载荷对线接触热弹流温度场的影响

在弹流动力润滑中,尤其是在重载的情况下,润滑剂呈现出强烈的非牛顿流体的流变特性,添加剂以及合成润滑剂的使用,也会导致润滑油呈现非牛顿性质,从而使得润滑剂的流变行为成为润滑设计中必须考虑的重要因素;对于高速或重载的情况,众多的研究结果表明:热效应是明显的、不可忽略的。选用Ree-Eyring模型,通过联立求解雷诺方程、弹性变形方程、能量方程而得到线接触非牛顿流体热弹流的温度场,分析了不同载荷下的温升变化,从而找出油膜内温度场随载荷的变化规律。     

非牛顿流体微挤出过程动态建模与数值仿真

非牛顿流体微挤出量难以在线测量,对挤出量的高一致性要求在很大程度上依靠预测模型的精度来保证。针对工程中常用的一类幂率非牛顿流体,在分析其动态特性的基础上,建立了非牛顿流体微挤出时的流速、流量及微挤出量的物理预测模型,且模型中保留了驱动压力、时间等挤出过程主要影响因素的物理量,使得模型便于与控制算法集成。通过数值仿真,验证了模型的有效性。研究结果表明,所建模型对幂率非牛顿流体微挤出量的预测精度达到了1%以上,为非牛顿流体定量微挤出过程的精确控制提供了基础。     

基于液晶引流的微驱动器试验研究

进行了液晶引流微驱动试验,设计制作了具有二、四方向驱动功能的液晶微驱动器,运用自制波形发生器进行驱动,利用偏光显微镜测试分析系统对驱动器上玻璃板运动情况进行观察和记录,计算上玻璃板运动的速度和位移。当驱动电场为幅值40 V、频率1 Hz、占空比10%的方波时,驱动速度可达约20μm/s。试验结果表明:利用液晶的引流效应实现微流体驱动是可行的,液晶微驱动器的设计方式比较简单、灵活,有利于微机械系统的微型化。     

炉排片熔模铸造充型与凝固过程的数值模拟及验证

基于有限元直接差分的方法.对炉排片熔模铸造充型过程流场的分布和凝固过程温度场的变化进行数值模拟,预测铸造过程中缩孔、缩松缺陷产生的位置,解释了这些缺陷产生的原因.通过对实际炉排片铸件取样及其在光学显微镜下微观组织的观察,对模拟结果进行了进一步的验证.     

半固态镁合金连续铸轧过程的数值模拟

根据半固态镁合金连续铸轧工艺过程的特点,建立了二维不可压缩非牛顿流体流动与传热的耦合计算模型,并使用流体分析软件FLOW3D对连续性方程、纳维-斯托克斯方程及能量方程进行稳态求解。考虑到辊间半固态金属浆料流动特点及双辊的布置方式有利于保持熔池内的层流流动,选取层流模型来进行描述。最终获得了连续铸轧工艺过程的流场、温度场和固相体积分数的分布以及入口浆料温度、铸轧速度等工艺参数对铸轧过程的影响规律。模拟结果可以很好地对工艺参数进行优化,从而为半固态镁合金的连续铸轧工业化生产奠定基础。     

基于示踪粒子的摆动TIG填丝焊熔池行为数值分析

为了使TIG焊熔池液态金属分布更加均匀,在普通TIG填丝焊的基础上,研究焊枪摆动对熔池行为的影响.建立了焊枪摆动的TIG填丝焊的数学模型,并利用示踪粒子的方法,对比普通TIG填丝焊和摆动TIG填丝焊的熔池温度场、流场及熔滴质量分布.结果表明,普通TIG填丝焊与摆动TIG填丝焊熔池轮廓基本一样,但摆动TIG焊通过摆动电弧,导致熔池内流场行为发生了改变,进而影响了温度场的分布,使熔池内温度分布更加均匀;示踪粒子分布表明,在TIG填丝焊中,摆动TIG填丝焊能够使熔滴金属更加均匀的分布在熔池中.     

重力对全位置TIG焊熔池温度场与流场影响的有限元分析

建立了全位置TIG焊接中平焊、45°下向焊、立焊下向焊的熔池温度场、流场数值计算模型,计算了不同焊接位置以及不同重力加速度值情况下熔池的温度场和流场,分析了重力方向和大小对熔池内流场和温度场的影响规律;发现重力方向对熔池内液态金属的流场、温度场影响明显,液态金属的流动影响温度场分布明显,重力大小对熔池流场的分布形态没有影响,但是对流动速度影响明显. 得到的结论对于研究熔池失稳判定依据、指导设计全位置TIG焊乃至MAG焊工艺具有一定的参考价值.     

