Gas-assisted displacement of a viscoelastic fluid in capillary geometries

The effect of fluid viscoelasticity on the fraction of liquid deposited on the walls of capillary geometry and the pressure drop at the capillary static region are theoretically investigated using the Criminale-Ericksen-Filbey (CEF) constitutive equation to describe a non-Newtonian fluid displaced by the pressurized gas in a capillary. The singular perturbation method is used to determine the residual liquid film thickness of a viscoelastic fluid on the walls of a circular tube or a rectangular channel when displaced by another immiscible fluid. Inner and outer expansions are developed in terms of both a small parameter Ca^1/3 and a small parameter De/Ca^1/3. The method of matched asymptotic expansion is used to match the inner and outer solutions by means of a transition region between the advancing meniscus and the entrained film where the fluid rheology has its greatest effect. A detailed analysis indicates that the residual liquid film thickness of the viscoelastic fluid tends to decrease and the pressure drop across the bubble front tends to increase as the fluid becomes more viscoelastic. The theoretical results presented in this paper are in agreement with some of the experimental data and theoretical analyses available in the literature.     

Effect of the contraction ratio upon viscoelastic fluid flow in three-dimensional square–square contractions

In this work we investigate the laminar flow through square–square sudden contractions with various contraction ratios (CR=2.4, 4, 8 and 12), using a Newtonian fluid and a shear-thinning viscoelastic fluid. Visualizations of the flow patterns were carried out using streak line photography and detailed velocity field measurements were performed using particle image velocimetry. The experimental results are compared with numerical predictions obtained using a finite-volume method. For the Newtonian fluid, a corner vortex is found upstream of the contraction and increasing flow inertia leads to a reduction of the vortex size. Good agreement is observed between experiments and numerical simulations. For the shear-thinning fluid flow a corner vortex is also observed upstream of the contraction independently of the contraction ratio. Increasing the elasticity of the flow, while still maintaining low inertia flow conditions, leads to a strong increase of the vortex size, until an elastic instability sets in and the flow becomes time-dependent at De≈200, 300, 70 and 450 for CR=2.4, 4, 8 and 12, respectively. At low contraction ratios, viscoelasticity brings out an anomalous divergent flow upstream of the contraction. For both fluids studied the flow presents a complex three-dimensional helical vortex structure which is well predicted by numerical simulations. However, for the viscoelastic fluid flow the maximum Deborah number achieved in the numerical simulations is about one order of magnitude lower than the critical Deborah number for the onset of the elastic instability found in the experiments.     

Flow of a viscoelastic shear-thinning fluid between two concentric rotating spheres

The flow of a viscoelastic fluid, which is simulated by a Criminale-Ericksen-Filbey model, between two concentric rotating spheres is analyzed theoretically. This problem has wide applications in practice. The assessment of the performance of a polymerization reactor, for example, requires knowledge about the rheological properties of the reacting fluids and flow patterns around agitators. We show that the shear-thinning effects of both the viscosity and the elasticity of a fluid may lead to double-vortex flows. The origin of these vortices are discussed, and the effect of the relative sizes of the inner and the outer spheres on the vortex flows is investigated. The influence of the elasticity on the torque required to rotate the spheres is also evaluated.     

A computationally-cost effective model for fluid flow in redox flow batteries

Redox flow batteries (RFBs) hold great potential for large-scale, extended-duration stationary energy storage. Here, a novel computationally cost-effective hydraulic-electrical analogous model (HEAM) for fluid flow in RFBs is developed. The HEAM demonstrated that lowering the electrode compression and enhancing the channel area lowers the pump power loss independent of the flow fields and electrodes. Additionally, the HEAM helped elucidate the deficiencies of flow distribution in interdigitated flow fields (IFFs) and suggested designing wider manifolds and/or shorter channels improve the flow distribution. Moreover, the HEAM suggested shallower and/or wider channels, and more permeable electrodes enhance the flow penetration rate above the channels. Finally, the HEAM showed that the average penetration depth in the electrode above the ribs (h_pen) was the critical parameter in the fluid-flow modeling of IFFs and was inversely proportional to the permeability. Hence, there is a trade-off between the pump power loss and h_pen when configuring electrode permeability.     

