枝晶尺度溶质再分配对连续铸造凝固过程的影响

阐述了凝固微细尺度结构上的溶质再分配与宏现尺度传热传质现泉之问的关联，总结比较了多种徽现健析模型，并针对反向凝固工艺和传统的薄板坯连铸连轧(CSP)工艺的传热传质现泉进行了数值模拟，与实验数据进行对比分析。结果表明，微细尺度固—液相界面上的溶质再分配对凝固宏观传输过程的影响不可忽略。     

45#钢连铸坯凝固过程数值模拟与验证

根据实际生产的工艺参数,通过ProCAST商业软件对45#钢连铸坯的坯壳厚度以及凝固过程进行数值模拟,并进行现场射钉实验对模拟结果验证。结果表明,数值模拟与现场二级模型相比其结果更接近于射钉实验所得坯壳厚度,说明数值模拟相对于现场二级模型更能有效地反映出铸坯不同位置坯壳厚度,为末端电磁搅拌提供有效的参考。     

电渣重熔过程中传热及凝固组织的数值模拟

在凝固传热数学模型基础上,采用正态分布形核模型和二维偏心生长模型,模拟了钢锭重新凝固过程温度场及钢锭凝固组织的生长情况.数值结果表明：电渣重熔钢锭以倒“V”形的柱状晶为主,中心和底部为等轴晶,模拟结果与实验结果符合良好.随着渣温的提高,熔池变得深且宽;随着侧壁换热系数的增大,熔池深度变浅;随着重熔速度的减小,熔池深度也逐渐变浅;较低的渣池温度、较大的对流换热系数有利于等轴晶形成,而重熔速度对凝固组织的影响不大.     

蓄能互联热泵系统相变装置熔化与凝固过程特性分析

介绍了一种新型的蓄能互联热泵系统。利用数值模拟的方法对填充石蜡C17的球型蓄热单元的熔化与凝固过程进行研究,分析了球壁温度、相变单元尺寸和相变材料初始温度三种影响因素对熔化过程和球壁温度对凝固过程的影响。通过对两个过程对比发现相变单元尺寸对相变过程影响最大,在相同温差条件下完全熔化时间少于完全凝固时间,熔化过程中始终存在的石蜡-壁面与液相石蜡-固相石蜡之间的对流换热过程增加了熔化速率。     

双燃料内燃机引燃油混合气形成与燃烧过程模型研究

对于双燃料发动机，引燃油蒸发、混合与燃烧是在天然气与空气混合物为介质的情况下，通过采用离散液滴模型，模拟了引燃油的蒸发与混合过程；采用经过修正的Shell模型，对引燃油的着火过程进行了数值模拟；以阿伦纽斯公式为基础，综合湍流对化学动力学的影响，提出了一个新的燃烧模型．经过模拟计算与实验对比，验证了该数值模型的模拟效果．     

合金凝固传热传质宏微观耦合数值研究及验证

建立了描述二元合金凝固的平面枝晶一维微观偏析数学模型,考虑溶质在固相中有限扩散,在液相中完全扩散。通过数值模拟,分析比较了A l-Cu和F e-C合金的微观偏析特性。同时,进一步将微观数值模型与宏观凝固实验的传热传质数学模型相耦合,实现了凝固宏微观复合尺度的全数值模拟。研究表明,数值计算结果与实验数据吻合良好,证明微观模型能较准确地反映微观质量传输并能可靠地与宏观相变传热传质模型相耦合。此外,从微观到宏观的计算结果都说明F e-C合金的凝固过程几乎接近平衡凝固。     

方形槽内水平圆管外相变蓄热过程的数值模拟

对以水为热载体,方形槽内水平圆管外石蜡的相变蓄热过程进行了数值模拟.通过合理的分析与假设,建立数学模型及其定解条件,并利用实验数据进行验证.引入无量纲管壁温度Ste数和无量纲相变材料初温G数,分析了Ste和G数对相变材料熔化、凝固过程的影响,给出了不同Ste教的熔化过程固液相图,结果表明:Ste数对熔化和凝固过程有显著影响,与S=0.098时的熔化时间相比,Ste =0.1875时熔化时间将会缩短将近1/2,而Ste =0.277时比Ste=0.1875时熔化时间缩短了1/3.与Ste=0.0804时的熔化时间相比,Ste =0.170时凝固时间缩短1/2,而Ste =0.259时比Ste =0.170时凝固时间缩短了1/3.G数对相变过程的影响比较小,凝固时甚至可以忽略不计.     

