双螺杆挤出中粒子运动轨迹的可视化模拟

在双螺杆挤出机的理论研究中，了解粒子的运动轨迹对于全面理解物料在挤出机中输送和混合的机理至关重要。本文使用POLYFLOW软件，对聚合物熔体在啮合同向和异向双螺杆挤出机中的三维等温流场分别进行了数值计算，并在此基础上对双螺杆挤出机中物料粒子的运动轨迹进行了可视化模拟，给出了运动粒子的运动轨迹。可视化模拟与传统的实验方法相比有着极大的优越性。本文的工作为研究各种新型螺杆元件的输送和混合机理提供了一种新的方法。     

双螺杆挤出机模具过渡和稳流段流场的数值模拟

采用Carreau模型描述熔体的黏度特征,使用POLYFLOW软件数值模拟了异向旋转双螺杆挤出机模具过渡和稳流段的三维等温流场。在不同的工艺参数下,分析比较了模具过渡和稳流段内的速度场、压强场、剪切速率场、剪切应力场以及黏度场。结果表明在模具过渡和稳流段内,螺杆的旋转是该聚合物熔体流变性能的最大影响因素,螺杆头对高黏度熔体的影响范围不大,只限于螺杆头的周围;进入稳流段后,在稳流段作用下熔体的流动逐渐变得稳定均匀;在本文讨论的转速和流量范围内,转速和流量的增加,分别会在不同的区域使熔体受到的剪切速率和剪切应力明显地增大。研究结果为某企业设计双螺杆模具和优化工艺条件提供理论依据。     

Ω形双螺杆泵流场仿真与实验研究

研究螺杆转子转动过程中的流场变化有助于对双螺杆泵运行参数的优化。采用计算流体力学(CFD)方法,对双螺杆泵流场进行三维瞬态动网格仿真分析。针对Ω形双螺杆泵,建立内流场数值模型,通过仿真得到一个转动周期内泵内压力分布,同时研究了不同粘度下泵外特性的变化规律。仿真结果表明,泵内压力由吸入端到排出端逐级增大,与容积腔推挤增压规律相吻合;在吸入端容积腔截面和螺杆啮合缝隙内均存在负压,且由于泄露的原因,密封腔两端的压力分布并非完全一致,存在轻微差异;流量随着压差的增大而呈近似线性下降趋势,粘度越大,流量受压差的影响就越小;泵效随压差的变化曲线呈抛物线形,达到峰值后先逐渐减小,最后趋于平稳。实验结果与仿真一致,证明了仿真方法的有效性。     

小型发动机高空模拟试验流量系数确定的数值分析与试验研究

流量系数在小型航空发动机高空模拟试验的空气流量计算中十分关键。建立了流量管模型,通过数值计算获取流量管各截面的流场分布,对比另外试验与数值模拟的总、静压分布和流量系数,结果与高空模拟试验进气空气流量测量结果一致。     

单螺杆大振幅振动对挤出影响的数值模拟研究

借助CFD专用模拟软件PolyFlow,本文模拟了动态塑化单螺杆挤出机计量段中,聚合物熔体在单螺杆大振幅振动条件下的三维非等温输送流场,研究了大振幅振动力场对挤出产量、功率及螺杆特性的影响。模拟结果显示:振动参数有一个可选范围,当螺杆振动幅值超过该范围而落入横流反向区时,挤出流率将急剧下降,产能比严重降低,螺杆特性变软。     

微流控芯片中电渗流的数值模拟与仿真研究

从电渗流形成的基本理论入手,推导了电场和流场双物理场耦合的控制方程.运用多物理场数值计算分析软件建立了长为1000μm,宽为100μm的二维流道,在微流道中间250~750μm的区域施加了直流电压,并在数值模拟中还原了微流道内壁和微流体的物理属性,计算得出了各段流体的速度场,进而得出了各段流体的流型.通过二维流道压力分布分析了微流道中各段产生不同流型的原因.对微流控芯片中的电动流动的功能原理分析及优化设计具有借鉴意义.     

反应堆流场分析的数值模拟

运用CFD分析工具对反应堆内的流场分布进行数值模拟,给出反应堆燃料组件入口处流场的速度和流量分配情况.通过对计算结果的比较分析,对核电厂反应堆内的流动特性有比较全面的了解,从而为反应堆堆内构件的设计和优化提供分析依据.结果表明反应堆内流场采用CFD技术进行模拟计算是可行的.     

基于CFD的通信机房流场模型建立与分析

介绍了一种利用计算流体动力学技术分析通信机房流场分布的方法,找出合理的气流组织形式以达到节能的目的.对机房三维物理模型进行非结构化网格划分,选用标准的k-ε湍流模型,设定相应的边界条件,针对不同的空调风速工况,利用Fluent软件对机房内的温度场和速度场进行数值模拟与分析,研究得出合理的风速以降低能耗.该方法能够有效地建立通信机房的流场模型,对制定机房节能方案有重要的参考作用.     

