磨料池内气固两相流模型的建立与分析

为研究磨料池内复杂的气固流动现象和规律,采用全尺度直接数值模拟法。基于改进的内嵌边界法,通过流场控制方程、数值求解、并行计算等方法,建立气体湍流与磨粒的运动耦合模型,以分析磨料池内的气固流动特性。通过分析入口气体速度、磨粒高度对磨料池内气固两相流动状态的影响,选择合适的方案以提高系统稳定性。验证了磨料池内颗粒、壁面和拟序结构的耦合作用使得颗粒出现扩散、局部富集、壁面沉降及再悬浮现象。研究结果为磨料池结构的优化设计提供了参考。     

Numerical simulation of gas and particle flow in a high-velocity oxygen-fuel (HVOF) torch

A transient two-dimensional numerical simulation of Inconel spraying in a high-velocity oxygen-fuel (HVOF) torch barrel was performed. The gas flow is treated as a continuum multicomponent chemically reacting flow, whereas particles are modeled using a stochastic particle spray model, fully coupled to the gas flow. The calculated results agree well with experimental data and show important statistical aspects of particle flow in the torch.     

Numerical modeling of in-flight characteristics of inconel 625 particles during high-velocity oxy-fuel thermal spraying

A computational fluid dynamics (CFD) model is developed to predict particle dynamic behavior in a high-velocity oxyfuel (HVOF) thermal spray gun in which premixed oxygen and propylene are burnt in a combustion chamber linked to a long, parallel-sided nozzle. The particle transport equations are solved in a Lagrangian manner and coupled with the two-dimensional, axisymmetric, steady state, chemically reacting, turbulent gas flow. Within the particle transport model, the total flow of the particle phase is modeled by tracking a small number of particles through the continuum gas flow, and each of these individual particles is tracked independently through the continuous phase. Three different combustion chamber designs were modeled, and the in-flight particle characteristics of Inconel were 625 studied. Results are presented to show the effect of process parameters, such as particle injection speed and location, total gas flow rate, fuel-to-oxygen gas ratio, and particle size on the particle dynamic behavior for a parallel-sided, 12 mm long combustion chamber. The results indicate that the momentum and heat transfer to particles are primarily influenced by total gas flow. The 12 mm long chamber can achieve an optimum performance for Inconel 625 powder particles ranging in diameter from 20 to 40 μm. At a particular spraying distance, an optimal size of particles is observed with respect to particle temperature. The effect of different combustion chamber dimensions on particle dynamics was also investigated. The results obtained for both a 22 mm long chamber and also one with a conical, converging design are compared with the baseline data for the 12 mm chamber.     

Numerical analysis of a high-velocity oxygen-fuel thermal spray system

The fluid and particle flow field characteristics of a high-velocity oxygen-fuel (HVOF) thermal spray (TS) system are analyzed using a two-phase flow model and simulated using computational fluid dynamics (CFD) techniques. The model consists of a conservation equation and constitutive relations for both gas and particle phases. Compressible, turbulent flow is modeled by ak-ɛ turbulent model. A Lagrangian formulation is used to model particle trajectory, and heat and momentum transfer. The fluid velocity fluctuations resulting from gas turbulence are simulated by a stochastic model and the particle motion in the turbulent flow is calculated in a Lagrangian Stochastic-Deterministic (LSD) method. Details of gas flow field, particle temperature and particle velocity histories, and particle temperature and velocity profiles in the system are presented. For the validation of the numerical analysis, the computed results are compared with available experimental measurement. Excellent agreement between simulations and measurements is obtained for both gas and particle flow fields. A parametric study is also conducted for different particle sizes and different nozzle barrel lengths. The flow phenomena for different flow parameters are analyzed and explained as the result of gas dynamics and heat and momentum transfer between the two phases. The developed methodology provides a means to analyze, design, and optimize the TS process. The numerical analysis presents a first comprehensive, fundamental quantitative analysis for the HVOF TS system.     

