基于ANSYS Fluent的气流喷雾干燥研究与仿真（英文）

基于气液两相流理论,采用Fluent对气流喷雾干燥的过程进行了仿真,得到了液滴通入前后的温度、速度、湿度场云图的变化情况,以及颗粒停留时间及含水量分布。结果表明:为防止粘壁现象的产生,液滴喷入塔体的速度不宜过高。为提高产品干燥效果,应适当延长干燥时间。对其喷雾干燥过程的三维数值模拟,为喷雾干燥塔的设计与制造提供了参考依据。     

气旋型喷雾干燥塔干燥效率的模拟分析

基于压力－旋流雾化喷嘴模型,采用Fluent和Gambit软件对气旋型喷雾干燥过程进行了数值模拟,得到了干燥过程中干燥塔内颗粒运动质量与停留时间的变化规律。分析了进风温度、风速、料液温度及重力等因素对蒸发干燥效率的影响。分析结果为气旋型喷雾干燥塔的优化设计和干燥效率的提高提供了参考。     

离心式喷雾干燥制备纳米晶粉末的工艺原理及其影响因素

为了实现离心喷雾干燥纳米晶粉末生产的批量化和工业化。本文结合离心式喷雾干燥法的工艺特点 ,论述了离心式喷雾干燥法的三种雾化机理 :料液直接分裂成液滴、丝状割裂成液滴和膜状分裂成液滴三种情况 ;考察了制备复合氧化物粉末的影响因素 ,其制粉效果与分散盘的形状、直径、转子转速、进料量、进出口温度、料液浓度、料液表面张力和粘度等因素密切相关 ;给出了喷雾干燥法制备超细粉末的应用前景。     

斜锥表面液滴的定向流动特性

目的 研究液滴在斜锥表面的定向流动特性，揭示液滴在斜锥表面的定向输运机理。方法 以斜锥表面液滴运动行为为研究对象，通过数值模拟技术提取液滴动力学参数，探讨不同斜锥结构参数对液滴自输运行为的影响。结果 液体内部速度的不均匀分布导致液滴内部产生速度涡旋，在泰勒毛细升力的作用下，斜锥间隙产生了涡量较大的速度主涡旋，液滴内部产生了涡量较小的速度次涡旋。在液滴定向输运过程中，2种涡旋的旋转方向与液滴的输运方向保持一致。液滴自输运过程伴随着表面能、动能的相互转化。在铺展收缩阶段，液滴的形变量较大，固–液接触面积和液滴的表面能先增大再减小，液滴运动速度却先减小再增大。在稳定输运阶段，液滴的表面能和速度基本保持不变。泰勒毛细升力和斜锥间隙中的不平衡毛细力驱使液体不断填充斜锥间隙。液滴在斜锥表面受到不平衡的钉扎阻力，使得液滴左侧更易脱钉，保证整个液滴向右无损输运。结论 可为揭示液滴在斜锥表面的流动特性和自输运机理提供理论支持，指导研制以斜锥结构为仿生原型的机械功能表面。     

喷雾干燥法生产颗粒保护渣的水分控制

喷雾干燥法生产保护渣的实际生产试验表明进风温度是影响水分含量的重要因素.基于此,计算了保护渣料浆雾化后的液滴在干燥室中的停留时间和干燥所需的时间,并比较不同进风温度下与不同直径的液滴间的关系.结果表明,进风温度越高,颗粒保护渣中水分含量越低,进风温度大于500℃后,水分含量变化平缓;液滴直径越小,水分含量越低.     

熔渣中金属液滴沉降过程流体力学模拟（英文）

建立熔炼渣中金属液滴沉降过程的数学模型，研究不同粘性模型下金属/熔渣界面张力对液滴沉降速度以及拖曳力的影响。结果表明：对于较小直径的液滴，Laminar模型和RNG k-ε湍流模型能够准确预测液滴最终沉降速度；界面张力对液滴沉降速度有影响，且影响作用随着液滴尺寸增大而逐渐增大；“RNG+CSS”模型可以准确描述液滴沉降过程和液滴沉降速度的变化。结合数学模型和实验数据，推导出耦合界面张力作用下拖曳力系数计算公式。该模型揭示熔炼渣中金属液滴沉降过程机理，为实际生产中降低渣中有价金属含量提供理论指导。     

