Phase field modeling of multiple dendrite growth of Al-Si binary alloy under isothermal solidification

Phase field method offers the prospect of being able to perform realistic numerical experiments on dendrite growth in metallic systems. In this study, the growth process of multiple dendrites in AI-2-mole-%-Si binary alloy under isothermal solidification was simulated using phase field model. The simulation results showed the impingement of arbitrarily oriented crystals and the competitive growth among the grains during solidification. With the increase of growing time, the grains begin to coalesce and impinge the adjacent grains. When the dendrites start to impinge, the dendrite growth is obviously inhibited.

MICRO-DESCRIPTION OF THE SOLUTE-FIELD AND THE PHASE-FIELD MODEL FOR ISOTHERMAL PHASE TRANSITION IN BINARY ALLOYS

A new phase field method for two-dimensional simulations of binary alloy solidification was studied. A model basing on solute conservative in every unit was developed for solving the solute diffusion equation during solidification. Two-dimensional computations were performed for ideal solutions and Ni-Cu dendritic growth into an isothermal and highly supersaturated liquid phase.

Phase field modeling of dendritic coarsening during isothermal solidification

Dendritic coarsening in AI-2mol%Si alloy during isothermal solidification at 880K was investigated by phase field modeling. Three coarsening mechanisms operate in the alloy： （a） melting of small dendrite arms; （b） coalescence of dendrites near the tips leading to the entrapment of liquid droplets; （c） smoothing of dendrites. Dendrite melting is found to be dominant in the stage of dendritic growth, whereas coalescence of dendrites and smoothing of dendrites are dominant during isothermal holding. The simulated results provide a better understanding of dendrite coarsening during isothermal solidification.

Multidimensional Stability of Subsonic Phase Transitions in a Non-Isothermal Van Der Waals Fluid

We show the multidimensional stability of subsonic phase transitions in a non-isothermal van der Waals fluid. Based on the existence result of planar waves in our previous work [1], a jump condition is posed on non-isothermal phase boundaries which makes the argument possible. Stability of planar waves both in one dimensional and multidi-mensional spaces are proved.

Phase field modeling of dendrite growth

Single dendrite and multi-dendrite growth for Al-2 mol pct Si alloy during isothermal solidification are simulated by phase field method. In the case of single equiaxed dendrite growth, the secondary and the necking phenomenon can be observed. For multi-dendrite growth, there exists the competitive growth among the dendrites during solidification. As solidification proceeds, growing and coarsening of the primary arms occurs, together with the branching and coarsening of the secondary arms. When the diffusion fields of dendrite tips come into contact with those of the branches growing from the neighboring dendrites, the dendrites stop growing and being to ripen and thicken.

Experimental studies and phase field modeling of microstructure evolution during solidification with electromagnetic stirring

Thixocasting requires manufacturing of billets with non-dendritic microstructure.Aluminum alloy A356 billets were produced by rheocasting in a mould placed inside a linear electromagnetic stirrer.Subsequent heat treatment was used to produce a transition from rosette to globular microstructure.The current and the duration of stirring were explored as control parameters.Simultaneous induction heating of the billet during stirring was quantified using experimentally determined thermal profiles.The effect of processing parameters on the dendrite fragmentation was discussed.Corresponding computational modeling of the process was performed using phase-field modeling of alloy solidification in order to gain insight into the process of morphological changes of a solid during this process.A non-isothermal alloy solidification model was used for simulations.The morphological evolution under such imposed thermal cycles was simulated and compared with experimentally determined one.Suitable scaling using the thermosolutal diffusion distances was used to overcome computational difficulties in quantitative comparison at system scale.The results were interpreted in the light of existing theories of microstructure refinement and globularisation.

Thermodynamic and rate variational formulation of models for inhomogeneous gradient materials with microstructure and application to phase field modeling

In this work,thermodynamic models for the energetics and kinetics of inhomogeneous gradient materials with microstructure are formulated in the context of continuum thermodynamics and material theory.For simplicity,attention is restricted to isothermal conditions.The materials of interest here are characterized by（1） first- and secondorder gradients of the deformation field and（2） a kinematic microstructure field and its gradient（e.g.,in the sense of director,micromorphic or Cosserat microstructure）.Material inhomogeneity takes the form of multiple phases and chemical constituents,modeled here with the help of corresponding phase fields.Invariance requirements together with the dissipation principle result in the reduced model field and constitutive relations.Special cases of these include the wellknown Cahn-Hilliard and Ginzburg-Landau relations.In the last part of the work,initial boundary value problems for this class of materials are formulated with the help of rate variational methods.

