Low-flow zones

The low-flow zone can be mechanically avoided by increasing the release angle significantly beyond 90°, avoiding capillary action on the blade back entirely. Establishing p > 90° eliminates the low-flow zone and thus promotes tape consistency and uniformity. A balance is typically drawn between dollars and danger, since a p > 90° blade is more costly to manufacture and refinish than a p = 90° blade. In... 

Extra precautions must also be taken to avoid hot spots due to flow maldistribution. A study by Jaffe [37] showed that a shadowing effect in large reactors can cause low flow zones and very high local temperatures. 

Cavitation Loosely regarded as related to water hammer and hydrauhc transients because it may cause similar vibration and equipment damage, cavitation is the phenomenon of collapse of vapor bubbles in flowing liquid. These bubbles may be formed anywhere the local liquid pressure drops below the vapor pressure, or they may be injected into the hquid, as when steam is sparged into water. Local low-pressure zones may be produced by local velocity increases (in accordance with the Bernouhi equation see the preceding Conservation Equations subsection) as in eddies or vortices, or near bound-aiy contours by rapid vibration of a boundaiy by separation of liquid during water hammer or by an overaU reduction in static pressure, as due to pressure drop in the suction line of a pump. 

Damage will be confined to the bubble-collapse region, usually immediately downstream of the low-pressure zone. Components exposed to high velocity or turbulent flow, such as pump impellers and valves, are subject. The suction side of pumps (Case History 12.3) and the discharge side of regulating valves (Fig. 12.6 and Case History 12.4) are frequently affected. Tube ends, tube sheets, and shell outlets in heat exchanger equipment have been affected, as have cylinder liners in diesel engines (Case History 12.1). 

This is a low flow condition where the discharge flow of the pump is restricted and the product cannot leave the pump. The liquid is forced to re-circulate from high-pressure zones in the pump into low-pressure zones across the impeller. 

The high resistanee offered by die support sereens of Zone C leads to very low flow dirough dial zone, and also leads to a reeireulating zone in die annular spaee between die eatalyst bed and die reaetor shell. The non-uniform flow dirough die eatalyst bed also brings about signifieant reeireulation in die eentral pipe. In short, die model tells us that the mere removal of shroud and filling the eatalyst in Zones B and C may not lead to eapaeity enhaneements, due to the assoeiated problems of maldistribution. 

The demands and design of a local ventilation system (not only local ex hausts) should naturally start with the demanded target levels and the toxicity of the air contaminants (see Chapters 5 and 6). For best performance the exhaust should be close to the source and preferably enclose the source, there should be no disturbances of the flow, and at the same time it should have a low flow rate and be able to minimize the concentration of even quite dangerous air contaminants in the working zone. 

The possible advantages of this system over the equilibrium-limited reactor system are smaller catalyst beds, lower gas recycle requirements, and lower capital requirements. The possible disadvantages of this system are (a) practically no turn-down since any turn-down would be equivalent to decreased space velocities, closer approach to equilibrium, and higher temperature rises (b) maldistribution of gases across the bed would give rise to excessive temperature rises in zones of low flow and (c) considerably shortened catalyst life because of possible high local or zonal temperature and, concurrently, greater chances for carbon laydown. 

Rabinowicz M, Ceuleneer G, Monnereau M, Rosemberg C (1990) Three-dimensional models of mantle flow across a low-viscosity zone implications for hotspot dynamics. Earth Planet Sci Lett 99 170-184 Reid MR(1995) Processes of mantle enrichment and magmatic differentiation in the eastern Snake River Plain Th isotope evidence. Earth Planet Sci Lett, 131 239-254 Reid MR, Ramos FC (1996) Chemical dynamics of enriched mantle in the southwestern United States Thorium isotope evidence. Earth Planet Sci Lett, 138 67-81. 

Chen, Y. R., Tseng, M. C., Chang, Y. Z., and Her, G. R. (2003). A low-flow CE/electrospray ionization MS interface for capillary zone electrophoresis, large-volume sample stacking, and micellar electroklnetlc chromatography. Anal. Chem. 75, 503 — 508. 

In laminar flow of Bingham-plastic types of materials the kinetic energy of the stream would be expected to vary from V2/2gc at very low flow rates (when the fluid over the entire cross section of the pipe moves as a solid plug) to V2/gc at high flow rates when the plug-flow zone is of negligible breadth and the velocity profile parabolic as for the flow of Newtonian fluids. McMillen (M5) has solved the problem for intermediate flow rates, and for practical purposes one may conclude... 

A sharp decrease in wall friction in the flow of visco-elastic liquids (including chemically reactive liquids) leads to the formation of low pressure zones. The gas flows into these zones, which leads to disruption of the liquid stream in nozzle 5. Additional gas blown into the disrupted liquid stream through the ring gaps by pipelines 11 and 15 causes separation of the liquid into smaller droplets, thus improving spray dispersion. 

Because considerable time may be required to flush the fluid initially present in the influent zones of low flow experimental columns, Rabideau and Khandelwal (1998b) proposed the following mixing zone (MZ) boundary condition for the column entrance ... 

Eq. 4 is amenable to solution techniques based on the numerical inversion of Laplace-transformed equations these calculations can be performed rapidly and are therefore suitable for calibration. In Figure 1, typical soil/bentonite column predictions are shown to highlight the effect of the influent mixing zone on the spatial contaminant distributions for low-flow systems. The simulation results, which were generated for column conditions described by Khandelwal et al. (1998), indicate that the mixing zone has a significant influence on the shape of the spatial contaminant distribution and, therefore should be considered explicitly in estimating sorption parameters from spatial column data. 

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