquid

The effects of process parameters on ultrasonic separation of dispersed particles in a liquid using a standing-plane-wave field are discussed on the basis of experimental and theoretical results. Numerical solution of the equation of motion of a fine particle in a standing-wave field indicates that the inertia term can be neglected during conventional ultrasonic separation of fine particles. Analytical solutions for the particle speed, the position at which particles are coagulated, and the minimum power for separation, have then been derived to incorporate key process parameters. Experiments are carried out to observe transitional coagulation of polystyrene particles in an aqueous sugar solution with the incidence of standing ultrasonic plane wave, in terms of the density difference as well as the acoustic energy density exerted. Experimental results agree well with the theoretical predictions. The time required for coagulating and for the separation of particles is shortened in the case that particles coalesce.

Orientation of Fibers in Liquid by Ultrasonic Standing Waves

The orientation of fibers in a liquid irradiated with ultrasound is studied to realize the noncontact directional control of reinforcing fibers in the molten matrix of a composite material. The equations of translational and rotational motions of a fiber in a standing wave field are derived. The numerical solutions show the movement of the fibers at various initial positions and their stable positions and directions. Experiments are performed using polystyrene fibers of various lengths suspended in an aqueous sugar solution. Both numerical and experimental results indicate that polystyrene fibers shorter than one-fourth of the wavelength are constrained at the pressure node and are oriented in the direction perpendicular to that of wave propagation. On the other hand, fibers ranging from one-fourth to one-half of the wavelength have orientation either parallel to the direction of wave propagation at the pressure loop or perpendicular to that at the pressure node depending on their initial positions and directions.

Relationship between a Standing-Wave Field and a Sonoluminescing Field

The relationship between a sound field and a multibubble sonoluminescing field has been studied experimentally and analytically to clarify the sonochemical reaction field. The variation of each field in a rectangular glass cell filled with distilled water or a luminol solution was investigated while changing experimental conditions such as driving frequency, applied voltage to the transducer, and thickness of the cell bottom. As for the sonoluminescing field, the intensity of sonoluminescence (SL) was measured and its distribution was observed. As for the sound field, sound energy density was analyzed theoretically and sound pressure distribution was observed optically. Comparing them, it became clear that SL occurred at pressure antinodes of a standing-wave field in the cell. There are upper and lower thresholds of the sound pressure for SL to occur. This explains the amplitude dependence and spatial nonuniformity of SL in a standing-wave field.

Influence of High-Power Ultrasonic Irradiation on Primary Nucleation Process during Solidification

In this paper, we describe the effects of ultrasonically preformed cavitation multibubbles in molten metal on both the onset of nucleation and the grain refinement of the solidified metal. Woods alloy, that is, a bismuth-based quaternary eutectic alloy having a low melting point, was employed in the experiments. It was found that the irradiation of ultrasound at a frequency of 20 kHz and a power of 800 W for a prescribed time before solidification can affect the solidification microstructures of the alloy even if no ultrasound is additionally irradiated during solidification. Scanning electron microscope (SEM) image analysis showed that the microstructures were refined by irradiating ultrasound before solidification. This result suggests that numerous ultrasonically induced nanobubbles remained in the molten metal and provided many sites of nucleation. The size of a stably remaining cavitation bubble is estimated to be about 0.2 µm. The grain size of the metal after solidification decreased with an increase in the time elapsed between ultrasound irradiation and solidification. It is very interesting to note that the refinement of solidification microstructures by irradiating ultrasound occurs not during, but before solidification.

Visualization of Acoustically Induced Cavitation Bubbles and Microjets with the Aid of a High-Speed Camera

Under ultrasonic irradiation at a frequency of approximately 42 kHz, model experiments in water have been performed to visualize the dynamic behavior of acoustic cavitation bubbles with a high-speed digital camera. The clustered cavitation multibubble moves linearly in random directions. The behavior of the clustered cavitation multibubble is discussed in relation to the behavior of a microjet as well as the phase transition of water between a normal liquid state and a supercritical fluid state. Another experiment has also been carried out to visualize microjets that appeared near a water/tetralin interface and a water/air interface. The behavior of clustered cavitation multibubble is accompanied by the microjets, and the microjets impinge strongly impinging on those interfaces. On the basis of these experimental results, one can conclude that the behavior of microjets and acoustic cavitation bubbles and their related phenomena are quite important for the understanding and control of the intensified macro- and micromixing of liquids.

Acoustic-Cavitation-Based Production of Foamed Metallic Material

Two trial experiments on the ultrasonic solidification of a metal have been carried out to develop an innovative technique to produce foamed metallic materials by trapping acoustic cavitation multibubble in a solidified metal. Woods alloy that is a bismuth-based quaternary eutectic alloy having a low melting point is employed in the experiments. The first experiment is devoted to the continuous casting of Woods alloy under an indirect irradiation of ultrasound at a frequency of 20 kHz. Continuously refined microstructures of Woods alloy are observed by scanning electron microscopy (SEM). The second experiment is carried out for casting Woods alloy under a direct irradiation of ultrasound at a frequency of 20 kHz. It is confirmed by SEM and energy-dispersive X-ray spectroscopy (EDS) that acoustic cavitation multibubble can be trapped as uniformly distributed nanoscale voids only in Cd-rich eutectic microstructures. This result is attributed to the finding that the Cd-rich microstructure is the primary crystal and Cd is the element having the highest vapor pressure in the quaternary system. All results suggest that the production of foamed materials by the trapping of acoustic cavitation multibubble is possible with irradiating ultrasound at the appropriate frequency and power.

THE THEORETICAL ANALYSIS OF A COLD CRUCIBLE

Three theoretical models are developed to estimate the effects of design parameters of a cold crucible. The first one is a lumped parameter model, the concept of which is a transformer, and which is easier to calculate than the other two models. The second one is a filiform wire model which is an axisymmetrical three-dimensional model based on a magnetic potential method. The third one is a strict three dimensional model where the boundary element method is applied to solve three-dimensional magnetic field distribution around a cold crucible system.