Wall Slip Effects Research Articles

Influence of Coupling Agents on Melt Flow Behavior of Natural Fiber Composites

AbstractThe influence of coupling agents on the melt rheological properties of natural fiber composites has been investigated in this work using capillary and rotational rheometers. Scanning electron microscopy was also employed to supplement the rheological data. It was found that molecular weight and molecular weight distribution of the polymer matrix and coupling agent characteristics influence the filler wetting and the melt flow properties of the filled composites. Generally, low molecular weight and narrow molecular weight distribution polyethylene matrix provides relatively larger increase of the viscosity of the composites. Coupling agents tend to increase the resistance to shearing, but wall slip effects may interfere with the measured values, especially at very high filler loadings. Entrance pressure loss in capillaries is also influenced by polymer matrix and coupling agent used.magnified image

Wall slip of concentrated suspension melts in capillary flows

The effects of wall slip of concentrated suspension melts in capillary flows were investigated at elevated temperature. The modeled material is a mixture of polymer EVA (Ethylene Vinyl Acetate) and non-colloidal spherical powder (glass microspheres) with mean particle size within 53∼63 μm. The effect of particle concentration on wall slip was studied experimentally in a capillary rheometer. For suspensions with different particle loadings (35%, 40%, and 45% by volume), the slip velocity V s increased with an increase of particle concentration at the same testing temperature. A master slip curve can be obtained by plotting slip velocity versus the product of wall shear stress and square root of particle concentration. As such, a new particle concentration-dependent slip model is proposed. A theoretical approach coupled with the new slip model and flow equation is employed to characterize the flow behavior of concentrated suspension in a capillary rheometer, with reasonable agreement obtained with experimental observations.

Evaluation of wall slip effects in the lubricating grease/air two-phase flow along pipelines

Evaluation of wall slip phenomena during the horizontal pipeline flow of air/lubricating grease mixtures was investigated. With this aim, pressure drop measurements have been carried out along pipelines with different diameter and roughness. A modified Jastrzebski's equation for the slip velocity, based on the introduction of the relative roughness, has been used to correct wall slip effects for a lithium lubricating grease/air system. This expression has been introduced in the classical Rabinowitsch–Mooney treatment and applied to the superficial liquid velocity instead of the single-phase average velocity following a single-phase treatment analogy. Thus, the non-slip flow curve data for the two-phase mixture were obtained from roughened pipes and compared with those obtained from pipes with smooth internal surfaces. The effect of air on the extension of wall slip has been established as a function of air flow rate. Thus, the consideration of the reduction of the wetted pipe surface as air is injected allows an adequate explanation of this phenomenon, confirmed by the reduction of the effective slip contribution on the observed apparent shear rate. A power-law relationship between the slip velocity and the wall shear stress has been deduced, although this tends asymptotically to linearity as air flow rate is increased.

Non-Newtonian flow in parallel-disk viscometers in the presence of wall slip

Finite element computation is used to investigate the effects of wall slip on the slow viscous flow of non-Newtonian fluids in parallel-disk viscometers. A simplified Bird–Carreau constitutive equation is used to describe the general shear thinning behavior exhibited by non-Newtonian fluids. The linear relationship between wall shear stress and slip velocity proposed by Navier and two of its non-linear generalizations are investigated. The main objective here is to determine whether the kinematics of the flow field satisfies the key assumption implicit in the method of Yoshimura and Prud’homme for determining wall slip functions. The results of the finite element computation indicate that these assumptions are approximately met but the amplification of the non-linearity of the shear stress with respect to the radial coordinate means that the popular method of converting the measured torque into rim shear stress may incur significant error.

