Flow behavior in microchannel made of different materials with wall slip velocity and electro-viscous effects

The flow of a suspension system with glass microspheres in polymer EVA (Ethylene Vinyl Acetate) melts system was studied in a capillary rheometer. The slip velocity was determined by Mooney technique. A modified slip law describing the slip velocity as a function of the wall shear stress and particle concentration was proposed and employed to describe the flow behavior of the suspension system.

Download Full-text

Large Eddy Simulations of Strongly Non-Ideal Compressible Flows through a Transonic Cascade

Energies ◽

10.3390/en14030772 ◽

2021 ◽

Vol 14 (3) ◽

pp. 772

Author(s):

Jean-Christophe Hoarau ◽

Paola Cinnella ◽

Xavier Gloerfelt

Keyword(s):

Compressible Flows ◽

Flow Behavior ◽

Pressure Ratio ◽

Organic Rankine Cycle ◽

Ideal Gas ◽

Operating Conditions ◽

Large Eddy Simulations ◽

Working Fluid ◽

Viscous Effects ◽

Large Eddy

Transonic flows of a molecularly complex organic fluid through a stator cascade were investigated by means of large eddy simulations (LESs). The selected configuration was considered as representative of the high-pressure stages of high-temperature Organic Rankine Cycle (ORC) axial turbines, which may exhibit significant non-ideal gas effects. A heavy fluorocarbon, perhydrophenanthrene (PP11), was selected as the working fluid to exacerbate deviations from the ideal flow behavior. The LESs were carried out at various operating conditions (pressure ratio and total conditions at inlet), and their influence on compressibility and viscous effects is discussed. The complex thermodynamic behavior of the fluid generates highly non-ideal shock systems at the blade trailing edge. These are shown to undergo complex interactions with the transitional viscous boundary layers and wakes, with an impact on the loss mechanisms and predicted loss coefficients compared to lower-fidelity models relying on the Reynolds-averaged Navier–Stokes (RANS) equations.

Download Full-text

Quantification of shear viscosity and wall slip velocity of highly concentrated suspensions with non-Newtonian matrices in pressure driven flows

Rheologica Acta ◽

10.1007/s00397-021-01281-5 ◽

2021 ◽

Author(s):

Patrick Wilms ◽

Jan Wieringa ◽

Theo Blijdenstein ◽

Kees van Malssen ◽

Reinhard Kohlus

Keyword(s):

Shear Stress ◽

Liquid Phase ◽

Shear Rate ◽

Shear Viscosity ◽

Slip Velocity ◽

Volume Fraction ◽

Solid Volume Fraction ◽

Wall Slip ◽

Flow Index ◽

Concentrated Suspensions

AbstractThe rheological characterization of concentrated suspensions is complicated by the heterogeneous nature of their flow. In this contribution, the shear viscosity and wall slip velocity are quantified for highly concentrated suspensions (solid volume fractions of 0.55–0.60, D4,3 ~ 5 µm). The shear viscosity was determined using a high-pressure capillary rheometer equipped with a 3D-printed die that has a grooved surface of the internal flow channel. The wall slip velocity was then calculated from the difference between the apparent shear rates through a rough and smooth die, at identical wall shear stress. The influence of liquid phase rheology on the wall slip velocity was investigated by using different thickeners, resulting in different degrees of shear rate dependency, i.e. the flow indices varied between 0.20 and 1.00. The wall slip velocity scaled with the flow index of the liquid phase at a solid volume fraction of 0.60 and showed increasingly large deviations with decreasing solid volume fraction. It is hypothesized that these deviations are related to shear-induced migration of solids and macromolecules due to the large shear stress and shear rate gradients.

