Thermal and flow analysis of a magneto-hydrodynamic micropump

A study of transient fully developed laminar flow and temperature distribution in a magnetohydrodynamic (MHD) micropump is presented. The micropump is driven using the Lorentz force which is induced as a result of interaction between an applied electric field and a perpendicular magnetic field. The governing equations are solved analytically by an eigenfunction expansion method, and numerically by a finite-difference (ADI) method. The numerical and analytical results are found to be in good agreement with each other. The effect of different parameters on the transient velocity and temperature, such as aspect ratio, Hartman number, Prandtl number, and Eckert number is studied. The results obtained showed that controlling the flow and the temperature can be achieved by controlling the potential difference, the magnetic flux, and by a good choice of the electrical conductivity.     

Numerical simulation of AC electrothermal micropump using a fully coupled model

The classic model by Ramos et al. for numerical simulation of alternating current electrothermal (ACET) flow is a decoupled model based on an electrothermal force derived using a linear perturbation method, which is not appropriate for the applications, where Joule heating is large and the effect of temperature rise on material properties cannot be neglected. An electrically–thermally–hydrodynamically coupled (fully coupled) ACET flow model considering variable electrical and thermophysical properties of the fluids with temperature was developed. The model solves AC electrical equations and is based on a more general electrostatic force expression. Comparisons with the classic decoupled model were conducted through the numerical simulations of an ACET micropump with asymmetric electrode pairs. It was found that when temperature rise is small the fully coupled model has the same results with the classic model, and the difference between the two models becomes larger and larger with the increasing temperature. The classic decoupled model underestimates the maximum temperature rise and pumping velocity, since it cannot consider the increase in electrical conductivity and the decrease in viscosity with temperature. The critical frequencies where the lowest velocity occurs or pumping direction reverses are shifted to higher frequencies with the increasing voltage according to the fully coupled model, while are kept unchanged according to the classic model.     

Performance improvement of an AC electroosmotic micropump by hydrophobic surface modification

This paper describes the improvement of bi-directional micropump velocity by deposition of a hydrophobic nanocomposite monolayer. A polymer base nanocomposite coating consisting of a homogeneous mixture of silicon nanoparticles in polydimethylsiloxane (PDMS) is used to improve the hydrophobicity of the micropump surfaces. For hydrophobic nature of PDMS and the monolayer coating with nanoscale surface roughness, the hydrophilic surface of a biased AC electroosmotic micropump will transform to a hydrophobic surface. In our previous research the applied AC voltage, frequency, channel dimension, and electrode width were optimized (Islam and Reyna, Electrophoresis 33(7), 2012). Based on the prior results obtained for the biased AC electroosmotic micropump, the pumping velocity was 300 micron/s in 100-μm channel thickness for applied voltage of 4.4 V at 1 kHz frequency. Here in this work, improvement of the micropump velocity is investigated through a surface modification process. The highest velocity of 450 micron/s is observed by modifying the surface characteristics. This paper will also discuss the synthesis process and characteristics of the polymer base nanocomposite monolayer. In addition to hydrophobicity improvement, adding a thin nanocomposite monolayer will physically separate the electrodes from the pumping liquid, thus eliminating their reaction, which is usually observed due to the application of voltage. As a result, higher voltages can be applied to the electrodes and higher pumping rates are achievable.     

Numerical investigation of fully developed channel flow using shock-capturing schemes

