考虑壁面滑移的磁流变胶泥缓冲器设计理论与落锤冲击试验

为提高汽车碰撞磁流变缓冲器力学模型的准确性,实现冲击作用下磁流变缓冲器动态特性的高精度预测,基于Herschel-Bulkley模型,同时考虑表观滑移和壁面滑移,建立了缓冲器理论力学模型。通过分析表观滑移和壁面滑移对缓冲器阻尼通道内部压力梯度的影响,结果表明,载液黏度较低时,受表观滑移影响,阻尼通道内部压力梯度有所降低,且在低速下影响更加显著;随着载液黏度的增加,在表观滑移作用下压力梯度有所增加,但对总体影响不大;壁面滑移使通道内部压力梯度明显降低,且随着滑移系数的增加,压力梯度变化更为显著;不同电流、冲击速度下的缓冲器落锤冲击试验表明,理论模型能够较好地预测、表征磁流变缓冲器的力学特性;磁流变胶泥在通道内流动主要受壁面滑移的影响,未出现明显的表观滑移。     

平行平板微通道流场在双电层和边界滑移条件下的解析解

讨论了双电层和边界滑移对微通道液体流动系统的影响.利用双电层动电效应理论对控制方程进行修正,同时引入Navier滑移边界条件考虑边界滑移现象,建立了两种壁面效应同时存在时微流动控制模型,得到了流场的解析解.结果显示,双电层效应抑制流动的发展,使流体在固/液边界附近区域产生回流;边界滑移虽能促进流动发展,但当这两种效应同时存在时,边界滑移的作用变为加强抑制流动,使回流现象更加显著.     

Microfluidics and rheology of carbon black suspensions for In-Mold Coating applications: some insights into the slip flow phenomena

Microfluidics and rheology are critical to modeling material processes such as In-Mold Coating (IMC). In the IMC process, a carbon black suspension is injected onto the surface of the molded part while the part is still in the mold. Due to the microscopic length scale of the IMC flow (10–25 μm), a study of the slip flow and rheological properties at high shear rates of the coating liquid becomes significant. A customized microslit rheometer was developed and used to measure the viscosity of a coating material being considered for IMC in the channel gaps between 25 and 100 μm (C. Aramphongphun Ph.D. Dissertation, The Ohio State University, 2006). The results were interpreted assuming true slip at the wall. In this paper, we present further insights into the slip flow phenomena. It is assumed that there is a fluid layer of lower viscosity near the wall, which accounts for the appearance of slip. The previously obtained data is interpreted using this assumption.     

Transient Free Convection Fluid Flow in a Vertical Microchannel as Described by the Hyperbolic Heat Conduction Model

The transient hydrodynamics and thermal behaviors of fluid flow in an open-ended vertical parallel-plate microchannel are investigated analytically under the effect of the hyperbolic heat conduction model. The model that combines both the continuum approach and the possibility of slip at the boundary is adopted in this study. The effects of Knudsen number Kn and thermal relaxation time τ on the microchannel hydrodynamics and thermal behaviors are investigated using the hyperbolic and the parabolic heat conduction models. It is found that as Kn increases, the slip in the hydrodynamic and thermal boundary condition increases. Also, this slip increases as τ decreases.     

Asymptotic analysis of the viscous micro/nano pump at low Reynolds number

The steady viscous parabolic flow past an eccentrically placed rotating cylinder is studied in the asymptotic limit of small Reynolds number. It is assumed that the flow around the rotating cylinder undergoes boundary slip described by the Navier boundary condition. This involves a single parameter to account for the slip, referred to as the slip length ?, and replaces the standard no-slip boundary condition at solid boundaries. The streamlines for ? > 0 are closer to the body than for ? = 0, and it is discovered that the loss of symmetry due to the rotation of the cylinder is significantly reduced by the inclusion of slip. This arises as a result of a balance between the rotation velocity and the slip velocity on that portion of the cylinder which rotates opposite to the free-stream flow. Streamline patterns for nonzero eccentricity partially agree with Navier–Stokes simulations of the viscous pump; the small discrepancy is primarily due to the fact that here wall effects are not explicitly considered. Expressions for the frictional drag and the torque on the cylinder are obtained. The expression for the torque agrees well with the lubrication solution for the flow past a rotating cylinder placed symmetrically in a fully developed channel flow. The results presented here may be used to validate numerical schemes developed to study the viscous pump.     

