Flow Velocity, Rheometry

2 Flow Velocity, Rheometry. - Papers relating to the flow of hyperpolar-ized He or Xe gas are picked up in Sections 2 and 6 as well. Poiseuille flow in Xe gas phase was studied by dynamic NMR microscopy. The flow profile images are different for a short observation time than for a longer 

Visualization of gas transport in silica aerogels and zeolite molecular sieves was demonstrated by the polarization-weighted images.  

Instead of polarized noble gases, thermally polarized NMR microimaging was used to study of liquid and gas flow in monolithic catalysts. Two-dimensional spatial maps of flow velocity distributions for acetylene, propane, and butane flowing along the transport channels of shaped monolithic alumina catalysts were obtained at 7 T by NMR, with true in-plane resolution of 400 xm and reasonable detection times. The flow maps reveal the highly nonuniform spatial distribution of shear rates within the monolith channels of square cross-section, the kind of information essential for evaluation and improvement of the efficiency of mass transfer in shaped catalysts. The water flow imaging, for comparison, demonstrates the transformation of a transient flow pattern observed closer to the inflow edge of a monolith into a fully developed one further downstream. 

The flow through an abrupt contraction or a step stenosis was demonstrated. The flow of liquid crystalline hydroxypropyl cellulose aqueous solutions, where polymer was flowing through an abrupt contraction, was imaged by pulsed field gradient NMR techniques. Flow velocity was determined in the case of the flow through step stenosis.  

There is a need for improving the understanding of complex fluid and suspension flow behavior in mixing equipment. The concentration profiles obtained by MR imaging were used to monitor mixing with respect to axial position in a Kenics mixer, to which the two viscosity-matched test fluids were delivered at approximately the same volumetric flow rate.  

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Normal stress difference Complex fluid Rheometers Rheometry Viscometric flows Velocity gradient Rate of deformation Weissenberg effect Extrudate Die swell Melt fracture Hydrostatic pressure Shear direction Anisotropic... 

The title of the book, Optical Rheometry of Complex Fluids, refers to the strong connection of the experimental methods that are presented to the field of rheology. Rheology refers to the study of deformation and orientation as a result of fluid flow, and one principal aim of this discipline is the development of constitutive equations that relate the macroscopic stress and velocity gradient tensors. A successful constitutive equation, however, will recognize the particular microstructure of a complex fluid, and it is here that optical methods have proven to be very important. The emphasis in this book is on the use of in situ measurements where the dynamics and structure are measured in the presence of an external field. In this manner, the connection between the microstructural response and macroscopic observables, such as stress and fluid motion can be effectively established. Although many of the examples used in the book involve the application of flow, the use of these techniques is appropriate whenever an external field is applied. For that reason, examples are also included for the case of electric and magnetic fields. 

Funatsu K. and Sato M., "Measurement of slip velocity and normal stress difference of polyvinylchloride, Adv. in Rheol., 4 (Mexico 1984) 465-472. Knappe W. and E. Krumbock, "Evaluation of slip flow of PVC compounds by capillary rheometry," Adv. in Rheol., 3 (Mexico 1984) 417-424. de Smedt C., Nam S., "The processing benefits of fluoroelastomer appUcation in LLDPE," Plast. Rubber Process. Appl. 8,11 (1987). 

A tubular rheometry that is based on obtaining velocity profiles by NMR imaging and measuring pressure drop of the flow is used for polymer melts. This technique allows one to obtain viscosity data potentially over many... 

Here we have expressed the mean velocity in terms of the apparent shear rate j>o = 4v/R. For water (p = l()00kg/m, t] = 1 mPa-s) flowing in a O.S mm radius capillary, eq. 6.2.19 gives a maximum shear rate of 17,000 s for laminar flow. The addition of polymer or particles will tend to raise the critical Re. Therefore, with the small diameters typically used in capillary rheometry, turbulence is rarely an issue. 

Figure 2 Various types of unidirectional shear flow experiments used in rheometry. Here is the x-component of velocity and is the x-component of the displacement (adapted from R. B. Bird, R. C. Armstrong and O. Hassager, Dynamics of Polymeric Liquids , 1st edn., Wiley, New York, 1977, vol. 1, p. 142)...

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