基于DSMC-CFD方法的气固两相流冲蚀预测研究

目的 研究气固两相流中固体颗粒间碰撞对冲蚀的影响。方法 使用Eulerian-Lagrangian方法,将气相作为连续相,通过Navier-Stokes方程求解,颗粒平移运动由离散相模型（DPM）求解。颗粒间碰撞运动采用直接模拟蒙特卡罗（DSMC）方法进行模拟,用少量采样颗粒代替真实颗粒计算颗粒间碰撞,碰撞的发生条件通过修正的Nanbu方法判定,碰撞过程遵循颗粒间碰撞动力学模型,采用Grant-Tabakoff随机颗粒-壁面碰撞反弹模型,计算颗粒与壁面的碰撞运动。将颗粒运动信息导入5种不同的冲蚀模型,并将计算与未计算颗粒间碰撞的冲蚀预测模拟结果与实验数据进行对比。结果 颗粒间碰撞位置主要分布在90°弯头外拱侧的颗粒高浓度区,随着颗粒质量流量的增大,颗粒碰撞次数增加,且直管段中碰撞次数占比增大。随着入口速度的增大,颗粒碰撞次数减少。使用DSMC-CFD方法计算的最大冲蚀位置沿弯管外拱轴线向高角度方向偏移,且数值比忽略颗粒间碰撞的CFD方法约低5%~15%,总冲蚀率则两者区别不大。结论 引入DSMC方法计算颗粒间的碰撞,可以节省大量算力。弯管处发生颗粒间碰撞,DSMC-CFD冲蚀预测方法更符合实际,使用DSMC-CFD方法的Oka模型与实验测得值最贴近。     

Numerical Studies of the Application of Shock Tube Technology for Cold Gas Dynamic Spray Process

A new method for a combustion-free spraying is studied fundamentally by modeling and simulation in comparison with first experiments. The article focuses on the numerical simulation of the gas-particle nozzle flow, which is generated by the shock reflection at the end wall section of a shock tube. To study the physical fundamentals of this process, at present only a single shot operation is considered. The particles are injected downstream of the nozzle throat into a supersonic nozzle flow. The measurements of the particle velocity made by a laser Doppler anemometry (LDA) set up show that the maximum velocity amounts to 1220 m/s for stainless steel particles of 15 μm diameter. The CFD-Code (Fluent) is first verified by a comparison with available numerical and experimental data for gas and gas-particle flow fields in a long Laval-nozzle. The good agreement implied the great potential of the new dynamic process concept for cold-gas coating applications. Then the flow fields in the short Laval nozzle designed and realized by the Shock Wave Laboratory (SWL) are investigated. The gas flow for experimentally obtained stagnation conditions is simulated. The gas-particle flow without and with the influence of the particles on the gas flow is calculated by the Surface Engineering Institute (IOT) and compared with experiments. The influence of the injection parameters on the particle velocities is investigated, as well. This article is an invited paper selected from presentations at the 2007 International Thermal Spray Conference and has been expanded from the original presentation. It is simultaneously published in Global Coating Solutions, Proceedings of the 2007 International Thermal Spray Conference, Beijing, China, May 14-16, 2007, Basil R. Marple, Margaret M. Hyland, Yuk-Chiu Lau, Chang-Jiu Li, Rogerio S. Lima, and Ghislain Montavon, Ed., ASM International, Materials Park, OH, 2007.     

流动控制结晶器内磁场和吹氩对夹杂物粒子群运动的影响

利用数学模型求解包含电磁力项的Navier-Stokes方程得到流场的速度分布，以流场为基础，建立夹杂物粒子群运动的计算模型，利用水模型实验检验单一球体运动轨迹的计算结果。没有磁场作用时，所有粒子分两组分别进入上下回旋区作螺旋线运动，部分粒子在回流区内作螺旋线运动后又进入水口射流区，然后再进入反向回流区，处于上部回流区的夹杂物具有去除的可能性，吹入氩气能增加夹杂物粒子进入上部回流区的机会，从而提高夹杂物粒子的去除率，施加磁场后，夹杂物粒子的螺旋运动消失，同时粒子的运动速度明显降低，吹入氩气和施加磁场两者均能有效地控制夹杂物粒子群的运动。     