Download Full-text

Swirling Flows Characteristics in a Cylinder Under Effect of Buoyancy

Mechanika ◽

10.5755/j02.mech.27975 ◽

2021 ◽

Vol 27 (3) ◽

pp. 201-208

Author(s):

Mustafa FEKHAR ◽

Rachid SACI ◽

Renée GATIGNOL

Keyword(s):

Aspect Ratio ◽

Thermal Stratification ◽

Flow Behavior ◽

Flow Characteristics ◽

Swirling Flows ◽

Viscous Effects ◽

Thermal Buoyancy ◽

State Diagrams ◽

The Core ◽

Flow Stagnation

Thermal buoyancy, induced by injection or by differential heating of a tiny rod is explored to control breakdown in the core of a helical flow driven by the lid rotation of a cylinder. Three main parameters are required to characterize numerically the flow behavior; namely, the rotational Reynolds number Re, the cavity aspect ratio and the Richardson number Ri. Warm injection/rod, Ri > 0, is shown to prevent on-axis flow stagnation while breakdown enhancement is evidenced when Ri < 0. Results revealed that a bubble vortex evolves into a ring type structure which may remain robust, as observed in prior related experiments or, in contrast, disappear over a given range of parameters (Λh, Re, Ri > 0). Besides, the emergence of such a toroidal mode was not found to occur under thermal stratification induced by a differentially heated rod. Moreover, three state diagrams were established which provide detailed flow characteristics under the distinct and combined effects of buoyancy strength, viscous effects and cavity aspect ratio.

Download Full-text

Application of CFD-DSMC coupling method in micro-scale gas flow and combustion

Modern Physics Letters B ◽

10.1142/s0217984920503017 ◽

2020 ◽

Vol 34 (27) ◽

pp. 2050301

Author(s):

Shaoyi Suo ◽

Linsong Jiang ◽

Maozhao Xie

Keyword(s):

Boundary Layer ◽

Chemical Reaction ◽

Wall Temperature ◽

Slip Velocity ◽

Gas Flow ◽

Wall Slip ◽

Reaction Products ◽

Microscopic Method ◽

Flow Heat ◽

The Impact

The reversible elementary reaction mechanism of six components and seven steps of H2/O2 are applied by using a CFD-DSMC coupling iteration method to study the impact of boundary on flow, heat transfer and chemical reaction in a microtube. The microtube consists of a converging section and a straight section, which represents the gap on the contact surface of the pellets in porous media. It shows that after coupling, with the designed conditions in this paper, the influence of wall temperature is more obvious than that of wall slip velocity on the coupling results from the analysis of chemical reaction, yet the velocity field in the boundary layer is more affected by the wall slip velocity. In addition, the velocity in the central region of the flow decreases while the concentration of reaction products increases after coupling, due to the increasing of the velocity in the boundary layer and the influence of wall temperature, respectively. By the coupling of CFD-DSMC methods, more details and influence of the boundary can be considered, and the computational efficiency is higher than that of the single microscopic method.

Download Full-text

Experimental study and molecular dynamics simulation of wall slip in a micro-extrusion flowing process

Proceedings of the Institution of Mechanical Engineers Part C Journal of Mechanical Engineering Science ◽

10.1177/09544062jmes2335 ◽

2011 ◽

Vol 225 (5) ◽

pp. 1175-1190 ◽

Cited By ~ 7

Author(s):

D Zhao ◽

Y Jin ◽

M Wang ◽

M Song

Keyword(s):

