微流体流动中的分子模拟技术

综述了研究微流体流动的几种分子模拟方法:分子动力学方法,直接模拟MonteCarlo方法以及IP方法。主要介绍了各种模拟方法的基本原理、模拟的基本步骤、势函数、有限差分算法以及周期性边界条件。并指出了各种模拟方法的优缺点。

流体在纳米通道中流动分子动力学研究进展

分子动力学方法及分子动力学和计算流体力学混合的模拟方法是研究纳米通道中流体流动的重要手段。简要分析了纳米通道中流体流动的各种新特性及其出现的原因;针对不同流体模型的建立、不同流动过程的实现、不同模拟方法的使用和相近模拟过程结果的比较四个方面,介绍了应用分子动力学方法或混合方法模拟流体在纳米管道中流动的研究现状;最后从作用势函数、计算简化和模拟软件角度展望了分子动力学模拟技术的发展趋势,并从模拟方法、流体模型、热边界和势函数角度总结了分子动力学方法在模拟纳米通道中流体流动方面的改进趋势。

基于微流控芯片的悬臂梁传感器性能优化

以微悬臂梁传感器为核心,设计集混合、富集和检测等功能于一体微流控芯片,可有效弥补微梁传感器在集成化、便携式检测应用方面的不足。然而,片上微尺度条件下的液相环境将影响检测结果的准确性。建立流固耦合模型,研究了流体速度、芯片结构对微梁传感器的影响,确定了微流控芯片的结构参数。结果表明,进样速度小于0.375 mL/min、80μm圆形微沟道、梯形检测池及传感器0°固定能有效降低流体流动特性的影响,并提升检测结果准确性。

单相液体微槽散热研究进展

对单相液体微槽散热技术研究的三个主要方面:流体流动和传热特性研究、影响因素研究、系统匹配与优化研究进行了回顾与综述。这三方面密切相关,反映了单相液体微槽散热技术的理论和应用水平。阐述了微槽散热技术未来的发展方向。

Computational analysis of the flow of pseudoplastic power-law fluids in a microchannel plate

The flow of pseudoplastic power-law fluids with different flow indexes at a microchannel plate was studied using computational fluid dynamic simulation.The velocity distribution along the microchannel plate and especially in the microchannel slits,flow pattern along the outlet arc and the pressure drop through the whole of microchannel plate were investigated at different power-law flow indexes.The results showed that the velocity profile in the microchannel slits for low flow index fluids was similar to the plug flow and had uniform pattern.Also the power-law fluids with lower flow indexes had lower stagnation zones near the outlet of the microchannel plate.The pressure drop through the microchannel plate showed huge differences between the fluids.The most interesting result was that the pressure drops for power-law fluids were very smaller than that of Newtonian fluids.In addition,the heat transfer of the fluids through the microchannel with different channel numbers in a wide range of Reynolds number was investigated.For power-law fluid with flow index(n=0.4),the Nusselt number increases continuously as the number of channels increases.The results highlight the potential use of using pseudoplastic fluids in the microheat exchangers which can lower the pressure drop and increase the heat transfer efficiency.

Analytical Study of Magnetohydrodynamic Propulsion Stability

In this paper an analytical solution for the stability of the fully developed flow drive in a magneto-hydro-dynamic pump with pulsating transverse Eletro-magnetic fields is presented. To do this, a theoretical model of the flow is developed and the analytical results are obtained for both the cylindrical and Cartesian configurations that are proper to use in the propulsion of marine vessels. The governing parabolic momentum PDEs are transformed into an ordinary differential equation using approximate velocity distribution. The numerical results are obtained and asymptotic analyses are built to discover the mathematical behavior of the solutions. The maximum velocity in a magneto-hydro-dynamic pump versus time for various values of the Stuart number, electro-magnetic interaction number, Reynolds number, aspect ratio, as well as the magnetic and electrical angular frequency and the shift of the phase angle is presented. Results show that for a high Stuart number there is a frequency limit for stability of the fluid flow in a certain direction of the flow. This stability frequency is dependent on the geometric parameters of a channel.

