食品真空冷却的机制、特性与实验装置

本文分析了食品中水蒸汽压、比热容及单位质量失水1%获得温降的计算方法;建立了食品真空冷却过程的传热、传质微分方程与积分方程;分析了真空冷却技术的优点及适用性,并简要介绍了真空冷却实验装置及可深入研究的几个方向     

注塑成型冷却过程的数值模拟

采用循环平均假设，忽略模壁温度的周期变化，将模具的传热简化为三维稳态热传导总是，考虑到注射模的结构特点（型腔为狭缝面，冷却孔细长），推导出求解其温度场的边界积分方程；注塑件的传热简化为一维瞬态热传导，给出确定其冷却时间及表面循环平均热流的方法；通过模具及塑件传热的耦合迭代分析，使模具－塑料件界面的温度和热流满足相容条件，最终确定模具型腔的温度分布及塑件的冷却时间。最后通过一个例子说明数值模拟在冷却系统设计中的应用。     

定量刻画冷冻手术中生物组织损伤度的熵产理论

从不可逆热力学角度出发，建立了有一定通用性、能涵盖各类冻融损伤要素的熵产理论分析方法。基于经典Pennes生物传热方程，导出了描述活体组织传热过程熵产率的热力学模型，初步提出了用以刻画低温手术过程中组织冻融损伤程度的公式，在此基础上，采用有限元数值计算并借助熵产公式，就血液灌注率、代谢产热及手术时间等因素对手术疗效的影响进行了考察，为从组织层次定量刻画低温手术过程中的冻融损伤提供了新的理论途径。     

大拉深比薄壁筒形件充液成形过程数值模拟

目的利用充液成形工艺成形普通拉深工艺难成形的大拉深比筒形件。方法通过理论公式计算了冷冲压工艺成形该制件的道次,利用有限元软件Dynaform对充液成形过程进行了3个步骤模拟,并研究了第1步拉深时初始反胀高度对成形制件减薄率的影响规律。结果利用理论公式计算,传统冲压方法成形拉深比为3.2的筒形件至少需要5个道次,而采用被动式充液成形方法只需要3个道次。每个道次的最大减薄率都在8%以内,最后得到拉深制件的最大减薄率为8.53%,在安全范围以内;第1步充液拉深时,反胀高度分别为1.75,2.75,3.75,4.75,5.75 mm时,得到制件的最大减薄率分别为5.28%,5.08%,4.8%,5.03%,5.03%。结论充液成形工艺较传统冲压工艺可以大大提高板料的成形极限,减少成形道次,成形制件质量好;合适的初始反胀高度,可以减小成形制件壁厚的最大减薄率。     

基于有限差分法的金属复合板冷却过程温度分布预测

目的 研究金属复合板在冷却过程中的温度变化规律以及经过冷却后金属复合板的表面及内部温度分布规律.方法 利用二维有限差分法对金属复合板材冷却过程中的温度变化规律进行了分析,从理论上推导了使用等步长节点法求解温度的差分方程,通过设置合理的边界条件解决了金属复合板基板和复板接触界面互相影响的传热问题.结果 通过设置等步长节点的有限差分法计算得出金属复合板冷却过程的温度变化规律以及冷却后金属复合板的表面及内部温度分布,对比有限元软件模拟计算和有限差分程序计算结果,得出结论.结论 界面处温度高,越靠近两边温度越低,通过与有限元软件模拟计算作对比,验证了差分法计算的准确性.     

制造大直径厚壁管的工艺特点

研究了用各种原始毛坯生产大型厚壁管的工艺过程,分析了工艺参数对被获得制件的质量和金属利用系数的影响.确定了根据钢锭尺寸冷却冲孔钢锭的时间.     

自动铺放成型热塑性复合材料的非等温结晶动力学研究

利用差示扫描量热仪结合Avrami方程研究玻璃纤维增强聚丙烯复合材料自动铺放成型过程非等温结晶动力学,推导非等温结晶动力学模型,并通过构建冷压辊下方铺层的冷却模型,将结晶动力学模型和传热模型相结合,设定自动铺放成型过程中的冷却条件,探讨冷却速率及冷却时间对基体材料结晶行为的影响,求解出不同冷却速率下的最大铺放速率。研究结果表明:铺层树脂基体的结晶度随冷却速率的增大而依次减小;随着冷却速率的提高,树脂结晶起始温度和结晶完成温度均向低温方向移动,且树脂相对结晶度随温度变化规律接近反S形曲线;自动铺放成型实验件的压缩强度及层间剪切强度随着铺层结晶度的增大基本呈增大趋势,而冲击强度与铺层结晶度的变化趋势完全相反,随着结晶度的增大,材料韧性越差。     

真空渗透成型工艺制件厚度变化规律

为了解决真空渗透成型(VIP) 工艺中凝胶时间与充摸时间难以匹配的问题, 通过对不同层数的VIP 制件厚度变化规律的研究, 得到了厚度与树脂流动距离和增强纤维层数之间的经验公式, 并验证了公式的可行性。分析了试验过程中存在的问题和样品出现的缺陷, 探讨了改进试验的方法, 以提高工程实际应用中成型工艺效率及最终产品质量。      