The effect of the slip boundary condition on the flow of fluids in a channel

Summary The assumption that a liquid adheres to a solid boundary (no-slip boundary condition) is one of the central tenets of the Navier-Stokes theory. However, there are situations wherein this assumption does not hold. In this paper we investigate the consequences of slip at the wall on the flow of a linearly viscous fluid in a channel. Usually, the slip is assumed to depend on the shear stress at the wall. However, a number of experiments suggests that the slip velocity also depends on the normal stress. Thus, we investigate the flow of a linearly viscous fluid when the slip depends on both the shear stress and the normal stress. In regions where the slip velocity depends strongly on the normal stress, the flow field in a channel is not fully developed and rectilinear flow is not possible. Also, it is shown that, in general, traditional methods such as the Mooney method cannot be used for calculating the slip velocity.     

Dissipative heating under conditions of shear flow of liquid in a flat channel of finite length in view of temperature dependence of viscosity

A steady-state process of heat transfer is considered under conditions of Couette-type shear flow in a flat channel of finite length. The problem is solved in view of dissipation of mechanical energy and of temperature dependence of viscosity under symmetric boundary conditions of the third kind on the channel walls. A number of simplifying assumptions are made, and approximate solutions are obtained within two formulations of the initial problem. In the first case, the constant velocity of the moving channel wall is assigned. This problem is conventional and leads to quite predictable results. In the second case, it is assumed that it is the resultant force applied to the moving channel wall which is assigned. The wall velocity in the steady-state mode is not known in advance. It is found that, in this case, the dependence of kinematic and thermal characteristics of the process on Froude number exhibits a hysteretic pattern.     

A molecular dynamics simulation of TIP4P and Lennard-Jones water in nanochannel

Summary. The flow characteristics of the plane Poiseuille flow in the nanochannel driven by a constant external force are studied by the Lennard-Jones and TIP4P potentials. The problem is investigated by the leap-frog method in the field of molecular dynamics. In this work, the wall boundary condition is considered to be the situation that the water is absorbed on the metal wall and is then formed to be flat ice. Both global effect (effective channel width) and local effect (wall boundary types) are examined to demonstrate the features of the distributions of velocity and its gradient in the system. When the effective channel width is less than a critical value, the numerical results show that the Navier-Stokes theory would fail to predict the velocity distribution. Furthermore, the velocity profile at a virtual slip plane presents the slip condition. Finally, we can reason that the surface effect exists and will affect the shear stress in the nanochannel.     

Slip effects on the peristaltic flow of a non-Newtonian Maxwellian fluid

Summary In real systems there is always a certain amount of slip, which, however, is hard to detect experimentally because of the required space resolution. In this paper, we analyze the effect of slip boundary conditions on the dynamics of fluids in porous media by studying the flow of a Newtonian and non-Newtonian Maxwellian fluid in an axisymmetric cylindrical tube (pore), in which the flow is induced by traveling transversal waves on the tube wall. Like in peristaltic pumping, the traveling transversal waves induce a net flow of the liquid inside the pore. The viscosity as well as the compressibility of the liquid is taken into account. This problem has numerous applications in various branches of science, including stimulation of fluid flow in porous media under the effect of elastic waves and studies of blood flow dynamics in living creatures. The Navier-Stokes equations for an axisymmetric cylindrical pore are solved by means of a perturbation analysis, in which the ratio of the wave amplitude to the radius of the pore is small parameter. In the second order approximation, a net flow induced by the traveling wave is calculated for various values of the compressibility of the liquid, relaxation time and Knudsen number. The calculations disclose that the compressibility of the liquid, Knudsen number of slip flow and non-Newtonian effects in presence of peristaltic transport have a strong influence of the net flow rate. The effects of all parameters of the problem are numerically discussed and graphically explained.     

The Effect of Fluid Properties on Two-Phase Regimes of Flow in a Wide Rectangular Microchannel

We have experimentally studied a two-phase flow in a microchannel with a height of 150 μm and a width of 20 mm. Different liquids have been used, namely, a purified Milli-Q water, an 50% aqueous-ethanol solution, and FC-72. Before and after the experiment, the height of the microchannel was controlled, as well as the wettability of its walls and surface tension of liquids. Using the schlieren method, the main characteristics of two-phase flow in wide ranges of gas- and liquid-flow rates have been revealed. The flow regime-formation mechanism has been found to depend on the properties of the liquid used. The flow regime has been registered when the droplets moving along the microchannel are vertical liquid bridges. It has been shown that, when using FC-72 liquid, a film of liquid is formed on the upper channel wall in the whole range of gas- and liquid-flow rates.     