Molecular Dynamics ◽

Shear Stress ◽

Molecular Dynamics Simulation ◽

Slip Velocity ◽

Polymer Melts ◽

Critical Shear Stress ◽

Wall Slip ◽

Dynamics Simulation ◽

Desorption Mechanism ◽

Adsorption Desorption

Wall slip is one of the most important characteristics of polymer melts’ elasticity behaviours as well as the most significant factor which affects the flow of polymer melts. Based on the traditional Mooney method, through a double-barrel capillary rheometer, the relationship between velocities of wall slip, shear stress, shear rate, diameters of dies, and temperature of polypropylene (PP), high-density polyethylene (HDPE), polystyrene (PS), and polymethylmethacrylate (PMMA) is explored. The results indicate that the velocities of the wall slip of PP and HDPE increase apparently with shear stress and slightly with temperature. Meanwhile, the rise of temperature results in the decrease of critical shear stress. The wall-slip velocities of PS and PMMA are negative which means that the Mooney method based on the adsorption–desorption mechanism has determinate limitation to calculate the wall-slip velocity. Based on the entanglement–disentanglement mechanism, a new wall-slip model is built. With the new model, the calculation values of velocity of PP and HDPE correspond to the experimental values very well and the velocities of PS and PMMA are positive. The velocities of PS and PMMA increase obviously with the rise of shear stress. The rise of temperature results in the increase of velocity and decrease of critical shear stress. Then, the molecular dynamics simulation is used to investigate the combining energy between four polymer melts and the inside wall. The results show that at the given temperature and pressure, the molecules of PS and PMMA combine with atoms of the wall more tightly than those of PP and HDPE which means when wall slip occurs, the molecules of PS and PMMA near the wall will adsorb to the surface of the wall. However, those of PP and HDPE will be easy to slip. Therefore, the wall-slip mechanism of PP and HDPE is the adsorption–desorption mechanism, and that of PS and PMMA is the entanglement–disentanglement mechanism. According to the different wall-slip mechanisms of four polymers, an all-sided calculation method of wall-slip velocity is raised which consummates the theory of wall slip of polymer melts.

Download Full-text

Development Length in Planar Channel Flows of Newtonian Fluids Under the Influence of Wall Slip

Journal of Fluids Engineering ◽

10.1115/1.4007383 ◽

2012 ◽

Vol 134 (10) ◽

Cited By ~ 7

Author(s):

L. L. Ferrás ◽

A. M. Afonso ◽

M. A. Alves ◽

J. M. Nóbrega ◽

F. T. Pinho

Keyword(s):

Slip Velocity ◽

Numerical Study ◽

Fluid Flows ◽

Wall Slip ◽

Newtonian Fluids ◽

Planar Channel ◽

Development Length ◽

Navier Slip ◽

Slip Boundary ◽

Low Reynolds Number Flows

This technical brief presents a numerical study regarding the required development length (L=Lfd/H) to reach fully developed flow conditions at the entrance of a planar channel for Newtonian fluids under the influence of slip boundary conditions. The linear Navier slip law is used with the dimensionless slip coefficient k¯l=kl(μ/H), varying in the range 0<k¯l≤1. The simulations were carried out for low Reynolds number flows in the range 0<Re≤100, making use of a rigorous mesh refinement with an accuracy error below 1%. The development length is found to be a nonmonotonic function of the slip velocity coefficient, increasing up to k¯l≈0.1-0.4 (depending on Re) and decreasing for higher k¯l. We present a new nonlinear relationship between L, Re, and k¯l that can accurately predict the development length for Newtonian fluid flows with slip velocity at the wall for Re of up to 100 and k¯l up to 1.

Download Full-text

WALL SLIP EFFECTS MEASURING THE RHEOLOGICAL BEHAVIOR OF ELECTRORHEOLOGICAL (ER) SUSPENSIONS

International Journal of Modern Physics B ◽

10.1142/s0217979211102150 ◽

2012 ◽

Vol 26 (01) ◽

pp. 1250006 ◽

Cited By ~ 3

Author(s):

STEFFEN SCHNEIDER

Keyword(s):

Shear Stress ◽

Wall Shear Stress ◽

Flow Behavior ◽

Wall Slip ◽

Hydraulic Test ◽

Slip Effects ◽

Measured Flow ◽

Wall Shear ◽

Er Fluids ◽

Electrorheological Suspensions

In this work, a new method to determine the wall shear stress was developed step by step. To determine the wall shear stress, methods of the suspension rheology are being used for the first time to characterize ER fluids. This work focuses on investigations of the flow behavior of electrorheological suspensions in flow channels with different geometries at different electrical field strengths. Careful interpretation of the results with respect to different gap geometries has shown that the measured flow curves should undergo a combination of corrections. As a result it can be shown that wall slip effects can be measured under application like conditions on a hydraulic test bench.

Download Full-text