The Influence and Interpretation of Surface Parameters for Wetting Transitions in Ternary Mixtures

Starting from a microscopic Hamiltonian defined on a semi-infinite cubic lattice, and employing a mean-field approximation, the surface parameters relevant for wetting in confined ternary mixtures are derived. These are found in terms of the microscopic coupling constants, and yield a physical interpretation of their origins. In comparison with the standard expression for the surface free-energy density, several new terms arising from the derivation are identified. The influence of the surface parameters on a predicted unbinding transition in a mixture of oil, water, and amphiphile demonstrate that existing results are robust to the addition of the extra surface terms.     

Studies of instability of two-phase flows in parallel channels

Experiments were carried out to determine boundaries of unsteady-state flows of a two-phase water-air mixture in three parallel vertical channels. It is shown that under certain conditions of parallel operation of individual channels self-sustained fluctuations of the pressure drop can appear and that interchannel interations can lead to unstable two-phase flow regimes with stable hydrodynamic characteristics of the individual channels and the whole system. It is shown that these fluctuations are caused by nonlinear characteristics of the two-phase flow.     

Simulations of pressure-driven flows through channels and pipes with unified flow solver

The aim of this paper is to demonstrate the benefits of direct methods of solving kinetic equations and adaptive kinetic-fluid solvers for vacuum science and technology. We consider pressure-driven flows through short channels for a wide range of gas rarefaction degrees. Our Unified Flow Solver combines Adaptive Mesh Refinement (AMR) with automatic selection of kinetic and fluid solvers in different parts of computational domain. The discrete velocity method is used for direct numerical solution of Boltzmann and model kinetic equations. The advantages of adaptive hybrid method are demonstrated for compressible flows at large pressure gradients. For small pressure drops, direct solutions of the Boltzmann equation provide accurate solutions of rarefied flows not achievable by the traditional direct simulation Monte Carlo methods.     

Heat transfer to two-phase flows of helium and nitrogen in vertical channels

Equations are presented for calculation of heat transfer in two-phase convection and well-developed bubble boiling of helium and nitrogen in vertical channels.Translated from Inzhenerno-Fizicheskii Zhurnal, Vol. 51, No. 6, pp. 885–892, December, 1986.     

Application of the parameter-expansion method to calculation of two-phase flows in channels with injection

Velocity and concentration distributions of the condensed phase have been obtained based on the parameter-expansion method. The characteristics of two-phase flows in channels with strong and weak injections have been investigated. The factors exerting an influence on the velocity of nonequilibrium phase motion have been determined; the range of applicability of the solution obtained has been established and its qualitative behavior has been elucidated; the possibility of applying such a solution to calculation of the concentration of the condensed phase has been shown. __________ Translated from Inzhenerno-Fizicheskii Zhurnal, Vol. 79, No. 6, pp. 119–127, November–December, 2006.     

Development of applied software for analysis of gas flows in vacuum devices

The paper presents some results of computing gas mixture flows in Knudsen and Holweck pumps. Transient flow development processes and attainment of the steady state are considered. The calculations were performed using a problem solving environment developed by the authors. A conservative projection method of solving the Boltzmann kinetic equation upon which the problem solving environment is based is described. The paper is aimed to demonstrate the capabilities of the software at the actual stage of its development.     

Magnetohydrodynamic (MHD) flows of viscoelastic fluids in converging/diverging channels

The present work is a theoretical investigation of the applicability of magnetic fields for controlling hydrodynamic separation in Jeffrey-Hamel flows of viscoelastic fluids. To achieve this goal, a local similarity solution was found for laminar, two-dimensional flow of a viscoelastic fluid obeying second-order/second-grade model as its constitutive equation with the assumption being made that the flow is symmetric and purely radial. These assumptions enabled a third-order nonlinear ODE to be obtained as the single equation governing the MHD flow of this particular fluid in flow through converging/diverging channels. With three physical boundary conditions available, Chebyshev collocation-point method was used to solve this ODE numerically. Results are presented in terms of parameters such as Reynolds number, Weissenberg number, channel half-angle, and the magnetic number. It was found that these parameters all have a profound effect on the velocity profiles in Jeffrey-Hamel flows. The effect of magnetic field was found to be more striking in that it is predicted to force fluid elements near the wall to exceed centerline velocity in converging channels and to suppress separation in diverging channels. Interestingly, the effect of the magnetic field in delaying flow separation is predicted to become more pronounced the higher the fluid’s elasticity.     

瓶体内表面制备类金刚石膜流场结构的格子波尔兹曼模拟

类金刚石膜(Diamond-like Carbon,DLC)具有优异的气体阻隔性能.在PET瓶体内表面制备DLC阻隔涂层时,阻隔涂层的均匀性会受到瓶体内流场结构(气压分布、速度分布等)的显著影响.本文应用格子波耳兹曼方法(Lattice Boltzmann Method/Model,LBM)对PET瓶体内制备DLC阻隔涂层时的流场结构进行模拟,研究了送气速度、气体运动粘度系数和装置结构变化时瓶体内部流场结构的变化.研究结果表明,降低进气速度和提高气体运动粘度系数有利于减弱回流而获得层流结构;装置结构的调整能够改变瓶体内的流场结构,采用恰当的异型装置可以获得较理想的流场结构,对实验工作具有一定的指导意义.     

