Oleg P. Solonenko

Stabilization of pulverized coal combustion by plasma assist

Ignition and stabilization of pulverized coal combustion by plasma assist is investigated with a 10 kW plasma torch for three different kinds of coal, such as high, medium and low volatile matter coals. Not only high volatile matter coal but also low quality coal can be successfully burned with plasma assist. Research for volatile component of coal shows that a higher temperature field is necessary to extract the volatile matter from inferior coal, while their compositions are almost the same.

Plasma processing and deposition of powdered metal oxides consisting of hollow spherical particles

Features of the plasma processing of agglomerated metal oxide powders and the deposition of coatings of these powders, which consist of hollow spherical particles are considered in a particular case of powdered zirconium dioxide. It is shown that the formation of zirconia splats (melt drops spread over and solidified on a substrate surface) during the collision of hollow drops with the substrate has a more stable character as compared to the case of continuous (dense) particles of the same mass. Advantages of using hollow powders in the thermal spraying of coatings are discussed.

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

Spreading and Solidification of Hollow Molten Droplet under Its Impact onto Substrate: Computer Simulation and Experiment

The peculiarities of plasma treatment of agglomerated (spray dry) YSZ powder and subsequent plasma spraying of produced HOSP powder (consisting of the hollow spherical particles) are analyzed. Formation of splats of hollow YSZ droplets deserves a special attention within thermal spraying. In this case, immediately prior to particle—substrate collision, we have a droplet that consists of a liquid shell enclosing a gas cavity heated to a temperature close to the melt temperature. The paper presented includes the results of computer simulation and model experiments carried out under full control of the key physical parameters: temperature, velocity, external diameter of droplet, thickness of its shell, and temperature of polished substrate. It was shown that formation of splats of hollow droplets proceeds in a manner more stable compared to the case of “dense” molten particles obtained from fused and crushed compacts, and this provides superior coating‐substrate interface.

Hydrodynamic features of the impact of a hollow spherical drop on a flat surface

Physical model experiments devoted to visualization of the collision of a hollow liquid drop with a flat surface are described. A new phenomenon is discovered, according to which the impact leads to the formation of a central counter jet of the liquid. A simple theoretical model is proposed for predicting the dynamics of the hollow liquid drop spreading on the surface. The results of calculations satisfactorily agree with the data of model experiments.

Optimization for plasma spraying processes by numerical simulation

Numerical simulation of plasma spraying processes from the injection of metallic particles to formation of the coating was conducted by integrating plasma thermofluid, and splat and coating formation models. The first model clarifies the electromagnetic thermofluid fields of a particle-laden plasma flow impinging onto the substrate, as well as the characteristics of the fine particles injected in the plasma flow. The second model clarifies the splat thickness and diameter of a particle using the melted particle impact velocity and temperature obtained by the first model. Furthermore, the optimal particle impact velocity and temperature were derived from the stable splat criterion given by the integrated model.

Preparation of Carbon-Doped TiO2 Nanopowder Synthesized by Droplet Injection of Solution Precursor in a DC-RF Hybrid Plasma Flow System

Carbon-doped titanium dioxide nanopowder has received much attention because of its higher photocatalytic performance, which is practically activated not only by UV, but also by visible light irradiation. In the present study, C-TiO2 nanopowder was synthesized by droplet injection of solution precursor in a DC-RF hybrid plasma flow system, resulting in higher photocatalytic performance even under visible light irradiation. In-flight C-TiO2 nanoparticles reacted with the high concentration of carbon in plasma flow and were then deposited on the surfaces of two quartz tubes in the upstream and downstream regions of this system. The collected C-TiO2 nanopowder contained anatase-rutile mixed-phase TiO2 and TiC, the contents of which depended on the location of the powder collection, the temperature, and the duration of plasma treatment. Highly functional C-TiO2 nanopowder collected in the downstream region exhibited a higher degradation rate of methylene blue than that of single-phase anatase TiO2, even under visible light irradiation, in spite of being TiC.

Formation of Splats from Suspension Particles with Solid Inclusions Finely Dispersed in a Melted Metal Matrix

A theoretical model has been developed to describe the splats formation from composite particles of several tens of micrometers in size whose liquid metal binder contains a high volume concentration of ultra-fine refractory solid inclusions uniformly distributed in the binder. A theoretical solution was derived, enabling evaluation of splat thickness and diameter, and also the contact temperature at the particle-substrate interface, under complete control of key physical parameters (KPPs) of the spray process (impact velocity, temperature, and size of the particle, and substrate temperature) versus the concentration of solid inclusions suspended in the metal-binder melt. Using the solution obtained, the calculations performed demonstrate the possibility of formulating adequate requirements on the KPPs of particle-substrate interaction providing a deposition of ceramic-metal coatings with predictable splat thickness and degree of particle flattening on the substrate, and also with desired contact temperature during the formation of the first coating monolayer.

Numerical simulation of a particle-laden plasma flow in a complex configuration under an electromagnetic field ☆

Numerical simulation was conducted to clarify the electromagnetic thermofluid fields of a particle-laden compressible plasma flow in a complex configuration, as well as the characteristics of the injected fine particles in the plasma flow. The effects of an applied radio-frequency (RF) electromagnetic field, particle size and particle material on in-flight particle characteristics were clarified by use of a numerical model which took plasma compressibility and variable transport properties into consideration. Particle velocity was found to be strongly influenced by particle diameter. The heating rate of the Ni particle was the highest due to its lowest latent heat of melting. It was possible to efficiently control particle temperature by heating of the plasma flow with an RF electromagnetic field applied to the nozzle. Finally, particle phase condition was drawn in terms of the injection velocity and particle diameter for optimization of particle properties.

Evaluation of Highly Functional TiO2 Synthesized by Solution Precursor Spraying in a DC-RF Hybrid Plasma Flow System

Carbon doped TiO2 synthesized in a DC-RF hybrid plasma flow system by the solution precursor spraying method was evaluated. The effects of mass fraction of TTB in solution precursor on contents and photocatalytic activity of carbon doped TiO2 were investigated. Highly functional carbon doped TiO2 at 11 wt% TTB had higher UV-visible absorption characteristics, which showed higher degradation rate of methylene blue solution under visible light irradiation.