Fluid shear, effect

Fluid shear, effect of secondary for each particular system, prevent incrustation... 

As observed from Figure 27, the cake removal by fluid shear is also aided by centrifugal force. Other arrangements include stationary filtration media and rotating disks to create the shear effects, and rotating cylindrical elements it has also been shown how such filters can be used for cake washing. 

Drop breakage occurs when surrounding fluid stresses exceed the surface resistance of drops. Drops are first elongated as a result of pressure fluctuations and then spHt into small drops with a possibiUty of additional smaller fragments (Fig. 19). Two types of fluid stresses cause dispersions, viscous shear and turbulence. In considering viscous shear effects, it is assumed that the drop size is smaller than the Kohnogoroff microscale, Tj. 

The value for n is often given as 2/3, but polymer melts have shown a wide range of values. The constant d is associated with mpture of the linkages in the stmcture of the fluid. The effect of different values of a, ie, at the same values of T q and, is shown in Figure 4. As d increases, breakdown occurs at lower and lower shear rates. 

When we consider many particles settling, the density of the fluid phase effectively becomes the bulk density of the slurry, i.e., the ratio of the total mass of fluid plus solids divided by the total volume. The viscosity of the slurry is considerably higher than that of the fluid alone because of the interference of boundary layers around interacting solid particles and the increase of form drag caused by particles. The viscosity of a slurry is often a function of the rate of shear of its previous history as it affects clustering of particles, and of the shape and roughness of the particles. Each of these factors contributes to a thicker boundary layer. 

Fig. 8. Sustained damage in Daucus carota suspensions, as a function of total energy expended, under laminar flow conditions in a Couette viscometer. Redrawn from Dunlop et al. (1994) Effect of fluid shear forces on plant cell suspensions. Chem Eng Sci 49 2263 - 2276, with permission of Elsevier Science...

Shah, S.N. and Watters, L.T. "Time and Shear Effects on Rheological Properties of Crosslinked Fluids Evaluation Method," SPE Prod. Eng.(January 1986) 55 61. 

If colloids are sufficiently large or the fluid shear rate high, the relative motion from velocity gradients exceeds that caused by Brownian (thermal) effects (orthokinetic agglomeration). 

Heterodisperse Suspensions. The rate laws given above apply to monodisperse colloids. In polydisperse systems the particle size and the distribution of particle sizes have pronounced effects on the kinetics of agglomeration (O Melia, 1978). For the various transport mechanisms (Brownian diffusion, fluid shear, and differential settling), the rates at which particles come into contact are given in Table 7.2. 

Acoustic cavitation (AC), formation of pulsating cavities in a fluid, occurs when a powerful ultrasound is applied to a non-viscous fluid. The cavities are formed when the variable acoustic pressure in the rarefaction phase exceeds the cohesive strength of the fluid. Under acoustic treatment (AT), cavities grow to resonance dimensions conditioned by frequency, amplitude of oscillations, stiffness properties and external conditions, and start to pulsate synchronously (self-consistently) with acoustic pressure in the medium. The cavities undergo significant strains (compared to their dimensions) and their size decreases under compression up to collapsing. This nonlinear behavior determines the active, destructional character of the cavities near which significant shear velocities, local pressure and temperature bursts occur in the fluid. Cavitation determines the specific character of acoustic treatment of the fluid and effects upon objects resident in the fluid, as well as all consequences of these effects. 

In experiments with cultured cells it has been shown that osteocytes, but not periosteal fibroblasts, are extremely sensitive to fluid flow, resulting in increased prostaglandin as well as nitric oxide production [104, 105], Three different cell populations, namely osteocytes, osteoblasts, and periosteal fibroblasts, were subjected to two stress regimes, pulsatile fluid flow and intermittent hydrostatic compression [104], Intermittent hydrostatic compression was applied at 0.3 Hz with a 13-kPa peak pressure. The pulsatile fluid flow was a fluid flow with a mean shear stress of 0.5 Pa with cyclic variations of 0.02 Pa at 5 Hz. The maximal hydrostatic pressure rate was 130 kPa/sec and the maximal fluid shear stress rate was 12 Pa/sec. Under both stress regimes, osteocytes appeared more sensitive than osteoblasts, and osteoblasts more sensitive than periosteal fibroblasts. However, despite the large difference in peak stress and peak stress rate, pulsatile fluid flow was more effective than intermittent hydrostatic compression. Osteocytes, but not the other cell types, responded to 1 hour pulsatile fluid flow treatment with a sustained prostaglandin E2 upregula-... 

To further sort out the effect of mixing, it is usually desirable to vary the impeller diameter. For example, if a 100-mm impeller had been used in a 300-mm diameter tank for the original runs, and if it were thought that pumping capacity would be more helpful in fluid shear rate, a series of runs with 125- or 150-mm diameter im-... 

First ask yourself if there is any role for fluid shear stresses in determining and obtaining the desired process result. About half of the time the answer will likely be no. That is the percentage of mixing processes where fluid shear stresses either have no effect or seem to have no effect on the process result. In these cases, mixer design can be based on pumping capacity, blend time, velocities and other matters of that nature. Impeller type location and other geometric variables are major factors in these types of processes. 

However, if the answer to this first question is yes there is an effect of fluid shear stresses on the process, then there needs to have a second question asked. Is it at the micro-or macroscale that the process participants are involved And, of course, it may be both. 

The process of particle collision is governed by physical factors such as diffusion, temperature, fluid shear, particle and fluid density, and the size of particles and aggregates. Whether particles will adhere when they collide is considered to be a function of conditions at the interface between the two solid particles and the fluid medium. Chemical interactions at the solid-liquid interface are responsible for the development of surface charge and potential, the electric diffuse layer, and hydration and hydrophobic effects which determine the probability of particle attachment. 

The cone-and-plate viscometer is an in vitro flow model used to investigate the effects of bulk fluid shear stress on suspended cells. Anticoagulated whole blood specimens (or isolated cell suspensions) are placed between the two platens (both of stainless steel) of the viscometer. Rotation of the upper conical platen causes a well-defined and uniform shearing stress to be applied to the entire fluid medium as described by Konstantopolous et al. (1998). The shear rate (y) in this system can be readily calculated from the cone angle and the speed of the cone using the formula i/ = where y is the shear rate in sec-1, mis the... 

Mechanical cell disruption techniques are based on high shear effects as fluid is forced through a narrow orifice, or a chamber containing rotating disks and glass beads to break up the cellular material. Both techniques enable some control over the extent of cell disruption. 

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