Kazuyoshi Aoki

Study on Internal Flow and Surface Deformation of Large Droplet Levitated by Ultrasonic Wave

Abstract:  It is expected that new materials will be manufactured with containerless processing under the microgravity environment in space. Under the microgravity environment, handling technology of molten metal is important for such processes. There are a lot of previous studies about droplet levitation technologies, including the use of acoustic waves, as the holding technology. However, experimental and analytical information about the relationship between surface deformation and internal flow of a large levitated droplet is still unknown. The purpose of this study is to experimentally investigate the large droplet behavior levitated by the acoustic wave field and its internal flow. To achieve this, first, numerical simulation is conducted to clarify the characteristics of acoustic wave field. Second, the levitation characteristic and the internal flow of the levitated droplet are investigated by the ultrasonic standing wave under normal gravity environment. Finally, the levitation characteristic and internal flow of levitated droplet are observed under microgravity in an aircraft to compare results with the experiment performed under the normal gravity environment.

Interfacial stability and internal flow of a levitated droplet

Under the microgravity environment, production of new and high quality material is expected. Large droplet is preferable for such a containerless processing in microgravity environment. There are a lot of previous studies for droplet levitation [1]. However, effect of surface instability and internal flow appear remarkable when the droplet becomes large. Elucidation of effect of surface instability and internal flow of the levitated droplet is required for the quality improvement of new material. The objective of present study is to clarify critical conditions of the occurrence of the internal flow and the surface instability. At first, the condition between the stable region and the unstable region of the droplet levitation was evaluated by using the existing critical Weber number theory. The experimental result agreed well with the theory. It was suggested that the stability of droplet can be evaluated by using the theory for the interfacial instability. Finally, two-dimensional visual measurement was conducted to investigate the internal flow structure in a levitated droplet. The effect of physical properties on the internal flow structure of the droplet is investigated by Particle Image Velocimetry (PIV) technique. As the result, it is indicated that the internal flow structure is affected by the physical property such as viscosity.

Study on Interfacial Stability and Internal Flow of a Droplet Levitated by Ultrasonic Wave

For a microgravity environment, new and high‐quality material is expected to be manufactured. However, the effect of surface instability and the internal flow become significant when the droplet becomes large. Elucidation of internal flow and surface instability on a levitated droplet is required for the quality improvement of new material manufacturing in a microgravity environment. The objectives of this study are to clarify the interfacial stability and internal flow of a levitated droplet. Surface instability and internal flow are investigated with a large droplet levitated by the ultrasonic acoustic standing wave. The experiment with a large droplet is conducted both under normal gravity and microgravity environments. In the experiment, at first, the characteristics of the levitated droplet are investigated; that is, the relationships among the levitated droplet diameter, the droplet aspect ratio, the displacement of the antinode of the standing wave, and the sound pressure are experimentally measured. As a result, it is clarified that the levitated droplet tends to be located at an optimal position with an optimal shape and diameter. Second, the border condition between the stable and the unstable levitation of the droplet is evaluated by using the existing stability theory. The experimental results qualitatively agree with the theory. It is suggested that the stability of the droplet can be evaluated with the stability theory. Finally, multidimensional visual measurement is conducted to investigate the internal flow structure in a levitated droplet. It is suggested that complex flow with the vortex is generated in the levitated droplet. Moreover, the effect of physical properties of the test fluid on the internal flow structure of the levitated droplet is investigated. As a result, the internal flow structure of the levitated droplet is affected by the surface tension and viscosity.

Heat Removal Capability of Core-Catcher With Natural Circulation

Toshiba has developed a core-catcher system. It is to be installed at the bottom of the lower drywell in order to stabilize a molten core flowing down from a reactor vessel. It consists of a round basin made up of inclined cooling channels arranged axisymmetrically, and the structure including risers, downcomers and a water chamber to get natural circulation of the flooding water. So it can cover entire pedestal floor and can work in passive manner.In order to confirm the heat removal capability of the core catcher with natural circulation, we have conducted full scaled tests in several conditions. Some important dimensionless numbers obtained from fundamental equations of the natural circulation are used for the tests.Using dimensionless number and to compare with several analysis, we can verify that the experiment is adequate to simulate the actual plant.Copyright

Simulation for Forces on Levitated Droplet in Ultrasonic Standing Wave

Under the micro-gravity environment, holding technology of liquid is important to manufacture new materials and noncontact material property measurement methods. There are previous studies about droplet levitation by the ultrasonic standing wave for the holding technology. However it is still unknown experimentally and analytically how the ultrasonic standing wave acts on the levitated droplet. In the present study, the technology to handle the material in space by the ultrasonic wave is developed and the simulation technique to evaluate the ultrasonic standing wave field and the movement of the droplet in the ultrasonic standing wave. At first, the characteristics of droplets holding by the ultrasonic standing wave under normal gravity environment and micro-gravity environment are investigated experimentally. Secondly, pressure field by ultrasonic standing wave is measured with probe microphone. The measurement shows that 2-dimensional pressure distribution is arisen between the horn and the reflector, and positions where droplets are held are near nodes of the ultrasonic standing wave. Thirdly, numerical simulation considered for compressibility of gas is conducted to clarity the characteristics of ultrasonic standing wave. The 2-dimensinal pressure distribution obtained by this simulation agrees with the measurement result by probe microphone quantitatively. Finally, droplet movement is solved using results of pressure field simulation. It is shown that 2-dimensinal pressure distribution causes horizontal holding force.Copyright