Wall Slip Coefficient Research Articles

Influence of nozzle structure and process parameters for 3D printing HTPB-based propellant

This paper studies the influence of nozzle structure and process parameters on the solid propellant 3D printing process, so as to better guide nozzle structure optimization and process improvement, and realize continuous extrusion and molding of solid propellant. Four characteristic parameters such as nozzle diameter, nozzle shrinkage angle, process temperature, and wall slip coefficient are selected, and the influence of these four parameters on the extrusion speed and pressure of solid propellant slurry in the nozzle is discussed by using the orthogonal test principle and ANSYS/POLYFLOW. The results show that the most significant factor affecting the outlet velocity uniformity index and outlet pressure is the wall slip coefficient, followed by the nozzle diameter, and the influence of temperature and contraction angle is small.

Scrutinization of second law analysis and viscous dissipation on Reiner-RivlinNanofluid with the effect of bioconvection over a rotating disk

In comparison to Newtonian fluids, non-Newtonian fluids have fascinating features in heat transportation. Here, newly type of Reiner-Rivlinnanoliquid flow over the revolving disk for viscous dissipation (VD) is being explored in a multiple-slip effect. The inclusion of gyrotactic microorganisms in the nanoliquid enhances the tendency of the nanoparticles. The idea of the intended model is enhanced by considering in the impact of activation energy, thermal radiative, heated convective conditions and entropy minimization. The system of nonlinear PDE is constructed into nonlinear ODE's by applying the von-Karman similarity method and later solved numerically using the BVP4c solver which is considered to study the complicated ordinary differential equations. TheInfluence of various parameters is elaborated and plotted physically through the graphical illustration. By contrasting the reported data in the restricted form to a previously published article, the accuracy of the current model has examined. The impact of a non-Newtonian fluid parameter over the velocity field appeared to showdpreciation in it. The results elucidate that when the wall slip coefficient is larger more torque is needed to maintain constant disk revaluation. Surface heat transmission and wall skin friction are computed for a wide variety of factors. These flows have several real world-applications, including modeling cases that occur in oceanography and geophysics, various industrial fields (such as lumber production).

Effects of gas-assisted technology on polymer micro coextrusion

Abstract In this paper, a three-dimensional viscoelastic flow model of double-layer micro coextrusion with circular cross-section was established. The simulations were carried out by the finite element method and the influence of wall slip coefficient on micro coextrusion was studied. The results show that gas-assisted technology is suitable for polymer micro coextrusion forming. The differences between the gas-assisted and conventional micro coextrusion process were compared by analyzing the distribution of melts velocity, pressure, shear rate and first normal stress difference. The research results show that in the conventional micro coextrusion process there are pressure drop, shear rate, first normal stress difference and secondary flow. And there is a gradient distribution of melts velocity. The die swell and deformation vary with the forming process parameters, and it is difficult to control the product quality. But in the gas-assisted micro coextrusion process, there is no pressure drop, shear rate, first normal stress difference and secondary flow. The velocity of polymer melts is evenly distributed and the melts are extruded in a plunger shape. The melts do not swell and deform and it is independent of forming process parameters. It is easy to ensure the products quality and the inherent problems of polymer conventional micro coextrusion are well solved.

Optical investigation of dense suspensions with non-Newtonian matrix under pulsating pumping

Pressure-driven Poiseuille flow of a bidisperse, non-colloidal, dense granular suspension with non-Newtonian matrix under pulsatile pumping is studied experimentally. The flow properties are recorded using a transmitted light optical system. The setup allows to derive velocity profiles with a high spatial and temporal resolution. Statements can be made about the near-wall region, which are difficult to achieve experimentally in dense suspensions. The results are compared in detail with velocity profiles based on theoretical models from the field of suspension rheology and particle migration under the conditions present in the experiment. • Spatially and temporally resolved velocity profiles of a pulsating pipe flow. • Complete comparison of experimental data with rheological and particle migration models. • Determination of wall slip coefficients and comparison with predictions of theoretical models. • Evaluation of velocity profiles during acceleration and deceleration phases (pulsating pumping). • Investigation of different particle distributions with regard to the evolving velocity profiles and wall slip.

Numerical study for slip flow of Reiner-Rivlin nanofluid due to a rotating disk

The analysis of heat augmentation through non-Newtonian fluid is found promising and suitable arena for technological applications than the Newtonian fluid. Herein, a new kind of non-Newtonian (Reiner-Rivlin) fluid has been examined by a rough rotating disk under various slip conditions. A coupled nonlinear differential system is obtained due to constitutive relations in Reiner-Rivlin liquid. An advantageous numerical treatment is summoned to explain the subsequent similitude conditions for wide scopes of non-Newtonian liquid parameter and slip coefficients. Our principle here is to foresee the von-Kármán flow problem under the practices of liquid viscosity and wall slip coefficients. From our calculations, it is found that to keep consistent rotation of disk we need higher torque when wall slip is high. For wide range of parameters calculations are made for surface heat transfer and wall skin friction.