面向控制的一维非等温两相流燃料电池模型研究

经验模型不能反映电池内部复杂的物理化学耦合过程及其导致的响应迟滞,这给燃料电池系统精确控制策略的开发带来了一定困难。针对此问题,建立了面向控制的一维非等温两相流模型,考虑了流道内气体瞬态效应、电池内部水相变,研究了电流密度对气体浓度以及水热分布特性的影响,分析了运行条件和模型参数对电池输出电压的影响,探究了电流阶跃下该模型相比于集总参数模型在输出性能方面的优势。结果表明,该模型具有更好的适用性,可为燃料电池系统层面的模型优化及控制策略设计提供可靠依据。

电场作用下气泡上升和破碎特性

利用数值模拟方法研究均匀垂直电场下单个气泡在液体中的上升和破碎特性。采用相场方法捕捉气泡形状变化,考察电场强度对气泡上升速度、液体射流速度和破碎时间的影响。结果表明:电场对气泡运动有显著影响,电场强度越大,气泡上升速度和液体射流速度越大。考察电场作用下气泡初始形状对上升和破碎特性的影响。结果表明:在电邦德数Bo_(e)为0—20内,初始水平轴与竖直轴比为1.3和1.0的椭球形气泡,电场强度的增大缩短了其破碎时间,而初始水平轴与竖直轴比为0.8的椭球形气泡,破碎时间随电场强度增大而变长。

平原河网地区稻田农药脉冲输出迟滞分析研究

针对目前有关稻田施药后短期内农药的脉冲式输出过程及运移机理尚不十分明确的问题,以扬州市江都区农田水利科学研究站的试验稻田为研究区,针对稻田施药后农药面源污染物与排水的双脉冲输出过程开展高频监测,捕捉到毒死蜱、阿维菌素、三环唑和噻呋酰胺等4种水稻常用农药的脉冲式输出过程,结果显示:上述4种农药的输出浓度和负荷率峰值均发生在施药后2.5~5h内,输出脉冲特征明显.农药和排水双脉冲的相位关系受人为因素影响较大,呈现一定的随机性.对于淋溶性较好、受田间水文状况影响较大的农药(以杀菌剂三环唑和噻呋酰胺为代表),其迟滞指数分别为0.419和0.326,首次观察到较为明显的c-Q迟滞现象,该现象的发生与稻田施药先后顺序以及农药施入稻田后随水流运移到农沟水体的路径长短有关;而对挥发性较大的农药(以杀虫剂毒死蜱为代表),其迟滞系数仅为0.083,由于易受风向等不确定性因素影响未能观察到明显的迟滞现象.上述3种农药的FI值均等于或接近于1,说明排水脉冲对农药以富集作用为主.

对撞射流下通风空间的流场结构实验研究

为了研究送风口和回风口位置对室内流场结构的影响,本文搭建了采用多条缝对置撞击送风的通风缩比模型实验平台,利用激光粒子测速技术研究了等温和非等温工况下多条缝通风空间中不稳定气流场的速度和湍流信息。结果表明:等温和非等温工况下,送风口截面的流场速度、湍动能和涡量的空间分布类似,最大值分别可以达到1.3 m/s、0.1 m^(2)/s~2和60 s^(-1),射流碰撞形成2个大尺度涡旋,造成流场结构不稳定;CS4.5截面,流场速度、湍动能和涡量最大值分别可以达到0.9 m/s、0.04 m^(2)/s~2和30 s^(-1);CS3.5截面,速度与涡量最大值均出现在近壁面附近,分别为0.42 m/s、8 s^(-1),湍动能最大值出现在截面中间位置,为0.13 m^(2)/s~2,且流场中形成了大规模的涡旋;非等温工况下,送风口截面和CS3.5截面中小尺度涡旋增加,大尺度涡旋减少,热羽流抑制了大尺度流场结构,增加了小尺度流场结构。

超声相控阵在隧道衬砌精细化检测中的应用

针对隧道衬砌精细化检测的需求,引入超声相控阵技术,该技术融合了超声波和相控阵的基本原理,通过加密布设测点,实现检测断面图像精确重构,并具备三维可视化成像功能。与传统超声波无损检测技术相比,该技术具有分辨力高、信噪比大、检测效率高、解译方便等优势。经在西部某高速铁路隧道开展超声相控阵检测技术试用,检测结果准确,成像效果好,可分辨钢筋分布等衬砌内部构造,并能用于衬砌背后脱空等缺陷的判断,是一种高精度的无损检测技术。

Preliminary study of non-isothermal phase change phenomena in vertical Bridgman crystal growth

Axisymmetric dual reciprocity boundary element method (DRBEM) with augmented items is extended to simulate the heat and mass transfer problems in the vertical Bridgman method (VBM) crystal growth of HgCdTe and CdZnTe. Axial solute concentration redistribution of three regions numerically reap-pears, and the influence of the pulling rate of the ampoule on it is further studied. Secondly, one di-mensional transient phase change phenomena is studied, and non-isothermal phase change phe-nomena is obtained from the initial transient region through the steady growth region to the final tran-sient region. Thirdly, the two-dimensional axisymmetric phase change interface position, interfacial shape and the temperature field of the melt and the crystal are numerically captured under the condi-tion to arrive at the steady state with zero pulling rate of the ampoule. Finally, the study of transient axisymmetric non-isothermal phase change phenomena is stressed and the results are compared with those in isothermal phase change. The influence of the pulling rate on non-isothermal phase change phenomena is revealed.