错流射流混合过程的基本特性及影响因素

错流射流已被证明是强化流体热质传递最有效的方法之一。介绍了错流射流的结构、基本特性以及在混合过程中的影响因素,并展望了其发展前景。     

Completed double layer boundary element method for periodic fibre suspension in viscoelastic fluid

A numerical method to simulate the periodic fibre suspension in viscoelastic fluid is developed with the completed double layer boundary element method (CDLBEM). The periodic summations that arise in the formulation were well handled by Ewald summation technique to speed up the convergence rate in the computation. The formulation for velocity field in periodic fibre suspension in viscoelastic fluid is derived and is used to simulate multiple fibres suspended in a viscoelastic shear flow. Simulations are carried out for various fibre aspect ratios and volume fractions ranging from dilute to concentrated regimes. Numerical results of macroscopic rheological properties of the system are compared to available experiments on viscoelastic fibre suspensions, and are found to agree reasonably well with the experimental data.     

Oscillatory convection in a horizontal porous layer saturated with a viscoelastic fluid

When a horizontal porous layer saturated with a viscoelastic liquid is heated from below, the onset conditions of thermal convection are found to be functions of Darcy-Rayleigh number, wave number, and viscoelastic properties. In this study, the linear stability was studied analytically in order to investigate the viscoelastic effects of saturated liquids on the onset conditions in connection with oscillatory instabilities at the threshold of stationary convection. It is suggested that the resulting oscillatory instabilities occur at lower values of Darcy-Rayleigh number than the critical value for the stationary convection. From the occurrence of oscillatory instabilities of viscoelastic liquid, it is expected that the periodic motion should be replaced by stationary modes in a horizontal porous layer.     

Pulsatile flow of discotic mesophases

This paper presents an analysis using computational and scaling methods of pulsatile capillary Poiseuille flow of a model anisotropic viscoelastic discotic liquid crystalline material. The analysis shows that pulsatile pressure drops applied to anisotropic materials can result in flow-rate enhancement or reduction, defined as the relative flow-rate change with respect to the steady-state flow rate, for a given average pressure drop, amplitude, and frequency. It is found that flow-rate modification in pulsatile flow is a direct result of orientation-dependent viscosity. The role of average pressure drop, oscillation amplitude, and frequency on flow enhancement is characterized and explained. The new mechanism of flow-enhancement in liquid crystals subjected to pulsatile pressure is expected to be useful to the fundamental understanding of pulsatile flows of anisotropic suspensions and anisotropic biological fluids.     

Numerical study of condensing bubble in subcooled boiling flow using volume of fluid model

In this numerical study, the behavior of condensing bubble was investigated using the volume of fluid (VOF) model in the FLUENT code. In order to simulate the condensing bubble with the FLUENT code, the bubble condensation was modeled using the user-defined function (UDF). For the validation of the UDF of bubble condensation, the results of CFD simulation were compared with the results of a bubble condensation experiment performed in Seoul National University (SNU). Simulation results showed good agreements with the experimental data. Moreover, the fundamental behavior of the condensing bubble was investigated in various conditions. The effects of condensation on bubble behavior were analyzed by comparing the behavior of condensing bubbles with that of adiabatic bubbles. It was found that the behavior of the condensing bubble was different from that of the adiabatic bubble in many respects including the bubble shape, velocity, rise distance and moving trajectory.     

