Modeling and simulation of heat transfer phenomena during investment casting

Determining the heat transfer phenomena during casting processes is an important parameter for measuring the overall performance of process. It gives information about the properties of the metal being casted and its possible behavior in the mold during casting process. Improper determination of heat transfer phenomena and use of improper molding materials and casting conditions leads to defects such as misruns, cold shuts, shrinkage, pin holes, air holes and porosity in final product. A mathematical model was developed using standard transport equations incorporating all heat transfer coefficients to calculate the time for solidification of metal in casting and computer simulation of the model was carried out in C++ to validate the model. The metal used was pure iron casted in investment molds of silica sand with zircon coating. It was shown that airflow near the mold surfaces was partially restricted due to geometry of the molds and arrangement of the pieces around a tree. So, the changes in heat transfer coefficient also contribute towards time of solidification. The time calculated was found to be in good agreement with experimental values.     

边界透水条件下浇筑混凝土密实性分析

采用透水模板布(CPFL)渗透性测试装置,研究了在振动条件下新拌混凝土密实过程中内部孔隙水压力随时间的动态变化特征,并分析了拌合物的体积收缩率对新拌混凝土密实过程中孔隙水压力消散特性的影响。结果表明,新拌混凝土在振动密实过程中,其内部孔隙水压力随振动加载时间的变化先增大后减小,末端再反向增大;且内部孔隙水压力变化与拌合物水胶比及体积收缩率均呈良好的非线性负相关性。     

Inverse alloy solidification problem including the material shrinkage phenomenon solved by using the bee algorithm

In the paper we solve the one-phase inverse problem of alloy solidifying within the casting mould, including the shrinkage of metal which results from the difference between densities of the liquid and solid phases. The process is modeled by means of the solidification in the temperature interval basing on the heat conduction equation with the source element enclosed, whereas the shrinkage of metal is modeled by the proper application of the mass balance equation. The investigated inverse problem consists in reconstruction of the heat transfer coefficient on the boundary of the casting mould on the basis of measurements of temperature read from the sensor placed in the middle of the mould. Functional expressing the error of approximate solution is minimized with the aid of Artificial Bee Colony Optimization algorithm.     

Ductile fracture of duplex stainless steel with casting defects

Experimental and numerical studies were carried out on CF8M steel with special regard to the influence of casting defects i.e. shrinkage cavities on fracture behaviour. As-received and thermally aged materials were studied. Physical steps of damage in specimens containing casting defects were observed using in-situ tensile tests in a scanning electron microscope. Mechanical properties of duplex stainless steel were characterised using three-point bend tests of three types of specimens — defect free, pre-cracked, and containing shrinkage cavities. These mechanical tests were simulated using Gurson's model including crack nucleation. A modelling of the shrinkage cavities is proposed in order to study their influence on the mechanical response of the specimen.     

Modeling of transport phenomena in continuous casting of non-dendritic billets

A macroscopic model for simulating the phase change process and transport of solid fraction is developed for the case of solidification during direct chill continuous casting of a non-dendritic Al-alloy billet, in presence of electromagnetic stirring. Maxwell’s equations are solved to obtain the electromagnetic force field, which is incorporated in the momentum conservation equations as body force source terms. Thereafter, the complete set of equivalent single-phase governing equations (mass, momentum, energy, species conservation and transport of solid fraction) are solved using a pressure-based finite volume method. A variable viscosity approach is employed to model fluid flow in presence of phase change. The model is first validated against some experimental and numerical results available in the literature, pertaining to the case of conventional continuous casting without any externally imposed stirring. The model predicts the temperature, velocity, species and most importantly, the solid fraction distribution in the mold. These predictions are then used for studying the influence of initial superheat, stirring intensity and cooling rate on the macroscopic behavior of the system.     

