自然对流对于熔体生长中小面化的影响

应用基于边界保角变换技术的Ｇａｌｅｒｋｉｎ有限元方法，研究熔体生长中动力学效应和自然对流的耦合作用探究了对流对生长系统中的温场分布、相界面２以及界面过冷度的影响。研究结果表明：自然对不充可使相界面的弯曲度减小，界面的相对位置降低，同时，对流使得小面端点处的夹角变得圆滑，小面域和粗糙面域的界线变得不明显，相应地，小面尺寸有所减小。     

勾形磁场中直拉硅单晶浓度场的数值模拟研究

为有效控制晶体尺寸、金属杂质含量、掺杂元素及氧分布的均匀性,提出在非均匀轴对称勾形磁场中利用磁控提拉法生长硅单晶。用有限差分法对非均匀轴对称勾形磁场中直拉硅单晶体系中的浓度场进行数值模拟研究,基于直拉硅单晶生长系统的物理及数学模型,进行无量纲化处理,借助于相应的边界条件进行求解,并针对不同工艺条件下熔体中及界面处氧浓度分布情况进行模拟研究。结果表明：在勾形磁场作用下,通过改变磁场强度、晶体和坩埚转速及晶体半径可有效控制固一液界面处氧浓度及分布均匀性,从而在晶体中获得径向均匀的氧浓度。     

A level set based immersed boundary method for simulation of non-isothermal viscoelastic melt filling process

In this work, the polymer melt filling process is simulated by using a coupled finite volume and level-set based immersed boundary (LS-IB) method. Firstly, based on a shape level set (LS) function to represent the mold boundary, a LS-IB method is developed to model the complex mold walls. Then the non-isothermal melt filling process is simulated based on non-Newtonian viscoelastic equations with differ-ent Reynolds numbers in a circular cavity with a solid core, and the effects of Reynolds number on the flow patterns of polymer melt are presented and compared with each other. And then for a true polymer melt with a small Reynolds number that varies with melt viscosity, the moving interface, the temperature distributions and the molecular deformation are shown and analyzed in detail. At last, as a commonly used application case, a socket cavity with seven inserts is investigated. The corresponding physical quantities, such as the melt velocity, molecular deformation, normal stresses, first normal stress differ-ence, temperature distributions and frozen layer are analyzed and discussed. The results could provide some predictions and guidance for the polymer processing industry.     

纳米结构表面液体沸腾传热的分子动力学模拟

为了从纳米尺度了解表面结构和润湿性对池沸腾液体与固体壁面的传热性能,本文采用分子动力学方法研究了超亲水至超疏水不同润湿性的液体氩在光滑表面和含凹、凸半球纳米结构表面的沸腾传热过程,分析了三种表面上液氩在受热过程的形态、温度、热流密度等相关参数的变化情况。结果表明,液氩层沸腾过程大致可分为液氩层吸附于固体表面和液氩层从壁面脱离两个加热阶段,当液氩层吸附于固体表面时,温度升高、热流密度及气态氩原子产生速度均大于液氩层脱离壁面时的情况,在这两个阶段亲水表面上氩原子温度变化有明显的拐点,而疏水表面在两个阶段加热过程相差不大。亲水表面上的微结构能吸附更多液氩原子,促进了气泡产生及加速温度、热流密度的变化,而在疏水及超疏水微结构表面,微纳结构与液氩间的气膜层促进了气泡产生,计算结果为池沸腾传热及微结构选择提供了理论依据。     

散热参数对冷心放肩微量提拉法生长蓝宝石晶体影响的数值模拟

利用数值模拟分析的方法,研究了冷心放肩微量提拉法生长蓝宝石晶体过程中热交换器散热参数对晶体质量的影响趋势;分析了热交换器的散热参数变化对晶体生长的固液界面凸出率和晶体内温度分布、温度梯度的影响.结果表明,热交换器的对流换热系数和工作流体的温度变化对晶体生长的固液界面突出率和温度梯度具有相似的影响效果;且在晶体长到一定尺寸后,只靠加大热交换器的散热热能力,不足使晶体继续生长.     