Measurements of the viscoelastic moduli of an acrylate polymer in bulk and film form using a contact method

This paper describes a new method for the measurement of the viscoelastic properties of polymers using a contact mechanics approach. The latter is based on the determination of the tangential response of a macroscopic contact between a polymer specimen and a rigid sphere under small amplitude cyclic micro-motions. Using an acrylate polymer, it was found possible to achieve contact conditions where the tangential behaviour of the contact is strictly linear. Under such conditions, the measurement of the contact stiffness allowed to determine the viscoelastic moduli of the bulk polymer through the glass transition zone. In addition, it was also found to be possible to measure the damping properties of thin films (30 μm) using the same method. The results indicated a shift of the glass transition temperature of the films as compared to bulk specimens. This result was interpreted as an indication of the sensitivity of the glass transition of amorphous polymers to the hydrostatic pressure.     

On the relative importance of the kinetic and frictional contributions to granular motion in an annular Couette flow

A quantitative assessment is made on the relative importance of the kinetic and frictional contributions to the motion of dry granular materials under shear in an annular Couette flow configuration. The assessment is based on comparing the modelling results using the kinetic-frictional model with the experiments. It is shown that the kinetic-theory-based model with equal weight of the collisional and frictional contributions, commonly used in the literature, gives a great deviation from the experimental results in the point of view of the dominant solids motion, while an increase in the weight of the frictional contribution improves the modelling towards the experimental results. An increase in the weight of the frictional contribution by 25-50% leads to the best match, suggesting the current constitutive relationship with equal weight of the kinetic and frictional contributions need to be refined in order to reflect the real dense granular flows.     

A generalized reduced fluid dynamic model for flow fields and electrodes in redox flow batteries

High-dimensional models typically require a large computational overhead for multiphysics applications, which hamper their use for broad-sweeping domain interrogation. Herein, we develop a modeling framework to capture the through-plane fluid dynamic response of electrodes and flow fields in a redox flow cell, generating a computationally inexpensive two-dimensional (2D) model. We leverage a depth-averaging approach that also accounts for variations in out-of-plane fluid motion and departures from Darcy's law that arise from averaging across three-dimensions (3D). Our resulting depth-averaged 2D model successfully predicts the fluid dynamic response of arbitrary in-plane flow field geometries, with discrepancies of <5% for both maximum velocity and pressure drop. This corresponds to reduced computational expense, as compared to 3D representations (<1% of duration and 10% of RAM usage), providing a platform to screen and optimize a diverse set of cell geometries.     

Residence time distribution analysis from a continuous couette flow device around critical taylor number

This paper presents an experimental study of residence time distribution (RTD) analysis by pulse response technique in a continuous Couette flow device with rotating inner cylinder and stationary outer cylinder. Two kinds of experimental tests using pulses of tracer dye solution and particles resulting from a fast precipitation were performed in the region near the critical Taylor number characterizing boundary between laminar and laminar vortex flow. For most experiments performed in laminar and laminar vortex flow regime around the critical Taylor number over the ranges 0 < T_a < 120 and 0 < R_e < 5.5 the normalized response can be described by a dispersion model. The results of the critical Taylor number as characterized by the minimum dispersion number appear consistent with both theoretical predictions and other empirical observations.     

NMR microimaging of fluid flow in model string-type reactors

Magnetic resonance microimaging (MRM) was employed to obtain quantitative velocity maps of water flowing in the channels possessing unconventional cross-section shapes formed by a bundle of parallel fibers within a tubular string-type reactor. The maps obtained demonstrate the presence of large amounts of an almost stagnant liquid in the stretched corners of the cross-sections representing distorted triangles or squares. This fact together with the irregularity of the filaments packing in the model string-type reactor was demonstrated to lead to a broad residence time distributions (RTDs) for liquid flow. Next, the pulsed field gradient NMR (PFG NMR) technique was employed to compare transport of water with that of butane gas in the same model string-type reactor. The experimentally measured average propagators (travel distance probability density functions) have demonstrated that Taylor dispersion can lead to much better RTDs for gas as compared to liquid in channels with sub-millimeter equivalent diameters. The PFG NMR data were compared with the RTD obtained using the conventional tracer time-of-flight transient response method. It is concluded that due to the differences in the quantities actually measured by the two techniques, and the significant differences in the measurement length scales (microns to 1-2 cm for NMR/MRM, tens of centimeters for transient response methods), there is no reliable way of directly comparing these results. The information obtained by NMR/MRM and more conventional techniques such as time-of-flight should be considered as complementary. In particular, NMR/MRM can reveal the reasons for the observed overall reactor performance by providing access to the transport processes on short length scales inside the reactor and by revealing structure-transport interrelations.     