非常规体型大体积混凝土基础裂缝控制

非常规体型大体积混凝土基础浇筑后,水和水泥的用量是混凝土干缩裂缝的主要成因.根据性质主要包括干缩裂缝、塑性收缩裂缝、自身收缩裂缝、安定性裂缝、温差裂缝和碳化收缩裂缝等.介绍了浙北变电站工程构架基础大体积混凝土施工工艺,叙述了从控制施工原材料和改进施工工艺入手,以科学配比和强化监测为主,通过采取重视混凝土基础浇筑、浇筑后精心养护、成型后准确测温三大措施,防止了大体积混凝土基础出现塑性收缩裂缝和温度裂缝的成功经验.     

Effect of Al and Ce doping on the deformation upon sintering in sequential tape cast layers for solid oxide fuel cells

Water-based tape casting is an attractive production route for planar solid oxide fuel cells (SOFCs) due to its high productivity and reduced environmental issues. In this work planar anode supported SOFCs with thin electrolyte were prepared by water-based sequential tape casting and co-sintering. An in situ high temperature monitoring apparatus was assembled to allow the determination of free sintering shrinkage of thin green tape cast layers and to follow the curvature developed in multilayers during the entire sintering process.The instantaneous curvature developed upon co-sintering was studied as a function of the firing schedule and layer composition. It was found that by tailoring the electrode composition it is possible to reduce the shrinking rate difference between anode and electrolyte thus obtaining defect-free electrolyte, minimising the residual curvature of the half-cell and improving the electrochemical performances of the cell.     

Investigation into the flow behavior of high-temperature ash and low-temperature ash of high calcium coal

The flow behavior of high-temperature ash (HTA) and low-temperature ash (LTA) of high calcium coal in the heating process was investigated systematically. By means of the heating stage microscope, the behavioral changes of samples were studied visually. The composition and mineral matters transformations of HTA and LTA samples were analyzed by X-ray fluorescence and X-ray diffraction. The results show that the original composition of HTA sample is different from that of LTA sample. In addition, the HTA and LTA samples experience the shrinkage, fusion and spreading processes in succession. However, the volume change of HTA sample is different from that of LTA sample. The volume of LTA sample shows a slow change at the temperature lower than 800 °C, while the volume of HTA sample is unchanged. In the temperature range of 800°C–1100 °C, the remarkable shrinkage of HTA and LTA samples are demonstrated. The formation of srebrodolskite and gehlenite attributes to this volume change. Moreover, the sharp shrinkage of HTA and LTA samples is indicated at 1100°C–1300 °C. This is caused by the formation of eutectics. Because of the diverse content and species of mineral matters in LTA sample, the volume change of LTA sample is more remarkable than that of HTA sample. The maximal shrinkages of LTA and HTA sample are 57% and 53%, respectively.     

THERMOMECHANICAL BEHAVIOR OF THE SOLIDIFYING SHELL WITHIN CONTINUOUS-CASTING BILLET MOLDS-A NUMERICAL APPROACH

A two-dimensional numerical model is given for the analysis of the coupled thermal and mechanical behavior of the solidifying shell within the mold during continuous casting of steel. The influence of different mold wall profiles on gap formation and heat flow during casting of billets is investigated. The calculated temperatures, stresses, and strains in the shell are used to estimate the risk for formation of longitudinal cracks. The effect of an initiated and growing macroscopic subsurface crack on the shell behavior is studied. The genesis of surface cracks is discussed. The calculated results are shown to be in reasonable agreement with experimental observations reported in the literature.     

Analysis of melting of alloy powder bed with constant heat flux

Melting of an alloy powder bed with constant heat flux for application in selective laser sintering (SLS) is analyzed in this paper. Since melting of an alloy occurs in a range of temperatures, instead of at a single melting point, there will be a mushy zone – containing partially melted powders – between the unmelted region and the completely melted region. The mushy zone can be further divided into two sub-regions: (1) a lower part with constant porosity (shrinkage takes place), and (2) an upper part with constant volume (no shrinkage). Temperature distributions in different regions and locations of melting interfaces are obtained using an integral approximation method. The results show that increasing initial porosity and temperature of the powder bed accelerate the melting process. The melting slows down with increasing thermal conductivity of the interstitial gas.     

