Polycarpos K. Papadopoulos

Investigation on streamers propagating into a helium jet in air at atmospheric pressure: Electrical and optical emission analysis

The plasma produced due to streamers guided by a dielectric tube and a helium jet in atmospheric air is herein studied electrically and optically. Helium streamers are produced inside the dielectric tube of a coaxial dielectric-barrier discharge and, upon exiting the tube, they propagate into the helium jet in air. The axisymmetric velocity field of the neutral helium gas while it penetrates the air is approximated with the PISO algorithm. At the present working conditions, turbulence helium flow is avoided. The system is driven by sinusoidal high voltage of variable amplitude (0–11 kV peak-to-peak) and frequency (5–20 kHz). It is clearly shown that a prerequisite for streamer development is a continuous flow of helium, independently of the sustainment or not of the dielectric-barrier discharge. A parametric study is carried out by scanning the range of the operating parameters of the system and the optimal operational window for the longest propagation path of the streamers in air is determined. For this...

Electromagnetic Low-Frequency Dipolar Excitation of Two Metal Spheres in a Conductive Medium

This work concerns the low-frequency interaction of a time-harmonic magnetic dipole, arbitrarily orientated in the three-dimensional space, with two perfectly conducting spheres embedded within a homogeneous conductive medium. In such physical applications, where two bodies are placed near one another, the 3D bispherical geometry fits perfectly. Considering two solid impenetrable (metallic) obstacles, excited by a magnetic dipole, the scattering boundary value problem is attacked via rigorous low-frequency expansions in terms of integral powers (????)??, where ??=0, ?? being the complex wave number of the exterior medium, for the incident, scattered, and total non-axisymmetric electric and magnetic fields. We deal with the static (??=0) and the dynamic (??=1,2,3) terms of the fields, while for ??=4 the contribution has minor significance. The calculation of the exact solutions, satisfying Laplace’s and Poisson’s differential equations, leads to infinite linear systems, solved approximately within any order of accuracy through a cut-off procedure and via numerical implementation. Thus, we obtain the electromagnetic fields in an analytically compact fashion as infinite series expansions of bispherical eigenfunctions. A simulation is developed in order to investigate the effect of the radii ratio, the relative position of the spheres, and the position of the dipole on the real and imaginary parts of the calculated scattered magnetic field.

Influence of Atmospheric Pressure Guided Streamers (Plasma Bullets) on the Working Gas Pattern in Air

This paper is devoted to the study of gas flow fields related to helium atmospheric pressure guided streamer (plasma bullet) propagation in the air. For very weak up to moderate helium flows, the modification induced to the gas flow field by the plasma ignition is demonstrated; it is shown that the turbulent flow region is expanded and two conditions must be fulfilled regarding the working gas profile in the air for streamer propagation, i.e., laminar flow and high concentration in this laminar flow region.

Biomagnetic fluid flow in the presence of a line dipole

Purpose – The purpose of this paper is to investigate the effect of a magnetic field on the flow of a biomagnetic fluid.Design/methodology/approach – The flow takes place in a straight circular duct and the magnetic field is produced by a line dipole placed perpendicularly to the longitudinal axis of the duct.Findings – The numerical results show that the magnetic field affects the characteristics of the flow, the velocity components and the friction factor, even for medium field intensity. A relation is proposed for the maximum and minimum longitudinal pressure drop in the pipe.Originality/value – From the present results it is obtained that it is important to take into account the magnetic properties of blood in the various applications that involve blood flow in the presence of a magnetic field.

On the Perturbation of the Three-Dimensional Stokes Flow of Micropolar Fluids by a Constant Uniform Magnetic Field in a Circular Cylinder

Modern engineering technology involves the micropolar magnetohydrodynamic flow of magnetic fluids. Here, we consider a colloidal suspension of non-conductive ferromagnetic material, which consists of small spherical particles that behave as rigid magnetic dipoles, in a carrier liquid of approximately zero conductivity and low-Reynolds number properties. The interaction of a 3D constant uniform magnetic field with the three-dimensional steady creeping motion (Stokes flow) of a viscous incompressible micropolar fluid in a circular cylinder is investigated, where the magnetization of the ferrofluid has been taken into account and the magnetic Stokes partial differential equations have been presented. Our goal is to apply the proper boundary conditions, so as to obtain the flow fields in a closed analytical form via the potential representation theory, and to study several characteristics of the flow. In view of this aim, we make use of an improved new complete and unique differential representation of magnetic Stokes flow, valid for non-axisymmetric geometries, which provides the velocity and total pressure fields in terms of easy-to-find potentials. We use these results to simulate the creeping flow of a magnetic fluid inside a circular duct and to obtain the flow fields associated with this kind of flow.

Parametric study of thermal effects in a capillary dielectric-barrier discharge related to plasma jet production: Experiments and numerical modelling

In the present work, a capillary dielectric-barrier discharge of the coaxial electrode configuration, commonly employed to atmospheric-pressure cold plasma jet production, is studied in terms of thermal effects. The discharge is driven by sinusoidal high voltage in the kHz range and operates with helium gas channeled into a capillary dielectric tube having one end opened to the atmospheric air. The voltage amplitude and frequency, gas flow rate, and discharge volume are varied independently, and thermal effects are investigated by experimentally acquired results coupled with numerically determined data. The experiments refer to electrical power measurements, time-resolved temperature measurements, infrared imaging, and high resolution optical emission spectroscopy. The numerical modelling incorporates an electro-hydrodynamic force in the governing equations to take into account the helium-air interplay and uses conjugate heat transfer analysis. The comparison between experimental and numerical data shows that power is principally consumed in the dielectric barrier-helium interface resulting in the dielectric heating. A linear relation between steady state temperatures and supplied power, independent of the designing and operating conditions, is experimentally established. However, the gas flow rate affects the thermal effects differently compared to the other parameters, supporting the idea of a twofold nature of these systems, i.e., electrical and hydrodynamic. The main claim states the possibility of correlating (both experimentally and numerically) designing and operating parameters for evaluating heat distribution and gas temperature in capillary dielectric-barrier discharges used for plasma jet production. This is of high importance for processing temperature-sensitive materials, including bio-specimens.In the present work, a capillary dielectric-barrier discharge of the coaxial electrode configuration, commonly employed to atmospheric-pressure cold plasma jet production, is studied in terms of thermal effects. The discharge is driven by sinusoidal high voltage in the kHz range and operates with helium gas channeled into a capillary dielectric tube having one end opened to the atmospheric air. The voltage amplitude and frequency, gas flow rate, and discharge volume are varied independently, and thermal effects are investigated by experimentally acquired results coupled with numerically determined data. The experiments refer to electrical power measurements, time-resolved temperature measurements, infrared imaging, and high resolution optical emission spectroscopy. The numerical modelling incorporates an electro-hydrodynamic force in the governing equations to take into account the helium-air interplay and uses conjugate heat transfer analysis. The comparison between experimental and numerical data shows ...

Application of the CVP Method on 3D Internal Flows

The CVP computational method is applied to three‐dimensional flow problems. The CVP method is accurate, converges easily and it is easily implemented, particularly in the case of complex geometries associated with generalized curvilinear coordinate systems. The validity of the method is illustrated by presenting results for the laminar, incompressible, developing flow in curved ducts of square cross‐sections.