聚合物注塑成型内应力数值计算的非线性模型

为了提高注塑内应力计算的可靠性,利用粘弹性力学理论建立了新的注塑制品内应力计算的四元件串联力学模型,并推导了其瞬态粘弹性响应的非线性本构方程.通过求解流动及保压控制方程,得到内应力计算所需的温度场和压力场,利用回归分析得到了聚合物弹性模量和粘壶系数的计算公式.用新模型对PS平板注塑制件脱模前的内应力进行了模拟计算.计算结果与固体高聚物的结构和力学性能的相关研究结论相一致.     

铁路支距尺在自然条件与检定环境温度平衡的数值模拟

铁路支距尺温度平衡时间控制检定或校准开始时间,在传热条件中引入一些可接受的传热假设,研究支距尺全长范围内传热过程。尺身传热系数较高,是重点控制项目,其他各部件控制好传热方程与绝热边界,在传热系数较小材料中确定边界换热条件或软件模拟换热面为绝热边界。空调出风形式与支距尺摆放方位确定支距尺温度平衡时间,为快捷校准提供技术支持。     

Calculation of Residual Stress Development in Injection Moulding Using a Nonlinear Viscoelastic Model

The residual stress development in amorphous polymers iscalculated for rapid cooling under pressure during injection moulding.This is done primarily to predict the warpage and shrinkage behaviour ofcomponents. A nonlinear viscoelastic model based on the free-volumeconcept is used as a constitutive equation to describe polymerbehaviour. The cooling process in injection moulding can be divided intotwo stages: until ejection, the part is clamped into the mould, whilethe remaining cooling down to room temperature happens free ofconstraints. Both steps are calculated viscoelastically in thesimulation procedure. First, the stress profiles are determined in onematerial point of a plate-like component. In these calculations, theinfluence of different processing conditions, such as ejection time,packing pressure, or different wall temperatures, is investigated. Next,to be able to take more complex geometries into account, the nonlinearconstitutive equation is used in a commercially available finite-elementprogram. This is done by programming user-defined subroutines. Thesolution of the coupled thermal-mechanical problem allows the predictionof the residual stresses as well as the warpage and shrinkage behaviourfor components with arbitrary geometry. Finally, the calculated warpageis compared with the behaviour of an experimental component.     

An evolutionary approach for cooling system optimization in plastic injection moulding

The cooling process is of great importance in plastic injection moulding as it has a direct impact on both productivity and product quality. Cooling process optimization is a sophisticated task which includes not only the design of cooling channels but also the selection of process parameters. Most existing optimization systems focus on either cooling channel design or process parameter selection but not both. This paper explores an approach to optimize both cooling channel design and process condition selection simultaneously through an evolutionary algorithm. The prototype system proposed in this paper is an integration of the genetic algorithm and CAE (Computer-Aided Engineering) technology. The aim is to launch a computerized system that can guide the optimization of the cooling process in plastic injection moulding. The objective is to achieve the most uniform cavity surface temperature to assure product quality.     

应急生保试验舱副舱内胆体的成形与检漏

重点介绍了应急生保试验舱副舱内胆体的焊接、成形与检漏以及焊接材料、焊接方法的选择,焊接工艺规范和焊接工艺参数.采取循环水冷却提高熔池的冷却强度,加快散热速度,防止产生热裂纹的倾向.采用刚性固定防止工件焊接整体变形.通过对内胆体成形原理的分析,利用干燥的型砂填充中空的流体通道卷制成形等工艺措施.配置辅助真空抽气系统,对较长的流体通道进行氦质谱真空检漏.     

Experimental study of defect formation during processing of randomly-oriented strand carbon/PEEK composites

An investigation of the moulding defects formed during compression moulding of randomly-oriented strand carbon/PEEK composites is presented. The cause of defect formation was identified as non-uniform shrinkage due to a high coefficient of thermal expansion at the onset of crystallization. Panels with void content ranging from 0% to 1.3% were moulded by releasing the moulding pressure at specific temperatures during the cooling process. Mechanical tests showed a reduction in specimen compressive strength from 15% to 25% for a void content of 0.63–1.3%. It was concluded that the high concentration of porosity near the surface of the panels was the likely cause of the strength reduction.     

Zum Einfluss von Abkühlverlauf und Tauchdauer auf die Haftfestigkeit und das Bruchverhalten von Zinküberzügen nach DIN EN ISO 1461

The influence of cooling conditions and immersion time on the adhesive power and the nature of the break of zinc coatings in according to EN ISO 1461 Zinc coatings are exposed external and internal mechanical stresses The object of our investigation was to determine the influence on the adhesion test results of the parameters originally determining the structure and the properties of the zinc coatings, such as • chemical composition of steels (Si/P‐content) • galvanizing conditions (zinc melt composition, immersion time) • cooling conditions The results of the adhesion test by the modified pull‐off‐test in according to ISO 4624, metallographic, scanning electron microscope and EDX analytical investigations and also layer thickness measurements make it possible to state that the structure and constitution of zinc coatings are especialy depended on the immersion time and cooling conditions. The reasons for this would be found. The best results for adhesive power of zinc coatings were received by cooling in cold water immediately after the hot dip galvanizing process. Relevant informations for the galvanizing industry are given.     