Numerical technique for modeling conjugate heat transfer in an electronic device heat sink

A fast running computational algorithm based on the volume averaging technique (VAT) is developed to simulate conjugate heat transfer process in an electronic device heat sink. The goal is to improve computational capability in the area of heat exchangers and to help eliminate some of empiricism that leads to overly constrained designs with resulting economic penalties.VAT is tested and applied to the transport equations of airflow through an aluminum (Al) chip heat sink. The equations are discretized using the finite volume method (FVM). Such computational algorithm is fast running, but still able to present a detailed picture of temperature fields in the airflow as well as in the solid structure of the heat sink. The calculated whole-section drag coefficient, Nusselt number and thermal effectiveness are compared with experimental data to verify the computational model and validate numerical code. The comparison also shows a good agreement between FVM results and experimental data.The constructed computational algorithm enables prediction of cooling capabilities for the selected geometry. It also offers possibilities for geometry improvements and optimization, to achieve higher thermal effectiveness.     

Experimental Studies for Convective Drying of Potato

An experimental facility was built at the Indian Institute of Technology Delhi in order to examine the characteristics of convective drying of a moist object. The test facility consists of an inlet section, a divergent and convergent section, a settling chamber, a test section, and an outlet section. Initial moisture content and time-dependent moisture content of a rectangular shaped moist object (4 cm × 2 cm × 2 cm) are measured by this test facility. The potato slice was selected as a sample moist object. Moisture content was measured at different air temperatures of 40, 50, 60, and 70°C with an air velocity of 2 m/sec. The density of potato slice was determined for various drying temperatures. The volume shrinkage during drying decreased almost linearly with moisture content. The percentage air pores and porosity increased gradually with decreasing moisture content and increasing drying air temperature. Volumes of water, air, and solid content of potato were determined at different drying air temperatures. The results are validated with theoretical data.     

太阳能级硅的电磁悬浮熔炼

电磁悬浮与离心铸造技术(ELCC)开辟了低成本制备太阳电池用硅片材料的新途径。硅材料的无污染熔炼是进行ELCC硅片成形的重要前提。采用电阻与高频感应相结合的两级加热方法，成功地实现了太阳能级硅的电磁悬浮熔炼，为低成本太阳电池的研制奠定了基础。实验还发现，与传统的理论计算不同，悬浮力随电源频率的不断下降而提高。     

A foam melting model for lost foam casting of aluminum

In lost foam casting of aluminum the liquid metal normally decomposes the foam pattern by ablation. But sometimes polymer vapor bubbling through the liquid metal accumulates along an upward-facing flow front until it opens a finite gap between the liquid metal and the decomposing foam. This changes the foam decomposition mechanism along that front from direct ablation to melting. A mathematical model is formulated for heat conduction, convection, and radiation across the gap, coupled with the vaporization of the excess polymer liquid behind the metal front and the resulting buoyant movement of polymer vapor bubbles through the liquid metal. Both models are combined to obtain an analytical solution for one-dimensional bottom filling of a pattern with uniform thickness. The results from this solution not only compare well with available experimental data, but they also explain how part thickness, metal temperature, and pressure affect filling speeds in bottom-fill situations.     

Numerical Method for Calculation of Complete Casting Processes—Part II: Validation and Application

In this article, the numerical method proposed in Part I is validated by applying it to relatively simple validation test cases with phase change as well as to real-life problems. First, the results of the 1-D calculations are compared with the analytical solutions for the Stefan problem with mushy zone. Then, the 2-D and 3-D calculations of the Bridgman crystal growth are compared with available experimental results. The ability to predict the residual stresses is demonstrated on an academic 2-D example. Finally, the results of calculation for two real-life industrial cases, injection casting and solidification of a multicomponent metal drill head, are presented and discussed.     

Evaluation of effective material properties of spiral wound gasket through homogenization

In this paper, a homogenization methodology is proposed to determine the material properties of spiral wound gaskets (SWGs) using finite element analysis through representative volume elements (RVE) of the gaskets. The constituents of this RVE are described by elasto-plastic material properties. The RVE are subjected to six load cases and the volume averaged responses are analyzed simultaneously to predict the anisotropic properties. The mechanical behaviour is simplified to an orthotropic material model with Hill’s plasticity model and the properties are verified with micro-mechanical simulation and experimental results available in the literature. Reasonable agreement is obtained between the results. Formulae for elastic properties are also derived by a simplified analytical method based on lamination theory and compared with those obtained from homogenization.     