Stabilization of liquid interface and control of two-phase confluence and separation in glass microchips by utilizing octadecylsilane modification of microchannels

We demonstrated a liquid/liquid and a gas/liquid two-phase crossing flow in glass microchips. A 250-microm-wide microchannel for aqueous-phase flow was fabricated on a top glass plate. Then, as a way to utilize the surface energy difference for stable phase confluence and separation, a 250-microm-wide microchannel for organic-phase (or gas-phase) flow was fabricated on a bottom glass plate and the wall was chemically modified by octadecylsilane (ODS) group. The top and bottom plates were sealed only by pressure. A microchannel pattern was designed so that the two phases made contact at the crossing point of the straight microchannels. The crossing point was observed with an optical microscope. Results showed that the ODS modification of the microchannel wall clearly improved stability of the interface between the two fluids. Pressure difference between fluids was measured and the interface of water and nitrobenzene was stable for the pressure difference from +300 Pa to -200 Pa. The pressure drop in a countercurrent flow configuration was also estimated, and the pressure difference required to realize the counter current flow was less than the allowable pressure range. Finally, we discussed the advantages of utilizing this approach.     

Electroviscous effects in purely pressure driven flow and stationary plane analysis in electroosmotic flow of power-law fluids in a slit microchannel

This paper is a comprehensive work on flow of power-law fluids in a slit microchannel. The first part of the paper deals with study of electrokinetic effects in steady, fully developed, laminar pressure driven flow of power-law fluids. The second part of the work provides analysis of stationary plane that is formed during electroosmotic flow (EOF) of power-law fluids inside a closed slit microchannel. In the entire analysis, the flow of power-law fluid is characterized by the modified Navier-Stokes equation incorporating the electric body force term. The electric double layer (EDL) potential is described by the Poisson-Boltzmann distribution under Debye Hückel approximation. In pressure driven flow, analytical expressions for velocity profiles of various power-law fluids are obtained for n = 1, 1/2, 1/3. Numerical simulation is carried out for all values of n to find induced streaming potential without any approximations for entire range of flow behavior indices. The analytical solutions are compared with numerical results. The effects of flow behavior index, zeta potential and channel dimension on velocity distribution, streaming potential, apparent viscosity, volumetric flow rate and friction coefficient are discussed. In the analysis of EOF in closed slit microchannel, the positions of stationary planes for various flow behavior indices at different EDL thicknesses are found both analytically and numerically. It is found that the electroosmotic counter pressure developed inside the cell is strongly dependent on zeta potential and weakly dependent on EDL thickness.     

Nanofluidic bubble pump using surface tension directed gas injection

A new concept for liquid manipulation has been developed and implemented in surface-micromachined fluid channels. It is based on the surface tension directed injection of a gas into the liquid flow through micrometer-sized holes in the microchannel wall. The injected gas is directed to an exhaust by a cross-sectional asymmetry of the microchannel and thereby moves minute liquid volumes. Successful pumping experiments were performed with single stroke volumes of tens of picoliters at frequencies around 1 Hz. The minimum actuation pressure is 0.6 bar for a 2-microm channel height, in accordance with theoretical predictions.     

Heat and mass transfer characteristics of a constrained thin-film ammonia-water bubble absorber

A study of absorption of ammonia vapour bubbles into a constrained thin-film of ammonia-water solution is presented. A large-aspect-ratio microchannel constrains the thickness of the weak solution film and ammonia vapour bubbles are injected from a porous wall. A counter flowing coolant in a minichannel removes the generated heat of absorption. Experiments and a simple one-dimensional numerical model are used to characterize the absorber performance at a nominal system pressure of 6.2 bar absolute. Effect of varying the mass flow rate of the weak solution, vapour flow rate, solution inlet temperature, and coolant inlet temperature on absorption heat and mass transfer rates and exit subcooling are discussed. Two absorber channel geometries, each of 600 μm nominal depth, are considered: 1) a smooth-wall channel, and 2) a stepped-wall channel that has 2-mm deep trenches across the width of a channel wall. Results indicate that the reduction in coolant inlet temperature significantly enhances the mass transfer rates in both absorber geometries. While the stepped-wall geometry exhibits higher mass transfer rates at lower coolant inlet temperatures of 30 °C and 40 °C, the smooth-wall channel shows higher mass transfer rates at the highest coolant inlet temperature of 58 °C. Both absorption limited and residence time limited conditions are observed with variation of weak solution flow rate at fixed vapour flow rates.     

Slip flow in converging and diverging channels

The classical problem of Jeffery-Hamel flow is considered in which the fluid is allowed to slip along the walls of the channel. The problem is solved analytically and the volumetric flow rate is computed and compared with that of the corresponding no-slip flow. In the converging channel case, it is found that the slip boundary condition enhances flow rates through the channel, although the effect is minimal when the Reynolds number is large.In the case of the diverging channel, the slip boundary condition in some instances actually lowers the flow rate from its no-slip value. In other instances, a stable velocity profile does not even appear to exist. These cases aside, when the mean pressure in the channel is adverse, slip flow solutions exist and increase the flow rate through the channel by at most 15.7%.     