Mathematical modeling of the gas and powder flow in HVOF systems

A mathematical model was developed to describe the gas dynamics and heat-transfer mechanism in the gas/particle flow of high- velocity oxyfuel (HVOF) systems. A numerical solution was carried out using a PC- based computer program. One- dimensional predictions of the temperature and velocity profiles of gas and particles along the axis of flow were obtained to conduct cost- effective parametric studies and quality optimization of thermal spray coatings produced by HVOF systems. The numerical computer model allows for the variation of the HVOF system parameters, such as air/fuel ratio and flow rates, cooling water inlet temperature and flow rate, barrel length, standoff distance, particle size, and gun geometry. Because of the negligible volume of the powder relative to the gas, the gaseous phase was modeled as continuous nonadiabatic, and friction flow with variable specific heats and changing cross- sectional areas of flow. The generalized continuity, momentum, and energy equations with the influence parameters were used to model the gaseous flow regime and predict its thermodynamic properties. Empirical formulas for the mean axial decay of both velocity and temperature in the supersonic jet plume region were generated from published measurements of these parameters using laser Doppler velocimeter and Ray leigh scattering techniques, respectively. The particle drag and heat- transfer coefficients were calculated by empirical formulas in terms of Reynolds, Nusselt, and Prandtl numbers to evaluate both the momentum and heat transferred between the combustion gases and the powder particles. The model predictions showed good agreement with the particle and gas temperature and velocity measurements that are available in the literature.     

Numerical modeling of a low temperature high velocity air fuel spraying process with injection of liquid and metal particles

An analysis of a low temperature high velocity air fuel (LTHVAF) thermal spray process is presented using computational fluid dynamics (CFD). The originality of the process lies in the injection of liquid (water) upstream of the powder injection to control to gas temperature and, therefore, the heat transfer to the injected particles. First, computation fluid dynamic techniques are implemented to solve the mass, momentum, and energy conservation equations in the gas phase. A turbulence model based on the renormalized group theory (RNG) is used for the turbulent flow field. The gas dynamic data are, then, used to model the behavior of the liquid droplets and particles in the gas flow field. The calculated results show that the liquid flow rate should range between 20 and 30 kg/h to achieve the optimal gas characteristics for particle treatment. They also show that particle velocity and temperature are strongly affected by particle size. At the gun exit, the particle velocity and temperature range between 700 and 300 m/s and between 900 and 400 K, respectively, for Cu and Ni particles with size distributions of 1 to 50 μm. As expected, the smaller particles have higher velocity and temperature. The metal coatings (Nickel and copper) produced by the LTHVAF spray process are characterized by low oxide content, low residual stresses, high deposition rates, and good bonding to the substrate.     

CFD simulation on performance of new type umbrella plate scrubber

A new type of umbrella plate scrubber was developed to address the pollution due to the dust, dioxide sulfur and other harmful gases, which were emitted from coal-burning boilers. The performance of the new device was studied through computational fluid dynamics（CFD） simulation and experiment methods. Initial work included experimental measurement of inlet-velocity, and gas phase simulation using Reynolds stress model（RSM）. After gas phase was converged, particles were injected from the inlet of the new device. Discrete phase model（DPM） was used for particle trajectories determination. The pressure drop and the collection efficiency of the new device were predicted through simulation. The simulation results show that the pressure drop of the new devices is 230-250 Pa and the efficiency is 84%-86%, with the inlet velocity equal to 10.6 m/s and the dust concentration ranging from 2 to 22 g/m3. The CFD simulation results of the new device show good agreement with experimental data. The relative error of the pressure drop and the efficiency is approximately 4% and 10% respectively. The results obtained both from the numerical simulation and from the experiment demonstrate that CFD simulation is an effective method for this type of study.     

Assessment of CFD Modeling via Flow Visualization in Cold Spray Process

The two-phase flow properties of copper particle laden nitrogen are computationally modeled and compared with the data obtained from the experiments, determining the achievable degree of consistency between model and reality. Two common, commercial nozzles are studied. A two-way coupled Lagrangian scheme along with the RSM turbulence model is used to track the particles and to model the interactions between the gas and the particulate phase. Significant agreement is found for the geometrical gas flow structure, the resulting particle velocities, and the dependence of the two-phase flow on the particulate phase mass loading. The particle velocities decrease with increasing mass loading, even for modest powder feed rates of <3 g/s. The velocity drop occurs even when the gas flow rate is kept constant. Adiabatic gas flow models neglecting the energy consumption by the particles are thus inaccurate, except for very dilute suspensions with low technical relevance. For the cases modeled, the experiments evidence the high predictive power of the chosen CFD approach.     