雾化Al-Si合金液滴冷却凝固过程模拟

在群体动力学模型基础上，提出了描述雾化共晶合金液滴组织演变的数学模型，并将其与液滴的运动与传热方程相耦合，对合金的冷却凝固过程进行研究，探讨液滴尺寸对合金冷却凝固过程的影响，结合A390合金进行了计算。结果表明：随着液滴尺寸的减小，所获得的冷却速度及过冷度增大，而析出初生Si相的尺寸及体积分数减小，当液滴尺寸足够小时，液滴温度的变化趋势及微观组织将发生突变。     

液滴间距对均匀金属液滴流温度场影响的研究

针对均匀金属液滴喷射成形过程中的液滴流,建立了均匀液滴流的有限元模型.通过该模型考察了液滴间距对液滴流中各个液滴温度场的影响,并讨论了液滴流的传热系数和冷却速率.结果表明,当液滴间距与液滴直径比为2.8、5.6、8.4、11.1、13.9时,液滴的冷却速率、传热系数均远低于单个液滴,液滴间距对冷却过程的影响较大.随着液滴间距增大,液滴之间的相互影响减小.     

凹槽结构强化液滴合并弹跳的数值研究

目的 为研究凹槽结构对液滴合并弹跳现象的影响,提高液滴自弹跳的速度和能量转化率。方法 通过有限元方法模拟,研究了液滴在凹槽结构上的动态行为和槽深H、槽宽W及液滴尺寸对V型槽和矩形槽上液滴的弹跳速度、无量纲速度、表面能转化率、动量和总动能的影响。结果 凹槽内与槽外液滴间的合并弹跳包含液滴接触、产生液桥、三相线收缩和液滴弹离表面4个过程;宽高比相同时,液滴弹跳速度和表面能转化率均随液滴半径先增大后减小,凹槽宽高比为1.5,液滴合并弹跳半径为0.25 mm工况下表面能转化效率最高;矩形槽与V型槽,宽高比为1.5,液滴合并弹跳半径为0.25 mm时,液滴弹跳速度与表面能转化率随半径比先增大后减小,在半径比为1时达到峰值,二者表面能转化率最大值为27.87%和30.66%。槽宽恒定时,凹槽结构强化液滴合并弹跳存在最佳凹槽宽高比和最优弹跳半径,V型槽提升液滴弹跳效率比矩形槽高约12%。结论 合并后液滴撞击V型槽侧壁的反作用力使液滴旁瓣较矩形槽更早受到侧壁面抑制而发生回流,从而提升了其能量转化效率,减少了液滴的振荡损失与黏性耗散。研究结果在设计高效的自清洁功能表面、提高冷凝表面换热效率、预防和抑制换...     

雾化工艺对Al-Si合金液滴凝固组织的影响

将雾化合金液滴的传热与运动方程相耦合,对合金的冷却过程进行研究,探讨液滴尺寸、气体初始速度、熔体过热度等参数对合金冷却过程及粉末微观组织的影响,并结合A390合金进行了计算。结果表明,随着液滴尺寸减小,所获得的冷却速度及过冷度增大;当液滴尺寸足够小时,液滴温度的变化趋势及合金液滴的组织将发生突变;增加雾化气体的初始速度,降低熔体过热度,可以抑制初生相的析出,有利于细化合金的组织。     

浆滴参数对酸性烟气净化的影响分析

建立了SO2、HCl酸性烟气喷雾干燥净化的气相、颗粒相及传热传质数学模型,模拟了浆滴运行参数对酸性烟气净化的影响,研究了浆滴在塔内的位移随入口烟气速度的关系;实验分析了雾化加湿和浆滴浓度对烟气净化效率的影响.实验结果表明:喷雾干燥过程中加入雾化加湿水后,SO2、HCl的脱除效率都增加,SO2脱除效率增加明显;增加浆液浓度,SO2脱除率和HCl脱除率均略有下降,且SO2脱除率减小程度大于HCl.模拟结果表明:当入口烟气速度为5 m/s时,浆滴的位移范围较大;随着浆滴粒径的增加,浆滴的温度随着时间逐渐升高,30,60和9 0 μm粒径的浆滴温度分别在3,3,4 s时达到稳定;随着浆滴粒径的增加,脱除效率不断增加,30,60和90 μm粒径条件下 HCl的去除效率分别在1.5,1.7和2 s达到平衡,SO2的去除效率分别在2.5,3.5和4 s达到平衡,可以看出, HCl 与SO2相比,其达到最大脱除效率的时间更短;随着入口烟气温度升高,SO2和HCl的脱除效率都在增大,HCl脱除效率比SO2脱除效率达到稳定的时间更短.     

