Slip-enhanced reverse electrodialytic power generation in ion-selective nanochannels

Power generation by reverse electrodialysis in ion-selective nanochannels is numerically investigated. Especially,in the present study, the influence of hydrodynamic slip at the surface of nanochannels is investigated. The current-potential characteristics of the nanochannels are calculated by solving several governing equations:Nernst-Planck equation for the ionic concentrations, the Poisson equation for the electric potential, and the Navier-Stokes equation for the diffusioosmotic flow. Hydrodynamic slip is applied as the boundary condition at the surface of nanochannels. As the slip length increases, the diffusioosmotic flow velocity and electrical conductance of ions increase because the friction at the surface of nanochannels decreases. It is shown that the power generation is enhanced by 44% with a moderate 100nm slip length by using a nanochannel with 10nm height.     

Three-Dimensional Molecular Dynamic Study on Accommodation Coefficients in Rough Nanochannels

The tangential momentum and energy accommodation coefficients (TMAC and EAC) are parameters to characterize the velocity slip and temperature jump in the gas–solid interface. To understand the wall effects on fluid flow and heat transfer in nanochannels, the accommodation coefficients for argon gas molecules and platinum wall atoms were calculated according to a proper statistical algorithm using a three-dimensional molecular dynamic method. Isothermal flows and thermal conductions were simulated in smooth and rough nanochannels, in which the roughness ranged from 0.2 nm to 1.4 nm. From the atomic viewpoint, different lattice arrangements of smooth walls would induce atomic roughness to different extents on the surface, which affected the momentum and energy exchange in gas–wall interactions and resulted in different accommodation coefficients. In channels with nanoscale roughness, the possibility of multiple gas–wall interactions were further increased so that the TMAC and EAC became much larger with more roughness.     

DSMC simulation of subsonic flow through nanochannels and micro/nano backward-facing steps

In this study, we use direct simulation Monte Carlo method to simulate subsonic flow in nanochannels and micro/nanoscale backward-facing (BF) step considering a wide range of Knudsen number regimes. The nanochannel flow simulation indicates that the nanoscale flow through the nanochannel resembles unique features such as encountering negative pressure deviation behavior and observing flat velocity profiles at higher Knudsen number regimes. On the other hand, the micro/nano BF step flow simulations demonstrate that the length of separation region considerably decreases as the flow becomes more rarefied and approaches the transition regime. Meanwhile, the variations in the flow properties are much slower in the mid-transition and free-molecular regimes compared with the slip and early transition regime cases.     

Mass transfer at smooth and rough surfaces in a circular couette flow

An extended set of mass transfer measurements at smooth and rough surfaces in a circular Couette flow has been carried out in the range 20000 < Re < 900000 and 770 < Sc < 8000. Mass transfer measurements for smooth rotating cylinders are in agreement with previous results and confirm that the Sherwood number depends on the power of the Schmidt number. Mass transfer coefficients at rough cylinders show a similar behaviour with Reynolds number as that observed for duct flows and the experimental results for both systems can be generalized in the fully rough region for equally rough surfaces when the maximum velocity of the flow and the surface roughness height are taken as the characteristic velocity and length scale in the Stanton and Reynolds numbers.     

Effects of wall roughness and velocity slip on streaming potential of microchannels

Microchannels made of polymeric materials usually have wall velocity slip. Sometimes, these microchannels are not smooth. Wall roughness is introduced by manufacturing processes or caused by adhesion of biological particles from the liquids. For a proper design and operation of these microchannels, it is important to secure an accurate value of zeta potential, which determines the volumetric flow rate of electroosmotic flows. Zeta potential of microchannels is usually determined by measuring the streaming potential. In the present investigation, we have derived a simple formula that predicts accurately the streaming potential of microchannels having wall roughness and Navier velocity slip, and investigated how the streaming potential is affected by wall roughness, slip coefficient, zeta potential and bulk ionic concentration. The simple formula, which requires numerical solution of three sets of Stokes equation sequentially using a small grid number, yields very accurate results as compared with the numerical solution of nonlinear partial differential equations employing a very large number of grids. It is found that the streaming potential per pressure drop increases with respect to slip coefficient and decreases with respect to wall roughness hight. On the other hand, the streaming potential per volumetric flow rate increases as the wall roughness increases.     

Fully developed hydrodynamic and thermal transport in combined pressure and electrokinetically driven flow in a microchannel with asymmetric boundary conditions

Thermally and hydrodynamically fully developed combined pressure-driven and electroosmotic flow through a channel has been simulated for isoflux wall boundary conditions. Effects of asymmetries in wall zeta potential and heat flux have been considered and closed form expressions have been obtained for transverse distribution of electric potential, velocity and temperature. The results indicate that both flow and heat transfer characteristics are significantly affected by the asymmetries in wall boundary conditions for both purely electroosmotic and combined pressure-driven and electroosmotic flow. These findings have important implications for flow and heat transfer control in microfluidics through alteration of surface conditions.     