Non-isothermal Dissolution Modelling of Sigma Phase in Duplex Stainless Steels

In this work, the non-isothermal dissolution kinetics of the sigma phase in duplex stainless steels has been studied and modelled. A semi-empirical model is proposed to describe the kinetics of sigma phase precipitation/dissolution during continuous heating starting from the isothermal transformation kinetics. The proposed model, which presumes validity of the additivity rule, is validated by means of experimental investigations. A good agreement is found between experimental and analytical results.

非等温包覆机头胶料流场挤出均匀性的研究

对非等温包覆机头胶料流场挤出均匀性进行研究。结果表明:非等温包覆机头胶料流场压力、温度和流动速度的模拟变化趋势与实际相似,模拟试验是分析包覆机头胶料流场的一种有效方法;确定了影响胶料包覆均匀性的包覆机头流道参数(进料口位置、进料口直径和芯线牵引速度)的相关变化规律,为包覆工艺及包覆机头流道参数的合理选择提供了理论依据;确定了包覆过程中芯线牵引速度与进料口胶料流量匹配性的评价方法。该研究为包覆工艺及包覆机头流道参数的确定提供了理论指导。

固空枝晶微结构生长演化非等温定量相场模拟研究

为了探究热-溶质耦合驱动下固空枝晶生长演化和氧溶质分布规律,建立了非等温定量相场模型,应用该模型研究了不同过冷度及边界热流条件下固空枝晶的形态演化、生长速度、氧溶质分布。结果表明:相比等温模拟,考虑凝固潜热的非等温模拟可以更真实再现固空枝晶的生长过程,氧质量分数峰值相比等温模拟减少了8.5%~14.6%;枝晶的生长速度、最大氧质量分数和初始过冷度呈现正相关性,初始过冷度为4 K时,固液糊状区氧质量分数最大可达到0.401;边界热流的存在可显著改变计算域温度分布进而影响固空枝晶的生长模式;热输入减缓了枝晶的生长,而热提取使枝晶臂生长得更为繁盛粗壮,热提取、热输入条件下的固相率分别是无边界热流时的138%、69%,最大氧质量分数分别是无边界热流时的146%、94%。该研究可增进对液氢中固空枝晶生长行为的理解,为液氢系统的安全使用提供一定的理论指导。

基于近端电场积分法的耦合误差分析研究

近年来,非侵入式电压测量因具备安全性高、便于安装和维护等优势而受到国内外电力工作者们的青睐,但以电场为中间量反演电压的过程中,电场存在串扰问题。为此,对一种新的电场反演方法进行电场耦合误差分析。首先,通过探究10 kV架空线路空间电场分布情况,提出近端电场积分方法;其次,利用三相电场耦合计算模型,对10 kV架空线路三相电场之间的串扰进行误差分析;最后,通过Comsol有限元软件搭建10 kV三相输电线路仿真模型,获取其电场分布并进行验证。结果表明:三相导体下方电场强度均主要来自待测相,传感器距离待测导体0.03 m、0.05 m和0.07 m时,其他相的耦合分量非常小,其占比小于1.5%,基本可忽略不计。仿真结果与理论计算结果相一致,两者之间的误差控制在0.2%以下,因此,通过调整传感器的安装部署位置,近端电场积分法对不同电压等级均有适用性。本文的研究成果可为非侵入式电压在线监测装置的研发提供理论指导。

Dirac equation with a magnetic field in 3D non-commutative phase space

For a spin-l/2 particle moving in a background magnetic field in noncommutative phase space, the Dirac equation is solved when the particle is allowed to move off the plane that the magnetic field is perpendicular to. It is shown that the motion of the charged particle along the magnetic field has the effect of increasing the magnetic field. In the classical limit, matrix elements of the velocity operator related to the probability give a clear physical picture. Along an effective magnetic field, the mechanical momentum is conserved and the motion perpendicular to the effective magnetic field follows a round orbit. If using the velocity operator defined by the coordinate operators, the motion becomes complicated.

二元合金非等温凝固枝晶生长的相场法模拟

利用相场模型与溶质场、温度场进行耦合计算,以Al 4 5%Cu合金为例模拟了二元合金非等温凝固和等温凝固的等轴枝晶生长过程。研究了扰动对过冷合金熔液中的等轴枝晶生长以及溶质场和温度场的分布的影响。结果表明:扰动的引入可以促进枝晶的二次晶臂生长,并使固相中的最高温度提高,但基本不影响枝晶尖端稳定行为;在枝晶生长过程中枝晶尖端的浓度和温度的梯度最大;在非等温凝固时枝晶没有等温凝固时发达,并且凝固界面的溶质浓度也会降低。

二元合金非等温凝固相场法模拟

利用耦合温度场与溶质场的相场模型对Al-Cu二元合金非等温凝固时枝晶生长过程进行了模拟。研究了人为变化的不同热扩散系数对枝晶生长行为的影响。计算结果表明:随着热扩散系数的减小,潜热释放对熔体内原有温度场的影响也减小,熔体内过冷度能够更好地保持;而枝晶尖端生长速度将增大,枝晶呈现出更发达的侧向分支;同时,溶质偏析程度也越小。将计算模拟结果和实验结果进行了比较,两者吻合良好。