微型直接甲醇燃料电池CO2气泡排除问题研究

利用CFD方法模拟了微型直接甲醇燃料电池阳极二氧化碳气体在沟道中的流动与分布.结合两相流理论,研究了不同流场布局、沟道尺寸和燃料入口流速对两相流压力降及气体分布的影响,得到了最大残余体积含气率与流道宽度及入口速度的关系.结果表明蛇形流场排泡效果要优于平行流场,尤以流道宽度为600μm时最佳.根据计算结果,分别对蛇型流场、平行流场和三通道蛇型流场中气体聚集的位置进行了预测.研究结果为流场的参数设计和结构优化提供了参考.     

螺杆挤出机螺杆元件性能数值研究的进展

随着计算机和计算技术的飞速发展,数值计算与实验研究和理论研究成为科学研究的3种重要方法. 近年来,越来越多的人使用成熟的计算流体力学(Computational Fluid Dynamics, CFD)软件,数值仿真动态模拟聚合物螺杆挤出过程,深入研究螺杆结构、工艺条件和物性之间的规律,研究各类参数对聚合物挤出过程的影响和聚合物螺杆挤出机理. 数值计算作为新的研究手段之一,不但能大大缩短研发周期和降低成本,而且可得到实验手段无法测量的数据. 为了推动数值计算在该领域的应用,综述国内外数值研究螺杆挤出机螺纹元件性能的进展.     

Effect of inlet and outlet boundary conditions on swirling flows

Numerical simulations are conducted for both three-dimensional, turbulent flow in a multi-channel swirler and axisymmetric, isothermal, turbulent flow in combustion chambers using the standard κ−ε turbulence model. Calculations are first carried out for three-dimensional, isothermal and turbulent flow inside the swirler channels in order to derive the velocity profiles of both air and gas at the swirler outlets, which are used as inlet boundary conditions of the model combustor and can also be used in future studies for different combustors with the same type of swirler. In order to study the sensitivity of swirling flow inside the chamber to the inlet and outlet boundary conditions, different inlet velocity profiles and outlet boundary conditions are also employed. The results show that in the cases considered, the flow behaviour in the chamber is not very sensitive to the actual shape of the inlet velocity profiles provided the averages of the inlet axial, radial and azimuthal velocity components are separately preserved. Other conditions being equal, we find that the swirling flow performance in the combustor depends not only on the inlet swirl number, but also strongly on the relative magnitude of the radial velocity component at inlet and introduce a new dimensionless number N_r, analogous to the swirl number, to measure the relative importance of this quantity. Outlet boundary conditions have some influence near the outlet, but nearly no effect further upstream for the cases investigated.     

On a method for direct numerical simulation of shear layer/compression wave interaction for aeroacoustic investigations

Direct numerical simulation (DNS) of a spatially developing mixing layer was performed. The compressible three-dimensional Navier-Stokes equations were solved for pressure, velocities and entropy for this flow using a compact finite-difference scheme of sixth-order accuracy in space, combined with Runge-Kutta three-step time advancement. On one of the transverse boundaries of the box-shaped domain, a compression wave profile was imposed in pressure and velocity components via a wave decomposition of the governing equations, in order to study the interaction of an isolated weak shock wave entering the domain with the mixed subsonic/supersonic shear layer. This flow situation is found along the shear layer of supersonic, imperfectly expanded jets containing a shock cell structure. In the present work, an isolated compression-expansion structure constitutes the model problem. The domain setup and the boundary conditions were chosen such as to allow analysis of the sound field generated by the turbulent flow and the shock-turbulence interaction. The numerical method used to impose the boundary conditions and solve the compressible Navier-Stokes equations, and the choice of numerical parameters, are described in detail. Some results on the two-dimensional and three-dimensional flow field computed are presented as well.     

Numerical boundary conditions at the interface in a confined flow computation

This study investigates the effects of the numerical zonal boundary conditions required for the computation in a confined flowfield which has been split into several zones (because of its complex geometry or computer capacity limitations) for reasons of computation convenience. A practical engineering problem of isothermal mixing flow is selected and the domain is split purposely into two computational subdomains for demonstration purposes. Three different approximations of the boundary conditions at the interface are examined and compared with the baseline predictions obtained with the overall computational domain. Comparison shows that application of the conventional, empirical inlet and outlet boundary conditions as the required zonal boundary conditions to the present practical problem gives poor prediction. A one-way coupling (parabolic) approximation slightly improves prediction accuracy in comparison with the use of conventional, empirical boundary conditions. A two-way coupling (zonal approaching) approximation yields the most satisfactory solution.     