Generalized Couette flow in channels of irregular cross‐section

Abstract

The flow behavior of non‐Newtonian power‐law fluids in channels of irregular cross‐section is examined. The driving force of the flow may be a constant pressure gradient (Poiseuille flow), a moving boundary (Couette flow) or the combination of the two (generalized Couette flow). There are three factors that influence the fluid motion in a channel, namely, the power‐law index n, the channel geometry and a dimensionless quantity E which can be viewed as the ratio of drag flow to pressure flow. The effects of these variables on velocity distributions and volumetric flow rates for various channel geometries are analyzed. The direct application of the numerical results on extruder design and operation is discussed.     

The Transport Properties of Hot Electrons in a Xenon/Methane Mixture

The results are given of numerical solution of the Boltzmann equation in a binomial approximation in view of elastic and inelastic electron collisions in Xe+CH₄ mixtures and in pure methane. The electron energy distribution functions obtained are used to calculate the electron transport coefficients for E/N values of up to several Townsends, i.e., the drift velocity, mobility, average and characteristic energies, diffusion coefficient. The results of calculation for pure methane fit the available experimental data. A similarity rule is found for the electron transport coefficients in a Xe+CH₄ mixture with different concentrations of methane molecules, which enables one to determine the values of transport coefficients in a mixture with a minor (less than 30 percents) methane content.     

Improving mass conservation in simulation of incompressible flows

We propose a technique for improving mass‐conservation features of fractional step schemes applied to incompressible flows. The method is illustrated by using a Lagrangian fluid formulation, where the mass loss effects are particularly apparent. However, the methodology is general and could be used for fixed grid approaches. The idea consists in reflecting the incompressibility condition already in the intermediate velocity. This is achieved by predicting the end‐of‐step pressure and using this prediction in the fractional momentum equation. The resulting intermediate velocity field is thus much closer to the final incompressible one than that of the standard fractional step scheme. In turn, the predicted pressure can be used as the boundary condition necessary for the solution of the pressure Poisson equation in case a continuous Laplacian matrix is employed. Using this approximation of the end‐of‐step incompressible pressure as the essential boundary condition considerably improves the conservation of mass, specially for the free surface flows of fluids with low viscosity. The pressure prediction does not require the resolution of any additional equations system. The efficiency of the method is shown in two examples. The first one shows the performance of the method with respect to mass conservation. The second one tests the method in a challenging fluid–structure interaction benchmark, which can be naturally resolved by using the presented approach. Copyright © 2012 John Wiley & Sons, Ltd.     

模板法制备聚乙烯咔唑纳米结构中的浸润转变

采用溶液浸润多孔阳极氧化铝模板(AAO)的方法,在孔径为200nm的AAO模板中制备功能聚合物聚乙烯咔唑(poly(vinyl carbazole),PVK)一维纳米结构.SEM和TEM测试结果表明,2.5%和3.5%(质量分数)的PVK溶液可制得纳米管,其外径约为200nm,管壁厚度分别为30与70nm;而5.0%与10.0%(质量分数)的PVK溶液可制得纳米线,其直径约为200nm.提出了溶液浸润模板法中的完全浸润体系和部分浸润体系以及临界浸润浓度Cw.     

Asymptotic modelling of flows in microchannel by using Navier-Stokes or Burnett equations and comparison with DSMC simulations

The modelling of isothermal gas flows driven by pressure drops, in coplanar microchannels is investigated. The goal is to construct an asymptotic model deduced from Navier-Stokes or Burnett equations for this type of flow, assuming slip boundary conditions along the walls. The dimensionless balance equations are written, taking into account a geometrical parameter introduced in this study. The application of the Principle of Least Degeneracy produces models with small Mach numbers and small or moderate Knudsen numbers and allows the development of asymptotic models. The first and second approximations associated with the Navier-Stokes or Burnett equations are presented and discussed. Navier-Stokes and Burnett second approximations reduce the mass flow rates.Additionally, first asymptotic solutions as against Direct Simulation Monte Carlo (DSMC) simulations show overall satisfactory agreements.     

Identification and characterization of solids in sand-water two-phase flows via vibration multi-sensor approaches

The monitoring of tiny particles in multiphase pipe flow is widely encountered in industry. In this paper, the identification and characterization of solid particles suspended in sand-water flow were developed based on vibration multi-sensor approaches. Verification experiments were conducted, and good agreement was found between the concentrations (0–0.1 wt% with an interval of 0.02 wt%) of varisized sands (from 86 to 180 μm) and the monitored vibration signal characteristics by multi-sensor approaches. A quadratic relationship between the particle concentration and vibration energy was obtained. In sand-carrying flow measurement experiments, the sand’s characteristic frequency bands were found at 22–23.6 kHz and 24.8–26.6 kHz. Additionally, the sand particle identification effect was evaluated from two positions at bends, that is, the outer wall of the 45-degree bend on the elbow and the exit of the 90-degree bend. Compared with these two monitor positions, the sand vibration energy from the outlet of the elbow had a higher signal-to-noise ratio with obvious energy variations. In addition, the accuracies of the detected sand vibration features were mutual confirmation. Consequently, the above methods are applicable for little solid detection in sand-water flow, which lays the foundation for particles monitoring in the complexed multiphase flow.     