纳米复合相变材料固液相变储能过程研究进展

对纳米复合相变材料固液相变储能过程的若干最新研究进行了回顾,从相变储能系统的动态性能和典型的凝固、熔化传热过程两方面总结了相关研究的进展,并重点评述了数值模拟研究中纳米复合相变材料有效热物性预测方法的适用性及其与实验结果之间的偏差,最后对纳米复合相变材料固液相变储能过程的未来发展和重点研究方向进行了展望。     

石墨泡沫/石蜡储热装置的凝固过程分析

石蜡相变材料的导热系数较小,严重影响了其传热速率和凝固速率。通过对填充石墨泡沫/石蜡的储能系统进行凝固过程的模拟,确定了石墨泡沫对相变储能系统性能的影响。研究结果表明石墨泡沫不仅大大缩短了相变凝固时间,也使储能系统的温度分布更加均匀;通过分析冷却水进口速度和温度对复合相变材料的凝固过程的影响,说明随着冷却水进口速度的增大和温度的降低,传热速率加快,凝固时间缩短。分析了复合材料相变区的自然对流对相变过程的影响,模拟结果证明自然对流能在一定程度上加快相变材料的凝固过程。     

汽油机工作过程的数值模拟

本文根据Benson等人提出的汽油机燃烧模型,考虑了燃烧室挤气面积对火焰传播速度的影响,对汽油工作过程进行了数值模拟,并结合EQ6100发动机实测数据找出了紊流火焰传播速度因子与发动机转速的关系。实验表明模拟结果是合理的,建立的汽油机燃烧过程模拟是可行的。     

Effect of macro scale phenomena on microsegregation

A metal solidification system consists of solid, mushy and liquid regions. In many systems the two phase mushy region has a fine scaled columnar dendritic morphology. Microscopic models of metal solidification systems focus on the mass diffusion (microsegregation) and movement of the solid/liquid interface in the dendrite arm spaces. In this paper the effects of macroscopic variables on the microsegregation predictions are studied. In particular, the effect of cooling and macrosegregation histories on the solid solute profile, the eutectic fraction formed and solid/liquid interface movement in the arm spacing will be investigated.     

A numerical study of the combined effects of microsegregation, mushy zone permeability and fllow, caused by volume contraction and thermosolutal convection, on macrosegregation and eutectic formation in binary alloy solidification

Numerical simulations of the columnar dendritic solidification of a Pb-20 wt% Sn alloy in a square cavity cooled from one side and fed by a rectangular riser are reported. Overall macrosegregation patterns predicted using Scheil and lever-rule type microsegregation models are found to be similar, although the predicted eutectic fraction is significantly higher with the Scheil-type model. The choice of mushy zone permeability function significantly affects the predicted number, length and orientation of segregated channels. The inclusion of shrinkage-driven flow leads to the prediction of the well-known inverse macrosegregation pattern. However, macrosegregation caused by thermosolutal convection readily masks the inverse segregation. The microsegregation models predict different solid concentrations and eutectic fractions, leading to different solid density distributions which, in turn, cause differences in the extent of contraction-driven flow.     

An explicit–implicit time stepping scheme for solidification models

A numerical scheme with hybrid explicit and implicit time stepping in solidification problems is presented. An explicit time stepping scheme is used for solving coupled temperature and concentration fields, while an implicit scheme is used for solving equations of motion. The explicit approach results in a local point-by-point coupling scheme for the temperature and concentration fields that uses constitutive model for back diffusion in solid. The present method offers distinct advantages of simplicity and flexibility in incorporation of micro-scale models, as demonstrated by the use of a back diffusion parameter in the microsegregation model. Results from the present method are compared with those in the literature using a fully implicit method, and they show significant improvement in final macrosegregation prediction.     

Design of hydrogen permeable Nb–Ni–Ti alloys by correlating the microstructures,solidification paths and hydrogen permeability

The compositional window in Nb–Ni–Ti alloys leading to the crystallization of primary α-Nb phase and the eutectic (α-Nb + NiTi) phase is of high technical relevance due to its favorable properties with respect to hydrogen permeation. The solidification behavior of Nb–Ni–Ti alloys in the primary α-Nb phase region is investigated to reveal the potential solidification paths. The study is based on the characterization of as-cast microstructures in combination with numerical calculations of solidification paths using the CALPHAD method coupled with a microsegregation model. Four different kinds of solidification paths depending on initial composition and cooling rate are found. Correspondingly, a new compositional window appropriate for hydrogen permeation is established in the primary α-Nb phase region. The variation of the hydrogen permeability of the alloys in this window is surprisingly high. High Nb content and Ni/Ti ratio lead to a high permeability. Nb₅₅Ni₂₀Ti₂₅ shows the highest permeability at 673 K, particularly 2.9 × 10⁻⁸ mol H₂ m⁻¹ s⁻¹ Pa^−1/2. This is about 1.8 times higher than that of pure Pd.     