T形接头角接静轴肩搅拌摩擦焊三维流动特征

在T形接头横截面方向预置0.1 mm铜箔作为示踪材料，进行了角接静轴肩搅拌摩擦焊，焊后对T形接头角焊缝进行X-ray 2D透射和X-ray 3D扫描，得到了角焊缝示踪材料的2D流场和3D流场. 在前进侧热塑性材料主要以摩擦剪切为主，材料流动主要向焊接方向流动，并流向前进侧后方，后退侧材料主要以挤压为主，由于T接搅拌针螺纹的存在，使得材料整体向后退侧下方流动. 同时发现，在T接静轴肩后沿附近下方存在示踪材料"堆积区"，表明T接静轴肩对附近的材料塑性流动存在一定的阻碍作用. 根据所获得的观察结果，建立了T形接头角焊缝的三维流动主要特征模型.     

大型铸铝件铸造工艺有限元分析与优化

先对大型铸铝件设计两种浇冒口系统方案,然后利用有限元数值模拟方法对其充型流场及凝固过程温度场进行耦合分析,预测铸件铸造过程中缩孔缩松缺陷的产生,并对不同浇注速度条件下铸件的充型流场和充型初期流动前沿进行分析,优化出合理的浇冒口系统设计方案以及浇注速度,最终得到更为可行的铸造工艺方案.     

基于CCD的高炉火焰温度测量系统的设计

为克服传统测温方法的缺陷,提高温度测量的精度及操作的便捷性,设计了基于CCD的非接触式测温系统。采用比色测温的方法,并利用Nios Ⅱ设计CCD传感器控制和数据处理电路,提高测量设备便携性和测量结果的准确性。通过与多点接触式精确测温的测量结果对比,此测温系统具有很高的测量精度,能在不破坏温度场的条件下实现精确测量,最终的测量误差为5%。     

激光熔覆反应合成TiCP／Al复合材料过程的模拟研究

针对激光熔覆反应过程中的熔化和凝固问题，采用对流／扩散单相统一控制方程组，统一处理固相、固液两相和液相中的传热和流动问题；对运动引起的动量传输和能量传输，采用源项法分别在动量方程和能量方程中加以处理；在商用软件PHOENICS基础上增添相应模块，开发了激光熔覆三维瞬态温度场数值模拟软件。并以ZL104合金激光熔覆原位合成TiCp／Al复合材料的工艺为实例，进行数值模拟，模拟结果与实验结果基本一致。     

铝液充型过程水模拟计算机可视化技术

采用可视观察方法研究了铝液充型过程的二维流动状态,并与水模拟实验进行了对比分析,通过调整水粘度来保证水模拟过程与铝液充型过程的相似性.以PIV技术为基础,运用数字视频处理中的运动补偿技术,选用NCCF准则和三步搜索法对低压铸造浇注系统水模拟充型过程的内部二维流场进行了计算和分析;运用边缘检测及提取技术,选用Roberts边缘检测算子对低压铸造浇注系统水模拟充型过程的二维表面流场进行了计算和分析,实现了水模拟过程的量化分析.     

圆形微通道结构多相流流动形态试验及数值模拟研究北大核心

利用高速摄像观测系统对不同挡板结构的圆形微通道结构进行全流场流动显示实验。通过对重力影响、气液流量比、流动方向等变量的控制进行多相流流动形态分析;在此基础上进行数值模拟研究,得到气液多相流流动特性沿流道的压力、速度场等流动特性。结果表明:微通道复杂结构会显著影响气液多相流的流动形态,复杂结构微通道加剧了流体剪切流动特性,大大促进了气液两相流混合效果。     

三维搅拌摩擦焊接传热与塑性流动分析模型

为研究搅拌摩擦焊接过程热流相互作用下的温度场、速度场和粘度场,将材料看成是层流、粘性、非牛顿流体,基于流体力学理论,建立了搅拌摩擦焊接过程的三维热流分析模型.给出了焊接过程热输入与搅拌头的旋转频率、工件运动速度、搅拌头尺寸及材料发生屈服时的剪应力的数学关系式,并将其作为热边界条件加入到了模型中.结果表明,搅拌头前部温度低于后部,温度梯度前部大于后部;受搅拌头周围材料流动的影响,接近搅拌针的区域,后退侧温度高于前进侧;材料上部速度、粘度受轴肩影响较大,下部主要受搅拌针影响;计算得到的热力影响区与试验结果有较好的对应关系.     

