High-temperature Plasma Flow Research Articles

О математической модели взаимодействия высокотемпературного потока плазмы с поверхностью древесины

In this study, the processes of thermal decomposition of wood during its treatment with a plasma flow are considered. To develop a mathematical model of these processes, a differential thermogravimetric analysis of the heating of larch samples in an argon atmosphere at a rate of 10–20 degrees/min is carried out. Based on the results obtained, a mathematical model of thermal decomposition is proposed, including four stages. The wood during heating is represented by a mixture of six components. At each stage, the kinetic parameters of the reactions are determined by processing measurements at a heating rate of 20 degrees/min. The equations of chemical kinetics describing changes in the mass of wood components are numerically solved using the finite-difference implicit Euler method and the obtained reaction parameters. The numerical solution to the equations of chemical kinetics with these parameters shows satisfactory agreement with the data from the corresponding experiment. The calculation performed at a heating rate of 10 degrees/min with the same kinetic parameters also shows satisfactory agreement with the measurements. Thus, the obtained reaction parameters do not depend on the heating rate in the considered range. The proposed model can be used in the mathematical description of changes in the structure and thermal state of wood samples exposed to high-temperature plasma flow treatment.

Emission spectroscopy diagnosis and discharge characteristics distribution analysis of high-frequency, high-enthalpy inductive coupling plasma generator

To study the electromagnetic properties through reentry plasma on ground, a large-sized inductive coupled plasma generator is designed to produce a long-duration, large size and high temperature plasma flow in a low-pressure vacuum tank. Among all plasma parameters, the spatial distribution of electron density and electron temperature is of utmost importance for analyzing the electromagnetic scattering properties. In this study, we used the emission spectroscopy diagnosis method to explore the aforementioned parameters at two locations: upflow and downflow of the discharge coil in the plasma generator. Experiments were conducted under various discharge conditions for argon, with the panel power ranging from 28 kW to 85 kW. The results showed that, at the upflow of the discharge coil, the electron temperature at the center was higher than that at the edge, exhibiting an arch-shaped profile. However, at the downflow of the discharge coil, the electron temperature at the center was lower than that at the edge, exhibiting a saddle-shaped profile. The electron density exhibited a completely opposite distribution compared to the electron temperature, with higher values at the edge for upflow and higher values at the center for downflow. By varying the panel power, we observed the similar phenomenon: the electron temperature decreased, while the electron density increased with the increase in power. Finally, brief analysis was provided at the end of the paper. This study offered valuable insights into the transition of plasma distribution in a plasma generator, providing a reference for future research on the propagation of electromagnetic waves in plasma sheaths.

Numerical Study of the Effects of Twin-Fluid Atomization on the Suspension Plasma Spraying Process

Suspension plasma spraying (SPS) is an effective technique to enhance the quality of the thermal barrier, wear-resistant, corrosion-resistant, and superhydrophobic coatings. To create the suspension in the SPS technique, nano and sub-micron solid particles are added to a base liquid (typically water or ethanol). Subsequently, by using either a mechanical injection system with a plain orifice or a twin-fluid atomizer (e.g., air-blast or effervescent), the suspension is injected into the high-velocity high-temperature plasma flow. In the present work, we simulate the interactions between the air-blast suspension spray and the plasma crossflow by using a three-dimensional two-way coupled Eulerian–Lagrangian model. Here, the suspension consists of ethanol (85 wt.%) and nickel (15 wt.%). Furthermore, at the standoff distance of 40 mm, a flat substrate is placed. To model the turbulence and the droplet breakup, Reynolds Stress Model (RSM) and Kelvin-Helmholtz Rayleigh-Taylor breakup model are used, respectively. Tracking of the fine particles is continued after suspension’s fragmentation and evaporation, until their deposition on the substrate. In addition, the effects of several parameters such as suspension mass flow rate, spray angle, and injector location on the in-flight behavior of droplets/particles as well as the particle velocity and temperature upon impact are investigated. It is shown that the injector location and the spray angle have a significant influence on the droplet/particle in-flight behavior. If the injector is far from the plasma or the spray angle is too wide, the particle temperature and velocity upon impact decrease considerably.

Basic Characteristics of In-Liquid Plasma Jet and Electrode Damage

The most progress towards a practical method of fusing municipal waste incineration ash has been in the use of a plasma jet that employs arc discharge, a form of thermal plasma. However, a remaining problem is that stable plasma generation is prevented by melting of the nozzle of the plasma-jet torch by the high-temperature plasma flow. With the objective of developing high-speed fusion treatment for waste materials using an in-liquid plasma jet, basic research was conducted on plasma stability and the durability of plasma-jet torches, including electrodes and nozzles. Basic plasma jet characteristics such as the discharge voltage, current, and power value at the time of plasma jet generation were investigated experimentally. The relationship between the temperature distribution near the tip of a plasma jet torch and electrode damage was investigated by fluid-heat coupled analysis using the finite element method.