勾形磁场下重力水平和温度梯度对CdZnTe分离结晶生长的影响

为了了解勾形磁场下相关参数对CdZnTe晶体生长的影响,利用有限元法对坩埚内的热量和动量传递过程进行了全局数值模拟。分析了不同的磁场强度下重力水平、壁面温度梯度对CdZnTe晶体生长过程的影响。结果表明：重力水平存在1个临界值,此时CdZnTe熔体内最大流函数最小,流动最弱。随着温度梯度逐渐增大,熔体内最大流函数也逐渐增大,熔体的流动越来越强,不利于晶体稳定生长。通过施加勾形磁场,能有效抑制熔体内的流动,有利于晶体的稳定生长,为地面条件下制备大尺寸CdZnTe晶体创造了条件。     

Numerical analysis for the dynamics of the oxidation-induced stacking fault in czochralski-grown silicon crystals

The continuum model of point defect dynamics to predict the concentration of interstitial and vacancy is established by estimating expressions for the thermophysical properties of point defects and the point defect distribution in a silicon crystal and the position of oxidation-induced stacking fault ring (R-OiSF) created during the cooling of crystals in Czochralski silicon growth process are calculated by using the finite element analysis. Temperature distributions in the silicon crystal in an industrial Czochralski growth configuration are measured and compared with finite volume simulation results. These temperature fields obtained from finite volume analysis are used as input data for the calculation of point defect distribution and R-OiSF position. Calculations of continuum point defect distributions predict the transition between vacancy and interstitial dominated precipitations of microdefects as a function of crystal pull rate (V). The dependence of the radius of R-OiSF (R_OiSF) on the crystal length with fixed growth rate for a given hot zone configuration is examined. The R_OiSF is increased with the increase of crystal length. These predictions from point defect dynamics are well agreed with experiments and empirical V/G correlation qualitatively, where G. is the axial temperature gradient at the melt/crystal interface.     

The role of an SiC interlayer at a graphite–silicon liquid interface in the solution growth of SiC crystals

SiC crystal growth using the top seeded solution growth (TSSG) method involves the precipitation of solid SiC from carbon that is dissolved in a silicon melt. The growth rate of SiC is strongly influenced by the solubility of C in liquid Si, which is quite low. In this study, the dissolution of C from graphite to the Si melt was explored by observing the formation of an SiC interlayer at a graphite – Si liquid interface. The SiC interlayer was observed to become thickened during the several hours needed to reach a certain thickness at 1500 °C. Assuming that the SiC interlayer is a direct C source, a pre-formed SiC layer was coated on the graphite crucible to evaluate its effect on the concentration of C in the Si melt. As a result, the concentration of C in the Si melt increased within a short time, especially at low temperatures. By applying the SiC coated crucible to the TSSG process for SiC crystal growth, we confirmed that the development of a pre-formed SiC layer enhanced the growth rate of SiC crystals, especially at the initial stage of crystal growth at low temperatures.     

Flow Fields in Stirred Vessels Depending on Different Internal Heat Exchangers and Vessel Bottoms

This study provides the influence of different heat exchanger internals (helical pipe coils, heating plugs, pipe registers) and reactor bottom shapes (torispherical/dished (Kloepper‐shape), hemispherical, and flat) on the flow field, the turbulent kinetic energy, and the ratio of tangential flow in stirred vessels, based on extensive stereo PIV measurement series in refractive index‐matched, optically completely accessible systems. The investigations impressively show advantages and disadvantages of the various equipment, which have a massive influence on both heat transport and the flow.     

Analysis of Heat Transfer in the Corroding Sidewall of a Furnace

A mathematical formulation is developed to calculate the rate of corrosion of a refractory sidewall of a furnace in the presence of free convection in the melt. The free-convection phenomena considered are caused by the dependence of the melt density on its temperature and composition. The formulation involves the statement of equations for heat conduction in the refractory wall and movement of the solid-melt interface. The governing equations and the boundary conditions are transformed into nondimensional forms, and the dimensionless parameters which characterize corrosion in the presence of free convection are identified. The equations are solved by using coordinate transformation and a finite-difference method. Calculated results are presented on the shape of the solid-melt interface, temperature distribution in the corroding wall, and the amount of cooling at the outside surface of the wall. The results show how free convection in the melt interacts with heat conduction in a refractory wall to determine the rate of corrosion of the wall.     

