颗粒动力学与湍动能耦合的稠密两相流动数学模型

在气相湍流流动的k-ε模型基础上,建立了颗粒动力学与湍动能耦合的稠密两相流动数学模型。颗粒相的有效粘性系数取决于颗粒之间相互碰撞而引起的层流粘性以及颗粒微团的湍流脉动而形成的湍流粘性,其中颗粒的碰撞行为以及所形成的颗粒的层流特性用颗粒动力学模型来描述,颗粒的湍流特性采用颗粒湍动能输运方程模型来描述。利用所建立的模型对提升管内气固两相流动过程进行了数值模拟,可以合理地预报出提升管内气固两相的环核流动结构。     

颗粒动力学与湍动能耦合的稠密两相流动数学模型

在气相湍流流动的κ-ε模型基础上，建立了颗粒动力学与湍动能耦合的稠密两相流动数学模型。颗粒相的有效粘性系数取决于颗粒之间相互碰撞而引起的层流粘性以及颗粒性以及颗粒微团的湍流脉动而形成的湍流粘性，其中颗粒的碰撞行为以及所形式的颗粒的层流特性用颗粒动力学模型来描述，颗粒的湍流特性采用颗粒湍动能输运方程模型来描述。利用所建立的模型对提升管内气固两相流动过程进行了数值模拟，可以合理地预报出提升管内气固两相的环核流动结构。     

气固两相流模拟的随机离散模型

§1.引言 气固两相流动形式是最复杂的两相流动实例,其系统中的颗粒浓度较高,颗粒间的碰撞经常发生,从而导致细观层次上的颗粒运动具有复杂性,对于两相流动系统,拟流体模型以其大规模模拟的可行性在数值模拟领域中居重要地位。但是,拟流体模型的连续性假设     

基于DSM湍流模型的气固圆湍撞击流模拟

采用改进的DSM大涡模型模拟气相湍流流动,采用颗粒轨道模型模拟颗粒运动,在双向耦合气固湍流数理模型基础上,采用蒙特卡洛方法Tanaka模型进行颗粒碰撞计算,取得相同颗粒数量下不同粒径的固体颗粒随湍射流运动对气相射流的调制规律及颗粒弥散规律.结果 表明,较大粒径的颗粒加强了气流刚性,由于颗粒惯性较大,对冲碰撞使颗粒在碰撞滞止点聚集,使流场中颗粒相浓度分布不均;中等粒径的颗粒对气相耗散较小,颗粒受到离心力主导影响,碰撞后仍沿涡的外围扩散;较小粒径的颗粒对气相耗散严重,颗粒跟随性好,大多聚集在涡核内,碰撞后仍随气体向外扩散,在流场中分布均匀.     

粉末注射成形充模流动的数值模拟

论文将粉末注射成形喂料在薄壁模腔中的流动视为二维流动,以流变学的基本方程为基础,建立了从动量方程、连续方程和热传递方程得到的描述粉末注射成形充模流动的数学模型。在无滑移边界的条件下,推导了喂料熔体流导率的计算公式和压力场的控制方程,得到的压力场控制方程是一非线性椭圆偏微分方程,从而可用Galerkin方法进行数值求解,使模型的数值求解成为可能,为进一步对粉末注射成形进行计算机模拟和数值分析奠定了数学基础。     

颗粒物性对水平气力输送管中颗粒浓度分布影响的数值模拟

在考虑气-固流体的双向耦合、颗粒与颗粒的碰撞、颗粒与壁面的碰撞以及滑移摩擦的基础上,对气体相湍动能采用修正的k-ε二方程模型,颗粒相湍动动能采用颗粒动力学方法,发展建立了水平气力输送的数学模型和相应的计算方法,数值研究了颗粒粒径和密度对悬浮颗粒的浓度分布的影响。结果发现在水平气力输送中,在颗粒湍动、颗粒自身重力、颗粒与颗粒的碰撞以及颗粒与壁面的碰撞的共同作用下,颗粒浓度分布不均匀,其垂向分布存在着两种不同的形态。颗粒粒径越小、密度越低,越容易出现Ⅰ型分布：即颗粒浓度呈现出从管底部到管上部会先由小变大,到某位置时达最大值,尔后又向小变化的趋势。     

