L.L. Zheng

Studies of splat morphology and rapid solidification during thermal spraying

The splat morphology and substrate melting and re-solidification have been studied numerically and experimentally for molybdenum droplets impacting on molybdenum, mild steel, and glass substrates. The effects of the droplet size, velocity, temperature, and substrate material and temperature on the splat morphology and the rate of solidification have been investigated. The correlation between the splat flattening ratio and the Reynolds and Jakob numbers has been developed. The substrate melting and re-solidification strongly depends on the impinging temperature of the droplet, substrate temperature and its thermophysical properties. A two-dimensional model considering free surface deformation and rapid solidification has been developed and used to predict the splat morphology.

Three-dimensional simulation of plasma spray jet

A three-dimensional computational model has been developed to describe the compressible, multi-component, turbulent, reacting plasma jet coupled with the orthogonal injection of carrier gas and particles. This model has been applied to plasma spray process that includes physical phenomena such as heating, melting, accelerating, and evaporation of in-flight particles. The entrained particles, NiCrAlY and ZrO2 , are discretely treated in a Lagrangian coordinate and stochastically generated by sampling from the probability distributions of the particle size and its velocity at the injection nozzle. In this study, special attention has been directed to the effects of carrier gas injection on the characteristics of plasma jet. The computational results show that the injection of carrier gas from the orthogonal injector above the plasma jet introduce the 3-D phenomena of plasma gas flow. The plasma jet is defected and the thermo-fluid flow near the injector is locally deformed.Copyright

An integrated model for interaction between melt flow and non-equilibrium solidification in thermal spraying

Abstract In this paper, a micro/macro-integrated model based on the VOF scheme is presented that accounts for free surface movement, thermal contact resistance, and fluid instability. A sub-model is developed to include the non-equilibrium solidification phenomena at the solid/liquid interface. The melt flow is incorporated into the microscopic model through prescribing a velocity profile that is obtained from the interpolation of melt velocities on the macroscopic grids near the interface. To improve the efficiency of the integration between the melt flow and the microscopic model, a relational database is developed and applied to the integrated micro/macro model. Three velocity profiles, e.g., linear, parabolic, and cubic velocity profiles, are considered and the results are compared with those obtained from the diffusion model.

Revised form of Jackson–Hunt theory: application to directional solidification of MnBi/Bi eutectics

A generalized eutectic theory, which defaults to the prior theories by Jackson–Hunt and Trivedi et al. under their boundary conditions, has been developed to consider phase relations, solidification temperature and interface velocity over the entire parametric range, and for both lamellar and rod eutectic morphologies. The analytical solution reveals that λ2V and λΔT, though varying with the Peclet number, are virtually constant at very low Peclet number (crystal growth conditions). These constant values, however, may vary with the phase reaction and the degree of undercooling.

Thermoelectric effects on interface demarcation and directional solidification of bismuth

This paper investigates thermoelectric effects on interface demarcation during directional solidification of bismuth. A complete description of thermo-electric effects is presented and calculations of related thermo-electric coefficients are elucidated. Numerical simulations for directional solidifications in bismuth were carried out by using an in-house computer code - MASTRAPP, in which a non-orthogonal curvilinear coordinate system is adopted and a multi-zone adaptive grid generation scheme is used. Numerical results with and without current pulsing are presented.

Melting Behavior of In-Flight Particles and Its Effects on Splat Morphology in Plasma Spraying

Effects of particle size, velocity, and temperature on the splat morphology have been well studied in the open literature. Effects of melt fraction on deposition efficiency and splat morphology are, however, not well understood. In this paper, we will focus on the melting behavior of in-flight particles and their impact on splat morphologies. A group parameter, “melting index”, has been derived to correlate the melting status of in-flight particles with particle size, velocity, and temperature which can be measured experimentally. Numerical simulations have been used to determine the unknown parameters in the melting index. The effects of initial particle diameter on the melting behavior have also been investigated.© 2002 ASME

