Investigation of viscosity effect on droplet formation in T-shaped microchannels by numerical and analytical methods

Both numerical and analytical models have been developed to explore the viscosity effect of the continuous phase on drop formation at a T-shaped junction in immiscible liquids. The effects of the generalized power law coefficient, the power law exponent and the yield stress on the mechanism of drop breakup, final drop size and frequency of drop formation are studied by using the numerical three-dimensional volume of fluid model. Droplets coalescence in Bingham fluids is observed in the beginning transient period. The effect of yield stress on drop extension is also discussed. Predictions of drop size by using an analytical force balance show satisfactory agreement with simulation results for Newtonian and power law fluids with different viscosity ratios. The approximation error associated with the analytical model for Bingham fluids is also acceptable. This analytical model can greatly shorten the prediction time as compared with the numerical model, which is helpful for on-line control.     

Microphotometric characterization of fluid segment populations generated in different simple microfluidic networks

The stability of fluid segments is limited by deformation stress and by coalescence events. Both factors are typical for the passage of fluid segments through microfluidic networks. Therefore, the coalescence behaviour of micro fluid segments in simple network structures in dependence of flow rate ratios was investigated and characterized by the composition of obtained segment populations. Series of segments of different size and distance were generated either in a double T- or in a triple T-arrangement. PTFE elements were used for the micro fluid network. Nearly pulsation-free fluid actuation was realized by syringe pumps. The flow conditions in the input streams of carrier liquid and injected solutions remained constant during the experiments. Segment sequences become divers by different injection, stacking and coalescence events. The resulting segment sequences were characterized by on-line micro photometry. The populations of obtained micro fluid segments during each experiment were characterized by the distribution of segment size and segment distance or segment period, respectively. Microfluidic experiments as well as the numerical simulations support the assumption that the character of segment populations is mainly determined by the flow rate ratios and by the coalescence sensitivity beside the topology of the fluidic network.     

向下鼓气泡进入水或磁性液体的数值模拟与实验验证

应用数值与实验的方法分析了气泡向下鼓入水和磁性液体的过程,通过数学模型应用PLIC-VOF方法仿真重现了气泡的形成与破裂机理,通过数值模拟获得了气泡的形状与生长周期,用CCD摄像头完成实验测量,实验与模拟达到了数值上的吻合。     

MEMS微通道流动压力试验关联式仿真及修正

针对宏观流体理论计算MEMS微流体运动的结果与试验不一致问题,提出一种工程实用的微流体压力计算的修正方法。通过引入当量粘度系数,对微流体的流动压力进行修正,并仿真出了当量粘度系数曲线及获取途径,为微流体的分析提供了理论指导。     

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.     

Double-diffusive and Soret-induced convection of a micropolar fluid in a vertical channel

This paper reports an investigation of the fully developed natural convection heat and mass transfer of a micropolar fluid in a vertical channel. Asymmetric temperature and concentration boundary conditions are applied to the walls of the channel. The cases of double diffusion and Soret-induced convection are both considered. The governing parameters for the problem are the buoyancy ratio and the various material parameters of the micropolar fluid. The resulting non-dimensional boundary value problem is solved analytically in closed form using MAPLE software. A numerical solution of the time dependent governing equations is demonstrated to be in good agreement with the analytical model. The influence of the governing parameters on the fluid flow as well as heat and solute transfers is demonstrated to be significant.     

Highly resolved dose–response functions for drug-modulated bacteria cultivation obtained by fluorometric and photometric flow-through sensing in microsegmented flow

The application of the micro fluid segment technique for the investigation of highly resolved dose–response relationship at the physiological parameters segment–internal pH and growth of the cell culture is introduced. The method was used to study the concentration-dependent response of an Escherichia coli culture for the effectors 2,4-dinitrophenol (DNP) and an antibacterial peptide amide (KKVVFKVKFK-NH₂). Large sequences with up to 250 micro fluid segments containing gradually varying concentrations of the effectors were generated using a PTFE micro fluidic arrangement, including a 7-port-manifold and PC-controlled syringe pumps. The response of the cell culture was characterized by a double sensor system allowing for simultaneous read out of metabolism-related changes as well as changes in cell density. A twin arrangement of a micro flow-through photometer and a micro flow-through fluorometer based on LED devices in connection with the application of pH-sensitive polymer sensor particles was applied. This experimental setup allows a detailed determination of drug-related changes in fluorescence intensity by the E. coli culture and the polymer particles as a function of time by tracking changes in pH and cell density. The application of the segmented flow technique for multi-parameter drug screenings provides new insights into the biological answer of bacteria cultures cultivated at the nanoliter scale.     