Influence of void space on microscopic behavior of fluid flow in rock joints

Advanced microfluidic technology was used to examine the microscopic viscous and inertial effects evolution of water flow in rock joints. The influence of void space on fluid flow behaviour in rock joints under different flow velocities was experimentally investigated at the micro scale. Using advanced fabrication technology of microfluidic device, micro flow channels of semicircular, triangular, rectangular and pentagonal cavities were fabricated to simulate different void space of rock joints, respectively. Using the fluorescence labelling approach, the trajectory of water flow was captured by the microscope digital camera when it passed over the cavity under different flow velocities. The flow tests show that the flow trajectory deviated towards the inside of the cavity at low flow velocities. With the increase in flow velocity, this degree of flow trajectory deviation decreased until there was no trajectory deviation for flow in the straight parallel channel. The flow trajectory deviation initially reduced from the void corner near the entrance. At the same time, a small eddy appeared near the void corner of the entrance. The size and intensity of the eddy increased with the flow velocity until it occupied the whole cavity domain. The gradual reduction of flow trajectory near the straight parallel channel and the growth of eddy inside the cavity reflect the evolution of microscopic viscous and inertial forces under different flow velocities.The eddy formed inside the cavity does not contribute to the total flow flux, but the running of the eddy consumes flow energy. This amount of pressure loss due to voids could contribute to the nonlinear deviation of fracture fluid flow from Darcy's law. This study contributes to the fundamental understanding of non-Darcy's flow occurrence in rock joints at the micro scale.

Fluid Flow Characteristics in Micro-Pump with the Aid of Peltier Devices and Thermal Expansion Material

Experimental study is performed to design and develop a cylindrical micro-pump driven by expansion and contraction of the heat deformation material, whose variation is caused with the aid of heating and cooling properties of Peltier devices. The pump consists of the diffuser valve unit, the heat deformation material unit, the nozzle valve unit, the Peltier devices and the cover. The input current of the Peltier devices is controlled by the bipolar power supply so that the Peltier devices are heated and cooled periodically. The working fluid flow in the micro-pump is caused by the periodical thermal deformation of material which is caused by the periodical heating and cooling of the Peltier devices. In order to measure the fluid flow in the micro-pump, micro air bubbles are employed as a tracer. The corresponding movement is recorded by X-ray apparatus and its velocity is measured by PIV （particle image velocimetry）. It is found that, the micro-pump developed here can make the working fluid flow. The corresponding fluid flow in the micro pump is confirmed by the numerical method.

Experimental investigation on control and reduction of vibration induced by fluid flow in turbine governing valve

Governing valve is the necessary passage through which steam enters into the steam turbine. The instability of the gas flow inside valve is the main reason that can induce the valve vibration, especially the valve rod vibration. In order to reduce the vibration and improve the performance of the governing valve such as the security and economy of the steam turbine, we try to find the method by experimental investigation. As to commonly used governing valve such as ball governing valve in this paper, a number of micro pressure sensors that have high frequencies and nice dynamic capability are employed successfully. The micro sensors are inserted directly in key positions of the valve, such as positions of valve seat throat, valve disc top and so on. The collection and measurement of many different working conditions are carried out and the conclusion of the valve instability is obtained. Therefore, vibration induced by fluid flow is controlled and reduced by means of regulating operation conditions and valve structure. Meanwhile, by numerical simulation of ball governing valve, valve disc adhered flow and asymmetric collision force are considered as main factor to cause oscillation under the condition of small lift as well as small and middle pressure ratio.

Effects of gas compressibility and surface roughness on the flow in microfluidic devices

With the development of the micro-electro-mechanical system (MEMS),the flow characteristics in micro-channels have drawn increasing attention.In this paper,numerical simulations are conducted to investigate the flow characteristics of compressible flow through micro-channels and micronozzles.An improved surface roughness viscosity model is used to simulate the effect of surface roughness on micro-channels flow characteristics.Using this model,better agreements between the computational results and the experimental data are found.The result indicates that the surface roughness is one of the important factors affecting the flow characteristics of gas through micro-channels.The numerical investigation on the expansion channel shows that by using the laminar model that considers surface roughness,the computational results and experimental data are consistent when Re<450,whereas deviation increases when Re>450.Based on the synthetic model with considerations of turbulence viscosity and surface roughness,the numerical results and the experimental data are identical.