氧化锆空心球粉体制备及其涂层性能研究进展

粉体是决定等离子喷涂热障涂层性能的基本因素,本文主要介绍了高性能等离子喷涂热障涂层用氧化锆空心球粉体的制备方法及其涂层性能的相关研究。等离子球化和喷雾干燥造粒是制备氧化锆空心球粉体的两种不同方法;氧化锆空心粉体在等离子喷涂过程中更易熔化,并且在熔化后形成空心液滴;空心液滴与基体碰撞铺展过程中存在的"反溅"使其铺展凝固时间更长,形成的扁平粒子直径小,厚度均匀,缺陷少;相对于其它类型粉体,采用氧化锆空心球粉体制备的热障涂层具有适中的孔隙率,低热导率和低模量,抗高温烧结,符合高性能热障涂层的性能要求。     

Modelling the shape and thermal dynamics of Ni superalloy rings during spray forming Part 1: Shape modelling – Droplet deposition,splashing and redeposition

A numerical model has been developed to simulate the dynamic shape evolution of Ni superalloy rings during spray forming. The model comprises: (1) a droplet primary deposition model, simulating droplet primary deposition at a deposition surface; (2) a droplet splashing model, simulating the droplet splashing/scattering behaviour; and (3) a droplet redeposition model, simulating the redeposition of the scattered droplets onto the deposition surface. The model has been validated against experiments of spray forming large diameter IN718 alloy rings, and for the first time, the effects of droplet splashing and redeposition on the dynamic shape evolution of spray forming IN718 alloy rings and the deposition yields have been investigated and quantified. The model serves as the basis for a thermal dynamic model that is described in Part 2 of this publication.     

铝合金脉冲MIG焊接熔滴过渡行为的声发射信号时频域表征

通过在线检测铝合金脉冲MIG焊接过程的结构负载声发射信号,研究熔滴过渡模式和行为的时频域表征.结果表明,熔滴过渡声发射信号波形以及熔滴过渡声发射事件的有序性和周期性可以反映脉冲MIG焊接过程熔滴过渡的稳定性和周期性.随着脉冲频率和焊接热输入的增大,熔滴过渡模式由短路过渡转变为射滴过渡,过渡熔滴体积得到逐步细化,表现出递减的声发射能量释放特征.当熔滴过渡为短路过渡模式时,熔滴过渡声发射信号的频域范围较宽,且较为集中在高频部分;当熔滴过渡为射滴过渡模式时,熔滴过渡声发射信号的频率分布范围更窄,且集中在低频部分.随着熔滴过渡模式由短路过渡模式逐渐转变为射滴过渡模式,由熔滴过渡引入的能量变化呈现出先减小再增大的趋势.     

钛-铝双丝超音速电弧喷涂过程中熔滴粒子几何特性研究

为了研究工艺参数对钛-铝双丝超音速电弧喷涂熔滴粒子尺寸的影响规律,通过对喷枪出口处气流速度、气体质量流率和金属熔滴质量流率的分析测定,以Nukiyama-Tanasawa模型为基础,建立了喷涂粒子的平均直径与喷涂电压和电流间的解析关系,并对喷涂电压和电流对熔滴粒子平均直径的影响进行了计算机模拟仿真.结果表明:在喷枪结构和雾化气体压力一定时,雾化粒子的平均直径随喷涂电流的增大而增大,随喷涂电压的升高而减小,但总的变化幅度却较小.利用激光粒度分析仪对一定喷涂工艺条件下所得的粒子平均直径进行实验分析,结果与计算值之间的相对误差为9.52%,扫描电镜观察表明粒子形态以球形颗粒为主.     

Metal transfer instability in gas metal arc welding

Abstract

This work presents a simplified model of metal transfer in gas metal arc welding. The model incorporates key features of metal transfer including the change in droplet diameters as welding moves from the globular into the spray metal transfer region, and the increase in welding voltage that is observed to occur as the pendant droplet grows. The model predicts that an instability arises in the globular metal transfer region, which leads to deterministic chaos and complex limit cycles with many droplet sizes. The instability also causes deterministic chaos with a characteristic gap in droplet diameters at the transition to spray mode metal transfer. The model explains observed features of metal transfer in some detail, including the existence and location of preferred bands of droplet sizes. Whether the instability is present or not defines the boundary between chaotic globular metal transfer and the stable drop spray transfer mode. The identification of deterministic chaos in gas metal arc welding metal transfer opens the way for new approaches to welding control.     

