Internal re-circulation

Vaporization cavitation, also called inadequate NPSHa cavitation Internal re-circulation cavitation. 

Internal Re-circulation Operation / Design / Maintenance Internal Re-circulation results from something in the discharge side of the pump strangled valve, clogged filter, over-pressurized header, check-valve installed backwards. 

Internal re-circulation of 5 - 10 % of the core sand, which otherwise would be disposed of. 

Of all the possible parameters which may effect the equilibration time, the only ones which could easily be controlled to some extent are the flow rates of the re-circulated air and water. In our previous experimental work with Rn, we had relied on the internal air pump of the RAD-7 which is fixed at a flow rate of about 1 L/min. Since the very short half-life of thoron dictates that we perform the analysis as quickly as possible, we have now installed an external air pump so we can vary the flow rate of the air as well as the water (Fig. 3). We report here our observations on the effect of different water and air flow rates on the response to a constant source of thoron. 

High temperature re-carbonation can occur in lime kilns under abnormal conditions in which some kiln gases enter the cooling zone. It has been observed when re-circulating exhaust gases (to moderate calcining zone temperatures), when shaft kiln internal structures, such as arches, have failed and when crot-ching has occurred. 

The technique uses unbonded sand, which results in no binder-related emissions upon pouring and shake-out and which may be internally re-used without major treatment. EPS pyrolysis results in a slow build-up of organic material in the sand. This may be prevented by de-dusting and partial renewal of the circulating sand. A 5 % renewal is sufficient to maintain sand quality. The evacuated sand may be thermally regenerated for internal re-use. 

The biofilter produces a waste water flow. This may be re-circulated internally after treatment, yielding a sludge cake for disposal. 

Effective conductivity models which take the internal drop re-circulation into account via adjustment of the internal liquid conductivity... 

Reforming of natural gas for solid oxide fuel cells is achieved either internally as described above or externally by a pre-reformer reactor [40,41]. Further processing of the fuel is not required, because of the unlimited tolerance of the fuel cell to carbon monoxide. The re-circulation of anode off-gas to the pre-reformer [42] is an interesting option for solid oxide fuel cells. Through these means, addition of water is omitted, which clearly decreases the complexity of the system and reduces cold start problems (see also Section 3.5). 

The derivations of Hadamard and of Boussinesq are based on a model involving laminar flow of both drop and field fluids. Inertial forces are deemed negligible, and viscous forces dominant. The upper limit for the application of such equations is generally thought of as Re 1. We are here considering only the gross effect on the terminal velocity of a drop in a medium of infinite extent. The internal circulation will be discussed in a subsequent section. 

When a fluid sphere exhibits little internal circulation, either because of high K = Pp/p or because of surface contaminants, the external flow is indistinguishable from that around a solid sphere at the same Re. For example, for water drops in air, a plot of versus Re follows closely the curve for rigid spheres up to a Reynolds number of 200, corresponding to a particle diameter of approximately 0.85 mm (B5). In fact, many of the experimental points used in Section II to determine the standard drag curve refer to spherical drops in gas streams, where high values of k ensure negligible internal circulation. 

Experimenters who have observed asymmetry of internal circulation patterns have generally attributed this to accumulation of surface-active materials at the rear, causing a stagnant cap (see Chapter 3). It seems likely that at least part of the asymmetry results from the forward shift of the internal vortex at nonzero Re, as predicted numerically. 

All the work discussed in the preceding sections is subject to the assumptions that the fluid particles remain perfectly spherical and that surfactants play a negligible role. Deformation from a spherical shape tends to increase the drag on a bubble or drop (see Chapter 7). Likewise, any retardation at the interface leads to an increase in drag as discussed in Chapter 3. Hence the theories presented above provide lower limits for the drag and upper limits for the internal circulation of fluid particles at intermediate and high Re, just as the Hadamard-Rybzcynski solution does at low Re. 

The flow and shape transitions for small and intermediate size bubbles and drops are summarized in Fig. 7.13. In pure systems, bubbles and drops circulate freely, with internal velocity decreasing with increasing k. With increasing size they deform to ellipsoids, finally oscillating in shape when Re exceeds a value of order 10. In contaminated systems spherical and nonoscillating ellipsoidal... 

These flow transitions lead to a complex dependence of transfer rate on Re and system purity. Deliberate addition of surface-active material to a system with low to moderate k causes several different transitions. If Re < 200, addition of surfactant slows internal circulation and reduces transfer rates to those for rigid particles, generally a reduction by a factor of 2-4 (S6). If Re > 200 and the drop is not oscillating, addition of surfactant to a pure system decreases internal circulation and reduces transfer rates. Further additions reduce circulation to such an extent that shape oscillations occur and transfer rates are increased. Addition of yet more surfactant may reduce the amplitude of the oscillation and reduce the transfer rates again. Although these transitions have been observed (G7, S6, T5), additional data on the effect of surface active materials are needed. 

Fleischacker M, The 2nd International Symposium on Circulating Nucleic Acids in Plasma and Serum (CNAPS-2), Hong Kong, February 20-21, 2001, Eur. J. Med. Res., 6 364-368, 2001. 

Cholesteryl esters that are internalized via the LDL receptor are hydrolyzed to produce cholesterol and an acyl chain. Cholesterol, in (urn, activates the enzyme acyl-CoA cholesterol acyl-transferase (ACAT) which re-esterifies cholesterol. In an apparently futile cycle, the cholesteryl esters are hydrolyzed by cholesteryl ester hydrolase. The cholesterol moiety has several fates it may leave the cell and bind to an acceptor such as high-density lipoprotein (HDL), it may be converted to steroid hormones, or it may be reesterified by ACAT. When the cellular cholesterol concentration falls, the activity of HMG-CoA reductase is increased, as is the number of LDL receptors, which results in an increase of cellular cholesterol, due both to de novo synthesis and to the uptake of cholesterol-rich lipoproteins in the circulation. An increase in cellular cholesterol results in the rapid decline in the mRNA levels for both HMG-CoA reductase and the LDL receptor. This coordinated regulation is brought about by the presence of an eight nucleotide sequence on the genes which code for both proteins this is termed the sterol regulatory element-1. 

Up — Uc represents the resultant slip velocity between the particulate and continuous phase. Some other commonly used drag coefficient correlations are listed in Appendix 4.2. For fluid particles such as gas bubbles or liquid drops, the drag coefficient may be different than that predicted by the standard drag curve, due to internal circulation and deformation. For example, Johansen and Boysen (1988) proposed the following equation to calculate Cd, which is valid for ellipsoidal bubbles in the range 500 < Re < 5000 ... 

Kirpotin DB et al (2006) Antibody targeting of long-circulating lipidic nanoparticles does not increase tumor localization but does increase internalization in animal models. Cancer Res 66 6732-6740... 

As the Reynolds number increases, a wake is formed behind the fluid sphere or ellipsoid [22, 45]. The formation of a wake behind a fluid particle is delayed compared to a solid sphere due to the internal circulation of the gas. The recirculating wake may be completely detached from the fluid sphere. A secondary internal vortex will then not be formed. For smaller particle Reynolds numbers the wake is symmetrical, but as the Reynolds number increases further the vortex sheet breaks down to vortex rings. Further increase of the Re molds number cause the vortex rings to shed asymmetrically, and the drop or bubble will show a rocking motion. This is one of the two types of secondary motion defined. The other is oscillations (shape dilations), and is also thought to be due to the vortex shedding. 

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