Stochastic clustering of the surface at the interaction of a plasma with materials

The inhomogeneous stochastic clustering of the surface with the self-similarity of the granularity structure from nano- to macroscales is observed in various materials after the action of intense high-temperature plasma flows in nuclear fusion facilities. The spectral and statistical characteristics of hierarchical granularity and scale invariance are estimated. They qualitatively differ from the properties of the simplest Brownian surface roughness and clustering under other conditions possibly because of universal mechanisms of the formation of stochastic clustering of materials under the action of a high-temperature plasma.

Modeling and Analysis of a Dual-Channel Plasma Torch in Pulsed Mode Operation for Industrial, Space, and Launch Applications

Dual-channel thermal plasma torch can operate with air, argon, or combustible gases to produce high-temperature plasma flow. This plasma torch can be used in various important applications such as metal industry recycling, surface coating and hardening, space operations using controlled thrust, and macroparticle acceleration based on the electrothermal nature of thermal torches and electrical-to-thermal energy conversion. Power for this torch is supplied from the electric mains and the voltage is stepped up to 6 kV. However, the torch can also operate on dc or pulsed mode. The electrical operation is characterized by the voltampere relationship to determine the power rating of the torch as well as diagnosing the dynamic behavior of the plasma. Experiments on the torch using air and argon have shown plasma temperatures in the range of 0.4–0.6 eV with plasma number density in the range of $10^{24}$ – $10^{25}/\text{m}^{3}$ , indicating a dense plasma regime with the plasma tends to be weakly nonideal. Plasma kinetic temperature and electron number density were obtained from optical emission spectroscopy using the relative line method as the plasma is near local thermodynamic equilibrium condition. Plasma temperature has its peak for low flow rates and decreases for increased flow rates. The torch modeling was conducted using an electrothermal plasma code to simulate and predict the parameters for pulsed mode operation. Simulation was conducted on a single channel as the dual torch is symmetric. Code results for extended pulselength show a plasma temperature between 0.6 and 0.8 eV for nitrogen, oxygen, and helium; which are in good correlation with plasma temperatures obtained from optical emission spectra and measured plasma resistivity. A set of computational experiments using short pulses at higher discharge currents has shown temperature in the range of 2.0–2.5 eV for nitrogen and helium.

The role of radiative reabsorption on the electron energy distribution functions in H2/He plasma expansion through a tapered nozzle

A collisional-radiative model for the H2/He plasma, coupled to a Boltzmann solver for the free electron kinetics is used to investigate the non-equilibrium conditions created in the expansion of an high-temperature plasma flow through a converging-diverging nozzle, starting from the steady state composition at T0=10 000 K and p0=1 atm in the reservoir. It is shown that the plasma optical thickness plays a major role on the evolution of macroscopic quantities and internal distributions along the nozzle axis. Structured electron energy distribution functions, characterized by long plateaux and humps, are created due to superelastic collisions of cold electrons and electronically excited atomic hydrogen. The magnitudes of the plateaux are orders of magnitude higher in an optically thick plasma compared with a thin plasma, while the electron-electron collisions play a role in smoothing the peaks created by superelastic collisions between cold electrons and H(n>2).

Oxidation behavior of graphene nanoplatelet reinforced tantalum carbide composites in high temperature plasma flow

Graphene nanoplatelets (GNP) reinforced tantalum carbide (TaC) composites are exposed to a high temperature plasma flow in order to evaluate the effects of GNP on the oxidation behavior of TaC at conditions approaching those of hypersonic flight environments. The addition of GNP is found to suppress the formation of the oxide layer by up to 60%. The high thermal conductivity of GNPs dissipates heat throughout the sample thereby reducing thermal gradients and reducing the intensity of heating at the surface exposed to plasma. In addition, GNPs enhance oxidation resistance by providing toughening which suppresses crack formation and bursting that accelerates oxidation. Scanning electron microscopy (SEM) and high resolution transmission electron microscopy (HR-TEM) reveal that GNPs have the ability to survive the intense high temperature of the plasma. GNPs are believed to seal oxide grain boundaries and hinder the further influx of oxygen. GNPs also provide nano sized carbon needed to induce the localized reduction of Ta2O5 to TaC. Micro computed X-ray tomography (MicroCT) validates that the above mechanisms protect the underlying unoxidized material from the structural damage caused by thermal shocks and high shear forces, by reducing thermal gradients and providing toughness.