Analysis and optimization of the HVOF process by combined experimental and numerical approaches

Thermal spraying with the HVOF technology is a well known approach to dense metallic, ceramic and cermets coatings with good mechanical properties. Any attempt for improving HVOF coating properties requires a fundamental understanding of the mechanisms that occur during HVOF spraying. Thermal spray processes are not only optimized by empirical testing and by correlation analysis between process parameters and coating properties but also with numerical approaches. Recent attempts to understand the momentum and heat transfer mechanisms between flame and particles, and thus improve the control of the thermokinetic deposition process by analysis of fundamental thermophysical and fluid mechanical processes, have led to computational modeling of the spraying process and verification of simulation results by in-flight particle analysis.This paper focuses on modeling (tracking) of the particle properties during HVOF spraying using alumina powder. The particle properties are sensitive to a large number of process parameters (e.g., gas temperature, gas expansion velocity, pressure, spraying distance, spray powder particle diameter, nozzle geometry, etc.). Variation of the operating parameters of the HVOF process (gas flow rates, stoichiometric oxy/fuel ratio, nozzle design, etc.) is performed during modeling and simulation. The SprayWatch® system for particle in-flight measurement is used for verification of the numerical analysis result.     

Computer modeling of particle pushing and clustering during matrix crystallization

Two-dimensional cellular automaton computer simulations were carried out to model the geometric interaction between mobile, equiaxed particles and growing matrix grains, thus simulating crystallization (respectively, recrystallization, phase transformation or solidification) of a matrix material containing a mobile second phase (e.g. solid particles, liquid droplets or gas bubbles). The model allows the study of particle pushing by growing grains, which leads to particle accumulation and clustering at grain boundaries and triple points, and concomitant particle depletion within grains. Parameters explored are particle area fraction, particle settling speed, particle cluster mobility and grain nucleation rate under continuous nucleation conditions. These parameters are found to strongly affect the particle spatial distribution and clustering during and after crystallization. Conversely, the particles have no measurable effect on the grain shape or size. Finally, site-saturated nucleation at the boundaries of the simulation field is investigated, simulating e.g. solidification from crucible walls or recrystallization from sample edges. Pronounced clustering of particles takes place at grain boundaries and is further accentuated by particle settling.     

主气流温度对高压冷喷涂粉末速度和温度的影响

目的研究高压冷喷涂中,送粉气流在室温时主气流温度对冷喷涂粒子速度和温度的影响。方法利用计算流体力学软件FLUENT对冷喷涂流场进行数值模拟,分析不同送粉压差、不同喷管喉部直径的情况下,主气流温度对气体流场、粉末速度和温度的影响状况。结果送粉压差为0.1 MPa且喷管喉部直径为2 mm时,进入喷管的送粉气流流量占总气流流量的比值超过50%,此时提升主气流温度对冷喷涂粒子撞击速度和温度的提升幅度十分有限。在不改变送粉气流流量的情况下,增加喷管喉部直径可有效削弱送粉气流对粒子加速的不利影响。结论考虑送粉气流时,主气流温度对冷喷涂粉末沉积效果较弱,为了提高冷喷涂粉末沉积效率应保证顺利送粉的前提下尽可能地减小送粉气流流量,并且在设计喷管时应适当增加喷管喉部直径。     

Theoretical and Experimental Investigation of Gas Flows,Powder Transport and Heating in Coaxial Laser Direct Metal Deposition (DMD) Process

The results of theoretical and experimental investigations of direct metal deposition (DMD) processes involving a CO₂-laser with the power up to 5 kW and wave length of 10.6 μm are presented. The physical and mathematical model of multi-layer gas flows with gas-jet transportation of metal powder particles has been developed. To simulate the flows of carrier and shaping gases in annular channels of a triple coaxial nozzle, Navier-Stokes equations were applied for an axisymmetric flow. Thermodynamics and powder particles transport are calculated from a discrete-trajectory model with due regard to particle collision with solid walls of the transport nozzle. It is shown that particles may overheat on their way between the nozzle and substrate; the overheating depends on the trajectories by which particles move, on their size, and time of their retention in the laser-radiation region. The results of performed experimental researches on DMD processes visualization are presented. Some results of numerical simulation and experimental data are compared and analyzed.     