A two-layer model for buoyant displacement flows in a channel with wall slip

We study theoretically buoyant displacement flows of two generalized Newtonian fluids in a two-dimensional (2-D) channel with wall slip. We assume that a pseudo-interface separates two miscible (immiscible) fluids at the limit of negligible molecular diffusion (negligible surface tension). A heavy fluid displaces a light fluid at near-horizontal channel inclinations, implying that a stratified flow assumption is relevant. We develop a classical lubrication approximation model as a semi-analytical framework that includes a number of dimensionless parameters, such as a buoyancy number, the viscosity ratio, the non-Newtonian properties and the upper and lower wall slip coefficients. For specified interface heights and slopes, the reduced model can furnish the flux and velocity functions in displacing and displaced phases. We numerically solve the interface kinematic condition for four different wall slip cases: no slip (Case I), slip at the lower wall (Case II), slip at the upper wall (Case III) and slip at both walls (Case IV). The solutions for these cases deliver the interface propagation in time, for which leading and trailing displacement front heights, shapes and speeds and several key displacement features, such as front characteristic spreading lengths and short time behaviours, can be directly predicted by simplified analyses. The results reveal in detail how the presence of a channel wall slip may significantly affect the overall displacement flow and the interface evolution characteristics, for both Newtonian and non-Newtonian fluids. Regarding the latter, our analysis quantifies in particular the appearance and removal of static residual wall layers of the displaced phase, versus the wall slip cases.

A numerical treatment for partial slip flow and heat transfer of non-Newtonian Reiner-Rivlin fluid due to rotating disk

In this article, partial slip flow of Reiner-Rivlin fluid induced by a rough rotating disk is modeled. Heat transfer is also addressed by assuming more general temperature jump condition. The constitutive relations in Reiner-Rivlin fluid lead to a coupled and strongly non-linear differential system. A convenient numerical treatment is invoked to solve the resulting similarity equations for broad ranges of non-Newtonian fluid parameter and slip coefficients. Our main interest is to predict the behaviors of fluid elasticity and wall slip coefficients on the von-Kármán flow problem. Expressions of wall skin friction and surface heat transfer are calculated and deliberated for broad parameter values. Different from radial and axial velocities, tangential velocity appears to increase as Reiner-Rivlin fluid parameter (K) increases. Reduction in surface drag coefficient, which is vital in some applications, can be accomplished by increasing the parameter K. Volumetric flow rate is also inversely proportional to the parameter K. However, heat transfer rate diminishes when parameter K enlarges. We also conclude that larger torque would be required to keep steady rotation of the disk for higher values of wall slip coefficients.

Confined viscoplastic flows with heterogeneous wall slip

The steady, pressure-driven flow of a Herschel-Bulkley fluid in a microchannel is considered, assuming that different power-law slip equations apply at the two walls due to slip heterogeneities, allowing the velocity profile to be asymmetric. Three different flow regimes are observed as the pressure gradient is increased. Below a first critical pressure gradient G 1, the fluid moves unyielded with a uniform velocity, and thus, the two slip velocities are equal. In an intermediate regime between G 1 and a second critical pressure gradient G 2, the fluid yields in a zone near the weak-slip wall and flows with uniform velocity near the stronger-slip wall. Beyond this regime, the fluid yields near both walls and the velocity are uniform only in the central unyielded core. It is demonstrated that the central unyielded region tends towards the midplane only if the power-law exponent is less than unity; otherwise, this region rends towards the weak-slip wall and asymmetry is enhanced. The extension of the different flow regimes depends on the channel gap; in particular, the intermediate asymmetric flow regime dominates when the gap becomes smaller than a characteristic length which incorporates the wall slip coefficients and the fluid properties. The theoretical results compare well with available experimental data on soft glassy suspensions. These results open new routes in manipulating the flow of viscoplastic materials in applications where the flow behavior depends not only on the bulk rheology of the material but also on the wall properties.

Two differet self-propulsion types of Janus microspheres: from the comparative experiments and driving mechanisms

A Janus particle is a general term for a non-uniform particle that has different properties on different sides of particle. For a Pt-SiO2 type of Janus microsphere, Pt side serves as the catalysis surface to decompose H2O2 solution, leading to the self-propulsion motion of particle. In this paper, the relevant experimental phenomena in two driven modes are compared first. The results show that under the same concentration of solution, the microsphere with a diameter of about 1 m experiences self-diffusiophoresis propulsion; whereas, the one with an about 20 m diameter experiences bubble self-propulsion. Significant differences in motional trajectory and propulsion velocity are found between them. Then, the dominated physical factors are analyzed and the multi-field coupling numerical model is constructed based on the simplified force balance analysis. Subsequently, the velocity field distribution and O2 concentration distribution around Janus microsphere are also studied. According to these studies, we explain the position and size of the bubble generated. Further more, we infer that the wall slip coefficient is a key matching parameter in the numerical model, and two slip coefficients with a difference of an order of magnitude are given corresponding to the two types of self-propulsion modes. Then we explain the possible mechanism for the changes of wall slip coefficient under different particle sizes. The present study is beneficial to the in-depth exploration of the self-propulsion mechanism and also provides the theoretical foundation for improving the performance of self-propellant device.