The ability to simulate wall-bounded channel flows with second- and third-order shock-capturing schemes is tested on both subsonic and supersonic flow regimes, respectively at Mach 0.5 and 1.5. Direct numerical simulations (DNSs) and large-eddy simulations (LESs) are performed at Reynolds number 3000.In both flow regimes, results are compared with well-documented DNS, LES or experimental data.At Ma₀=0.5, a simple second-order centred scheme provides results in excellent agreement with incompressible DNS databases, while the addition of artificial or subgrid-scale (SGS) dissipation decreases the resolution accuracy giving just satisfactory results. At Ma₀=1.5, the second-order space accuracy is just sufficient to well resolve small turbulence scales on the chosen grid: without any dissipation models, such accuracy provides results in good agreement with reference data, while the addition of dissipation models considerably reduces the turbulence level and the flow appears almost laminar. Moreover, the use of explicit dissipative SGS models reduces the results accuracy.In both flow regimes, the numerical dissipation due to the discretization of the convective terms is also interpreted in terms of SGS dissipation in an LES context, yielding a generalised dynamic coefficient, equivalent to the dynamic coefficient of the Germano et al. [Phys. Fluids A 3(7) (1991) 1760] SGS model. This new generalised coefficient is thus developed to compare the order of magnitude of the intrinsic numerical dissipation of a shock-capturing scheme with respect to the SGS dissipation.     

Unsteady flow behaviors in an obstacle-type valveless micropump by micro-PIV

In this paper, a PZT micropump excited by amplified squarewave signals with various frequencies was used to study the transient flow behaviors in an obstacle-type valveless micropump. A micro-particle-image-velocimetry (micro-PIV) with an external trigger was developed to obtain flow fields at the outlet and around the obstacle with various phases in a cycle. In comparison with previous studies on the pump performance, such as pump pressure and volume flow rate, more detailed information about the pump was obtained. The velocity profiles and periodic sectional mean velocities exhibited the unsteady flow nature. The total net flow generation efficiency per cycle was obtained experimentally by integrating the phase-dependent velocities. The flow recirculation around the obstacle was observed and quantified to investigate the influence on the pump performance. The duration, circulation, and the size of the recirculation regions indicated that this flow behavior could enhance the flow-directing capability. These results are very useful for the design and improvement of obstacle-type valveless micropumps.     

Theoretical and numerical investigations of an electroosmotic flow micropump with interdigitated electrodes

In this work, an electroosmotic flow micropump is proposed and investigated using theoretical analysis and numerical simulations. The micropump comprises an array of interdigitated electrodes on the top and the bottom surfaces of a rectangular microchannel. Theoretical analysis and extensive numerical simulations are performed to predict the pressure-flow characteristics of the micropump. The results of the model and simulations are compared which show good agreement with each other. The effects of various geometrical parameters including spacing between a pair of electrodes, gap between adjacent pairs of electrodes, width and height of the electrodes, and width of the microchannel and operating parameter including applied voltage on the performance of the micropump in terms of flow and pressure capacity is investigated.     

Lattice BGK direct numerical simulation of fully developed turbulence in incompressible plane channel flow

Over the last decade, lattice Boltzmann methods have proven to be reliable and efficient tools for the numerical simulation of complex flows. The specifics of such methods as turbulence solvers, however, are not yet completely documented. This paper provides results of direct numerical simulations (DNS), by a lattice Boltzmann scheme, of fully developed, incompressible, pressure-driven turbulence between two parallel plates. These are validated against results from simulations using a standard Chebyshev pseudo-spectral method. Detailed comparisons, in terms of classical one-point turbulence statistics at moderate Reynolds number, with both numerical and experimental data show remarkable agreement.

Consequently, the choice of numerical method has, in sufficiently resolved DNS computations, no dominant effect at least on simple statistical quantities such as mean flow and Reynolds stresses. Since only the method-independent statistics can be credible, the choice of numerical method for DNS should be determined mainly through considerations of computational efficiency. The expected practical advantages of the lattice Boltzmann method, for instance against pseudo-spectral methods, are found to be significant even for the simple geometry and the moderate Reynolds number considered here. This permits the conclusion that the lattice Boltzmann approach is a promising DNS tool for incompressible turbulence.     