微通道气体滑移流动的压力分布

针对微通道中发生滑移流动的气体提出了一种滑移边界条件,用一个关于克努森数（Knudsen）的函数取代了传统滑移边界条件中的克努森数.此函数满足两个必要条件,首先当克努森数较小时,它与克努森数同阶;其次当克努森数趋于无穷时,函数值趋于1.采用该滑移边界条件推导了微通道中气体滑移流动时的压力分布解析式,并与文献中报导的实验值进行了比较.由于压力分布以通道出口处的压力作为均一化的标准,故出口处的无量纲压力值为1,进而对推导出的压力分布公式进行了修正,由修正公式计算出的结果与实验结果吻合得更好.     

Roughness effect on flow and thermal boundaries in microchannel/nanochannel flow using molecular dynamics‐continuum hybrid simulation

Our molecular dynamics‐continuum hybrid simulation method is further validated in terms of flexibility in domain decomposition. The roughness effects on the flow and thermal boundaries in liquid channel flow is studied. The results indicate that the molecules in the wall‐neighboring area can be firmly confined in the concaves due to geometric structure and strong liquid–solid interaction and cause locking boundary in the velocity profile and linear gradient in the temperature profile. The locking boundary can further lead to negative slip length, which varies in power law with channel height. The linear temperature gradient, as well as nearly constant temperature jump, can lead to obviously increasing Kapitza length versus channel height. Analysis on flow friction shows that the confinement on the molecules will equivalently narrow the channel, where larger pressure gradient is needed. Therefore, the roughness should be strictly restricted within a range shrinking correspondingly with channel height if the flow condition is supposed to be maintained unchanged. Finally, our hybrid simulation is compared with full molecular dynamics simulation in terms of computational efficiency. Great advantages of the hybrid simulation, such as exclusively flexibility and combination characteristics, demonstrate its potential values and promising applications in the field of microfluidics/nanofluidics. Copyright © 2011 John Wiley & Sons, Ltd.     

Slip effect on shear-driven evaporating liquid film in microchannel

The development of compact, advanced cooling technology leads to problems involving two-phase flows at micro-scales. We investigate the effect of slip on heated liquid film driven by its own vapor in microchannel. The macroscopic interface shape is found to be sensitive to slip length comparable with the initial film thickness. The slip at the wall tends to elongate the transition film, and can have an effect on the mass flow rate. Calculations reveal that the maximum of the slip velocity is located in the transition region. The present work is a part of the preparation of the SAFIR experiment of the European Space Agency onboard the International Space Station.     

Slip in Quantum Fluids

In this review we describe theoretical and experimental investigations of general slip phenomena in context with the flow of the quantum liquids³He,⁴He and their mixtures at low temperatures. The phenomenon of slip is related to a boundary effect. It occurs when sufficiently dilute gases flow along the wall of an experimental cell. A fluid is said to exhibit slip when the fluid velocity at the wall is not equal to the wall’s velocity. Such a situation occurs whenever the wall reflects the fluid particles in a specular-like manner, and/or if the fluid is describable in terms of a dilute ordinary gas (classical fluid) or a dilute gas of thermal excitations (quantum fluid). The slip effect in quantum fluids is discussed theoretically on the basis of generalized Landau-Boltzmann transport equations and generalized to apply to a regime of ballistic motion of the quasiparticles in the fluid. The central result is that the transport coefficient of bulk shear viscosity, which typically enters in the Poiseuille flow resistance and the transverse acoustic impedance, has to be replaced by geometry dependent effective viscosity, which depends on the details of the interaction of the fluid particles with the cell walls. The theoretical results are compared with various experimental data obtained in different geometries and for both Bose and Fermi quantum fluids. Good agreement between experiment and theory is found particularly in the case of pure normal and superfluid³He, with discrepancies probably arising because of deficiencies in characterization of the experimental surfaces.     

Sample dispersion for segmented flow in microchannels with rectangular cross section

Hydrodynamic dispersion in microchannels can be significantly reduced by segmentation with a second immiscible phase. We address the effect of microchannel cross section on the dispersion of analytes in a segmented gas-liquid flow of alternating bubbles and liquid segments. Channels of square or nearly square cross section are considered. A significant fraction of the liquid surrounds the bubbles and wets the channel walls in the form of films or menisci. This stagnant fraction of the liquid remains when gas and liquid segments flow by, and it is connected to the liquid within the liquid segments by diffusion only and it effectively increases dispersion. We design and fabricate a microchip with integrated analyte injection and detection to investigate the effects of the influence of the stagnant liquid in segmented flow through square microchannels on the analyte bandwidth. The measured data and a corresponding model confirm the experimental trends and suggest operating conditions at which the unwanted effect of dispersion in segmented microchannel flow is minimized. Dispersion is least when the liquid flow rate is greater than the gas flow rate, and the optimum ratio of the two flow rates slightly increases with increasing bubble velocity.