Model techniques for testing heated concrete structures

The experimental techniques described in this paper may be used in the laboratory to measure strains of model concrete structures representing to scale actual structures of any shape or geometry, operating at elevated temperatures, for which time-dependent creep and shrinkage strains are dominant. These strains could be used to assess the distribution of stress in the scaled structures and hence to predict the actual behaviour of concrete structures used in nuclear power stations. Similar techniques have been employed in an investigation by the author to measure elastic, thermal, creep and shrinkage strains in heated concrete models representing to scale parts of prestressed concrete pressure vessels^1–6 for nuclear reactors.The experimental techniques, the model specimens and the materials described in the paper refer to the above investigation.The paper starts by describing the models tested, the equipment used and the experimental procedures and ends by presenting, in graphical form, the experimental results. Analytical procedures developed to predict the model behaviour are briefly described in the Appendix.     

Flammability profiles associated with high-pressure hydrogen jets released in close proximity to surfaces

This paper describes experimental and numerical modelling results from an investigation into the flammability profiles associated with high pressure hydrogen jets released in close proximity to surfaces. This work was performed under a Transnational Access Agreement activity funded by the European Research Infrastructure project, H2FC.The experimental programme involved ignited and unignited releases of hydrogen at pressures of 150 and 425 barg through nozzles of 1.06 and 0.64 mm respectively. The proximity of the release to a ceiling or the ground was varied and the results compared with an equivalent free-jet test. During the unignited experiments concentration profiles were measured using hydrogen sensors. During the ignited releases thermal radiation was measured using radiometers and an infra-red camera. The results show that the flammable volume and flame length increase when the release is in close proximity to a surface. The increases are quantified and the safety implications discussed.Selected experiments were modelled using the CFD model FLACS for validation purposes and a comparison of the results is also included in this paper. Similarly to experiments, the CFD results show an increase in flammable volume when the release is close to a surface. The unstable atmospheric conditions during the experiments are shown to have a significant impact on the results.     

A Numerical Study of 3D Turbulent Melt Flow and Solidification in a Direct Chill Slab Caster with a Porous Combo Bag Melt Distributor

A 3D turbulent melt flow and solidification of an aluminum alloy (AA-1050) for an industrial-sized direct chill slab casting process is modeled. The melt is delivered through a rectangular submerged nozzle and a non-deformable combo bag fitted with a bottom porous filter. The non-Darcian model, incorporating the Brinkman and Forchheimer extensions, is used to characterize the turbulent melt flow behavior passing through the porous filter. The casting speed and the effective heat transfer coefficient at the metal–mold contact region within the mold are varied. The above two parameters are found to have significant influence on the solidification process.     

Design of experiment approach applied to reducing and oxidizing tolerance of anode supported solid oxide fuel cell. Part II: Electrical, electrochemical and microstructural characterization of tape-cast cells

One of the major limitations of the nickel (Ni) - yttria-stabilized zirconia (YSZ) anode support for solid oxide fuel cells (SOFC) is its low capability to withstand transients between reducing and oxidizing atmospheres (“RedOx” cycle), owing to the Ni-to-NiO volume expansion. This work presents results on different anode supports fabricated by tape casting. Three compositions are prepared, as the outcome of a preceding design of experiment approach. The NiO proportion is 40, 50 and 60 wt% of the anode composite.The anode support characteristics like shrinkage during sintering, in-situ conductivity at high temperature, electrochemical performance and tolerance against RedOx cycles have been measured. Performance up to 0.72 W cm⁻² (0.62 V, 800 °C) is recorded for the 60 wt% NiO sample on small cells. The open circuit voltage is maintained within ±5 mV after 10 full RedOx cycles at 800 °C and one at 850 °C. Performances tend to be stabilized after one or multiple RedOx cycles. The microstructural observations show round Ni particles after the first reduction; after a RedOx cycle, the Ni particles include micro-porosities that are stable under humidified reducing atmosphere for more than 300 h.