气固两相流中颗粒间碰撞传热进展

稠密气固两相流中,颗粒间的碰撞成为流动、传热过程的重要作用机理,它们对热交换过程有重要的贡献。文章概述了气固两相流中,颗粒间碰撞传热机理的研究现状;评述了欧拉-拉格朗日和欧拉-欧拉模型框架下,颗粒间碰撞传热研究的进展。基于颗粒动力学理论和随机碰撞频率的概念,建立了适用于欧拉-欧拉方法的颗粒间碰撞传热的数学模型。     

跃移层内沙颗粒空中碰撞的模型仿真

研究风沙运动的碰撞性建模问题,对控制雾霾污染有着重要的意义.风沙运动时,风沙呈现空间的非均匀分布,存在动态波动,受到外界影响,呈现较强不稳定状态.针对风沙运动碰撞分析的模型,通常假设风沙可以均匀分布且稳定状态,模型中的参数无法随着风沙流结构等因素不断变化,无法对跃移层内沙颗粒的碰撞进行有效分析,存在一定的缺陷.提出了一种颗粒流体动力学的跃移层中沙颗粒空中碰撞仿真模型,通过颗粒流体动力学方法,分析沙粒与床面碰撞、沙颗粒的空中碰撞及沙粒与气流的耦合作用,获取跃移沙粒在空中发生碰撞的概率,按照跃移层内沙颗粒空中碰撞运动的特征,塑造基于单颗沙粒动力学的沙颗粒空中碰撞仿真模型,对沙颗粒在风力作用下发生空中碰撞和达到动态稳定的过程进行仿真,为分析风沙运动提供可靠的依据.实验结果表明,所提模型获取跃移层沙颗粒运动轨迹同已知实测结果具有较高的匹配度,能够实现跃移层沙颗粒空中碰撞的有效仿真.     

水平通道内颗粒团聚的数值模拟

在研究现有的两相流数学模型的基础上,引入多点同时碰撞的概念,提出了1种新的颗粒碰撞数学模型,并结合直接Monte-Carlo(DSMC)方法对水平通道内气固两相流进行了数值模拟,再现了颗粒团的存在.     

三相鼓泡床反应器中固体浓度分布与数值模拟

在气、液两相为连续相固相为颗粒流(granular)的基础上进行模拟,建立了描述三相浆态鼓泡床中湍流流动的模型,在模型中考虑了气相在穿过液相中所形成的气泡直径以及固体颗粒大小对系统的影响,同时对任意两相之间的连续性方程和动量方程进行耦合。以三相浆态鼓泡床固体浓度分布为例进行数值模拟,模拟值与实验值吻合较好,表明模型可靠,模拟方法正确,为研究三相浆态床流态特征提供了一种便捷的方法,也为三相浆态床反应器的设计、开发、放大开辟了一条新的道路。     

Ceramic micro parts produced by micro injection molding: latest developments

Powder injection molding is a preferred technology for the production of micro parts or microstructured parts. Derived from the well known thermoplastic injection molding technique it is suitable for a large-scale production of ceramic and metallic parts without final machining. To achieve good surface quality and control the part size and distortions is an important goal to allow mass production. This means that all process steps like part design adjusted for MIM/CIM-technology, appropriate choice of powder and binder components and injection molding simulation to design the sprue are required. Concerning the injection molding itself high quality mold inserts, high-precision injection molding with suitable molding machines like Battenfeld Microsystem50 or standard machine with special equipment like variotherm or evacuation of the molding tool and an adjusted debinding and sintering process have to be available. Results of producing micro parts by powder injection molding of ceramic feedstock will be presented.     

A lattice Boltzmann method for immiscible two-phase Stokes flow with a local collision operator

We propose a lattice Boltzmann model for immiscible two-phase Stokes flow with a local collision operator. The model is based on two different lattice Boltzmann automata, one for the flow field and one for an indicator function for the two different phases. The model is described in detail and verified by the following test-cases: a static bubble for the surface tension, a closed capillary tube for the contact angle and two phase flow in a concentric annulus for the viscosity ratio. In the appendix an asymptotic analysis for the derivation of the two-phase Stokes equation is given.     

Fabrication of barrier ribs of PDP via powder injection molding

In this paper, an attempt was made to explore a possibility of powder micro injection molding process in manufacturing ceramic microstructures such as barrier ribs of plasma display panel. The barrier ribs are glass matrix composites with ceramic powder (alumina and/or titania) filler. In this molding process, a thermosetting paste was molded into polydimethlsilosane soft molds prepared by replication of thick film resist (SU-8) molds. The SU-8 mold was patterned with UV-lithography. The effects of powder content in the paste on paste viscosity and sintering characteristics of molded samples were examined. In addition, effect of molding speed on pore trapping in the microstructure was studied. These results indicated that the powder micro injection molding process at ambient temperature has merits of low-pressure injection molding process with superior mold release characteristics.     