化工原理流体流动过程的计算机辅助计算

流体输送过程为化工原理课程中的基础部分,其计算过程繁琐、图示过程众多,采用传统的板书讲解方式难以达到较好的授课效果。本文利用MATLAB语言强大的运算与图形表达功能,剖析了流体流动过程中的典型例题,给出了计算机辅助计算的具体思路和方法。此外,本文也给出了利用计算机对本单元实验数据进行自动处理的方法。这些计算机辅助计算过程,大大增强了教学过程的直观性和可视性,加深了学生对流体流动过程单元的理解。     

Natural convection boundary layer of a non-Newtonian fluid about a permeable vertical cone embedded in a porous medium saturated with a nanofluid

An analysis was performed to study the effect of uniform transpiration velocity on free convection boundary-layer flow of a non-Newtonian fluid over a permeable vertical cone embedded in a porous medium saturated with a nanofluid. The model used for the nanofluid incorporates the effects of Brownian motion and thermophoresis. The governing partial differential equations are transformed into a set of non-similar equations and solved numerically by an efficient implicit, iterative, finite-difference method. Comparisons with previously published work are performed and excellent agreement is obtained. A parametric study of the physical parameters is conducted and a representative set of numerical results for the velocity, temperature, and volume fraction profiles as well as the local Nusselt and Sherwood numbers is illustrated graphically to show interesting features of the solutions.     

Design of suspension and swing arm for improved shock resistance in the micro optical disk drives

One of the trends on information storage device is focused on the development of micro optical disk with optical flying head (OFH). The OFH in the small form factor ODD consists of optical slider, suspension, swing arm, and so on. In order for a slider with OFH to be successfully implemented in the system, the shock performance of the head-gimbal assembly should be guaranteed for mobile applications. We design development model of suspension and swing arm for improved shock resistance using topology optimization and shape optimization. The objective of the suspension design is to decrease the equivalent mass with increasing first torsional frequency. The objective of the swing arm design is mass reduction for reducing power consumption and fast access time. In this paper, we suggest a design method for the improvement of the dynamic performance of suspension and swing arm for the OFH considering shock characteristics.     

Transient response analysis and modeling of near wall flow conditions in a micro channel: evidence of slip flow

An experimental tool for determination of the near wall transport parameters in a micro channel, supported by flow simulation, is presented. The method is based on the transient flow response due to convective diffusion, in absence of specific adsorption. An approximately step-function type temporal solute concentration variation serves as the input signal. The associated response signal of a surface plasmon resonance sensor, acting as an integral part of a micro channel, has been taken as the output signal. It provides the flow-dependent change of the NaOH solute concentration in the channel within the optical detection and near wall distance interval 0 < d < 0.5 μm. The temporal signal evolution and response time, until an initially plain aqueous solution is replaced by the solute, varies inversely with solute concentration and flow rate. In the asymptotic limits, the near wall forced convective and diffusive channel transit times, along with the associated velocities, can be extracted and separated. A low convective near wall flow speed would account for 100% adsorption efficiency. The validity of the scaling relation for Fickian diffusive transport has been confirmed by experiments. Convective near wall flow reveals a distorted parabolic flow profile. This indicates relaxation of the no-slip condition, and presence of slip flow. Neither boundary layer formation, nor near wall micro turbulences have been observed. Eventually, a compact mathematical transient flow model, outlined in the Laplace domain for the electrical equivalent analogue circuit and applicable to the convective diffusion equation, has been developed for the flow transients.     

流体运动估计光流算法研究综述

对流体图像序列进行运动分析一直是流体力学、医学和计算机视觉等领域的重要研究课题。从图像对中提取的密集精确的速度矢量场能够为许多领域提供有价值的信息,基于光流法的流体运动估计技术因其独特的优势成为一个有前途的方向。光流法可以获得具有较高分辨率的密集速度矢量场,在小尺度精细结构的测量上有所改进,弥补了基于相关分析法的粒子图像测速技术的不足。此外,光流方法还可以方便的引入各种物理约束,获得较为符合流体运动特性的运动估计结果。为了全面反映基于光流法的流体运动估计算法的研究进展,本文在广泛调研相关文献的基础上,对国内外具有代表性的论文进行了系统阐述。首先介绍了光流法的基本原理,然后将现有算法按照要解决的突出问题进行分类：结合流体力学知识的能量最小化函数,提高对光照变化的鲁棒性,大位移估计和消除异常值。对每类方法,从问题解决过程的角度予以介绍,分析了各类突出问题中现有算法的特点和局限性。最后,总结分析了流体运动估计技术当前面临的问题和挑战,并对未来基于光流法的运动估计算法的研究方向和研究重点进行了展望。     

