Hideya Nishiyama

Three-dimensional effects of carrier gas and particle injections on the thermo-fluid fields of plasma jets

A numerical model is developed to clarify three-dimensional effects of the radial injection of carrier gas and particles on the thermo-fluid fields of the plasma jet with and without swirl. The plasma-particle two-way interactions are modelled by coupling a Lagrangian approach for particle behaviour with an Eulerian approach for plasma flow under dense loading. The effect of radial injection of the carrier gas on the flow and temperature fields of the plasma jet with and without swirl is clarified by numerical simulation. The deformations of the plasma jet thermo-fluid fields caused by dense particle loading with and without turbulent dispersion of the particles are presented. It is shown that the high mass flow rate of the carrier gas affects plasma jet fields strongly and particle turbulent dispersion can control the deformation of the plasma jet fields. The given swirl flow reduces the momentum and energy transfers between plasma and particles.

Spreading behavior of an impacting drop on a structured rough surface

The spreading of water drops impinging on structured rough surfaces is studied experimentally. The rough surfaces are specially prepared with a regular pattern of surface asperities. The arrangement of the square-shaped surface asperities creates channel-like grooves on the surface. A video microscope along with a controlled light exposure system is used to construct the image sequences of the spreading process. The images are digitally analyzed to measure the temporal variation of the spreading drop diameter 2R. Results are obtained for three rough surfaces with varying asperity heights in the range of 100-500 \mu m and for different impact drop conditions with Weber number We in the range of 35-225. The results on the temporal variation of 2R show that, on the structured rough surfaces, the spreading occurs simultaneously both inside and above the texture pattern of the surfaces. For a given surface geometry, the volume of liquid flowing inside the grooves of the surface increases with increasing We.Consequently, the values of 2R measured inside the texture pattern are larger than those measured above the texture pattern, and their difference increases with increasing We. The arrangement of the surface asperities influences the spreading pattern of an impacting drop spreading axis symmetrically. For the texture geometry used in the present study, the spreading pattern resembles a regular rhombus shape for the impact of low We drops and becomes complex at high We. The spreading distances, measured both inside and above the texture pattern of the structured rough surfaces, are nearer to the measurements recorded on the smooth surface if the asperity height of the rough surface is smaller than the thickness of the spreading liquid lamella;however, the surface asperities influence the spreading pattern drastically and create a liquid splash.

Numerical Analysis of Metallic Nanoparticle Synthesis Using RF Inductively Coupled Plasma Flows

A thermal plasma flow is regarded as a multifunctional fluid with high energy density, high chemical reactivity, variable properties, and controllability by electromagnetic fields. Especially a radio frequency inductively coupled plasma (RF-ICP) flow has a large plasma volume, long chemical reaction time, and a high quenching rate. Besides, it is inherently clean because it is produced without internal electrodes. An RF-ICP flow is, therefore, considered to be very useful for nanoparticle synthesis. However, nanoparticle synthesis using an RF-ICP flow includes complicated phenomena with field interactions. In the present study, numerical analysis was conducted to investigate the synthesis of metallic nanoparticles using an advanced RF-ICP reactor. An advanced RF-ICP flow is generated by adding direct current (DC) discharge to a conventional RF-ICP flow in order to overcome the disadvantages of a conventional one. The objectives of the present work are to clarify the formation mechanism of metallic nanoparticles in advanced RF-ICP flow systems and to detect effective factors on required synthesis. A two-dimensional model as well as a one-dimensional model was introduced for nanoparticle growth to investigate effects of spatial distributions of thermofluid fields in RF-ICP flows on synthesized nanoparticles. In an advanced RF-ICP flow, a characteristic recirculation zone disappears due to a DC plasma jet. Larger numbers of nanoparticles with smaller size are produced by using an advanced RF-ICP flow. Thermofluid fields in RF-ICP flows can be controlled by applied coil frequency by means of skin effect. Larger numbers of nanoparticles with smaller size are produced near the central axis. Dispersion of particle size distributions can be suppressed by higher applied coil frequency through control of RF-ICP flows. Applied coil frequency can be a remarkably effective factor to control nanoparticle size distribution.