Analysis of dynamic plasma behaviours in gas metal arc welding by imaging spectroscopy

For gas metal arc welding, the effect of CO₂ mixture in a shielding gas on a metal transfer process was investigated through the observation of the plasma characteristics and dynamic behaviour at the droplet’s growth-separation-transfer by the temperature measurement methods which were suitable, respectively, to the argon plasma region and the metal plasma region. At the present experimental conditions, the metal transfer process was a spray transfer type with 100%Ar shielding gas. On the other hand, with 85%Ar + 15%CO₂ shielding gas, the metal transfer process was a globular transfer type in which the arc length was shorter, the width was narrower and the time interval of the droplet separation was longer. For both shielding gases, the metal plasma region near the arc central axis exhibited 6500–7500 K, which was lower than the argon plasma region. With 85%Ar + 15%CO₂ shielding gas, when the metal droplet grew below the electrode wire, the region below the droplet has a high plasma temperature and a high concentration of iron vapour which surrounded the droplet. The region also exhibited a remarkably high electron number density. At the spray transfer process, the argon plasma region had an electron number density twice as high as the metal plasma region. Meanwhile, at the globular transfer process, the metal plasma region had a higher electron number density than the argon plasma region, which corresponded to a higher electrical conductivity near the arc axis. This means that the electric current goes through the arc axis easier than the spray transfer process. This condition increases the temperature below the droplet. The thermal expansion increases the force preventing the droplet from falling down. In consequence, the metal transfer takes the globular transfer type.     

Effect of Using Liquid Feedstock in a High Pressure Cold Spray Nozzle

This study investigates the effect of water injection in the high pressure chamber of a cold spray nozzle. A De Laval nozzle geometry with constant back pressure and temperature is modeled numerically using Reynolds Stress Model coupled equations. Water spray with a droplet size of 10-100 ??m is modeled using both uniform and Rosin-Rammler size distributions. The two-phase flow of gas-liquid is modeled using an unsteady discrete phase mass source with two-way coupling with the main gas flow. Upon injection, the droplets in the water spray evaporate while travelling through the nozzle due to momentum and energy exchange with the gas flow. The evaporation behavior in the presence of water content is modeled and a correlation between the initial diameter and the diameter just before the throat is obtained. As a result, the proper droplet size distribution with a fully evaporative spray can be used as a carrier of nano-particles in cold spray nozzles. Having the results, guides us to substitute the un-evaporated part of the droplet with an equal diameter agglomerate of nano-particles and find a minimum fraction of nano-particles suspended in the liquid which guarantees fully evaporative liquid spray injection.     

Droplet splashing during arc spraying of steel and the effect on deposit microstructure

The mechanism by which droplet deposition occurs is important when filling substrate features for the electric arc spray forming of steel tooling. Particle image velocimetry and high-speed video imaging techniques have been used to observe droplet deposition, particularly with regard to the behavior of droplets originating from splashing. Droplet splashing on deposition has been seen to be significant, and splash droplets form a large proportion of the overspray. The splash droplets are smaller and, when first created, move slower than the parent droplet. When spraying into deep features, the lateral and upward movement of splash droplets acts as a mechanism for deposit formation onto surfaces in shadow from the main spray. Microstructural study has shown that oxidation of the splash droplets before redeposition leads to a deposit with a high fraction of oxide. Simultaneous growth of deposit formed directly from the spray, and from splash droplets, results in a banded microstructure containing elongated macropores. A mechanism for such microstructural evolution is proposed.     

Droplet transition for plasma-MIG welding on aluminium alloys

The synchronous acquisition system of droplet image inspection and arc electric signals were established and the droplet transition characteristics of aluminum alloys were researched in the plasma-MIG welding process.Typical droplet transition modes include globular transfer mode,short circuiting transfer mode,metastable spray transfer mode and projected transfer mode.The result indicates that MIG droplet transfer frequency and droplet transfer modes are changed by introducing the plasma arc in the plasma-MIG welding process compared with the MIG welding on the aluminum alloys,which broadens the range of welding parameters when the stable welding process proceeds.The metastable spray transfer and projected transfer mode are proved to be the most optimal modes by comparing the stability of electronic signal,droplet transition,weld appearance and weld penetration.