An analytical solution for thermally fully developed combined pressure – electroosmotically driven flow in microchannels

An analytical solution is presented to study the heat transfer characteristics of the combined pressure – electroosmotically driven flow in planar microchannels. The physical model includes the Joule heating effect to predict the convective heat transfer coefficient in two dimensional microchannels. The velocity field, which is a function of external electrical field, electroosmotic mobility, fluid viscosity and the pressure gradient, is obtained by solving the hydrodynamically fully-developed laminar Navier–Stokes equations considering the electrokinetic body force for low wall zeta potentials. Then, assuming a thermally fully-developed flow, the temperature distribution and the Nusselt number is obtained for a constant wall heat flux boundary condition. The fully-developed temperature profile and the Nusselt number depend on velocity field, channel height, solid/liquid interface properties and the imposed wall heat flux. A parametric study is presented to evaluate the significance of various parameters and in each case, the maximum heat transfer rate is obtained.     

Effects of interface wettability on microscale flow by molecular dynamics simulation

Non-equilibrium molecular dynamic simulations have been carried out to study the effect of the interface wettability on the pressure driven flow of a Lennard-Jones (LJ) fluid in a nanochannel. The results show that the hydrodynamic boundary condition at the solid-liquid interface depends on both the interface wettability and the magnitude of the driving force. For a LJ fluid in a nanochannel with hydrophilic surfaces, the velocity profiles have the traditional parabolic shape. The no-slip boundary condition may break down when the driving force exceeds a critical value that overcomes the interfacial resistance. In such a case, the MD results show a pattern of an adsorbing layer sliding along the solid wall. For a LJ fluid in a nanochannel with hydrophobic interfaces, the results show that a gap exists between the liquid and the surface, resulting in almost frictionless resistance; the velocity shows a plug flow profile and the slip length is not constant but depends on the driving force. Furthermore, it is found that the non-uniform temperature and pressure profiles near the solid walls are owing to the effect of interface wettability.     

Numerical investigation of the influence of roughness on the laminar incompressible fluid flow through annular microchannels

The main objective of this work is to investigate, by means of numerical simulation, the effects of the surface roughness on the laminar fluid flow through annular microchannels, and to propose a method to take into account the surface roughness effects in the calculation or simulation of the fluid flow through these microchannels. This method is based on the classical viscous flow equations, and consists in building an equivalent smooth channel with the same flow resistance as the “rough” one.     

Convective nanofluid flow over a vertical cone with a rough surface

In recent literature, the analysis of a combined convective flow over a cone has received a lot of attention. To explore the convection effects of flow over a cone in greater detail, in this investigation, we have considered a cone with a rough surface, which is entirely a new flow problem. Recent studies have shown the influence of roughness on fluid flow over several geometries, but flow over a rough conical surface has not been studied so far. In addition, we have analyzed the effects of nanoparticles, magnetohydrodynamic (MHD), and suction/blowing, which could have significant impacts on characteristics of fluid flow over the cone with a rough surface. Initially, the governing equations, which are partial differential equations with a high degree of nonlinearity, are nondimensionalized through Mangler's transformations. Later, linear equations are obtained via the method of quasilinearization, which is then solved numerically through finite difference approximations. The roughness of the cone's surface has notable effects on fluid flow, that too away from the origin. In fact, the roughness increases the friction at the cone's surface. Furthermore, the magnetic field applied at the wall increases the surface friction. Thus, the combination of roughness and MHD helps delay the boundary layer separation. On the other hand, the suction reduces the temperature of the fluid and increases the energy transport strength, while the thermophoresis parameter exhibits the contrary nature. Therefore, the combined consideration of these two could enhance energy transport strength in several industrial applications.     

Application of Lubrication Theory and Study of Roughness Pitch During Laminar,Transition, and Low Reynolds Number Turbulent Flow at Microscale

