熔渣颗粒空冷相变换热的三维数值模拟

利用VOF方法结合凝固和熔化模型对熔渣颗粒在空气流中的冷却相变过程进行了三维数值模拟,讨论了熔渣颗粒直径和空气速度对冷却凝固过程演变的影响。结果表明：空冷方法能够实现熔渣颗粒表面的快速凝固成型,但同时也造成了颗粒内部的非均匀凝固。熔渣直径越小,完全凝固时间越短;空气流速越大时, 其表面换热越强, 完全冷却时间越短。颗粒初温为1673.15 K、直径为0.5~2 mm,风速为1~5 m·s^-1条件下熔渣颗粒在2 s内释放出全部凝固热,后续空气最高温度能达到900 K以上。     

Die Fettreifbildung von Schokolade in Abhängigkeit von der verfahrenstechnischen Durchführung der Vor- und Erstarrungskristallisation

Dependence of Bloom-Formation in Chocolate from Technical Operation of Pre-Crystallization and Solidification The undesired bloom-formation in chocolate is caused by post-crystallization of cocoa fat during storage. Experiments were carried out systematically in order to reveal, in what manner the bloom-formation is dependent on pre-crystallization and solidification operations. It was found that chocolate is more resistent to bloom-formation, higher the amount of fat that crystallizes out during pre-crystallization. Increased stability of strongly pre-crystallized chocolate with respect to bloom-formation is due to transformation of cocoa fat into a stable crystalline state, already during pre-crystallization and solidification.     

Expansion of solidifying saturated fats

From literature it appears that simple saturated triglycerides and commercial fats contract to a considerable extent when they undergo transformation from liquid to solid state and from an instable to a more stable crystalline form. In spite of these facts, the simple saturated even triglycerides and some fully hydrogenated fats exhibit violent solidification expansion by voluntary cooling. Solidification experiments carried out with several saturated even triglycerides and fats under various solidification conditions have shown that the solidification expansion tendency depends upon the chemical composition of the fat, as well as on the solidification conditions. The solidification expansion tendency is increased by various more or less independent factors, namely decreasing iodine value, increasing fatty acid and triglyceride uniformity, increasing triglyceride symmetry, increasing tendency to rapid formation of the stable β crystals, seeding with β crystals, voluntary cooling, moderate cooling velocity, and increasing bulk amount of fat. All the expanded solidified fats ended up being in the β crystalline form, whereas the nonexpanded fats ended up with the β’ form. The solidification expansion is avoidable by careful control of the cooling procedure.     

Beitrag zur Physik der Fette - Über ein neues verbessertes Meßverfahren zur Erfassung und Bewertung der Erstarrungseigenschaften von Fetten

Contribution to the Physics of Fats - A New Improved Method for Measurement and Evaluation of the Solidification Properties of Fats The physical behaviour of solidified fats can vary widely (e.g. consistency, melting behaviour, sensoric) depending on solidification conditions. The reason for this is mainly the polymorphism, but also other physical effects like e.g. fractionated cristallization. Reliable information about the solidification properties of fats and fat blends are to be expected only if reproducible and usefully selected test conditions are kept. Up to now only indirect procedures have been available which were related to the solidification enthalpy (Shukov. Jensen a. o.). New impulses were brought by the “thermorheography” (“TRG-procedure”, according to Baenitz/Kleinert), in this method the moving resistance (torque) is controlled during solidification. Initial disturbations caused by the device could be removed in the meantime by a varied construction of the measuring device. By this an additional improvement of enthalpy recording was possible. For this examples of determination are shown and explained.     

Relationship between characteristics of oil droplets and solidification of thermally treated creams

The relationship between the characteristics of oil droplets and the change in appearance of cream was investigated. The model creams (40 wt% oil-in-water emulsion), similar to commerical products, were prepared with vegetable fat, milk protein, and emulsifier. The thermal treatment, in which the cream was exposed to a certain temperature and subsequently recooled, was performed on the assumption that the temperature was temporarily elevated during transportation and storage of commercial products. Solidification of the cream was observed when it was exposed to a temperature where there was a small percentage in solid fat content (SFC) of fat in oil droplets and recooled, whereas the cream remained in the liquid state when it was exposed to the temperature where SFC was zero and recooled. When the SFC of oil droplets was 0% at the treated temperature, greater supercooling prior to fat crystallization occurred and the crystallization rate after the initial formation of crystals was much higher. On the other hand, the polymorphism of fat in the droplets was not directly affected by the thermal treatment. These results indicate that the crystallization in oil droplets at the heating temperature may be closely connected with the destablization of oil droplets via a partial coalescence mechanism, which will cause the solidification of cream.     