Numerical Analysis of Wall Slip Effects on Flow of Newtonian and Non-Newtonian Fluids in Macro and Micro Contraction Channels

We performed numerical simulation to investigate the effects of wall slip on flow behaviors of Newtonian and non-Newtonian fluids in macro and micro contraction channels. The results show that the wall slip introduces different vortex growth for the flow in micro channel as compared to that in macro channel, which are qualitatively in agreement with experimental results. The effects of slip on bulk flow behaviors depend on rheological property of the fluid. For Newtonian fluid, the wall slip always reduces the vortex length, while for non-Newtonian fluid, the strength of the slip determines whether the vortex length is reduced or increased. Analyses on the velocity and stress fields confirm the channel size dependent phenomena, such as the reduction of wall shear stress with the decrease in channel size. With the increase in average shear rate, the Newtonian fluid shows the reduction of wall shear stress that increases in the same trend with slip velocity-wall shear stress function, while for non-Newtonian fluid, the effect of the slip is suppressed by shear thinning effect and, therefore, the reduction of wall shear stress is less sensitive to the change in average shear rate and slip velocity-wall shear stress function.

Streamwise structures and density patterns in rapid granular Couette flow: a linear stability analysis

A three-dimensional linear stability analysis has been carried out to understand the origin of vortices and related density patterns in bounded uniform-shear flow of granular materials, using a kinetic-theory constitutive model. This flow is found to be unstable to pure spanwise stationary perturbations ( $k_z\,{\neq}\, 0$ , $k_x\,{=}\,0$ and $\partial/\partial y(.)\,{=}\,0$ , where $k_i$ is the wavenumber for the $i$ th direction) if the solid fraction is below some critical value $\nu\,{ . The growth rates of these spanwise instabilities are an order of magnitude larger than those of the two-dimensional ( $k_z\,{=}\,0$ ) streamwise-independent ( $k_x\,{=}\,0$ ) instabilities that occur if the solid fraction is above some critical value $\nu\,{>}\,\nu_{2D}$ ( ${>}\nu_{3D}$ ). The spanwise instabilities give birth to new three-dimensional travelling wave instabilities at non-zero values of the streamwise wavenumber ( $k_x\,{\neq}\, 0$ ) in dilute flows ( $\nu \,{ ). For moderate-to-large densities with $k_x\,{\neq}\, 0$ , there are additional three-dimensional instability modes in the form of both stationary and travelling waves, whose origin is tied to the corresponding two-dimensional instabilities. While the two-dimensional streamwise-independent modes lead to the formation of stationary streamwise vortices for moderately dense flows ( $\nu\,{>}\,\nu_{2D}$ ), the pure spanwise modes are responsible for the origin of such vortices in the dilute limit ( $\nu\,{ ). For more general kinds of perturbations ( $k_x\,{\neq}\, 0$ and $k_z\,{\neq}\, 0$ ), ‘modulated’ streamwise vortices are born which could be either stationary or travelling depending on control parameters. The rolling motion of vortices will lead to a major redistribution of the streamwise velocity and hence such vortices can act as potential progenitors for the mixing of particles. The effect of non-zero wall slip has been investigated, and it is shown that some dilute-flow instabilities can disappear with the inclusion of the wall slip. Even though the streamwise granular vortices have similarities to the well-known stationary Taylor–Couette vortices (which are ‘hydrodynamic’ in origin), their origin is, however, tied to ‘constitutive’ instabilities, and hence they belong to a different class.

Effect of temperature, moisture and lipid content on the rheological properties of rice flour

The rheological properties of rice flour (RF) with 21%, 25% and 29% moisture (wet basis) and 3%, 5% and 7% lipid contents were studied using a capillary rheometer in a wide temperature and apparent shear rates ranges. The material exhibited a pseudoplastic flow behaviour, described by the power law model and its viscosity decreased with the increase of temperature, moisture and lipid content (up to 5%). Samples with 7% lipid content show wall-slip effects for all temperature and moisture content conditions. The consistency coefficient decreased with the increase of temperature, but not with the moisture or lipid content. The flow behaviour index did not exhibit a defined trend with the increase of any variable, i.e., temperature, moisture or lipid content. Two rheological models for the prediction of viscous behaviour of rice flour, both with a 96% accuracy, were determined with a multivariable linear regression method. The first model takes into account temperature, moisture content and shear rate while the second includes the lipid content. Finally, the degree of transformation induced by the processing of rice flour in the capillary rheometer, at different temperatures and moisture contents, was studied using a differential scanning calorimeter (DSC).

