Hong-bing Xiong

Study of droplet deformation, heat-conduction and solidification using incompressible smoothed particle hydrodynamics method

This paper presents a numerical model based on incompressible smoothed particle hydrodynamics (SPH) method to simulate the deformation, heat-conduction and solidification process of a droplet impinging on a substrate. Continuum, momentum and energy equations of the fluid flow are solved using SPH method, with van der Waals force accounting for the surface tension and Fourier’s law for heat conduction. Incompressible smoothed particle hydrodynamics method (ISPH) is used here instead of weakly compressible smoothed particle hydrodynamics method (WSPH), to satisfy the fluid incompressibility. Effects of rough and smooth substrates are also taken into account. Results show that the droplet begins to deform and spread, followed by solidification. Also, the temperature of droplet falls faster in the rough one, and this may attribute to the roughness.

Smoothed particle hydrodynamics modeling of free surface flow

Abstract A method for simulating free-surface flow is presented in this paper. The approach is based upon smoothed particle hydrodynamics (SPH). The fully Lagrangian nature of SPH maintains sharp fluid-fluid interfaces without employing high-order advection schemes or explicit interface recons- truction. The SPH code is validated by the benchmark case of the shear driven cavity flow; the SPH results are compared with a grid-based finite difference method and an excellent agreement has been achieved. Using this code, the process of a water droplet impacting and spreading on a solid surface is investigated as an example of free surface flow

Simulation of Effervescent Atomization and Nanoparticle Characteristics in Radio Frequency Suspension Plasma Spray

In this paper, a comprehensive model was developed to investigate the suspension spray for a radio frequency (RF) plasma torch coupled with an effervescent atomizer. Firstly, the RF plasma is simulated by solving the thermo-fluid transport equations with electromagnetic Maxwell equation. Secondly, primary atomization of the suspension is solved by a proposed one-dimensional breakup model and validated with the experimental data. Thirdly, the suspension droplets and discharged nanoparticles are modeled in Lagrangian manner, to calculate each particle tracking, acceleration, heating, melting and evaporation. Saffman lift force, Brownian force and non-continuum effect are considered for nanoparticle momentum transfer, as well as the effects of evaporation on heat transfer. This model predicts the nanoparticle trajectory, velocity, temperature and size in the RF suspension plasma spray. Effects of the torch and atomizer operating conditions on the particle characteristics are investigated. Such operating conditions include gas-to-liquid flow ratio, atomizer orifice diameter, injection pressure, power input level, plasmas gas flow rate, and powder material. The statistical distributions for the multiple particles are also discussed for different cases.

Effects of Atomization Injection on Nanoparticle Processing in Suspension Plasma Spray

Liquid atomization is applied in nanostructure dense coating technology to inject suspended nano-size powder materials into a suspension plasma spray (SPS) torch. This paper presents the effects of the atomization parameters on the nanoparticle processing. A numerical model was developed to simulate the dynamic behaviors of the suspension droplets, the solid nanoparticles or agglomerates, as well as the interactions between them and the plasma gas. The plasma gas was calculated as compressible, multi-component, turbulent jet flow in Eulerian scheme. The droplets and the solid particles were calculated as discrete Lagrangian entities, being tracked through the spray process. The motion and thermal histories of the particles were given in this paper and their release and melting status were observed. The key parameters of atomization, including droplet size, injection angle and velocity were also analyzed. The study revealed that the nanoparticle processing in SPS preferred small droplets with better atomization and less aggregation from suspension preparation. The injection angle and velocity influenced the nanoparticle release percentage. Small angle and low initial velocity might have more nanoparticles released. Besides, the melting percentage of nanoparticles and agglomerates were studied, and the critical droplet diameter to ensure solid melting was drawn. Results showed that most released nanoparticles were well melted, but the agglomerates might be totally melted, partially melted, or even not melted at all, mainly depending on the agglomerate size. For better coating quality, the suspension droplet size should be limited to a critical droplet diameter, which was inversely proportional to the cubic root of weight content, for given critical agglomerate diameter of being totally melted.

Modeling of Micro- and Nanoparticle Characteristics in DC Suspension Plasma Spray

Suspension plasma spray is a promising technology for surface coatings. In this work, a comprehensive numerical model was developed to investigate the multiphase flow of suspension droplets and nanoparticles in direct-current (DC) plasma spraying. A three-dimensional computational model was developed to describe the plasma jet flow fields coupled with the axial injection of suspension droplets in which the zirconia micro- and nanoparticles were dispersed. The suspension droplets were tracked using Lagrangian coordinates, considering particle heating, melting, and evaporation. After evaporation of the solvent surrounding the particle, the nanoparticles were discharged into the plasma flow. In addition to the viscous force exerted by the flow on the micrometer-sized particles, the Brownian force and the Saffman lift force were taken into account. The effects of the noncontinuum on particle momentum transfer and evaporation on heat transfer were also considered. The numerical predictions of gas flow temperature were compared with experimental data and numerical data obtained with a different computational fluid dynamics code. The agreement was reasonable. The trajectories, velocity, and temperature of nanoparticles were calculated, and compared with those of microparticles. The results showed that the Brownian force plays a major role in acceleration and heating of nanoparticles. Compared with the conventional plasma spray process with micrometer-sized feedstock, the nanoparticles in suspension plasma spraying were found to have a wider spatial distribution and higher temperature. The effects of operating parameters, such as the power input to the plasma gas and plasma gas composition, on the gas velocity and temperature were investigated. The parameters that have a significant effect on the heat and momentum transfer to the particles injected in the plasma jet were identified.

Numerical research on the hydrodynamic stability of Blasius flow with spectral method

Abstract Orr-Sommerfeld equation is solved numerically using expansions in Chebyshev polynomials. The numerical results show a good agreement with Howarths solution, with relatively low computational cost. This method is then applied to the stability of flat plate boundary layer flow compared with the finite difference method; our study shows that the expansions in Chebyshev polynomials are more suitable for the solution of hydrodynamic stability problems than the expansions in finite difference method.