Laser heating of moving solid: Influence of workpiece speed on melt size

Laser heating of a moving solid is considered and phase change in the irradiated region is modeled. Three‐dimensional heating situation is considered in the analysis and phase change is modeled using the enthalpy‐porosity technique. Temperature field and melt zone are predicted for different workpiece velocities. The predictions are compared with the experimental results. It is found that the maximum temperature in the irradiated region and the size of heat affected zone increase significantly at low workpiece speeds. The prediction of melt depth agrees well with the experimental results. © 2010 American Institute of Chemical Engineers AIChE J, 2010     

温度和速率对磷酸区域熔融过程的影响

区域熔融法是一种有效的制取高纯物质的方法,可被应用于高纯磷酸制备过程中。为了获得最佳的磷酸净化效果,考察了区域熔融过程中冷凝、加热温度和熔区移动速率对于区熔过程的影响。结果表明:在冷凝温度为5℃,加热温度为65℃时,可以得到合适的熔区大小,且此时形成稳定熔区的时间为33 min。冷凝或加热温度过高,熔区明显变大,凝固界面不稳定;冷凝或加热温度过低,熔区变小甚至无法形成熔区,凝固界面晶体生长呈针状。在考察范围内,随着熔区移动速率的增加,Cr,Ca,Mg,Zn,Al和Fe 6种杂质在区域熔融后的脱除效果有不同程度的降低。当移动速率小于25 mm/h时,6种杂质均具有较好的脱除效果,但是实验需要较长的时间,不利于实际应用;当移动速率高于25 mm/h时,Cr,Ca,Zn脱除效果明显下降。移动速率为25 mm/h是最佳移动速率。     

预应力固定管板换热器预变形与应力特性的数值分析

为了实施预应力换热器技术,对预变形下固定管板式换热器中变形相互约束的构件之间所产生的温差应力与变形分布特性进行了数值研究。借鉴“分段建模,整体综合”的换热器流体力学与传热的数值模拟新方法,利用CFD得到换热器温度场并作为ANSYS结构分析边界条件,采用预拉伸单元进行预变形量的施加与控制,从而获得了热-结构耦合分析结果。数值模拟结果与实验数据吻合得很好,说明所采用的研究方法是合理和可行的。此外,实验和数值分析结果均证实,通过合理地施加和控制预变形量,可以有效缓解和协调固定管板换热器在运行中各构件（管板、壳体、管束）之间的变形约束,从而创造良好的运行环境,最终提高换热器的运行可靠性和使用寿命。     

Filling of a complex-shaped mold with a viscoelastic polymer. Part I: The mathematical model

A model for the filling stage of injection molding of viscoelastic thermoplastics in cavities of complex shape is presented. The model considers two-dimensional melt flow, with converging and diverging flow patterns induced by complex boundary shape and by the presence of an obstacle. The model is non-isothermal (with the melt loosing heat to the mold walls as it travels into the cavity) and handles a viscoelastic (following the White-Metzner model) material with properties that vary with temperature, shear rate, and pressure. The numerical method is based on finite differences, with boundary fitted curvilinear coordinates used in the mapping of the flow field (which has an arbitrary shape that evolves with time) into a time invariant rectangle. The numerical results reveal geometry-induced asymmetries in the flow and thermal fields as well as the effect of various process parameters on the pressure and temperature profiles in the cavity. The model admits variable cavity thickness, thus allowing for a treatment of the cavity thickness as a process parameter in the simulations.     

Wall slip heating

When molten plastic is extruded, the upper limiting throughput is often dictated by fine irregular distortions of the extrudate surface. Called sharkskin melt fracture, plastics engineers spike plastics formulations with processing aids to suppress these distortions. Sharkskin melt fracture is not to be confused with gross melt fracture, a larger scale distortion arising at throughputs higher than the critical throughput for sharkskin melt fracture. Sharkskin melt fracture has been attributed to a breakdown of the no slip boundary condition in the extrusion die, that is, adhesive failure at the die walls, where the fluid moves with respect to the wall. In this article, we account for the frictional heating at the wall, which we call slip heating. We focus on slit flow, which is used in film casting, sheet extrusion, curtain coating, and when curvature can be neglected, slit flow is easily extended to pipe extrusion and film blowing. In slit flow, the magnitude of the heat flux from the slipping interface is the product of the shear stress and the slip speed. We present the solutions for the temperature rise in pressure‐driven slit flow and simple shearing flow, each subject to constant heat generation at the adhesive slip interface, with and without viscous dissipation in the bulk fluid. We solve the energy equation in Cartesian coordinates for the temperature rise, for steady temperature profiles. For this simplest relevant nonisothermal model, we neglect convective heat transfer in the melt and use a constant viscosity. We arrive at a necessary dimensionless condition for the accurate use of our results: Pé?1. We find that slip heating can raise the melt temperature significantly, as can viscous dissipation in the bulk. We conclude with two worked examples showing the relevance of slip heating in determining wall temperature rise, and we show how to correct wall slip data for this temperature rise. POLYM. ENG. SCI., 55:2042–2049, 2015. © 2014 Society of Plastics Engineers     