Numerical analysis of non Newtonian fluid flow in a low voltage cascade electroosmotic micropump

In microfluidic devices, many fluids have non-Newtonian behaviors, especially biofluids. The viscosity of these fluids mostly depends on the shear rate. Sometimes the non-Newtonian fluids should be transferred by micropumps in lab-on-chip devices. Previous researchers investigated the flow rate in simple electroosmotic flow micropumps which have a simple channel geometry. In the present study, the effects of non-Newtonian properties of fluid in a low voltage cascade electroosmotic micropump are numerically investigated using the power law model. The micropump is modeled in two dimensional with one symmetric step and has a more complex geometry than previous studies. The numerical results show that, the non-Newtonian behavior of fluid affects flow rate in the micropump. The flow rate decreases if the fluid is dilatant. Also, it increases if the fluid is pseudoplastic. Moreover, the pressure which is needed to stop the electroosmotic flow rate in the micropump is calculated. Results show that, the back pressure has a slight change as the fluid has non-Newtonian behavior.     

Modeling steady axis-symmetric thermal plasma flow of air by a parallelized magneto-hydrodynamic flow solver

This paper discusses a parallelized magneto-hydrodynamic flow solver for modeling axis-symmetric thermal plasma flow using Cartesian grid system and taking the induced electrical and magnetic effects into account, where the magneto-hydrodynamic equations, including the continuity equation, momentum equations, energy equation, current continuity equation and turbulence transport equations are solved by a finite volume discretization in a segregated manner. The thermal plasma flow of a 476 mm long, transferred well-type plasma torch operating with air is simulated for two power conditions, i.e. I = 432 A and 901 A, to demonstrate the capability of proposed numerical model to analyze the heat and mass transfer characteristics of axis-symmetric thermal plasma flow, where the location of cathode is determined by fixing the measured voltage drop between two electrodes. The numerical calculation suggests that the high-power case can deliver an axial velocity of 400 m/s and 15,000 K in temperature at the center of torch outlet, where a strong jetting vortex is expected emitting from the torch body. The low-power case is predicted with a longer electric arc than that of the high-power one, which clearly results in a large high-temperature region between the gas inlet and cathode and unfavourable to reduce the cathode erosion and to increase thermal efficiency.     

Flow field analysis in a spiral viscous micropump

The paper presents a stream function solution and a computational analysis for the flow field of a viscous spiral pump, which employs a rotating spiral channel to achieve pumping action. This pump is fabricated using surface micromachining technology. The stream function solution employs a simplified 2D model for the flow field in its spiral channel that neglects the curvature of the spiral, and replaces it with an equivalent straight channel. The effect of spiral wall height on flow rate is analyzed and discussed. 3D computational analyses are obtained and are compared with analytical predictions.     

Forced convection in electro-osmotic/Poiseuille micro-channel flows of viscoelastic fluids: fully developed flow with imposed wall heat flux

The analytical solution for heat transfer in a dynamic and thermally fully developed channel flow of the simplified Phan-Thien–Tanner fluid induced by combined electro-osmosis and pressure gradient was obtained assuming that material properties are independent of temperature. The flow forcing was quantified by an appropriate dimensionless parameter and its effect and that of all other relevant dimensionless numbers is presented and discussed. Specifically, the forced convection occurs under conditions of constant wall heat flux and the solution includes the effects of Weissenberg number, electric double layer (EDL) thickness, forcing ratio parameter, viscous dissipation as well as of Joule heating due to the electric currents and was obtained under the simplifying Debye–Hückel approximation. Generally speaking, the Joule effect is stronger than the viscous dissipation except in very narrow channels, but these fall outside the validity of the Debye–Hückel conditions. For pure electro-osmosis, viscous dissipation is restricted to the near-wall region and virtually nonexistent elsewhere, so it is irrelevant for thin electric double layers and Joule heating is more relevant. As the EDL thickens and/or the pressure gradient contribution increases, the role of viscous dissipation grows and shear-thinning effects also appear more clearly on the Nusselt number. Generally speaking, an increase in internal heating results in lower Nusselt numbers and this effect is stronger than the effect of shear-thinning, which is responsible for a slight increase in the Nusselt number.     

