Dynamic solids, flow pattern

Tsuji et al. (1990) have modeled the flow of plastic pellets in the plug mode with discrete dynamics following the behavior of each particle. The use of a dash pot/spring arrangement to account for the friction was employed. Their results show remarkable agreement with the actual behavior of real systems. Figure 28 shows these flow patterns. Using models to account for turbulent gas-solid mixtures, Sinclair (1994) has developed a technique that could have promise for the dense phase transport. 

As with the flow regimes in fluid dynamic theory, that is, the stagnation, laminar flow and turbulent flow, it is obvious that a solid phase can exhibit the corresponding flow pattern regimes, which herein are referred to as fixed, moving and mixed, respectively. The terms fixed, moving and mixed are defined as the relative motion of the particle phase with respect to a fixed coordinate system (see Figure 26). Examples of commercial PBC systems with different fuel-bed movement are found in section B.3.4. A comparison between theoretical and practical conversion systems. 

The operation of fluidized beds is connected to fluid mechanics within the beds (Nicastro and Glicksman, 1984). For example, heat and mass transfer are greatly influenced by the solid and fluid flow patterns, which are in turn affected by the bed geometry and internal configuration. Consequently, a thorough knowledge of fluid dynamics is a prerequisite to the successful design of a commercial unit. 

In solid-liquid mixing design problems, the main features to be determined are the flow patterns in the vessel, the impeller power draw, and the solid concentration profile versus the solid concentration. In principle, they could be readily obtained by resorting to the CFD (computational fluid dynamics) resolution of the appropriate multiphase fluid mechanics equations. Historically, simplified methods have first been proposed in the literature, which do not use numerical intensive computation. The most common approach is the dispersion-sedimentation phenomenological model. It postulates equilibrium between the particle flux due to sedimentation and the particle flux resuspended by the turbulent diffusion created by the rotating impeller. 

Petrov and Petrov (1998) developed a molecular hydrodynamic theory of film deposition during removal. Their theory correctly assumes a flow pattern - which we identified as a split streamline - between the solid substrate and the monolayer in Figure 10.5 (c). This pattern is indeed the necessary pattern for successful deposition during removal, but it is not the only flow pattern for solid removal at all dynamic contact angles. Petrov and Petrov (1998) address the kinetics of water removal between the solid and the monolayer and the formation of wet or dry monolayers depending on the amount of water entrained. 

Region IV is the window of operation for successful deposition of Y-type films. The flow pattern in this region is typical during removal of solids with dynamic contact angles 0 < < 90°. The split-ejection streamline is in the liquid phase and the interface... 

For very small dynamic contact angles, the liquid is not completely removed by the split streamline and it is entrained between the film and the solid surface, creating what is known as a wet LB film. Water trapped between the solid surface and the LB monolayer prevents adhesion and is a leading cause of monolayer instability. Petrov etal. (1980) sketched the flow pattern near the moving contact line. The flow pattern is the one described here for region IV. The authors, however, reference Huh and Scriven (1971)... 

The Gibbs elasticity characterizes the lateral fluidity of the surfactant adsorption monolayer. For high values of the Gibbs elasticity the adsorption monolayer at a fluid interface behaves as tangentially immobile. Then, if two oil drops approach each other, the hydro-dynamic flow pattern, and the hydrodynamic interaction as well, is the same as if the drops were solid particles, with the only differenee that under some conditions they could deform in the zone of contact. For lower values of the Gibbs elastieity the... 

Most of the fluid dynamic studies of gas-flowing solids-fixed bed contactors were devoted to countercurrent flow systems, because of the higher efficiency of countercurrent operations for most of the processes when compared with cocurrent operations. However, there is an upper limit for gas flow rate in countercurrent systems, due to flooding. Hence, the cocurrent operation system is an interesting alternative for higher gas flow rates, particularly for very small particles. Further, in some of the proposed applications, cocurrent contacting is a desirable flow pattern [22]. 

As with all separation principles involving particle dynamics, a knowledge of the flow pattern in the hydrocyclone is essential for understanding its function and subsequentiy for the optimum design and evaluation of the particle trajectories, which in turn allow prediction of the separation efficiency. A short account of the flow pattern within typical hydrocyclones and the known or probable behaviour of solid particles in the flow is given in the following section. This is done for the case of low viscosity liquids under conditions in which the particles cause little or no interference to the flow patterns (i.e. for low solid concentrations). 

An understanding of multiphase microflows is critical for the development and application of microstructured chemical systems in the chemical industry. As one of the most important meso-scientific issues, interfacial science could be a bridge connecting microscopic molecular components and macroscopic fluid behaviors in these systems. Working together with viscous and inertial forces, the interfacial force also dominates complicated multiphase flow patterns and well-controlled droplets and bubbles. In this review, the generation mechanisms of different flow patterns and the break-up rules for droplets and bubbles in microchannels are introduced first. The effects of the adjustable fluid/solid interfaces, or so-called wetting properties, of microchannels on multiphase flow patterns, as well as microchannel surface modification methods, are then discussed. The dynamic fluid/fluid interfaces in multiphase microflows with variable... 

Extrapolating continuous description of fluid motion to a molecular scale might be conceptually difficult but unavoidable as far as interfacial dynamics is concerned. Long-range intermolec-ular interactions, such as London-van der Waals forces, still operate on a mesoscopic scale where continuous theory is justified, but they should be bounded by an inner cut-off d of atomic dimensions. Thus, distinguishing the first molecular layer from the bulk fluid becomes necessary even in equilibrium theory. In dynamic theory, the transport in the first molecular layer can be described by Eq. (60), whereas the bulk fluid obeys hydrodynamic equations supplemented by the action of intermolecular forces. Equation (61) serves then as the boundary condition at the solid surface. Moreover, at the contact line, where the bulk fluid layer either terminates altogether or gives way to a monomolecular precursor film, the same slip condition defines the slip component of the flow pattern. 

Since the information on the velocity close to the solid wall is contained in the recovery dynamics and not in the absolute values of the fluorescence intensities, the intensity axis is scaled such that the recovery curves range from I(%) = 0 to I(%) = 100. The flow pattern of photobleached probes is strongly affected by the fast diffusion during the fluorescence recovery time (x 50 ms) a probe travels by... 

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