Numerical Simulation on the Whole Co-Extrusion Process of Tread Rubber

The Phan-Thinen and Tanner (PTT) model was used to describe the co-extrusion behavior of tread rubber which is made up of two components. To investigate the effects of upper and lower channels on simulation results, two kinds of Finite Element (FE) models were established. One of them contained the upper and lower channels, while another did not. Comparison between the computational results and corresponding test data illustrates that the difference of these two models is obvious, which suggests that the consideration of upper and lower channels play an important role in analysis on polymer’s co-extrusion process. Besides, we also analyzed the effects of wall slip coefficients on co-extrusion products. The results show that the decreasing of wall slip coefficients can reduce the extrusion swell phenomenon effectively, but it may bring out the distorting of the corner parts of co-extrusion section shape. Therefore, various factors need to be comprehensive taken into account in the selection of wall slip coefficients.

Three-dimensional viscoelastic simulation of the effect of wall slip on encapsulation in the coextrusion process

Abstract A three-dimensional viscoelastic numerical simulation was developed for a two-layer coextrusion through a rectangular channel by using the finite element method. The Phan-Thien and Tanner model was considered as viscoelastic constitutive equations. The generalized Navier’s law was adopted to found the slip boundary condition. The numerical results of the effects of the wall slip coefficient and the flow rate on the interface profile and the degree of encapsulation were compared with the experimental results of previous researchers. It was found that the interfacial offset and the degree of encapsulation increased with the increase of the wall slip coefficient and the flow rate, and the growing rate was large when the wall slip coefficient was between 106 and 108. We were able to control the interface shape and the degree of encapsulation at the die exit by varying the wall slip coefficient and the magnitude of the melt flow rate.

Rheology and Hydraulics of Polymer (HEC) Based Drilling Foams at Ambient Temperature Conditions

Summary An experimental investigation on polymer-based drilling foams was carried out. Rheology tests were performed with foams that have different concentrations of hydroxylethylcellulose (HEC) and 1% commercial surfactant. Experiments were conducted in a large-scale flow loop that permits foam flow through 2-, 3-, and 4-in. pipe sections, and a 6×3.5-in. annular section. During the experiments, frictional pressure losses across the pipe and annular sections were measured for different gas/liquid flow rates, polymer concentrations (0, 0.25, and 0.5%), and foam qualities (70, 80, and 90%). Significant rheological variations were observed between aqueous foams containing no polymers and polymer-thickened foams. Experimental data show three distinct flow curves for the 2-, 3-, and 4-in. pipe sections, which indicates the presence of wall slip. The Oldroyd-Jastrzebski approach was used to calculate the wall slip velocity and determine the true shear rate. It has been found that wall slip decreases as the foam quality or polymer concentration increases. Two foam hydraulic models, which use slip-corrected and slip-uncorrected rheological parameters, have been proposed. These models are applicable for predicting pressure loss in pipes and annuli. Model predictions for the annular test section are compared with the measured data. A satisfactory agreement between the model predictions and measured data is obtained. This paper will help to better design foam drilling and cleanup operations. Introduction The use of drilling foams is increasing because foams exhibit properties that are desirable in many drilling operations. In practice, aqueous and polymer-based foams have been used with commercial success. However, drilling-foam rheology and hydraulics are still not sufficiently understood to minimize the risk and costs associated with foam drilling. It is generally accepted that the addition of polymers to the liquid phase affects the viscosity and stability of foams. However, the degree to which the bulk properties of drilling foams are enhanced by polymers has not been well understood and is difficult to predict. For safe and economical foam drilling, accurate knowledge of bottomhole pressure is essential. However, foam rheology and pressure drop predictions are not accurate enough to provide adequate hydraulic design information such as equivalent circulation density. This problem is more pronounced when polymers are added, because the apparent foam viscosity of polymer-thickened foams can be significantly higher than aqueous foams. It becomes apparent that there is a need for polymer foam rheological characterization in order to improve the knowledge of foam rheology and hydraulics. Foam rheological characterization was carried out using large-scale, single-pass pipe viscometers (composed of 2-, 3-, and 4-in. pipe sections). Foam qualities were varied from 70 to 90%. Test pressure and temperature were 100 psig and 80°F. Two foam hydraulic models were considered, assuming both no-slip condition at the wall and slip condition at the wall. The first model assumes no-slip boundary conditions in both pipes and annulus. By assuming no slip condition at the wall, slip-uncorrected foam rheological parameters were obtained from the pipe viscometer measurements. It has been found that if we plot friction factors vs. Reynolds numbers for all test data, regardless of pipe diameters, foam qualities, and flow rates, a single curve is obtained. This curve is similar to that obtained for incompressible fluid flow. Pressure drop in the annulus is calculated with the proposed model, and satisfactory predictions are obtained. The second model is based on the assumption that there is wall slip in both pipes and annulus. Rheological parameters and wall-slip coefficient corrections were first obtained using Oldroyd-Jastrzebski approach. The annular pressure losses are predicted based on slip-corrected rheological parameters and wall-slip coefficient correlations.