HDPE材料冷却过程的结晶动力学模型及其微分方程

根据高密度聚乙烯(HDPE)高聚物冷却时结晶和晶体分解两种动力学趋势相互竞争的观点,建立了HDPE材料冷却结晶的物理模型,引入了结晶过程中的物质结构参数概念。从模型推导出了描述该结晶过程的微分方程,并将其转换成熟知的Ricatti方程来求得解析解。模型中参数可以由实验来确定,方程中的温度变化本身是时间的一般函数,故在应用中可以不必区分结晶系统是否等温。     

A BEM approach to model heat flow during crystallization

In most polymer processing applications such as injection moulding, fibre spinning and extrusion, crystallization plays an important role. The energy flow at the solid-liquid interface during crystallization controls the kinetics and subsequently influences the morphology of the transforming material. Our approach is based on the classical phase-change problem. The governing transient heat conduction equations in the liquid and the solid domains are solved using a boundary element method (BEM) with a time dependent fundamental solution to determine the temperature distribution. The boundary energy equation is used to predict the movement of the front. The crystallization kinetics are introduced through a mathematical model based on the cooling rate of a hypothetical control volume in the solid (crystalline) domain and the changes in the crystallinity are expressed in terms of the varying interface temperature. First, we verified the BEM formulation and implementation by comparing the results of two examples with an analytical and a finite difference method in one-dimension. Next, we investigated the effects of the crystallization kinetics by allowing the interface temperature to vary during the crystallization process. The results show that the crystallization front movement is slowed considerably when the kinetics are taken into account. Also, depending on the cooling rate and the parameters in the kinetics model, there may be undercooling during the process.     

Boundary element model of microsegregation during volumetric solidification of binary alloy

 In the paper the mathematical model of heat and mass transfer processes proceeding in the domain of casting is discussed. In particular, the volumetric solidification of a binary alloy under the assumption that the temperature is only time-dependent is analyzed. From the practical point of view such situation takes place when the casting is produced in typical moulding sand. The differential equation describing the course of solidification and cooling processes is presented in Sect. 1. In this equation the capacity of an internal heat source results from the Mehl–Johnson–Avrami–Kolmogorov theory [1, 2], at the same time the constant number of nuclei is accepted, while the rate of the solid phase growth is proportional to the second power of undercooling below the liquidus temperature. The macroscopic model is coupled with a microsegregation one (Sect. 2). This process is analyzed at the level of a single grain. The distribution of the alloy component in the control volume corresponding to the final grain radius is found on a basis of the boundary element method using discretization in time [3, 4]. The examples of numerical computations are also presented. Received 6 November 2000     

Manufacturing process of reproduction plate by nonmetallic materials reclaimed from pulverized printed circuit boards

The aim of this study was to present a new method for resource utilization of nonmetallic materials reclaimed from pulverized waste printed circuit boards. A reproduction nonmetallic plate (RNMP) was prepared by adding resin paste, glass fiber and additives into nonmetallic materials using self-made hot-press former. Principle of manufacturing process and effects of mould temperature and moulding time on the mechanical properties of RNMP were studied. The results showed that when moulding pressure was fixed at 6 MPa, the optimum conditions for the RNMP were as follows: 140/135 degrees C for top/bottom mould temperature, 5 min for moulding time. The maximum content of nonmetallic materials in RNMP was up to 40 wt%. When nonmetallic material content was 20 wt%, the RNMP moulded at optimum conditions had excellent mechanical properties, with impact strength of 5.8 kJ/m(2) and flexural strength of 65.1 MPa.     

Feasibility study of use of recycled High Density Polyethylene and multi response optimization of injection moulding parameters using combined grey relational and principal component analyses

Usability of recycled High Density Polyethylene (HDPE) as substitute for virgin HDPE is investigated. Optimization design of the injection moulding parameters for recycled HDPE products is presented. Tensile, compressive and flexural strengths are selected to evaluate the process performance and the corresponding moulding parameters are melt temperature, holding pressure, injection time, and holding time. Optimal combination of injection moulding parameters is determined using Grey relational analysis. The principal component analysis is applied to evaluate the weighting values corresponding to various performance characteristics. Tensile, compressive and flexural strengths of the recycled HDPE are found close to that of virgin HDPE. Thus, recycled HDPE is a good substitute for virgin HDPE. Optimal combination of the process parameters for the multi-performance characteristics of the recycled HDPE is the set with melt temperature at 240 °C, holding pressure at 255 N/m², injection time at 0.6 s and holding time at 30 s.