Solidification of thermoviscoelastic melts. Part I: Formulation of model problem

The solidification of a molten layer of amorphous thermoplastic between cooled parallel plates is used to model the mechanics of part shrinkage and warpage and the buildup of residual stresses in the injection molding process. Flow effects are neglected, and a thermorheologically simple thermoviscoelastic material model is assumed. The equilibrium thermomechanical properties of the material and the shift function can be temperature- and pressure-dependent. The model allows material to be added to fill the space created by the packing pressure applied during solidification; therefore, this model can be used to assess packing-pressure effects in injection molding. The model also accounts for freeze-off effects in which the cavity pressure is controlled by the solidification process and must therefore be determined as a part of the solution.     

Automatische Messung von Erstarrungspunkten

Automatic Measurement of Solidification Points An automatic laboratory apparatus for the determination of solidification point (titer) of fatty acid is described. The measurements are made according to DGF uniform method using the technique of Shukoff. The values obtained agree well. The apparatus enables the measurement of solidification points above and below room temperature. It does not require constant observation of the course of temperature change. The solidification point is digitally exhibited and registered. If desired, the temperature diagram can be registered by a recorder.     

Solidification of thermoviscoelastic melts. Part 3: Effects of mold surface temperature differences on warpage and residual stresses

The solidification of a molten layer of amorphous thermoplastic between cooled parallel plates is used to model the mechanics of part warpage in the injection-molding process. Flow effects are neglected, and a thermorheologically simple thermoviscoelastic material model is assumed. The model allows material to be added to fill the space created by the pressure applied during solidification so that this model can be used to assess packing-pressure effects in injection molding. Parametric results are presented on the effects of the mold temperatures and the packing pressure—the pressure applied during solidification to counteract the effects of volumetric shrinkage of the thermoplastic—on the in-plane and through-thickness shrinkages, on warpage, and on residual stresses in plaque-like geometries. The packing pressure is shown to have a significant effect on part warpage. While the results are presented in terms of normalized variables based on the properties of bisphenol-A polycarbonate, they can be interpreted for other amorphous thermoplastics, such as modified polyphenylene oxide, polyetherimide, and acrylonitrile-butadiene-styrene.     

Residual thermal stresses in injection molded products

Nonisothermal flow of a polymer melt in a cold mold cavity introduces stresses that are partly frozen-in during solidification. Flow-induced stresses cause anisotropy of mechanical, thermal, and optical properties, while the residual thermal stresses induce warpage and stress-cracking. In this study, the influence of the holding stage on the residual thermal stress distribution is investigated. Calculations with a linear viscoelastic constitutive law are compared with experimental results obtained with the layer removal method for specimens of polystyrene (PS) and acrylonitrile butadiene-styrene (ABS). In contrast to slabs cooled at ambient pressures, which show the well-known tensile stresses in the core and compressive stresses at the surfaces, during the holding stage in injection molding, when extra molten polymer is added to the mold to compensate for the shrinkage, tensile stresses may develop at the surface, induced by the pressure during solidification.     

Gradient-controlled freeze casting of preceramic polymers

Solidification is the foundation upon which freeze casting is built, and this work seeks to understand the solidification process in freeze casting, especially with respect to the growth of dendrites. To this end, two solidification parameters, freezing front velocity and temperature gradient, were independently controlled using a gradient-controlled freeze casting setup to investigate the effects of each parameter. Changes in dendritic pore size and morphology with solidification parameters were investigated by scanning electron microscopy and mercury intrusion porosimetry. The results agree with dendrite growth theory. The theory of constitutional supercooling is used to describe the morphological changes of dendritic pores to cellular pores by the control of freezing front velocity, temperature gradient, and preceramic polymer concentrations.     