The effect of the slip condition on Stokes and Couette flows due to an oscillating wall: exact solutions

Stokes and Couette flows produced by an oscillatory motion of a wall are analyzed under conditions where the no-slip assumption between the wall and the fluid is no longer valid. The motion of the wall is assumed to have a generic sinusoidal behavior. The exact solutions include both steady periodic and transient velocity profiles. It is found that slip conditions between the wall and the fluid produces lower amplitudes of oscillations in the flow near the oscillating wall than when no-slip assumption is utilized. Further, the relative velocity between the fluid layer at the wall and the speed of the wall is found to overshoot at a specific oscillating slip parameter or vibrational Reynolds number at certain times. In addition, it is found that wall slip reduces the transient velocity for Stokes flow while minimum transient effects for Couette flow is achieved only for large and small values of the wall slip coefficient and the gap thickness, respectively. The time needed to reach to steady periodic Stokes flow due to sine oscillations is greater than that for cosine oscillations with both wall slip and no-slip conditions.

Compressive Squeeze Flow of Generalized Newtonian Fluids with Apparent Wall Slip

Abstract The compressive squeeze flow is encountered in various processes like compression molding and in rheometry. It involves a complex deformation field driven by both shear and normal stresses. Here the flow and deformation field generated during the compressive squeeze flow of generalized Newtonian fluids, subject to wall slip, was analyzed using both numerical simulation based on the 2-D Finite Element Method and the 1-D analytical model, which is based on the lubrication assumption. The agreement between the analytical and numerical solutions deteriorates with increasing Navier's wall slip coefficient and the increasing yield stress value of the viscoplastic fluid. The lubrication assumption is valid under various squeeze flow conditions but numerical analysis in conjunction with experimental studies are necessary to determine the ranges of material parameters and operating conditions for which the lubrication assumption is valid.

Slip Flow in Partially Filled Screw Channel

In polymer extrusion, occurrence of wall slip can considerably alter the flow and mixing behavior. In most cases, it occurs in partially filled screw channels because starved feeding is very common in industrial processing. In this paper an analytical model was developed to study the extrusion flow of Newtonian fluids with wall slip in a partially filled single screw extruder. Based on this analysis, the change of flow due to wall slip phenomenon was analyzed. The effects of percent channel fill, wall slip coefficient and viscosity on the flow and mixing are investigated.

EXTRUSION AND LUBRICATION FLOWS OF VISCOPLASTIC FLUIDS WITH WALL SLIP

Abstract The simplest model flow which approximates the extrusion (shallow screw channels) and lubrication flow is the steady, laminar flow occurring between two infinitely long parallel plates i.e., the generalized plane Couette flow. Here we develop an analytical model of the generalized plane Couette flow of viscoplastic fluids. The deformation and flow behavior of viscoplastic fluids can be realistically represented with the Herschel-Bulkley constitutive equation, which we have utilized as the basis for the development of our analytical model. Furthermore, as also demonstrated here, the deformation behavior of viscoplastic fluids is generally complicated by the presence of wall slip at solid walls, which occurs as a function of the wall shear stress. The wall slip versus the wall shear stress behavior of viscoplastic fluids can be experimentally characterized using viscomelric flows, including steady torsional and capillary flows. Thus determined Navier's wall slip coefficient can then be utilized in ...

EFFECT OF DISPERSED PHASE ON THE SLIP COEFFICIENT OF APPLE SAUCE IN A CONCENTRIC CYLINDER VISCOMETER

Slip coefficients for apple sauce in concentric cylinder viscometers were determined using concentric cylinders of different radii according to the theory of Mooney. Wall slip was relatively low for highly shear‐thinning samples and for highly viscous samples; it also decreased with increase in pulp content until tunneling of the sample by the bob condition was reached. Wall slip coefficient varied with the applied torque so that any comparison of slip between samples must be made by determining the slip coefficients for the samples at the same magnitude of applied torque.