Application of lattice Boltzmann method in free surface flow simulation of micro injection molding

The filling flow in micro injection molding was simulated by using the lattice Boltzmann method (LBM). A tracking algorithm for free surface to handle the complex interaction between gas and liquid phases in LBM was used for the free surface advancement. The temperature field in the filling flow is also analyzed by combining the thermal lattice Boltzmann model and the free surface method. To simulate the fluid flow of polymer melt with a high Prandtl number and high viscosity, a modified lattice Boltzmann scheme was adopted by introducing a free parameter in the thermal diffusion equation to overcome the restriction of the thermal relaxation time. The filling flow simulation of micro injection molding was successfully performed in the study.     

Replication processes for metal and ceramic micro parts

To cover the demand for effective manufacturing of metal and ceramic micro components two process technologies are described. The first one can be regarded as a special variant of micro powder injection molding (MicroPIM): inmold-labeling using powder filled feedstocks. Its basic procedures are the backfitting of powder filled foils by an injected PIM-Feedstock and the subsequent co-debinding and co-sintering steps. For example, two-material ceramic parts with microstructured surfaces could be produced and compacted with mostly tight interfaces. The second process conduct combines two-component and insert injection molding with an electroforming process. Since all process steps involved are based on technologies suitable for series production, low product costs per unit will be realistic. Surface qualities and dimensional accuracies are comparable or even better than achieved if applying alternative processes like MicroPIM.     

Micro powder metallurgy for the replicative production of metallic microstructures

 Reproductive techniques like injection molding or embossing of feedstock provide microstructures of a wide variety of materials for a reasonable price to micro system technology. In this paper, the dependencies and barriers to produce high aspect ratio structures by micro metal injection molding are described; some results of embossing of metal powder based feedstocks are presented, too. The investigations show different influencing parameters for reaching high aspects ratios. The main factor is the used powder, finer powders allow higher aspect ratios. Moreover, the binder system, the feedstock (mixture of powder and binder) and the quality of the injection mold influence the reproduction process. Received: 10 August 2001/Accepted: 24 September 2001     

ANSYS二次开发在PIM充模流动模拟中的应用

论文介绍了ANSYS所具有的针对不同需求、特点和应用范围的多种二次开发技术;根据PIM充模喂料流体的幂律模型,基于ANSYS用户可编程特性中对其FLOTRAN模块流体特性中的粘度模型进行了二次开发,用所开发的幂律模型对I型拉伸试样的PIM充模二维流动进行了数值模拟,模拟结果与实验结果吻合很好;并将二次开发的方法推广到PIM喂料的密度、导热系数、比热等的修改,为将ANSYS强大的计算流体力学功能广泛应用于PIM充模流动模拟的研究奠定了基础。     

Micro powder injection molding: process characterization and modeling

The development of a new simulation tool for micro powder injection molding (MicroPIM) needs experimental material data and verification experiments to describe the process correctly. A new and innovative approach is to use dissipative particle dynamics (DPD) to describe the form filling process with respect to the interactions on a mesoscopic scale (Hoogerbrugge and Koelman in Europhys Lett 19(1):155–160, 1992). The individual parameters that enter DPD modeling of this process have to be adjusted using specially designed experiments for the injection molding process. The material properties in the standard injection molding process are primarily determined by the bulk material. In micro dimensions surface effects begin to dominate because of the large surface to volume ratio. Therefore, the surface interactions between feedstock and mold insert were studied. Finally, first observations of the injection molding experiments are shown and qualitatively compared to DPD simulation results.     

The effects of metal particle size and distributions on dimensional accuracy for micro parts in micro metal injection molding

This study aims to develop processing techniques to improve dimensional accuracy of micro-size parts produced by micro metal injection molding (μ-MIM). Micro dumbbell specimens were molded by a micro injection molding machine, which can monitor the cavity pressure in injection molding process. The effects of particle size and distribution of metal powder on dimensional accuracy of micro dumbbell specimens at both grip parts were investigated. As the results, it is confirmed that the powder properties and sintering conditions to improve the dimensional accuracy of micro-MIM parts.