電磁流体工学

Winslow's Effect is defined as an essentially instateneous reversible change in apparent viscosity when a fluid is subjected to an externally applied electric field. ER (Electro-Rheological) fluids that show an extreme increase in viscosity could be employed as new functional materials and are target for competitive development worldwide. Fundamentally, ER fluids are to drive clutches and hydraulic valves which can control torque and the pressure respectively only by application of a high voltage. In one recent application which received keen attention ER fluids were used in an engine mount with an active vibration control function. However, ER fluids should not be seen simply as a means of controlling uncomfortable ribration. They can be expected to have a vast range of applications as elements for highly functional servo control systems. Here, specific characteristics of ER fluids are reviewed, and new applications developed for servo systems are introduced. Regarding the effectiveness of ER fluids, the greatly improved capacity for control that can result from their use in servo systems is demonstrated by their application in the case of a robot with a pneumatic manipulation system.

Control of nano-micro particulate flow processes in plasma spraying, cold spray and fine particle synthesis using plasma flows is described by integrating numerical simulation and experiment. These processes can be regarded as particle laden plasma flows with complex interactions and phase changes. The in-flight particle heating can be effectively enhanced by Joule heating with applying RF electromagnetic field in plasma spraying to control the coating thickness distribution. Furthermore, the impact particle velocity can be effectively accelerated by electrostatic force with applying corona discharge in cold spray process to increase deposition efficiency. Finally particle size distribution and particle morphology can be changed by phase change control with various kinds of inlet gas flow rates, downstream quench gas and operating pressure in fine particle synthesis using RF induction plasma flow and DC-RF hybrid plasma flow system. It is clearly shown that control of dynamic behavior and phase change of in-flight nano-micro particles in plasma flows can be achieved by using electromagnetic field, discharge mode, inlet flow rates and operating pressure for coating process and fine particle synthesis.

This review describes the researches on flow phenomena at gas-liquid interface subjected to external oscillating force. Especially, surface disintegration, droplet formation, bubble cluster generation, and dynamic behaviour of bubbles in oscillating liquid container are described. And further, surface wave generation, liquid jet generation, jet breakup phenomena, droplet formation of magnetic fluid subjected to vertical vibration or alternating magnetic field are also described. These phenomena are related to interfacial instability of liquids subjected to external oscillating forces.

Solute control in boundary layer under a high temperature environment is important from the industrial viewpoint, because mass transfer in boundary layer is often rate-determining step of chemical reactions and chemical composition of solidified alloy is determined by solute concentration in the vicinity of solid-liquid interface. An alternating electromagnetic force is one of candidates to control the mass transfer in the boundary layer of molten metals and alloys since it can directly excite a flow in it. The alternating electromagnetic force also has a potential to enhance the removal of inclusions in molten metal because increase in their collision efficiency is expected. This paper reviews the recent investigations on possibility of the alternating electromagnetic force as a tool of solute concentration control and inclusion removal. A model experiment using an electrolyte aqueous solution and electrodes having a shape with triangles aligning in line was carried out to clarify the effect of the alternating electromagnetic force on solute concentration distribution in the vicinity of the boundary between the electrode and the electrolyte aqueous solution. The alternating electromagnetic force imposition changed the solute concentration distribution because of the flow excitation. Then, it was applied to solidification of an alloy. Local eutectic area ratio of a Sn-10mass% alloy was dependent on the imposing period of the alternating electromagnetic force during its solidification. This suggests that the alternating electromagnetic force might control the solute distribution. Theoretical investigation on collision frequency among the inclusions using the alternating electromagnetic force suggests that this method is suitable for small inclusions collision and the collision frequency increases as frequency of the alternating electromagnetic force becomes low.

The concept of plasma flow system by mixing particle, droplet and bubble in a plasma flow and utilizing chemical reaction at the gas-liquid interface and solid surface are proposed based on functional fluids engineering and multiphase flow principle. As the typical examples of advanced applications, nano powder transportation and cleaning in a plasma actuator tube, photo catalytic nano particle synthesis using DC-RF hybrid plasma flow system with precursor droplet injection and nano/micro pulse discharged bubble jet system for water purification are explained by author’s results.

A cavitation bubble plasma, which is a kind of plasma in solution, is a low-temperature plasma generated in a large number of microbubbles of a solvent component generated by a cavitation phenomenon. In this paper, the cavitation bubble plasma technology is explained by introducing the originally developed apparatus, the discharge phenomenon, and the effect of cavitation bubbles. As application examples of cavitation bubble plasma, the results of water dispersion of carbon nanotubes, synthesis of silver nanoparticles, sterilization of Escherichia coli, and decomposition of methylene blue are introduced.

Fine bubbles generated by plasma in water that we named as “Plasma Fine Bubbles,” have high potential to generate long-life ultrafine bubbles due to electrical charges. It is shown that the residual bubbles formed by collapse of streamer gas channels which show a similar process of cavitation bubbles in various points such as growth, shrink, collapse, rebound, and fragmentation. There are also introductions about ultrafine bubbles measured by a dynamic laser scattering method, amount of electrical charge of plasma fine bubbles estimated by analyzing their movements under application of an intense electric field strength, evidence of hydrogen gas generation in laser/spark induced bubbles and a distinguishing method for between gas bubbles and solid particles.

This article outlines the fundamentals of electric conducting fluid analysis and its numerical model for an electrically conductive droplet when alternating magnetic field is applied. The level-set method was applied for interfacial capture and the electromagnetic potential was introduced for electromagnetic field analysis. The pressure and electromagnetic potential were computed by the Red-Black SOR method using GPU (Graphics processing unit). It is exhibited that the solid, liquid and gas phases were able to be computed simultaneously and that an electromagnetic levitation phenomenon was successfully simulated.