基于BML模型的二维交通流系统的模拟分析

BML模型是专门用于模拟分析交通现象的元胞自动机模型,利用此模型通过计算机模拟二维城市交通流系统,找出车辆的平均速度与平均密度等参数的关系,通过编程模拟的交通流的时空图可以得出当车辆密度保持基本不变时,交通流由完全阻塞相恢复到运动相存在着自组织性,并且指出相变点的位置与平均密度有着密切的关系,同时分析了临界密度在交通控制中的实际意义。     

Thermal Simulation of AC Electromagnetic Contactor

Transient magnetic circuit method is adopted to calculate the power loss in winding and shading coil. Based on the analysis of heat transfer process in AC contactor, a thermal model is proposed and the temperature field distribution is simulated with 3-D FEM of ANSYS. Comparison of simulation results with measurements shows that the proposed method is effective.     

Slow-to-start effect in two-dimensional traffic flow

This paper studies slow-to-start effect in two-dimensional Biham–Middleton–Levine (BML) traffic flow model with traffic light periods $T=2\tau$. In most cases, the model exhibits free flow, jam, and phase separation phenomenon. Nevertheless, when the slow-to-start parameter $p=0$, and traffic light parameter $\tau =3$ or 5, it is found that phase separation phenomenon does not occur. We have explained this via the evolution process from a designed regular initial configuration. Moreover, it is also found that the free flow self-organizes into grid-like structure when τ is large and the slow-to-start parameter $0<p<1$.     

综采工作面供液系统负载流阻理论分析及试验

通过理论分析推导了环形供液模式下综采工作面供液系统负载流阻损失计算公式,并以黄陵煤矿1号工作面供液系统为例,得出了工作面不同液压支架处负载流阻损失分布;在工作面每隔20架支架安装1个压力传感器来测试供液系统管路压力,得到的压差与理论计算结果接近,验证了负载流阻损失计算公式的正确性;分别在采煤机速度为8,12m/min,单泵供液及双泵供液模式下,通过试验得出了供液系统泵站出口压力分布,并统计了系统卸载时间比与加载时间比,为后续跟机自动化过程中供液系统智能控制和优化提供了参考。     

Synthesis of two-dimensional bodies in potential flow

This paper describes the incorporation of simple potential flow theory with limited interactive graphics to produce a computer program for the potential flow analysis of a wide variety of two-dimensional bodies. The general program approach is explained and three particularly effective program techniques developed specifically for use in this program are described. These techniques allow the location of stagnation points, the closure of a streamline, and the tracing of streamline patterns for complex flow. Several possible applications of the program are cited and some examples of these applications are shown.     

AC electroosmotic flow in a DNA concentrator

Experimental velocity measurements are conducted in an AC electrokinetic DNA concentrator. The DNA concentrator is based upon Wong et al. (Transducers 2003, Boston, pp 20–23, 2003a; Anal Chem 76(23):6908–6914, 2004)and consists of two concentric electrodes that generate AC electroosmotic flow to stir the fluid, and dielectrophoretic and electrophoretic force fields that trap DNA near the centre of the inside electrode. A two-colour micro-PIV technique is used to measure the fluid velocity without a priori knowledge of the electric field in the device or the electrical properties of the particles. The device is also simulated computationally. The results indicate that the numerical simulations agree with experimental data in predicting the velocity field structure, except that the velocity scale is an order of magnitude higher for the simulations. Simulation of the dielectrophoretic forces allows the motion of the DNA within the device to be studied. It is suggested that the simulations can be used to study the phenomena occurring in the device, but that experimental data is required to determine the practical conditions under which these phenomena occur.     

方向选择性流速传感器研制

该文提出具有方向选择性并通过热平衡检测流速的方法,隔热套管与流体接触面之间平行安装,并保持有一特殊距离,实现方向选择性,通过检测流体接触面的温度来测量流体流速。用这种方法研制了一种单方向流速测量传感器,扩展用于构成三维风速传感器。该传感器可广泛应用于风力发电、气象预报、空间飞行器、工业锅炉等领域的气体和液体速度场、流量场测量。