A miniature pump with a fluid dynamic bearing

This paper describes the development of a miniature pump having an impeller with an exit diameter of 24 mm supported with the motor rotor by a fluid dynamic beating. Tests verify that the miniature pump is stable and quiet for rotational speeds larger than 4000 rain-1. The three-dimensional turbulent flow in the entire pump flow passage and the laminar flow in the fluid dynamic bearing were then simulated numerically. The average pump performance was well predicted by the simulations. Both the tests and the simulations show that there is no obvious Reynolds effect for the miniature pump within the tested range of rotational speeds. The numerical results also show that the beating capacity of the fluid dynamic bearing increases with the pump rotor rotational speed and the eccentricity ratio of the journal to the bushing. This pump is very compact, so it is a prom- ising device for surgical use.

Numerical simulation of immiscible liquid-liquid flow in microchannels using lattice Boltzmann method

Immiscible kerosene-water two-phase flows in microchannels connected by a T-junction were numerically studied by a Lattice Boltzmann (LB) method based on field mediators.The two-phase flow lattice Boltzmann model was first validated and improved by several test cases of a still droplet.The five distinct flow regimes of the kerosene-water system,previously identified in the experiments from Zhao et al.,were reproduced.The quantitative and qualitative agreement between the simulations and the experimental data show the effectiveness of the numerical method.The roles of the interfacial tension and contact angle on the flow patterns and shapes of droplets were discussed and highlighted according to the numerical results based on the improved two-phase LB model.This work demonstrated that the developed LBM simulator is a viable tool to study immiscible two-phase flows in microchannels,and such a tool could provide tangible guidance for the design of various microfluidic devices that involve immiscible multi-phase flows.

FLOW OF MICROPOLAR FLUID THROUGH CATHETERIZED ARTERY -- A MATHEMATICAL MODEL

In this paper, the flow of blood through catheterized artery with mild constriction at the outer wall is considered. The closed form solutions are obtained for velocity and microrotation components. The impedance （resistance to the flow） and wall shear stress are calculated. The effects of catheterization, coupling number, micropolar parameter, and height of the stenosis on impedance and wall shear stresses are discussed.

The effects of slip condition and fluid flow through a channel multiple stenoses on micropolar of non-uniform cross-section

In this paper, steady incompressible micropolar fluid flow through a non-uniform channel with multiple stenoses is considered. Assuming the stenoses to be mild and using the slip boundary condition, the equations governing the flow of the proposed model are solved, and closed-form expressions for the flow characteristics （resistance to flow and wall shear stress） are derived. The effects of different parameters on these flow characteristics are analyzed. It is observed that both the resistance to the flow and the wall shear stress increase with the heights of the stenoses and the slip parameter; but decrease with the Darcy number, b^rthermore, the effects of the wall exponent parameter, the cross-viscosity coefficient and the micropolar parameter on the flow characteristics are discussed.

Mathematical modeling of micropolar fluid flow through an overlapping arterial stenosis

In this study a mathematical model for two-dimensional pulsatile blood flow through overlapping constricted tapered vessels is presented. In order to establish resemblance to the in vivo conditions, an improved shape of the time-variant overlapping stenosis in the elastic tapered artery subject to pulsatile pressure gradient is considered. Because it contains a suspension of all erythrocytes, the flowing blood is represented by micropolar fluid. By applying a suitable coordinate transformation, tapered cosine-shaped artery turned into non-tapered rectangular and a rigid artery. The governing nonlinear partial differential equations under the imposed realistic boundary conditions are solved using the finite difference method. The effects of vessel tapering on flow characteristics consid- ering their dependencies with time are investigated. The results show that by increasing the taper angle the axial velocity and volumetric flow rate increase and the microrota- tional velocity and resistive impedance reduce. It has been shown that the results are in agreement with similar data from the literature.