In this paper, the combination of structural changes and diffusion in dispersion flows is considered. It is assumed that size and shape of microstructures generated in dispersions depend on flow properties, i.e., shear rate or shear stress. These structures can be transported by diffusive processes. Possible transport mechanisms are Brownian motion and hydrodynamic diffusion. The combination of structure formation and transport may have effects on the flow and thereby on the rheology of such dispersions. For example, an apparent wall slip or pseudo wall slip may occur due to a structural change near walls.
mensetzung als Funktion der experimentellen Bedingungen qualitativ beschrieben werden (s. Abb.).Mit Hilfe von Röntgenbeugungsexperimenten an einer Synchrotron-Strahlungsquelle konnte in situ die Phasenzusammensetzung während der Entstehung der unterschiedlichen Polymorphe bei Reaktionszeiten im Millisekundenbereich vermessen werden. Dadurch konnte die Annahme experimentell bestätigt werden, dass die Keimbildungsraten der Modifikationen die Produktzusammensetzung entscheidend bestimmen.
Abbildung.Massenanteil an Calciumoxalat-Dihydrat im Produkt als Funktion der anfänglichen Übersättigung.
K.110Turbulence-Induced Aerosol Coagulation: Rates and Size Distribution Kinetics D r. G. B a r t h e l m e s E-Mail: barthelmes@ptl.mavt.ethz.ch F. O. E r n s t P r o f. D r. S. E . P r a t s i n i s *
A simple model for aggregation and fragmentation in concentrated suspensions (up to solids volume fractions of 20 %) is developed, based on a previous detailed sectional model describing the behavior of aggregate size distributions [1]. The entrapment of matrix liquid within the fractal-like aggregates renders the viscosity dependent on the degree of aggregation. A new fragmentation kernel is proposed accounting for particle-particle interaction forces and aggregate structure. The transient behavior of aggregate size and viscosity is studied and special focus is put on the calculation of steady state flow curves. The model predictions are validated by comparison to the detailed sectional model and to experimental results. The flow curves can be approximated by power-laws and their simulation is much faster than with the sectional model, see Fig. [1] G. Barthelmes, S. E. Pratsinis, H. Buggisch, Particle Size Distributions and Viscosity of Suspensions Undergoing Shear-Induced Coagulation and Fragmentation, Chem. Eng. Sci. 2003, in press.Figure. Steady state flow curves for varied solids volume fraction: comparison between results by the sectional model ([1], points) and the simplified model (lines).
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