A nonequilibrium argon-oxygen planar plasma jet using a half-confined dielectric barrier duct in ambient air

A nonequilibrium argon plasma jet with oxygen addition, generated in a planar dielectric duct and issuing into ambient air with one edge stuck on a dielectric plane, is reported for the first time. This argon-oxygen plasma jet can be operated at low applied voltage as a filamentary discharge at atmospheric pressure. The addition of a small amount of oxygen results in the increase of produced ozone concentration and continuous emissions of centering at about 185 nm, 205 nm, 230 nm, and 253 nm. The synergistic generation of short wavelength ultraviolet emissions and active species is significantly important for plasma applications.

Preparation, viscosity and damping of functional fluids that respond to both magnetic and electric fields

A fluid that can be controlled both by magnetic and electric fields, will lead to applications such as dampers, separators, light scattering in a thin film, actuators, etc. In this paper, the preparation, viscosity and damping characteristics of three types of functional fluids that responses to magnetic and electric fields, are presented. These fluids have qualities of magnetic and electrorheological fluids.

Characteristics of Ozone Jet Generated by Dielectric-Barrier Discharge

The characteristics of ozone jet generated by atmospheric-pressure coaxial dielectric-barrier discharge has been experimentally clarified through the visualization of discharge and also by the measurement of ozone concentration for various operating conditions, such as applied voltage, frequency, and gas flow rate. It is shown that the ozone production is highly controllable with applied voltage, and 1500 ppm of ozone can be produced at the input power as low as 72 W.

Numerical simulation of MR fluid damping characteristics using a modified Bingham model

A numerical simulation is conducted to clarify the unsteady flow behavior and drag force around the oscillating flat plate immersed in MR fluids under the applied magnetic field. A modified Bingham model is adopted to take continuous viscous flow behavior at the very small shear rate and also magnetically variable yield stress into account. The measured yield stress is given to the model as a function of magnetic field intensity. The governing equations for the oscillating flat plate and unsteady MR fluids flow are derived by a modified Bingham model taking magnetic body force, yield stress and viscous force into account. The effects of magnetic field intensity, oscillation frequency and amplitude and further plate thickness on the fluid flow and damping characteristics around the oscillating flat plate are clarified by the numerical simulation.

Flow Around Two Elliptic Cylinders in Tandem Arrangement

Flow around two elliptic cylinders in tandem arrangement was experimentally investigated through measurements of the surface static pressure distribution and estimations of the flow parameters such as the drag, lift and moment coefficients. The elliptic cylinders examined had an axis ratio of 1:3 and they were aranged in tandem with an identical angle of attack. The angle of attack ranged from 0 to 90 deg and the nondimensional cylinder spacing l/c from 1.03 to 4.0, where l denotes the distance between the cylinder centers and c is the major axis. It has been found that the flow characteristics vary drastically with the angle of attack and also the cylinder spacing.

Visualization of arc and plasma flow patterns for advanced material processing

Results are presented for physical experiments that illustrate the possibilities and efficiency of visualization for studying the effect of operating conditions (backward-facing stepped forming nozzle, exit diameter of anode, mass flow, and composition of working gas) on plasma flows at low Reynolds numbers for advanced coating and powder processing. In particular, the shadow method, based on adaptive visualization transparency, is used for imaging electric arc and plasma jet flow patterns for different operating conditions. Because of visualization, the optimal geometrical characteristics of the backward-facing stepped forming nozzle, mass flow rate of the working gas, and its composition were found. These provide: (1) the absence of micro-shunting of the arc inside the backward-facing stepped nozzle for a transfer arc and twin arcs; and (2) compared to transient and turbulent jets, a higher density for the heat flux from a quasi-laminar flow to the surface of a flat substrate and the powder material to be treated, for nontransfer arc DC (direct current) torches and DC–RF (direct current and radio frequency) hybrid plasma flow system.Graphical Abstract

The control of gas temperature and velocity fields of a RF induction thermal plasma by injecting secondary gas

The present study describes the control characteristics of the gas temperature and velocity fields of a RF induction argon thermal plasma by injecting cold helium gas axially at atmospheric pressure. The flow and the gas temperature fields in the RF induction mixed gas plasma are obtained by solving the axisymmetric turbulent 2D MHD equations and the energy transport equation coupled with 2D Maxwells equations. The mixing rate of a secondary injected gas is also calculated using the species conservation equation. It is examined how the thermofluid and diffusion characteristics of a RF induction plasma are influenced by the input power and the positions of injection of secondary gas and swirling. The calculated gas temperature shows good agreement with the previously obtained experimental data.