05/02430 Experimental study of grease flow in pipelines: wall slip and air entrainment effects

The tribological performances of epoxy-based composites, filled with short carbon fibre, graphite, PTFE and nano-TiO2 in different proportions and combinations, were investigated. The patterns of frictional coefficient, wear resistance and contact temperature were examined by a pin-on-disc apparatus in a dry sliding condition under different contact pressures and sliding velocities. The experimental results indicated that the addition of nano-TiO2 could apparently reduce the frictional coefficient, and consequently reduce the contact temperature of fibre-reinforced epoxy composites. As a result, the wear resistance of the composites was significantly enhanced, especially at extreme wear conditions, i.e. high contact pressures and sliding velocities. Compared to traditional fillers, epoxy nanocomposites attained generally improved both wear resistance and load-carrying capacity.

Rheology of shear thickening suspensions and the effects of wall slip in torsional flow

The rheological characterisation of concentrated shear thickening materials suspensions is challenging, as complicated and occasionally discontinuous rheograms are produced. Wall slip is often apparent and when combined with a shear thickening fluid the usual means of calculating rim shear stress in torsional flow is inaccurate due to a more complex flow field. As the flow is no longer “controlled”, a rheological model must be assumed and the wall boundary conditions are redefined to allow for slip. A technique is described where, by examining the angular velocity response in very low torque experiments, it is possible to indirectly measure the wall slip velocity. The suspension is then tested at higher applied torques and different rheometer gaps. The results are integrated numerically to produce shear stress and shear rate values. This enables the measurement of true suspension bulk flow properties and wall slip velocity, with simple rheological models describing the observed complex rheograms.

Study on rheological behavior of polymer melt flowing through micro-channels considering the wall-slip effect

Micro molding is attracting more attention nowadays and determination of the rheological behavior of the polymer melt within micro structured geometry is considered to be very important for the accurate simulation modeling of micro molding. The lack of commercial equipment is one of the main hurdles in the investigation of micro melt rheology. In this study, the melt viscosity measurement system for PS (polystyrene) melt flowing through a micro-channel was established using a micro-channel mold operated at a mold temperature as high as the melt temperature. From measured pressure drop and volumetric flow rate both the capillary flow model and the slit flow model were used for the calculation of viscosity utilizing Rabinowitsch and Walters corrections. It was found that the measured viscosity values in the test ranges are significantly lower (decreased by a factor of about 1.4–4.1) than those obtained from the traditional capillary rheometer at a melt temperature of 200 °C using both the capillary flow model and the slit flow model. As the micro-channel size decreases, the reduction in the viscosity value increases when compared with data obtained from the traditional capillary rheometer. The ratio of slip velocity relative to mean velocity was also found to increase with decreasing size of micro-channels. It seems that wall slip plays a dominant role when melt flows through micro-channels and would result in a greater percentage in apparent viscosity reduction when the size of the micro-channel decreases. In addition, the wall-slip effect becomes more significant as the melt temperature increases. In the present study we emphasize that the rheological behavior of the melt in the microscopic scale is different from that of the macroscopic scale and that current simulation packages are not suitable for micro molding simulation without considering this difference.

Abnormal behavior of a hydrodynamic lubrication journal bearing caused by wall slip

Reynolds lubrication theory assumes that there is no wall slip on the interfaces between the solids and lubricant. During recent years, however, it is found that wall slip often happens. The present paper analyzes the wall slip occurring in a hydrodynamic lubrication journal bearing. If the two surfaces have the same adhesion property wall slip always decreases the oil film load support capacity. If there is wall slip over all of the lubricated surfaces, the hydrodynamic effect of the journal bearing vanishes, and no load support exists. If the two lubricated surfaces have different adhesion properties, the wall slip effect is more complex and may cause the journal bearing to operate in an instable manner. In order to avoid the wall slip, the limiting shear stress at the bearing surface should be higher than that at the journal surface.