制备板状玻璃炭工艺中固化曲线的确定

固化处理是在玻璃炭制备工艺中很关键的阶段．在差热分析和热失重分析的基础上．建立了玻璃炭（GC）固化升温曲线，并与等速升温法作了比较．     

BFFO的热安全性和凝固特性

为了探究双呋咱并[3,4-b:3′,4′-f]氧化呋咱并[3″,4″-d]氧杂环庚三烯(BFFO)的熔铸工艺性能,通过DSC实验并结合长时恒温试验,对BFFO的热安全性进行了研究;通过X光透射成像、扫描电镜形貌分析、DSC控制凝固测试等对其凝固性能进行了研究,并与3,4-二硝基呋咱基氧化呋咱(DNTF)进行了对比。结果表明,BFFO熔融峰温84.7℃,95~135℃的恒温长时加热未见变色发烟,显示良好的熔铸工艺热稳定性。BFFO的起始分解温度为251.8℃,分解峰温339.5℃,β→0时热爆炸临界温度(T_b)为270.8℃,显示出良好的熔铸工艺热安全性。BFFO具有一种特殊的凝固结晶特性,能以小时为单位,长时间保持熔融状态,显示出非常缓慢的凝固速率。BFFO熔融液相密度为1.659g/cm³(95℃),自然凝固体积收缩率为11.3%。其凝固缺陷集中于药柱顶部补缩区,成型部分整体均匀致密,凝固成型密度1.774g/cm³,达到理论密度的94.9%,显示出良好的熔铸凝固成型性能。相比DNTF,BFFO的熔融峰温降低24.8℃,具有更好的熔融工艺性;分解峰温提高62.6℃且分解过程相对缓和,具有更优的热安定性。BFFO的凝固速率显著低于DNTF,具有更好的凝固补缩性能,可以降低铸件的内部缺陷,使凝固成型相对密度较DNTF提高7.8%。     

Automatisches Gerät zur Messung von Erstarrungspunkten

Automatic Device for the Determination of Solidification Point An automatic monitoring device for the determination of solidification point is described. This device is suitable for process control in fatty acid distillation columns. The solidification point must lie between 30° and 80° C. The value obtained correspond to the results provided by the laboratory procedure prescribed in the DGF standard methods.     

Effect of scanning speed on the solidification process of Al2O3/GdAlO3/ZrO2 eutectic ceramics in a single track by selective laser melting

Al₂O₃/GdAlO₃/ZrO₂ ternary eutectic ceramics with fine microstructure were directly fabricated from mixed pure ceramic powders by selective laser melting. The specimen forming quality, molten pool morphology and microstructure characteristic were investigated as functions of scanning speed. Solidification defects such as cracks and pores were effectively suppressed when the scanning speed was 12 mm/min. The relative density of the as-solidified eutectic specimens decreased from 98.7% to 95.7% with increasing the scanning speed up to 48 mm/min. The melting in this study was governed by conduction mode, leading to a decrease tendency of both melting width and depth with the increase of the scanning speed. Different from ordinary cognition, the eutectic spacing in top zone of the molten pool first decreased and then increased with increasing the scanning speed from 6 mm/min to 48 mm/min. The transition point appeared at 12 mm/min, where the dominant factor affecting the solidification rate changed from the scanning speed to the value of the angle between microstructure growth direction and laser scanning direction.     

Die Bestimmung der Schmelz- und Erstarrungs- Temperaturen von Fetten

Determination of Melting and Solidification Temperatures of Fats Considerable fluctuations in results are observed when solidification temperatures, and, especially melting temperatures are determined by customary methods. In an apparatus developed by Walisch and Eberle, the sample is heated in an oven under linear increase in temperature, during which the temperature within the sample is measured. In pure glycerides and various fats, the resulting temperature-time curve enables detection of the polymorphic crystalline transformations and determination of the entire temperature region at which melting occurs. Reproducibility of the results was very good. In parallel studies using NMR-spectrometry, the amount of liquid part as a function of temperature was measured. The results agreed with melting diagrams. The cooling curve served for determining solidification point in a most reproducible manner and for detecting polymorphic transformations.     

Solidification of thermoviscoelastic melts. Part 4: Effects of boundary conditions on shrinkage and residual stresses

The solidification of a molten layer of amorphous thermoplastic between cooled parallel plates is used to model the mechanics of part shrinkage and the buildup of residual stresses in the injection-molding process. Flow effects are neglected, and a thermorheologically simple thermoviscoelastic material model is assumed. The model allows material to be added to fill the space created by the pressure applied during solidification, so that this model can be used to assess packing-pressure effects in injection molding. The interactions between the mold surfaces and the solidifying material are accounted for by modeling different types of constraints through different model boundary conditions. For several sets of boundary conditions, parametric results are presented on the effects of the packing pressure—the pressure applied during solidification to counteract the effects of volumetric shrinkage of the thermoplastic—on the in-plane and through-thickness shrinkages, and on residual stresses in plaque-like geometries. Plaques that can shrink in the in-plane direction while in the mold are shown to shrink more and to have higher residual stresses than plaques that are fully constrained while in the mold. Although the results are presented in terms of normalized variables based on the properties of bisphenol-A polycarbonate, they can be interpreted for other amorphous thermoplastics such as modified polyphenylene oxide, polyetherimide, and acrylonitrile-butadiene-styrene.     