Numerical analysis of off-track vibration of head gimbal assembly in hard disk drive caused by the airflow

The problem of flow-induced vibration of head gimbal assembly (HGA) in hard disk drive (HDD) is analyzed by means of numerical models. Flow field is calculated in a fraction of physical domain called extended wedge-like domain. For this purpose, appropriate inlet and outlet boundary conditions are specified. The aerodynamic disturbances generated by the slider are studied by performing flow calculations for the cases of with and without slider. It is observed that the slider blocks the airflow causing a high-pressure region in the proximity of the leading surface of the slider. In addition, significant vortical structures are found being generated by the side surfaces of the slider. A finite element model is developed for calculating the response of HGA to the aerodynamic excitations. It is found that the flow disturbances generated by the slider play a significant role in the off-track vibration of the HGA.     

A multiplicative decomposition of Poiseuille number on rarefaction and roughness by lattice Boltzmann simulation

Poiseuille number of rarefied gas flow in channels with designed roughness is studied and a multiplicative decomposition of Poiseuille number on the effects of rarefaction and roughness is proposed. The numerical methodology is based on the mesoscopic lattice Boltzmann method. In order to eliminate the effect of compressibility, the incompressible lattice Boltzmann model is used and the periodic boundary is imposed on the inlet and outlet of the channel. The combined bounced condition is applied to simulate the velocity slip on the wall boundary. Numerical results reveal the two opposite effects that velocity gradient and friction factor near the wall increase as roughness effect increases; meanwhile, the increments of the rarefaction effect and velocity slip lead to a corresponding decrement of friction factor. An empirical relation of Poiseuille number which contains the two opposite effects and has a better physical meaning is proposed in the form of multiplicative decomposition, and then is validated by available experimental and numerical results.     

Inlet conditions for LES using mapping and feedback control

Generating effective and efficient inlet boundary conditions for large eddy simulation (LES) is a challenging problem. The most accurate way of achieving this is to run a precursor calculation to generate a library of turbulence, either prior to the simulation or concurrently with it, and to transfer the data from the library simulation to the main domain inlet. In this paper, we investigate a variant of this, in which the precursor calculation is subsumed into the main domain, its function being adopted by a mapping of data from a specified plane downstream of the inlet back to the inlet. Within this inlet section of the main domain, the flow can be affected by a number of computational manipulations, including the introduction of artificial body forces, modification of the mapped data, and direct correction of the velocity data. These modifications can be linked to feedback control algorithms to drive the solution towards specified characteristics, including mean and turbulent flow profiles, and bulk properties of the flow such as swirl. Various variants of the basic technique incorporating different levels of complexity in the control are implemented and tested on simulation of flow in a rectangular channel and in a circular pipe.     

Numerical approximation of flow induced vibrations of channel walls

In this paper the numerical method for solution of an aeroelastic model describing the interactions of air flow with vocal folds is described. The flow is modelled by the incompressible Navier–Stokes equations spatially discretized with the aid of the stabilized finite element method. The motion of the computational domain is treated with the aid of Arbitrary Lagrangian–Eulerian method. The structure motion is described by an equivalent system with two degrees of freedom governed by a system of ordinary differential equations and discretized in time with the aid of an implicit multistep method and strongly coupled with the flow model. The influence of inlet/outlet boundary conditions is studied. The numerical analysis is performed and compared to the related results from literature.     

On numerical simulation of flow through oil filters

This paper concerns numerical simulation of flow through oil filters. Oil filters consist of filter housing (filter box), and a porous filtering medium, which completely separates the inlet from the outlet. We discuss mathematical models, describing coupled flows in the pure liquid subregions and in the porous filter media, as well as interface conditions between them. Further, we reformulate the problem in fictitious regions method manner, and discuss peculiarities of the numerical algorithm in solving the coupled system. Next, we show numerical results, validating the model and the algorithm. Finally, we present results from simulation of 3-D oil flow through a real car filter.     

Meshless local boundary integral equation (LBIE) method for the unsteady magnetohydrodynamic (MHD) flow in rectangular and circular pipes

The meshless local boundary integral equation (LBIE) method is given to obtain the numerical solution of the coupled equations in velocity and magnetic field for unsteady magnetohydrodynamic (MHD) flow through a pipe of rectangular and circular sections with non-conducting walls. Computations have been carried out for different Hartmann numbers and at various time levels. The method is based on the local boundary integral equation with moving least squares (MLS) approximation. For the MLS, nodal points spread over the analyzed domain, are utilized to approximate the interior and boundary variables. A time stepping method is employed to deal with the time derivative. Finally, numerical results are presented to show the behaviour of velocity and induced magnetic field.