Experimental study of grease flow in pipelines: wall slip and air entrainment effects

Prediction of pressure drop gradient and evaluation of wall slip and air bubbles entrainment effects observed during the piping flow of lubricating greases were investigated. With this aim, viscous flow tests in rotational rheometers and pressure drop measurements in pipelines were carried out using different geometries with both smooth and rough surfaces. The Sisko model was applied in the experimental range of flow rates for predicting pressure drop gradient. Air entrainment occurring when the pumping system was primed with a highly viscous material as lubricating greases significantly decreases pressure drop gradient. This air entrainment effect can be corrected using a modified expression to evaluate the drag ratio defined for non-Newtonian liquid/air intermittent flows. On the other hand, a new expression based on the internal relative roughness of pipelines was proposed to correct wall slip effect. Eliminating these two effects, the classical definition of the friction factor for a non-Newtonian fluid, f = 16/ Re′, can be applied to predict pressure drop of grease flow in pipelines.

Wall slip determination for coarse food suspensions in tube flow at high temperatures

An alternative method, developed based on variable particle concentration in conjunction with the wall slip correction method originally established by Mooney, was used to determine wall slip for model coarse food suspensions consisting of 1.5% CMC solution and green peas (15–30% v/v) in tube flow at high temperatures (85–135 °C). Within the wall shear stress range of 19.4–125 Pa, the slip coefficient varied from 3.09 × 10 −3 to 1.01 × 10 −2 m/Pa s and could be described well as a power function of wall shear stress. The rate of change in slip coefficient with wall shear stress decreased with the increase in the consistency coefficient of the carrier fluid. The opposite trend was observed with the increase in flow behavior index of the carrier fluid. It was also found that in general, contribution of the slip effect on the measured flow rate increased with the increase in particle concentration. The modified method presented here offers a simple alternative approach for computation of the wall slip effect of coarse food suspensions in continuous tube flow at high temperature, the scarce information of which is essential for further determination of their rheological properties.

Does Klinkenberg’s Law Survive Upscaling?

This work deals with the large-scale mathematical modelling of flow of gas at low pressure in porous media. At the pore scale, this type of flow is characterised by a wall-slip effect, which at the sample scale entails a dependence of permeability upon gas pressure. This latter property is described by Klinkenberg's law. The goal of the present work is to examine the robustness of this law, by determining whether it is still verified on a large-scale: upscaling is thus applied, starting with Klinkenberg's law at the local scale. A Klinkenberg's flow of gas in a two-constituent composite porous medium is considered, and the constituents are firstly assumed to be homogeneous. The cases of low and of high permeability contrast are successively examined. Upscaling is performed using the homogenisation method of multiple scale expansions. In both cases, the large-scale permeability tensor differs from its liquid counterpart. Except in the particular case of equal Klinkenberg factors, Klinkenberg's law is not verified at low permeability contrast. At high permeability contrast, the large-scale gas permeability verifies Klinkenberg's law. The case of heterogeneous constituents is then examined. It is shown that the large-scale permeability differs from its liquid counterpart, but it does not verify Klinkenberg's law.

New technique for reconstructing instantaneous velocity profiles from viscometric tests: Application to pasty materials

We present a new technique for reconstructing the instantaneous velocity profiles during creep, dynamic, or ramp tests under controlled stress in wide-gap Couette flows, from a series of similar tests under smaller stress amplitudes. This approach is based on a rigourous theory, and since it requires that the fluid does not flow close to the outer cylinder, it is particularly suitable for yield stress fluids. The interest of this reconstruction technique is that it is simpler than direct techniques (nuclear magnetic resonance, light scattering, particle imaging velocimetry, etc.) and has almost no limitations in time and space resolution. Thus, one can obtain the velocity profiles under steady-state and transient flows. We show that for a commercial hair gel the velocity profile obtained with this technique is in excellent agreement with that found from magnetic resonance imaging rheometry within the range of measurement (four decades of velocity). From other tests with a mustard and a kaolin–water suspension we demonstrate that the “viscosity bifurcation” effect observed with various pasty materials [Coussot et al., Phys. Rev. Lett. (2002a)] is directly associated with an abrupt change in the slope of the velocity profiles at the interface between the sheared and unsheared regions. We also show that the effect of wall slip on the reconstructed velocity profile is to shift the level of the unsheared region to a virtual, constant, finite, velocity level.