20405 遮熱コーティングの皮膜特性に及ぼす溶射条件と高温曝露条件の影響(材料加工(2))

Thermal Barrier Coating (TBC) is fabricated by thermal spray technology. In this technology, coating particles are melted in high-temperature plasma flow, impinged and deposited continuously to a substrate surface. Since a coating layer is formed by this way, it seems that the microstructure and the mechanical properties of TBC are affected by the spray condition and high-temperature environment in service. In this study, the substrate temperature and the particle velocity of the coating particle in plasma flow as the spray condition are varied. Coefficient of thermal expansion, elastic modulus, crack length, Vickers hardness, porosity, bending strength and residual stress of the ceramic layer are measured for as-sprayed specimen. In addition, thermal exposure tests are conducted to examine influence of exposure temperature on the mechanical properties of TBC.

Flattening Behaviour and Cohesion Strength of Molten Metal Particle Impacted onto High-Temperature Substrate

In a thermal spraying process, a lot of molten powders are impinged continuously onto a substrate due to a high-temperature plasma flow, and the coating is deposited consequentially. Thus, powder size, powder material, powder velocity, powder temperature and substrate temperature as the process parameter affect the mechanical properties and adhesion of coating. The understanding of fundamental physical principle for the spread and solidification of splat is essential to determine the relation between the coating properties and the process parameter. Relatively few experimental studies have been done to examine about a molten metal droplet impact. The simple model had been also developed to estimate quantitatively the flattening of a splat. However, there are no papers for the splat cohesion property, in spite of an essential one in characterizing the coating adhesion. In this study, the splat cohesion property for molten metal particle (Sn 60%-Pb 40%) dropped onto the substrate (SUS304 stainless steel plate) is examined based on a free fall experimental procedure, which could be simply simulate a powder deposition process in a thermal spraying. Influence of kinetic energy of the impact droplet and the substrate temperature on the flattening of splat and the cohesion is shown. The splat cohesion strength will be tried to estimate based on an energy balance consideration.

3402 溶射施工条件が遮熱コーティングの膜構造に及ぼす影響(S16-1 遮熱コーティング(TBC)(1),S16 コーティング材料の皮膜特性とその評価)

Thermal Barrier Coating (TBC) is fabricated by thermal spray technology. By plasma spraying coating particles are melted in high temperature plasma flow and impinged and deposited continuously to a substrate surface. Since a coating layer is formed in this way it seems that the structure and the mechanical properties of TBC are influenced by spray condition. In this study, we varied the substrate temperature and the particle velocity of the coating material in plasma flow about spray condition in plasma spraying and measured coefficient of thermal expansion, elastic modulus, crack length, Vickers hardness, porosity and bending strength of the ceramic layer.

Energy balance in the interaction of intense high-temperature plasma flows with solid targets

The energy balance in the interaction of intense (W≈7 MW/cm2, Q≈130 J/cm2) flows of a high-temperature (Te+Ti≈0.7 keV deuterium plasma with targets made of different materials (graphite, tungsten, copper, etc.) is studied experimentally. It is shown that radiation plays a decisive role in the interaction energy balance: a plasma layer arising near the surface of the eroded target reemits most of the plasma-flow energy into the surrounding space. No more than 50 J/cm2 reaches the surface. Then, this energy is expended primarily on the target heating and only a small fraction (less than 3 J/cm2) is spent on the evaporation of the target surface layer. It is shown that, for targets made of high-Z materials, the energy reaching the surface is transferred predominantly by radiation.

Recombining processes in a steady plasma flow penetrating into hydrogen gas

A numerical investigation of recombining process in a high-temperature plasma flow in a quasi-steady state (electron temperature about 10 eV, electron density about 1014 cm-3 and flow velocity about 106 cm s-1) was performed under gas contact cooling conditions using the collisional-radiative model, in which gas cooling is mainly due to surrounding gas particles freely coming into the plasma column. The calculation has shown that the electron temperature relaxes in accord with experimental results and also that the occurrence of the recombining region and the inverted populations almost agree with the experimental ones. Atomic processes responsible for gas cooling are discussed.

Ponderomotive pressure effects in laser driven high-temperature plasma flows

Gasdynamical equations taking radiation energy density and ponderomotive pressure into account are investigated. Conditions for these quantities to be important are stated: low absorption, reflexion at cut-off. The density ratio in a discontinuity is studied as a function of ponderomotive pressure, absorbed intensity and Mach number in the initial state. Chapman–Jouguet conditions are defined. Compressive (R type) flows and rarefaction (D type) appear. The structure of the latter is discussed including plasma effects.