MathCAD2001在复杂流体流动计算中的应用

在流体流动计算中经常会遇到求解非线性方程或非线性方程组等复杂的数学问题,使用传统的手工计算或计算机编程计算都是不方便的。本文开发了使用MathCAD2001解决复杂的流体流动计算问题的方法。首先对流体流动过程进行分析, 得出需要求解的数学模型。然后在MathCAD2001中选择适当的求解函数进行求解。该方法过程简便、概念清楚、使用方便, 便于教师课堂教学使用。     

多相流搅拌器流场数值模拟软件

为对搅拌器多相流体间的最终乳化结果进行模拟,结合FLUENT与ANSYS软件,以VB为前台程序开发搅拌器流场数值模拟软件.用VB开发用户交互界面,通过后台调用ANSYS的APDL完成前处理模块,实现搅拌器叶片、筒体的建模,网格划分和组元创建;通过后台调用FLUENT运行日志文件实现流体流场仿真运算与结果后处理,并以图形...     

催化裂化汽提器的流体动力学数值模拟

提出了一种新型填料式汽提器,并采用气固双流体模型模拟了新型汽提器和传统汽提器内气固流体流动行为.模拟结果显示新型填料式汽提器在改善颗粒流动,提高颗粒分布均匀性和汽提器空间利用率方面要优于传统盘环挡板汽提器.     

Impact of inlet design on mass transfer in gas–liquid rectangular microchannels

The length of the gas bubbles as well as of the liquid slugs in Taylor flow in rectangular microchannels was studied. At constant flow ratios of the gas and the liquid phase, we were able to vary the unit cell length, and therefore the gas bubble length as well as the liquid slug length by factor 4 solely by changing the inlet geometry. Based on literature and experimental data, we state a new correlation to predict the gas bubble length, the liquid slug length as well as the pressure drop for rectangular microchannels. The observed decrease in slug length for the investigated case corresponds to an increase in the Sherwood number and therefore of the mass transfer by a factor of 1.6. Short unit cell lengths, meaning enhanced mass transfer within the liquid slug and increased interfacial area, can be achieved for given flow rates using a small gas inlet channel compared to the main channel and injection in flow direction.     

固液两相非牛顿流体中水的质量分数检测系统

分析了检测系统的工作原理,对系统的硬件原理图进行介绍。通过对水泥料浆测试数据的分析,说明此系统检测高浓度料浆时具有很高的准确度,它的设计为解决固液两相非牛顿流体料浆实现在线连续实时检测这一技术难题奠定了基础。     

Parallel implementation of the fluid particle model for simulating complex fluids in the mesoscale

Dissipative particle dynamics (DPD) and its generalization—the fluid particle model (FPM)—represent the ‘fluid particle’ approach for simulating fluid‐like behavior in the mesoscale. Unlike particles from the molecular dynamics (MD) method, the ‘fluid particle’ can be viewed as a ‘droplet’ consisting of liquid molecules. In the FPM, ‘fluid particles’ interact by both central and non‐central, short‐range forces with conservative, dissipative and Brownian character. In comparison to MD, the FPM method in three dimensions requires two to three times more memory load and a three times greater communication overhead. Computational load per step per particle is comparable to MD due to the shorter interaction range allowed between ‘fluid particles’ than between MD atoms. The classical linked‐cells technique and decomposing the computational box into strips allow for rapid modifications of the code and for implementing non‐cubic computational boxes. We show that the efficiency of the FPM code depends strongly on the number of particles simulated, the geometry of the box and the computer architecture. We give a few examples from long FPM simulations involving up to 8 million fluid particles and 32 processors. Results from FPM simulations in three dimensions of the phase separation in binary fluid and dispersion of the colloidal slab are presented. A scaling law for symmetric quench in phase separation has been properly reconstructed. We also show that the microstructure of dispersed fluid depends strongly on the contrast between the kinematic viscosities of this fluid phase and the bulk phase. This FPM code can be applied for simulating mesoscopic flow dynamics in capillary pipes or critical flow phenomena in narrow blood vessels. Copyright © 2002 John Wiley & Sons, Ltd.