Flow pattern rolling

Droplets appeared on the surface of the pipe (Fig. 5.33b) after increasing the water flow rate up to I/ls = 0.007 m/s. Spedding et al. (1998) referred to this regime as film plus droplet pattern. When the water flow rate increased and superficial liquid velocity was Gls = 0.03 m/s (Fig. 5.33c) droplets began to roll back into the liquid film. Kokal and Stanislav (1989) identified such a regime as annular plus roll wave flow pattern. The experimental facility used in the present study allowed us to achieve values of superficial gas velocities up to 20 m/s in the 49.2 mm pipe. 

When the shear rate reaches a critical value, secondary flows occur. In the concentric cylinder, a stable secondary flow is set up with a rotational axis perpendicular to both the shear gradient direction and the vorticity axis, i.e. a rotation occurs around a streamline. Thus a series of rolling toroidal flow patterns occur in the annulus of the Couette. This of course enhances the energy dissipation and we see an increase in the stress over what we might expect. The critical value of the angular velocity of the moving cylinder, Qc, gives the Taylor number ... 

Regions in a liquid crystal having a specific cellular periodic flow-pattern in the form of long rolls induced by the application of an electric field perpendicular to a nematic layer with an initial planar alignment of the director. 

Fig. 6. (a) The flow pattern assumed in the physical model for the motion near the wall, (b) A more realistic flow pattern of the motion, (c) The roll cell pattern near the wall in a section normal to the main direction of flow. 

Inside the shell sits the tube bundle, in which the tubes are either brazed, welded, or rolled to a circular single or double tube sheet. Typical tube flow patterns are either one, two, four, or six passes. 

Figure 6.30 Flow pattern that develops in the nip region of a two-roll calendering system.

A more detailed study of material motion in the actual compaction zone has been made in the roll press simulator using marker beads, 16 mm cinematographic photography, and stereoanalysis. The flow pattern of particles in the cups of the roller press was recorded which enabled determination of strain distribution. Figure 251 shows examples of bead positions before and after partial compaction. 

A remarkable analysis of the role of hydrodynamics in LB depositions was done by de Gennes (1986). This analysis concerns only deposition during removal of the solid substrate, de Gennes recognizes that the only flow pattern that would allow Y-deposition is a split-ejection streamline in the liquid phase. However, he uses as a reference the work of Huh and Scriven (1971), but their hydrodynamic theory predicts a rolling motion in the liquid phase. 

Figure 10.4 Schematic representation of flow patterns near a moving contact line during immersion of a solid substrate into a pool of liquid, (a) Split-injection streamline in phase B and rolling pattern in phase A. (b) Transition flow pattern with motionless interface and rolling motion in phases A and B. (c) Rolling motion in phase B and split-ejection streamline in phase A...

It will be seen in the next and subsequent chapters that a wide variety of cell geometries (e.g. parallel plates, concentric cylinders, Swiss roll), types of electrode (e.g. plates, beds, porous, expanded metals and gauzes) and flow patterns are used in industrial electrochemistry. In most the flow is too complex to warrant a detailed fluid-mechanical calculation. Rather the normal approach to mass transport in electrolytic cells is to treat the cell as a unified whole and to seek expressions in terms of space-averaged quantities which permit some insight into the mass transport conditions within the cell. 

Initial die gaps are set to about 20 percent greater than the final film thickness, and then adjusted to accommodate changes in polymer flow which are resin and rate sensitive. Higher screw speeds increase extruder output, overall film thickness, the tendency toward melt fracture, and may alter the flow pattern. Thus, extruder speed is not a recommended control. In contrast, increased chill-roll speeds decrease film thickness, reduce film width due to increased neck in, increase uniaxial orientation, and alter the optimum air gap or drawdown distance. The optimum air gap, which produces the best orientation, crystallization, and surface properties, depends on the material and chill-roll speed. At 23 to 30 m/min (75 to 100 ft/min), the air gap for low-density polyethylene is about 100 mm (4 in), but when the line speed increases, the air gap is found by trial and error, Since the chill-roll speed controls film stretching, the take-off speed has little effect on the film dimensions. 

In the process of calendering a molten polymer enters usually as a sheet on one of the two rotating rolls and leaves on the other with a reduced thickness. The process is schematically shown in Figure 4.22. It is seen that due to the reduced area, a melt bank is created before the nip region. In this melt bank, a very interesting flow pattern develops with multiple recirculation regions, as shown in Figure 4.23 from... 

Taylor vortices A secondary fluid flow pattern that can occur in the gap or annulus of a concentric cylinder, or cup and bob system known as a Couette. For a rotating bob and stationary cup, a shear rate may reach a critical value such that a series of rolling toroidal flow patterns occur in the annulus of the Couette. In a cup and bob-type rheometer, this gives rise to inaccurate measurements of viscosity. 

In addition to these secondary boimdary layer flows, there is experimental evidence that a swiss roll type of secondary flow pattern exists in the core of the cyclone body, as indicated in Fig. 3.1.3. Such a flow pattern can cause particles to recirculate in the cyclone body. 

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