Couette system

The RMS-800 provides steady-shear rotational rates from 10 to 100 rad/s and oscillatory frequencies from 10 to 100 rad/s. An autotension device compensates for expansion or contraction. With the standard 25- and 50-mm parallel plates, the viscosity range is 50-10 mPa-s, and the shear modulus range is 8 x 10 to 10 N/m. These ranges can be expanded with nonstandard plates, cones, and a Couette system. The temperature range is 20-350°C (-150 0 optional). 

In some foods, a thin layer of low-viscosity fluid forms at the solid-fluid interface that in turn contributes to lower viscosity values. The boundary condition that at the solid-fluid interface the fluid velocity is that of the wall is not satisfied. This phenomenon is known as slip effect. Mooney (1931) outlined the procedures for the quantitative determination of slip coefficients in capillary flow and in a Couette system. The development for the concentric cylinder system will be outlined here for the case of the bob rotating and details of the derivation can be found in Mooney (1931). 

The Couette system (Fig. 9.2) is another concentric cylinder system with the advantage that the drive and sensor are separate. The motor drives the outer cylinder the inner, stationary cylinder is connected to the sensor. 

Figure 6 Viscosity as a function of the shear rate y for microemulsions containing DDABr-water-dodecane. Samples have a constant dodecane/DDABr weight ratio of 4 6. Weight percentage of water (x, +) 22% (O, ) 30% (A, A) 40% ( , ) 50% (O. ) 60%, Open symbols denote measurements in a Couette system, and closed symbols signify measurements taken with a Rheometrics RFX instrument. (From Ref. 110.)...

A few differential methods of rheometry have appeared in the literature over the past 50 years. In 1941 J. Pryce-Jones (1) reported on a twin Couette system in which the two outer cylinders rotated in opposite directions while the two inner cylinders were connected together and to a torsion wire. Sakamoto et al. (2) described another dual Couette system with two identical cylinder sets one above the other. The upper-outer and the lower-inner cylinders were connected together while the upper-inner was fixed to the support. The lower-outer cylinder was driven at a constant... 

According to DIN 53018, simple rotational viscosimeter have a coaxial cylinder system with a cup (inner radius R ) and a cylinder with an outer radius Rj and a length h, that is lowered into the cup. For the Couette system, the outer cup is moved and the measurement of the torque T takes place at the inner cylinder. For the Searle system on the other hand, the inner cylinder is moved. 

Because of centrifugal forces in the Searle System, crosscurrents can arise, that are called Taylor vortices. Couette Systems on the other hand have the disadvantage that temperature control of the rotating outer cylinder is achieved only with complicated equipment. Most rotational viscosimeters are therefore built in the Searle Type. 

Figure 4.21 Longitudinal ribs used on the inner rotating cylinder in aTaylor-Couette system.The flow is essentially axial (jeng et al., 2007).

Water purification Rotation plus photocatalysis -Taylor-Couette system... 

Andereck D, Liu S, Swinney H (1986) Row regimes in a circular Couette system with independently rotating cylinders. 1 Huid Mech 164 155-183... 

In shear measurements one expects the described solutions behave like normal Newtonian aqueous solutions. This is in fact the case for small shear rates (Fig. 11.32). In Fig. 11.32 the shear viscosity, which was measured in a capillary viscometer, is plotted vs.the shear rate. One observes a sudden rise of the viscosity at a characteristic shear rate and for y> % the solutions show some shear thickening behaviour. Obviously something dramatic has happened to the micelles in the solutions. Some conclusions about what has happened can be drawn from flow birefringence measurements. Some typical results of flow measurements from a Couette system are shown in Fig. 11.33. We note a sudden increase of the flow birefringence at a critical shear rate. For y < yc no flow birefringence could be detected. 

During the experiments, the solid concentration was increased to 20% by volume. Except for suspensions with plastic particles, the suspensions showed a Newtonian behavior up to volume contents of 15 %. Suspensions with glass beads and s = 0.2 as well as all examined suspensions with plastic particles showed a shear thinning behavior. Considering the non-Newtonian behavior of these suspensions in the calculation of the time steady flow based on Eqs. (5.9-5.21), the viscosity of the suspension had to be described by a model depending on the deformation speed y. A Carreau-Yasuda model according to Eq. (5.52) fitted well to measurements carried out with a Couette system. The parameters Hq, a, n, and X were determined by the rheological measurements. 

The change in the viscosity of the system upon nanosilica reinforcement was analyzed as a function of the filler content. The viscosity measurements were performed on a parallel plate viscometer from Couette-System/MetllCT-Toledo GmbH. The viscosity change was monitored between room temperature and 100°C. 