Water analogue validation of numerical model for fluid flow during slow shot phase in die casting process

Abstract

Air that becomes entrapped in the molten metal during the die casting process has a major effect on the formation of porosity in horizontal cold chamber die castings. In this study, a three-dimensional computational fluid dynamics model was developed to study the flow patterns of liquid metal in the injection chamber, in which the moving boundary conditions of the plunger movement was considered in detail. According to the principle of die casting machines, a water analogue system was designed and built to investigate the slow shot process. A colour high speed camera was used to record the fluid flow patterns under different plunger movement profiles. The numerical simulation results agreed well with the water analogue experimental results, which validated the numerical model of shot processing in the cold chamber of the die casting process.     

Effect of wetting behavior on generation of uniform aluminum droplets obtained by pneumatic drop-on-demand technique

Micro metal droplet is the basic building block of three-dimensional metal parts fabricated by micro droplet deposition manufacturing (MDDM) technique. In this paper, the effect of wetting behavior between liquid metal and spray nozzle on the generation of micro aluminum droplets produced by pneumatic drop-on-demand (DOD) technique was investigated by simulation and experiment. A finite element model of liquid–gas flow was established based on the improved level set method (LSM). Then the generation of micro liquid aluminum droplets under different wetting conditions was simulated. A series of spraying experiments were also performed on micro droplet deposition experiment platform. The results show that the generation and flight of micro aluminum droplets are influenced by wetting condition between liquid metal and the nozzle surface significantly. Additionally, the effect of wetting behavior on the droplet size was analyzed to achieve the smallest building block. It was found that the droplet radius decreased with the increase of contact angle exponentially, which agreed with the numerical calculation and experiment results. On this basis, a wettability criterion was proposed for selecting nozzle materials. These works would be helpful for the processing optimization and equipment improvement of MDDM technique.     

弹药化验废液电渗析脱盐研究

为了解决弹药化验废液盐度高、毒性大且对环境污染严重等问题,采用电渗析技术对弹药化验废液进行脱盐处理.通过实验,研究了电渗析时间、实验温度以及物料流量等因素对脱盐效能的影响,结果表明:电渗析技术用于弹药化验废液脱盐处理是可行的,最佳脱盐温度为30℃.     

Deformation and flow in bulk metallic glasses and deeply undercooled glass forming liquids—a self consistent dynamic free volume model

Bulk glass forming metallic liquids such as those of the Zr–Ti–Ni–Cu–Be Vitreloy alloy family have been shown to have Newtonian Viscosity which is well described by the Vogel–Fulcher–Tamann (VFT) equation over roughly 15 orders of magnitude from the high temperature equilibrium melt to the deeply undercooled liquid near and below the experimentally observed glass transition. Experiments have also shown flow becomes non-Newtonian and ultimately unstable against spatial localization into shear bands as the strain rate at a given temperature is increased. This transition from homogeneous to inhomogeneous flow and flow localization has been discussed by several authors and attributed to the influence of strain softening, strain rate sensitivity, and thermal softening.which collectively result in the destabilization of the uniform flow field. The present paper presents a simple self consistent model of uniform steady state flow which is based on the tradition Free Volume Model of the glass transition, the VFT-equation, and a simple treatment of free volume production and annihilation during flow. The model is used to analyze the flow data and shown to give a simple one-parameter fit to experimental steady state Newtonian and non-Newtonian flow data over a broad range of temperatures and strain rates. The model gives simple analytic expressions for the steady state constitutive flow law, the strain rate sensitivity exponent (SRSE), and an implicit equation for the strain rate and temperature dependent viscosity, which is solved numerically. An approximate analytic expression for the non-Newtonian effects is proposed. Generalizing the model to time dependent flow, it is argued that observed shear localization and serrated flow in bulk glass forming liquids arises primarily from transient response phenomena.     

Finite element simulation and experimental investigation of forming micro-gear with Zr–Cu–Ni–Al bulk metallic glass

Bulk metallic glasses exhibit some unique physical properties as compared to their corresponding crystalline alloys. Due to the superplasticity by behaving like a Newtonian fluid in their supercooled liquid region, the bulk metallic glasses can be used to make high strength microparts by net-shape forming. In this paper, the compressive tests of Zr–Cu–Ni–Al metallic glass are performed with different strain rates at a temperature of 683 K. According to the experimental results, the forming evolution of a metallic glass micro-gear is simulated using a finite element simulation software DEFORM 3D, and the forming load is predicted at different processing parameters. Meanwhile, the filling stages of bulk metallic glass in the micro-gear mold cavity are investigated by finite element simulation and experiment. The predicted workpiece geometry shows good agreement with experimental result. The forming experiments for micro-gear of Zr–Cu–Ni–Al metallic glass are carried out by hot embossing process, and the amorphous micro-gears are obtained successfully. It is found that the finite element simulation results are in reasonable agreement with the experimental observation.