Simulation study of effects of initial particle size distribution on dissolution

Dissolution kinetics of γ′ particles in binary Ni–Al alloys with different initial particle size distributions (PSD) is studied using a three-dimensional (3D) quantitative phase field model. By linking model inputs directly to thermodynamic and atomic mobility databases, microstructural evolution during dissolution is simulated in real time and length scales. The model is first validated against analytical solution for dissolution of a single γ′ particle in 1D and numerical solution in 3D before it is applied to investigate the effects of initial PSD on dissolution kinetics. Four different types of PSD, uniform, normal, log-normal and bimodal, are considered. The simulation results show that the volume fraction of γ′ particles decreases exponentially with time, while the temporal evolution of average particle size depends strongly on the initial PSD.     

Numerical model and experimental observation for distribution of SiCp in electromagnetic-centrifugally cast composites

A two-phase numerical model coupled with heat transfer was presented to describe the radial distribution of SiC particles on centrifugally-cast metal matrix composite,and a transverse static magnetic field was concurrently imposed to induce electromagnetic stirring of the melt as it revolved with the mold.Meanwhile,experimental observations were also carried out to examine the radial distribution of SiC particles in pure aluminum.The effects of the imposed magnetic field,particle size and the matrix metals were discussed.The computational results show that the particles tend to be congregated by the centrifugal force,and both increasing the imposed magnetic field and decreasing the particle size tend to result in even distribution of the particles.With the magnetic field varying from 0 to 1 T and the particle size from 550 to 180 μm,a uniform distribution of the particles in the aluminum matrix can be obtained among the computational results.The matrix metal can also influence the particle distributions due to the difference in physical properties of metals.Experimental observation shows similar tendency of particle distributions in aluminum matrix influenced by magnetic field and particle size.However,the chilling effect from the mold wall results in an outer particle-free zone,which is not involved in the numerical model.     

Simulation of PYSZ particle impact and solidification in atmospheric plasma spraying coating process

In this work a numerical model of the impact and solidification of partially yttria stabilized zirconia particles on flat and rough substrate surfaces under plasma spraying conditions and the simulation results are presented. Results of the numerical simulation showed the influence of particle diameter and particle state prior to impact on splats spreading behavior and final morphology. The particles have a diameter range from 20 µm to 60 µm. Particle initial conditions prior to impact: speed, temperature and melting state are taken from previous simulation approaches of particle acceleration and heating. Simulations of fluid dynamics, heat transfer and solidification during the particle impact were performed using computational fluid dynamics. Tracing of free surfaces was determined by the volume of fluid method. The simulation results are compared with several numerical and experimental studies of other scientists and showed good agreement. Simulated splat morphologies are compared with experimentally obtained splats. The numerical model shows good results under real coating conditions and is suitable for the implementation in industrial applications. This model builds a basis for calculation of microstructure during real coating processes and can be used not only for coating under atmospheric plasma spraying conditions but also for similar coating processes and diverse materials.     

Analysis of the two-phase flow in a de laval spray nozzle and exit plume

One method used in spray forming and coating technology involves transonic/supersonic gas-droplet two- phase flows through a de Laval nozzle and subsequent subsonic freejet flow from the nozzle to the sprayed surface. To the first- order approximation, this complex phenomenon can be treated in a quasione-dimensional manner to simulate the entire converging- diverging nozzle flow field (with particle injection at the throat) as well as the plume (freejet) region. The basic numerical technique and computer model solve the steady gas field equations through a conservative variable approach and treat the droplet phase in a Lagrangian manner, with full aerodynamic and energetic coupling between the droplets and the transport gas handled via source terms. These analyses are simple and economical to execute. The one- dimensional models are valuable in constructing algorithms for automated process control. Finally, these one- dimensional models give direction to two- and three- dimensional simulations and serve as a test bed for models based on particle dynamics and energetics.