If a fluid is placed between two concentric cylinders, and the inner cylinder rotated, a complex fluid dynamical motion known as Taylor-Couette flow is established. Mass transport is then by exchange between eddy vortices which can, under some conditions, be imagmed as a substantially enlranced diflfiisivity (typically with effective diflfiision coefficients several orders of magnitude above molecular difhision coefficients) that can be altered by varying the rotation rate, and with all species having the same diffusivity. Studies of the BZ and CIMA/CDIMA systems in such a Couette reactor [45] have revealed bifiircation tlirough a complex sequence of front patterns, see figure A3.14.16. 

Under steady-state conditions, as in the Couette flow, the strain rate is constant over the reaction volume for a long period of time (several hours) and the system of Eq. (87) could be solved exactly with the matrix technique developed by Basedow et al. [153], Transient elongational flow, on the other hand, has two distinctive features, i.e. a short residence time (a few ps) and a non-uniform flow field, which must be incorporated into the kinetics equations. In transient elongational flow, each rate constant is a strongfunction of the strain-rate which varies with time in the Lagrangian frame moving with the center of mass of the macromolecule the local value of the strain rate for each spatial coordinate must be known before Eq. (87) can be solved. 

At the 24th Combushon Symposium, Shy et al. [26] introduced an experimental aqueous autocatalytic reaction system to simulate the premixed turbulent combustion in a well-known Taylor-Couette (TC) flow field. By electrochemically inifiafing fhis reachon system, the... 

Couette Flow Simulation. MD typically simulate systems at thermodynamic equilibrium. For the simulation of systems undergoing flow various methods of nonequilibrium MD have been developed (Ifl iZ.). In all of these methods the viscosity Is calculated directly from the constitutive equation. 

We simulated two systems (1) bulk fiuld (no wall potential) at equilibrium and undergoing Couette fiow, and (2) fiuld confined between planar micropore walls at equilibrium and undergoing Couette fiow. 

The shear stress Is uniform throughout the main liquid slab for Couette flow ( ). Therefore, two Independent methods for the calculation of the shear stress are available It can be calculated either from the y component of the force exerted by the particles of the liquid slab upon each reservoir or from the volume average of the shear stress developed Inside the liquid slab from the Irving-Kirkwood formula (JA). For reasons explained In Reference (5) the simpler version of this formula can be used In both our systems although this version does not apply In general to structured systems. The Irvlng-Klrkwood expression for the xy component of the stress tensor used In our simulation Is... 

Flow systems. In this subsection we present the results of our Couette flow simulations. Most of these results were first presented In Reference ( ). 

Although only one density profile Is shown In each of Figures 7 and 8 the density profiles of the two systems both at equilibrium and In the presence of flow that have been determined. A conclusion of great importance that is suggested by the Couette flow simulations is that the density profiles of the two systems in the presence of flow coincide with the equilibrium density profiles, even at the extremely high shear rates employed in our simulation. A detailed statistical analysis that Justifies this point was presented In Reference ( ). 

In Couette cylinder systems, laminar flows extends to much higher Re or Ta numbers. The transition point for all radius ratios is above Re = oo2r2(r2-ri)/v >2000 [39]. 

Figure 11 shows the reference floe diameter for viscometers as a function of shear stress and also the comparison with the results for stirred tanks. The stress was determined in the case of viscosimeters from Eq. (13) and impeller systems from Eqs. (2) and (4) using the maximum energy density according to Eq. (20). For r > 1 N/m (Ta > 2000), the disintegration performance produced by the flow in the viscosimeter with laminar flow of Taylor eddies is less than that in the turbulent flow of stirred tanks. Whereas in the stirred tank according to Eq. (4) and (16b) the particle diameter is inversely affected by the turbulent stress dp l/T, in viscosimeters it was found for r > 1.5 N/m, independently of the type (Searle or Couette), the dependency dp l/ pi (see Fig. 11). 

One characteristic of shear banded flow is the presence of fluctuations in the flow field. Such fluctuations also occur in some glassy colloidal materials at colloid volume fractions close to the glass transition. One such system is the soft gel formed by crowded monodisperse multiarm (122) star 1,4-polybutadienes in decane. Using NMR velocimetry Holmes et al. [23] found evidence for fluctuations in the flow behavior across the gap of a wide gap concentric cylindrical Couette device, in association with a degree of apparent slip at the inner wall. The timescale of these fluctuations appeared to be rapid (with respect to the measurement time per shear rate in the flow curve), in the order of tens to hundreds of milliseconds. As a result, the velocity distributions, measured at different points across the cell, exhibited bimodal behavior, as apparent in Figure 2.8.13. These workers interpreted their data... 

The vortex flow reactor was a glass Couette cell driven by a Bruker RheoNMR system. The cell consisted of a stationary outer glass tube with an id of 9 mm and a rotating inner glass tube with an od of 5 mm, giving a gap of 2 mm. The Couette was filled with cylindrical bacterial cells, F. nucleatum ( 2 x 20 pm), suspended in water at a concentration of =10" cells mL-1. 

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