This work aims to experimentally examine the effects of different roughness structures on internal flows in high-aspect-ratio rectangular microchannels. Additionally, a model based on lubrication theory is compared to these results. In total, four experiments were designed to test samples with different relative roughness and pitch placed on the opposite sides forming the long faces of a rectangular channel. The experiments were conducted to study (i) sawtooth roughness effects in laminar flow, (ii) uniform roughness effects in laminar flow, (iii) sawtooth roughness effects in turbulent flow, and (iv) varying-pitch sawtooth roughness effects in laminar flow. The Reynolds number was varied from 30 to 15,000 with degassed, deionized water as the working fluid. An estimate of the experimental uncertainty in the experimental data is 7.6% for friction factor and 2.7% for Reynolds number. Roughness structures varied from a lapped smooth surface with 0.2 μ m roughness height to sawtooth ridges of height 117 μ m. Hydraulic diameters tested varied from 198 μ m to 2,349 μ m. The highest relative roughness tested was 25%. The lubrication theory predictions were good for low relative roughness values. Earlier transition to turbulent flow was observed with roughness structures. Friction factors were predictable by the constricted flow model for lower pitch/height ratios. Increasing this ratio systematically shifted the results from the constricted-flow models to smooth-tube predictions. In the turbulent region, different relative roughness values converged on a single line at higher Reynolds numbers on an f–Re plot, but the converged value was dependent on the pitch of the roughness elements.     

Experimental and numerical studies of liquid flow and heat transfer in microtubes

Experimental and numerical studies were conducted to reveal the flow and heat transfer characteristics of liquid laminar flow in microtubes. Both the smooth fused silica and rough stainless steel microtubes were used with the hydraulic diameters of 50–100 μm and 373–1570 μm, respectively. For the stainless steel tubes, the corresponding surface relative roughness was 2.4%, 1.4%, 0.95%. The experiment was conducted with deionized water at the Reynolds number from 20 to 2400. The experimental data revealed that the friction factor was well predicted with conventional theory for the smooth fused silica tubes. For the rough stainless steel tubes, the friction factor was higher than the prediction of the conventional theory, and increased as the surface relative roughness increased. The results also confirmed that the conventional friction prediction was valid for water flow through microtube with a relative surface roughness less than about 1.5%. The experimental results of local Nusselt number distribution along the axial direction of the stainless steel tubes do not accord with the conventional results when Reynolds number is low and the relative thickness of the tube wall is high. The numerical study reveals that the large ratio of wall thickness over tube diameter in low Reynolds number region causes significant axial heat conduction in the tube wall, leading to a non-linear distribution of the fluid temperature along the axial direction. The axial heat conduction effect is gradually weakened with the increase of Reynolds number and the decrease of the relative tube wall thickness and thus the local Nusselt number approaches the conventional theory prediction.     

Turbulent flow in a rectangular duct with a smooth-to-rough step change in surface roughness

Experimental study has been conducted on the response of turbulent flow in a rectangular duct to a smooth-to-rough step change in surface roughness on its short-side walls. The distributions of the primary flow velocity, secondary flow velocities, turbulence intensities, and turbulent shear stresses have been measured in seven cross-sections located from 2.0d_h to 37.5d_h downstream from the smooth-to-rough junction, and the relaxation process of this turbulent flow has been examined. It has been found that the relaxation process of this flow is considerably complex, but the flow characteristics except for the primary flow velocity in the core region as well as near the rough wall reach an almost equilibrium state, i.e., the fully developed rough-duct flow condition, within a distance of 37.5d_h from the change in the surface roughness.     

纳米通道内纳米流体流动特性的分子动力学模拟

利用分子动力学方法对铜-氩纳米流体和基础流体在不同剪切速度下的纳米尺度的Couette流进行模拟计算。结果表明:在纳米尺度通道内,纳米流体流动过程中颗粒存在旋转运动和平移运动,从而加强湍流效果,强化传热并影响整个流动区域内的流动速度分布,造成纳米流体速度呈非线性分布。壁面和纳米颗粒表面都会形成一层排布更为规则的液体原子吸附层,吸附层内液体分子在流体流动过程中一直伴随着壁面和纳米颗粒进行运动,且吸附层具有"类固"特性,可以增强纳米流体的传热能力。     

Introduction of a length correction factor for the calculation of laminar flow through microchannels with high surface roughness

The main objective of this work is to introduce a length correction factor, based on CFD simulation results, in order to extend the method for taking into account the surface roughness effects in the calculation of laminar flow through rough microchannels described in [J.R. Valdés, M.J. Miana, T. Pütz, Numerical investigation of the influence of roughness on the laminar incompressible fluid flow through annular microchannels, Int. J. Heat Mass Transfer 50 (2007) 1865–1878], to surface roughness values much larger than the ones considered in the previous study. The method proposed in Valdés et al. (2007) consisted in building an equivalent smooth channel with the same flow resistance as the rough one. As in this case, the new method is validated with results of CFD simulations of microchannels with different roughness values.     