Effect of wall slip on blown film thickness distribution

AbstractOne of the most important materials for blown film is high‐density polyethylene (HDPE) with wide molecular weight distribution. First, we computed a wall stress at the entrance of a spiral groove in a particular die during blown film processing on a particular condition, to which a similar condition is widely utilized in a film works. The computed value is about 170 kPa, while the HDPE melt slips at die wall at stresses above approximately 50 kPa. The stress of 170 kPa is sufficiently large for the slip occurrence of the melt. Then, we investigated the effects of wall slip and melt visosity on film thickness distribution in the circumferential direction; the distribution tends to decrease with decreasing wall slip and melt viscosity. This tendency is explained by considering flow distribution in a spiral mandrel die and polymer melt flow characteristics.

Liquid flow in microchannels: experimental observations and computational analyses of microfluidics effects

Experimental observations of liquid microchannel flows are reviewed and results of computer experiments concerning channel entrance, wall slip, non-Newtonian fluid, surface roughness, viscous dissipation and turbulence effects on the friction factor are discussed. The experimental findings are classified into three groups. Group I emphasizes ‘flow instabilities’ and group II points out ‘viscosity changes’ as the causes of deviations from the conventional flow theory for macrochannels. Group III caters to studies that did not detect any measurable differences between micro- and macroscale fluid flow behaviors. Based on numerical friction factor analyses, the entrance effect should be taken into account for any microfluidic system. It is a function of channel length, aspect ratio and the Reynolds number. Non-Newtonian fluid flow effects are expected to be important for polymeric liquids and particle suspension flows. The wall slip effect is negligible for liquid flows in microconduits. Significant surface roughness effects are a function of the Darcy number, the Reynolds number and cross-sectional configurations. For relatively low Reynolds numbers, Re < 2000, onset to turbulence has to be considered important because of possible geometric non-uniformities, e.g., a contraction and/or bend at the inlet to the microchannel. Channel-size effect on viscous dissipation turns out to be important for conduits with Dh < 100 µm.

A217 圧縮成形の数値解析による可視化

The compression process is one of the widely used process technologies and has been used for manufacturing many kinds of products in the rubber, elastomer and polymer industries. In some cases, unpredictable defects, which decrease the product quality, are found during the process. This leads to increase the importance of qualitative investigation of material flows during the process. Although most of the research has been done by the experiment, recently various numerical investigations relating to the compression process were reported in some literatures. In this study, we investigated the effects of wall slip on the defects generation in compression process. The results show that the wall slip caused the flow behavior complexly and might play an important rule in the defects generation in real manufacturing.

Global Concept for Describing and Investigation of Wall Slip Effects in the Extrusion Process

AbstractWhen it comes to the design of extrusion line components such as single‐screw extruders or extrusion dies, no one has yet succeeded in developing a well‐functioning concept for wall‐slipping materials. This article examines the fundamental influences of the wall slippage effect on flow behavior in the extrusion die and plastification unit. New and extended methods of calculation for describing this flow phenomenon are presented for both these extrusion line components, and the general possibilities for linking the two approaches are discussed. Diagram of the flow curve (τ = shear stress, $\dot \gamma$ = shear rate) of wall‐slipping plastics for constant pressure.magnified imageDiagram of the flow curve (τ = shear stress, $\dot \gamma$ = shear rate) of wall‐slipping plastics for constant pressure.

Slotted-Plate Device To Measure the Yield Stress of Suspensions: Finite Element Analysis

A slotted-plate technique has been used to measure the yield stress of suspensions. The wall slip effect is eliminated by opening a number of slots on the plate, which is dragged through the suspen...