Das Scaling‐Down‐Problem bei der Zweistoff‐Gaszerstäubung von Metallschmelzen

The production capacities of plants for metal pulverisation are frequently rated according to the minimum throughput of melt flowing from the distributor crucible to pulverisation. Especially in the case of high‐melting point metals and their alloys deficiencies occur in the thermal balance of the crucible in the exit section as a result of gas expansion at the pulverisation gas jets, which act as heat sinks. Minimum throughputs and associated limiting values of the convective heat transfer of the melt are stipulated in order to prevent "freezing" (solidification of the melt) in the crucible. The pertinent situation is illustrated for copper and steel melts and technical possibilities for compensating for heat deficiencies by inductive heating of the distributor exit are presented. In addition, the demand for minimum throughputs can be abandoned, and there result possibilities of scaling‐down and energy conservation as well as improvement of powder discharge.     

Finite Element Analysis of a Moving Boundary Value Problem

A finite element formulation and the solution of a set of nonlinear coupled heat and mass transfer equations for a two-phase system with a moving evaporation interface is presented. The interface condition takes into account the moisture transfer balance at the moving boundary. The finite element results were compared with existing results for a single phase system for model validation. In the two-phase system, the movement of evaporation front has an appreciable effect on the temperature and moisture distribution inside the porous medium during drying. The effect of the nondimensional heat of vapourization parameter γ on the evaporation front, temperature and moisture distribution in porous medium was studied. The higher the value of γ, the slower is the movement of the evaporation front. The temperature decreased and the moisture content increased as the nondimensional vapourization parameter γ increased. This model has potential applications in studying the heat and mass transfer characteristics in food and biomaterials.     

Coupled Part and Mold Temperature Simulation for Injection Molding Based on Solid Geometry

This paper presents a coupled method that determines the interface temperatures by filling and cooling analyses simultaneously to simulate the mold and part temperature distributions for injection molding. The mold temperature is assumed to be changing and is calculated with melt together at the filling stage instead of keeping constants as is usually done in conventional methods. The mold temperature is first determined with a 3-D finite element method by specifying the heat-flow rate at the interface between mold and part. Then the finite difference approach is employed to solve the melt thermal problem to get melt temperature distributions inside the cavity and the heat-flow rate at the interface. The under-relax scheme is used to correct the boundary condition and to resolve both mold and melt thermal problems until the solutions are convergent. This method can simulate transient and multicycle problems with more complex process conditions. The simulated results agree with experimental data.     

Effects of crystal growth rate and heat and mass transfer on solute distribution

The effects of crystal growth rate and heat and mass transfer on solute distribution during solidification of binary melt have been theoretically investigated on the basis of a new theory of solute distribution proposed by the present authors. The solute distribution factor f at the solid-liquid (SL) interface is in inverse proportion to the one-half power of the dimensionless growth rate U. The growth rate U is in proportion to the second power of the normalized concentration difference between the SL interface and bulk melt. A new transport factor K, which describes heat and mass transfer in melt, gives an important contribution to the crystal growth and the solute distribution at the SL interface. The transport factor is used successfully to control the solidification of melt. The flow structure in melt exerts essential influence on the solid purity.     

泡沫金属强化石蜡相变蓄热过程可视化实验

相变材料的低热导率是限制潜热蓄热广泛应用的重要原因。将相变材料石蜡真空条件下注入到泡沫金属铜内制备泡沫金属铜-石蜡复合相变材料,通过铜的高热导率及高孔隙材料的大面体比来强化相变换热过程。采用DSC示差扫描量热法对石蜡进行热物性测量获得准确的石蜡相变温度及相变潜热。以管壳式相变蓄热结构为对象,提取对称结构进行可视化设计,对比纯石蜡及泡沫金属铜-石蜡复合材料在相同运行条件下的相变过程,追踪二者熔化过程的相界面位置随时间的演化过程并布置热电偶准确测量材料内部的温度分布。结果显示加入泡沫金属后的复合材料的内部温差明显减小,温度分布均匀,蓄热热通量显著增大,有效缩短相变时间并缓解了自然对流造成的顶部过热和底部不熔化现象。