A comparative study on poiseuille flow of simple fluids through cylindrical and slit-like nanochannels

Poiseuille flow of simple fluids in cylindrical nanochannels is investigated by nonequilibrium molecular dynamics simulations. The effects of pore shape and size are studied by comparing the fluid behavior in cylindrical and slit-like pores. It is found that the cylindrical pores are involved with a stronger molecular interaction between the fluid and wall atoms due to larger surface-to-volume ratio, curvature and possibly greater molecular roughness. The fluid-wall interaction plays an important role in the fluid flow at nanoscale. As a result, the bulk fluid density in cylindrical pores is lower than in the slit-like pores because a larger number of fluid particles are apposed onto the wall surface. In addition, evident slip on the boundary is observed in the cylindrical pore while no slip occurs in the slit-like pore. The fluid in the cylindrical pores has lower temperature and more uniform temperature distribution. This suggests that the cylindrical pores perform better to conduct the viscous heat out of the channel than the slit-like pores.     

糙条应用于弯段溢洪道内水力特性试验研究

为研究溢洪道弯段泄槽内布置糙条后的水力特性,选取槽内无糙条方案与布置糙条方案,从水深均匀度、横断面弗劳德数、凹凸岸水位三方面分析糙条对弯道水流的影响。结果表明,在淹没糙条的流量条件下,糙条的置入不仅可改善溢洪道横断面水深分布不均问题,还可缩短下游直线段距离;一定工况下,糙条可将溢洪道流态由急流转变为缓流;糙条对水流的控导作用将弯道底部水流由凸岸强迫导向凹岸,不仅消减水面横比降,还可抑制水流冲击波。     

Estimation of inhomogeneous zeta potential in the electroosmotic flow by means of mode reduction

A reduced-order model is derived for electroosmotic flows in a microchannel of nonuniform cross-section. The model is constructed based on the Karhunen–Loève Galerkin (KLG) procedure which can reduce nonlinear partial differential equations to sets of minimal number of ordinary differential equations. The reduced-order model of the present investigation is carefully constructed such that it is not necessary to re-evaluate any coefficient of the model even though the inhomogeneous zeta potential ζ(x) and the dielectric constant ε vary. This feature reduces the computational time greatly when employed in the estimation and control of electroosmotic flows, where repeated solution of governing equations is required. Using the present reduced-order model, a practical method is devised to estimate inhomogeneous zeta potential ζ(x) from velocity measurements of the electroosmotic flow in the microchannel. The proposed method is found to estimate ζ(x) with reasonable accuracy even using noisy velocity measurements.     

Numerical study of the aerodynamic performance of blunt trailing-edge airfoil considering the sensitive roughness height

For rough wind turbine airfoil and its blunt trailing-edge modification, the aerodynamic performance has been numerically investigated to facilitate a greater understanding of the effects of the blunt trailing-edge modification on the aerodynamic performance enhancement of airfoil with sensitive roughness height. The S834 airfoil from National Renewable Energy Laboratory is used for the simulation. The lift and drag coefficients of S834 airfoil with smooth or rough surface are calculated by the k-ω SST turbulence model, and are compared with wind tunnel test results. The aerodynamic performance of airfoils with different roughness heights is studied to obtain the sensitive roughness heights of suction and pressure surfaces. The mathematical expression of the blunt trailing-edge airfoil profile is established using the coordinate's rotation combined with the zoom coefficient of coordinate. Then, the S834 airfoil with sensitive roughness height is modified to be symmetrical blunt trailing-edge modification, and the lift and drag coefficients and the lift-drag ratio are also calculated and analyzed. Results indicate that the sensitive roughness height of suction surface is 0.5 mm, and the pressure surface is insensitive to the roughness height. Through the blunt trailing-edge modification, the lift coefficient and the maximum lift-drag ratio obviously increase for rough airfoil, and the sensitivity of airfoil to roughness height is reduced. The research provides significant guidance for designing the wind turbine airfoil under conditions of rough blade.     

Roughness effects of gas diffusion layers on droplet dynamics in PEMFC flow channels

Water management remains one of the major challenges in optimising the performance of PEMFCs, in which liquid accumulation and removal in gas diffusion layers (GDLs) and flow channels should be addressed. Here, effects of GDL surface roughness on the water droplet removal inside a PEMFC flow channel have been investigated using the Volume of Fluid method. Rough surfaces are generated according to realistic GDL properties by incorporating RMS roughness and roughness wavelength as the main characteristic parameters. Droplet dynamics including emergence, growth, detachment, and removal in flow channels with various airflow rates are simulated on rough substrates. The influences of airflow rate on droplet dynamics are also discussed by comparing the detachment time and droplet morphology. The liquid removal efficiency subject to different surface roughness parameters is evaluated by droplet detachment time and elongation, and regimes of detachment modes are identified based on the droplet breakup location and detachment ratio. The results suggest that rough surfaces with higher RMS roughness can facilitate the removal of liquid inside flow channel. Whilst surface roughness wavelength is found less significant to the liquid removal efficiency. The results here provide qualitative assessments on identifying the key surface characteristics controlling droplet motion in PEMFC channels.