Numerical and experimental study of polysilicon deposition on silicon tubes

Abstract Computational and experimental methods were used to investigate the production of bulk polysilicon via a horizontal tubular CVD reactor. Experiments were conducted to study the effect of cooling gas on the system process while keeping the process gas flow rate at zero. A numerical model was also developed to simulate the process. Simulation results were compared to the experimental data to examine the effect of cooling gas on temperature distributions of the silicon tube and the inner quartz tube, as well as the effect of different process gas flow rates on heating power input and silicon tube temperature. Using the numerical simulation method, the investigation has also been conducted to reveal the correlation between the polysilicon production rate and the process gas flow rate, mass fraction of the silane gas in hydrogen and the temperature of the substrate silicon tube. This study has demonstrated the feasibility of high production rate for polysilicon in the new reactor.

Generic Correlation of Spray Distance With In-Flight Particle Behavior for Different Kinds of Materials in Air Plasma Spraying

This paper investigates the influence of spray distance on inflight particle status for Yttria Stabilized Zirconia (YSZ), Mo and NiCrAlY thermal spray powders in Ar-H2 plasma through modeling and experiments. Distributions of individual particle characteristics such as temperature, velocity and size at the point of the maximum particle flux and points of the squared grids were measured for a few plume cross-sections. Numerical simulations were performed using the experimental conditions, and parametric studies were also performed. In-flight particle characteristics and their distributions were presented at different standoff/spray distances to reveal the heating and melting behaviors of different kinds of materials such as metal, alloy and ceramics powders. It can be found that for ceramic, metal and alloy powders, the particle velocity and temperature all increase with spray distance and then decrease after reaching maximum. However the positions of maximum temperature and velocity depend strongly on the material and size of the particles. The statistical distributions of temperature and velocity at different spray distances were also presented. It can be observed that the temperature distribution becomes narrower as spray distance increases. The occurrence of such behavior can be explained by analyzing particle kinetic energy and molten status. The correlations between powder properties and spray conditions have been established and they can serve as guidance for spray distance optimization of air plasma spraying for metal, alloys and ceramics particles.Copyright

Smoothed Particles Hydrodynamics Method for Interaction Between Multi-Droplets and Substrate

The smoothed particle hydrodynamics (SPH) method, one of meshfree methods, is developed to simulate the interaction between multi-droplets and substrate with solidification. However, the SPH method for this complicated problem needs a large amount of computing time, since it has to use a large number of the SPH particles to represent multidrops and substrate. All-pair search and linked list algorithms are popular in the neighbor search, which is the most time consuming part of the SPH calculation. Both algorithms are tested in this paper. For the solidification process, since the volume of the melt is decreased continuously, a new method is proposed to speed up the SPH calculation. The new treatment is used to handle the particles near the free surface and near the solidification interface Multi-droplets impinging on a smooth substrate in 2D and 3D are simulated to demonstrate the capability of current numerical method on simulating the spreading and solidification of multi-droplets.Copyright

Transport Phenomena in Growth and Annealing of Laser Crystals

Cloudiness, bubble core defects, anomalous absorption, low-angle grain boundaries, and cracking are the main problems in the growth of high quality and large diameter (>7cm) Yb:S-FAP crystals utilizing the Czochralski method. The generation mechanism of these defects is highly related to transport phenomena in the growth system. In this paper, firstly, inductive, conductive and radiative heat transport phenomena are examined in the entire growth system. Then, an integrated modeling and experimental study is presented to determine an efficient means to eliminate or reduce crystal cracking during cooling. A process model has been developed to simulate the crystal cooling process. The effect of temperature distribution on thermal stress in the crystal during cooling is predicted by a simple but effective computational algorithm. With the relationship between the power change and crystal surrounding temperature change, the process model is further used to optimize power ramp-down profile to avoid cracking of the crystal during cooling-down process.Copyright