Plane thermal transpiration of a rarefied gas in the presence of gravitation

Plane thermal transpiration of a rarefied gas that flows horizontally in the presence of gravitation is studied based on the Boltzmann equation. Assuming that the temperature gradient along the walls is small, the asymptotic analysis for a slow variation in the flow direction is conducted. The semi-analytical solution that is valid for arbitrary values of the mean free path and the gravitational strength is derived, and the problem is reduced to solving the spatially one-dimensional Boltzmann equation. This reduced problem is solved numerically for a hard-sphere molecular gas for small values of gravitational strength, and the behavior of the flow is studied based on the numerical solution. The effect of weak gravitation is no longer negligible when the gas is so rarefied that the mean free path is comparable to the maximum range that the molecules travel along the parabolic path within the channel. This phenomenon has been observed in the plane Poiseuille flow of a highly rarefied gas, and a similar phenomenon also occurs in the plane thermal transpiration considered in the present paper.     

Analytical solution of combined electroosmotic/pressure driven flows in two-dimensional straight channels: finite Debye layer effects

Analytical results for the velocity distribution, mass flow rate, pressure gradient, wall shear stress, and vorticity in mixed electroosmotic/pressure driven flows are presented for two-dimensional straight channel geometry. We particularly analyze the electric double-layer (EDL) region near the walls and define three new concepts based on the electroosmotic potential distribution. These are the effective EDL thickness, the EDL displacement thickness, and the EDL vorticity thickness. We show that imposing Helmholtz-Smoluchowski velocity at the edge of the EDL as the velocity matching condition between the EDL and the bulk flow region is incomplete under spatial bulk flow variations across the finite EDL. However, the Helmholtz-Smoluchowski velocity can be used as the appropriate slip velocity on the wall. We discuss the limitations of this approach in satisfying the global conservation laws.     

Lattice Boltzmann simulation of dispersion in two‐dimensional tidal flows

A lattice Boltzmann method (LBM) is applied to problems of dispersion in two‐dimensional water flows. The water flow is modelled by shallow water equations. A two‐distribution lattice Boltzmann equation algorithm is presented to solve the pollutant transport problem within the framework of shallow water flow. One distribution models the shallow water flow. The other distribution models the pollutant transport. Flow characteristics and concentration profiles of dispersive species are obtained at various flow regimes. For fast water flow, the concentration profiles are highly affected by the flow advection and become completely different from those at slow water flow. Numerical results are presented for pollutant transport in bounded and open channel flows. The proposed LBM is also used to simulate a pollution event in the Strait of Gibraltar. The obtained results indicate that the present method is useful for the investigation of transport phenomena by shallow water flows in complex geometries and practical flow problems. Copyright © 2008 John Wiley & Sons, Ltd.     

Lattice dynamics and Raman spectrum of rutile TiO2: The role of soft phonon modes in pressure induced phase transition

Using the plane-wave pseudopotential technique based on the first-principles density functional perturbation theory (DFPT), we have studied the vibrational properties and Raman susceptibility tensor at ambient and high pressure of rutile phase of TiO₂. Full phonon dispersion curves and phonon densities of states with projected phonon density of states and Raman tensors at high pressures are calculated and given. It is found that rutile TiO₂ shows a pressure induced phase transition, especially when lattice dynamical instabilities are involved, like the soft phonon modes, at a hydrostatic pressure lower than 10 GPa. An analyses of the vibrational displacements is given. The possibility to use Raman line intensities as an additional tool in the study of phase transitions is also discussed.     

Magnetic field effects on force convection flow of a nanofluid in a channel partially filled with porous media using Lattice Boltzmann Method

In this paper the Lattice Boltzmann Method (LBM) is utilized to investigate the effects of uniform vertical magnetic field on the flow pattern and fluid–solid coupling heat transfer in a channel which is partially filled with porous medium. Al₂O₃–water nanofluid as a work fluid with temperature sensitive properties is forced to flow into the channel while the top and bottom walls of the channel is heated and kept at a constant temperature. In the present study, with respect to previous works and experimental data, a new correlation is presented for density of Al₂O₃–water nanofluid as a function of temperature. The result also shows that the step approximation which is used for the complex boundaries of porous medium is reliable. Finally, the effect of various volume fractions of nanoparticles (ϕ = 0%, 3%, 5% and 7%) and different magnitude of magnetic field (Ha = 0, 5, 10 and 15) on the rate of heat transfer are thoroughly explored. In accordance with the results, by raising the nanoparticle volume fraction, average temperature and velocity at the outlet of the channel increase and the average Nusselt number rises dramatically. In addition, the increase the Hartmann number leads to the slow growth in the average Nusselt number, although the outlet average temperature and velocity shows a little drop.