Vergleich von Milchfetten aus der Kristallisationsfraktionierung und einem kontinuierlichen Fraktionierungsverfahren mittels überkritischem Kohlendioxid

Comparison of Milk Fat Fractions Obtained by Crystallization with those Obtained in a Continuous Fractionation Process Using Supercritical Carbon Dioxide Several milk fat fractions obtained by crystallization from molten fat as well as milk fat fractions obtained in a new continuous fractionation process using supercritical carbon dioxide have been investigated by GC, HPLC und DSC. The influence of its fatty acid and triglyceride compositions on the crystallization and melting behaviour is discussed. The milk fat fractions obtained by crystallisation tend to crystallize in two distinct main triglyceride group. A separation even into three fractions is shown in the melting thermograms. The milk fat fractions obtained by the fractionation using supercritical carbon dioxide have much less tendency to separate into triglyceride groups. Obviously, this continuous fractionation process using supercritical carbon dioxide can be improved. Therefore, it seems to be possible to obtain milk fat fractions with different triglyceride compositions and better physical properties.     

Solidification and phase transformation behaviour of food fats — a review

Recent studies on physical behaviour of food fats have been reviewed, with an emphasis on solidification properties in bulk and emulsion states. It was shown that the use of synchrotron radiation X-ray beam highlighted the solidification behaviour of polymorphic fats, which was not unveiled by traditional X-ray beams on a laboratory scale as summarised in the following: (a) kinetic processes of molecular ordering of lamella-structured fat crystals, (b) mixing behaviour of different triacylglycerols forming miscible, eutectic, and molecular compound crystals, and (c) remarkable influences of shearing forces on the rapid solidification of the more stable forms of cocoa butter. These results have given some indications to the physical control of fat solidification, and the blending and fractionation of solid fats. The influences of hydrophobic emulsifiers on the solidification of fat crystals in O/W emulsions, which were in-situ monitored by an ultrasound velocity technique, have shown that the addition of highly hydrophobic emulsifiers having long-saturated acid moieties accelerated the nucleation of the fats at the interface areas of the emulsions. These effects are also indicative for the selection of the emulsifier for the control of the fat solidification in the emulsion phases.     

Slag Solidification Modeling Using the Scheil–Gulliver Assumptions

In this paper, the Scheil–Gulliver approach on solidification modeling is applied to slag solidification. The Scheil–Gulliver model assumes no diffusion in the solid phases, infinitely rapid diffusion in the liquid phase, and local equilibrium at the solid/liquid interface. For two distinct CaO–MgO–SiO₂ slags, solidification is simulated with the Scheil–Gulliver model and with the commonly used thermodynamic equilibrium model. The simulations show that in the Scheil–Gulliver model, as opposed to the equilibrium model, compositional gradients in the solid phases can exist and peritectic reactions do not occur. Solidification experiments are performed under laboratory conditions to validate the simulation results. The experimental results show a better correspondence with the Scheil–Gulliver model than with the equilibrium model. However, quantitative differences in mineralogy persist.     

Solidification of thermoviscoelastic melts. Part II: Effects of processing conditions on shrinkage and residual stresses

The solidification of a molten layer of thermoplastic between cooled parallel plates is used to model the mechanics of part shrinkage and the buildup of residual stresses in the injection-molding process. Flow effects are neglected, and a thermorheologically simple thermoviscoelastic material model is assumed. The model allows material to be added to fill the space created by the pressure applied during solidification, so that this model can be used to assess packing-pressure effects in injection molding. Parametric results are presented on the effects of the mold and melt temperatures, the part thickness, and the packing pressure—the pressure applied during solidification to counteract the effects of volumetric shrinkage of the thermoplastic—on the in-plane and through-thickness shrinkages, and on residual stresses in plaque-like geometries. The packing pressure is shown to have a significant effect on part shrinkage, but a smaller effect on residual stresses. Packing pressure applied later in the solidification cycle has a larger effect. Mold and melt temperatures are shown to have a much smaller effect. The processing parameters appear to affect the through-thickness shrinkage more than the in-plane shrinkage. While the results are presented in terms of normalized variables based on the properties of bisphenol-A polycarbonate, they can be interpreted for other amorphous thermoplastics such as modified polyphenylene oxide, polyetherimide, and acrylonitrile-butadiene-styrene.