Simulation and experimental study on carbide dissolution process of Fe‐1C‐1.42Cr‐bearing steel

In this paper, the distribution of C and Cr in the microsegregation of Fe‐1C‐1.42Cr‐bearing steel was measured by experimental and numerical simulation methods. Melting points of the low‐melting zones were calculated with the Thermo‐Calc system; and then the verifications were made by heat treatment experiments and the temperature and time required for the bulk carbide dissolution process of bearing steel were tested. Bulk carbides could be dissolved by soaking for 60 minutes above 1260°C.     

A Dual Scale Model for Macrosegregation in Alloy Solidification

A numerical approach for coupling the temperature and concentration fields using a micro/macro dual scale model for a solidification problem is presented. The dual scale modeling framework is implemented on a hybrid explicit-implicit solidification scheme. The advantage of this model lies in more accurate consideration of microsegregation occurring at micro-scale using a subgrid model. The model is applied to the case of solidification of a Pb–40% Sn alloy in a rectangular cavity. The present simulation results are compared with the corresponding experimental results reported in the literature, showing improvement in macrosegregation predictions. Subsequently, a comparison of macrosegregation prediction between the results of the present method with those of a parameter model is performed, showing similar trends.     

A numerical investigation of various phase change models on simulation of saturated film boiling heat transfer

There is a great variety of two‐phase models in numerical simulations. The performance of each model complicates the numerical simulation of boiling. The challenge of the right choice of heat and mass transfer models makes this type of problem more complicated. In this research work, the volume of the fluid two‐phase model has been used to simulate the film boiling of saturated liquid. The geo‐reconstruction method also reconstructs the interface of two phases. The models of the sharp interface, Lee and Tanasawa have been employed among the available models for calculating the phase change rate and the source terms of the equations. The Numerical solver of the phase‐change is verified through the Stefan one‐dimensional vaporizing problem. Correct empirical coefficients used in both Lee and Tanasawa models are presented. Bubble detachment time, flow pattern, the periodic Nusselt number, and the bubble form have been investigated in all three phase change models. Two Berenson and Klimenko experimental correlations have been used for verification of Nusselt number derived from simulations. The Nusselt number shows a proper fit with the Klimenko's Nusselt number. Obtained Nusselt number demonstrates the Lee model is more precise than other phase change models in simulating of film boiling on the flat plate.     

Effect of micro mass transfer through phase interface on numerical simulation of solidification process

Existing models for the solute redistribution during solidification have been reviewed. Some typical models are applied for the numerical simulation of heat and mass transfer with phase change under experimental condition of inverse casting. The results show that the effect of micro mass transfer models on the formation of the new phase cannot be omitted. © 2004 Wiley Periodicals, Inc. Heat Trans Asian Res, 33(6): 393–401, 2004; Published online in Wiley InterScience ( www.interscience.wiley.com ). DOI 10.1002/htj.20024     

Direct identification of absorption and scattering coefficients and phase function of a porous medium by a Monte Carlo technique

A general method of direct identification of absorption and scattering coefficients and phase function of porous media, assumed statistically homogeneous and isotropic, has been developed for wavelengths small in regard of the typical structure length (i.e. by neglecting diffraction). This method is here limited to a transparent fluid phase and an opaque solid phase. Diffuse and specular reflection laws and combination of these laws are considered. First results have been obtained for sets of Dispersed radius Overlapping Opaque Spheres (DOOS) in a transparent fluid phase, or sets of Dispersed radius Overlapping Transparent Spheres (DOTS) in an opaque solid phase. In the case of DOOS models, the absorption and scattering coefficients have the same analytical expressions as those characterizing the optically thin limit for any porous medium. For DOTS models, these coefficients have been, for porosity higher than 0.65, identified from Monte Carlo calculations versus the porosity or the specific area per unit volume of the fluid phase. For DOOS models, the phase functions have been expressed by a simple analytical expression in the case of a specular reflection law, and derived from Monte Carlo calculations in the case of the diffuse reflection law. For DOTS models, phase function have been numerically calculated from Monte Carlo calculations. The case of the combination of the two reflection laws is finally discussed.