Rheological properties and stability of magnetorheological fluids using viscoelastic medium and nanoadditives

In order to improve the stability of magnetorheological (MR) fluids, viscoelastic medium having 2.2 Pa yield stress has been used as a continuous phase and nanosized CrO₂ particles are added too. The rheological properties as well as the dispersion stability of MR fluids have been studied by using a stress-controlled rheometer and sedimentation test. The steady-shear MR response was independent of the continuous and nano additives and the fieldinduced yield stress increased subquadratically with the flux density. Since the constant stress is generated within the limit of zero shear rate, the plateau in the flow curve corresponds to the Bingham yield stress. Under an external field, the yield stress varied as B^3/2. The yield stress has an approximately linear relation with the particle volume fraction.     

Influence of the inorganic phase concentration and geometry on the viscoelastic properties of latex coatings through the glass-transition

The mechanical properties of artists’ acrylic (latex) paint films containing different volume fractions of TiO₂, CaCO₃ and kaolin were measured in uni-axial tension over a broad range of temperatures and crosshead speeds. Young’s modulus results in the glassy region were first compared with several micromechanics theories for particle-filled composites containing elastic phases. It was found that the Mori-Tanaka theory slightly under-predicted the modulus enhancement, while the Lielens approach provided the most accurate results. A nonlinear viscoelastic material model involving a Prony series and the neo-Hookean hyperelastic function was used to represent the tensile data up to relatively small strains of a few percent. From the experimental data, the material model was calibrated and the required parameters were determined. The derived parameters were then used to re-construct relaxation modulus plots, which were compared with the approximations given by Clements and Mas for the viscoelastic Mori-Tanaka theory in the time-domain. It was found that the experimentally observed modulus enhancement was much stronger than the predicted values in the rubbery region. Mechanisms such as constrained polymer at the inorganic particle interface, and the possible formation of a percolation network are discussed.     

利用一维波和段塞稳定性模型预测分层流向段塞流的转变

利用气液二相流一维波模型和段塞稳定性模型,对直径2.54 cm水平管内空气-水二相流出现段塞流时的各相临界表观速度和临界液层高度进行了理论预测。计算中发现,2种模型分别适用于不同的流速区域,在较低的气相流速下,一维波模型的预测结果比较理想,但是在较高的气速条件下不太适合,而利用段塞稳定性模型可以较好地获得高流速下分层流向段塞流的流型转变条件。因此,结合这2种模型对发生流型转变时的临界参数作了分析,并且应用于40 mm和50 mm水平管道的油气二相流实验。将理论计算的结果和实验测得的流型数据进行了对比,并且对影响流型的管径、流速等因素作了分析,结果表明计算得到的特征参数和实验数据比较吻合。     

A simplified couette flow solution of non-newtonian power-law fluids in eccentric annuli

Flow of non-Newtonian fluids in both the concentric and eccentric annuli is of great importance in extruders for molten plastics and wellbore fluid circulation for the removal of drilling cuttings. The steady laminar couette flow of non-Newtonian power-law fluids in eccentric annulus is employed in this study to analyze the problems of surge or swab pressures encountered when running or pulling tubular goods (pipes) in a liquid filled borehole. This is similar to the annular space created by two long co-axial cylinders with the inner cylinder in motion at a steady velocity, and a stationary outer cylinder. The solutions of the equations of motion are presented in both dimensionless form and as a family of curves for different pipe/borehole eccentricity ratios and power-law fluid index values for a more general application. The expected error in surge computation for concentric annulus as a result of eccentricity is evaluated.