Flow characteristics

Flow characteristics represent another way of describing viscoelastic properties involving rather complex models. Furthermore, variations are seen among manufacturers due to the use of individual techniques. 

Although flow mechanics are determined by their physical surroundings in more watery fluids, it is a substance s rheologic character which essentially determines flow characteristics among the more viscous and viscoelastic substances (Fig. 8). 

Molecular reformation, following deformation by an external force, is time-de-pendent (Fig. 9). An external force exerted briefly and quickly, causes a viscoelastic substance to go out of shape for a short time only, allowing the substance to regain its shape as soon as the force is removed. If force is applied more slowly, the viscoelastic molecules gradually adapt to the new form. This relationship is easily seen in the following two models which also take into account the direction of the external force  

Viscosity describes the tendency of fluids to resist reciprocal laminar displacement of two adjacent layers the so-called inner friction . Viscosity is therefore a measure of fluid flow resistance. 1 % solutions of sodium hyaluronate (e.g., Hea-lon or Proviso ) show a viscosity 400,000 times higher than aqueous fluid. They are dependent on concentration, molecular weight, addition of solvent, and temperature. Viscosity can usually be determined by placing a certain amount of viscous fluid between two parallel plates of equal size at a predetermined distance from one another and sliding them in the same direction, at different speeds (Fig. 10). 

The speed at which one plate is moved in relation to the other is called the shear rate, usually measured in radiant per second (rad/s) or in hertz (Hz, 1/s). The shear force, expressed in dynes/cm, is the force necessary per surface to displace the parallel-set plates. The velocity gradient is determined by the relative velocity of the plates divided by the fluid density. Dynamic viscosity is defined as the quotient of the shear force and the velocity gradient measured in N sec m.  

The pump number of a stirrer, Np, correlates in a dimensionless way the total volume flow F produced by the stirrer with the stirrer speed N and the stirrer diameter d. 

The value of Np is constant in the turbulent regime and is characteristic for a defined vessel/stirrer set-up. Since the pump number of a stirrer is independent 

The velocities calculated with CFX 5, using the SST turbulence model, are also shown in Fig. 5 a and b for the unbaffled system. Especially near the centerline, the calculated values differ from the measured ones. In general, the values of the measured mean velocities are somewhat underpredicted by the calculations, which is in accordance with literature [7]. In general, however, the measured trends are relatively well predicted by the computations both for water and SCCO2. 

About 10000 data points (N) are collected with LDV at a single point in the vessel with a data rate of around 200 Hz. From these data points the mean velocity is calculated (as given in Figs. 3 to 5) but also the fluctuating velocity V as defined by 

From the fluctuating velocities in the three Cartesian directions the local turbulent kinetic energy k is calculated by Eq. (3) assuming that the influence of periodic fluctuations is negligible. 

The major disadvantage of the mirror image model is that the interaction between the two opposing streams in impingement was not considered. In fact, even in laminar impinging streams, such an interaction cannot be negligible. 

When the jet on the right-hand side is at the top and that on the left-hand side is at the bottom, the twisting is clockwise while for opposite positions of the jets, it is anticlockwise. The deflection and twisting of the jets result in the formation of vortices on both sides of the outflow plane. Ihe intensities of which depend on the parameters of the jets in impingement. The researchers considered that the collision between the two jets is associated with retardation of the liquid, resulting in an increase in pressure in the impingement zone. The experimental measurements were correlated in terms of the period T of the auto oscillation as a function of the operation parameters. For two equal jets ejected from the nozzles at the same velocity, the relationship they obtained is 

L is the distance between the nozzles, n(l is the velocity of the jet just leaving the nozzles, i.e., impinging velocity, 8 is the transverse dimension of the nozzles, and Lis dynamic viscosity of the fluid. 

It is clear that such significant deflection as that shown in Fig. 1.1 must be related to the properties of the liquid employed. In comparison with liquid, both the density and viscosity of a gas are much smaller, so that such a strong deflection could not be observed with gas jets. However, in principle, it is possible that such deflection phenomena could occur in gaseous single-phase impinging streams, but the degree of deflection may differ greatly from that of liquor ones. 

A notable factor is that in the experiments made by both Denshchikov et al. [28, 29] and Oren et al. [301 the cross-section areas of the jets are considerably smaller. This may be an important reason for the deflection of the jets. 

Smith or concave- or hollow-blade with 6 blades 4.4 

For suspension of sinking solids, it is important to provide liquid velocities directed to the tank floor for an effective sweeping action. Hydrofoils perform well in this duty. However, if the solids have any tendency to be sticky and cling to the blades, the effectiveness of a hydrofoil can be reduced. This can reduce its versatility as a multipurpose impeller. 

In addition to suspending solids off the tank bottom, a process may require homogeneous suspension throughout the bulk. An additional axial flow impeller, perhaps an up-pumping one, may be needed at a higher level for this purpose. Radial flow impellers can be designed to suspend solids, especially if placed on the bottom of the tank, but are less efficient and provide relatively poor solids homogeneity in the bulk. 

Variations in flow patterns for axial flow impellers can be generated to advantage by changing the impeller position or by baffling. For example, axial flow impellers provide radial flows when placed near the tank bottom. For submergence of floating solids, the tank is often unbaffled in the top half to create a controlled vortex useful for pulling down solids. The same method can also be used for surface inducement of gas from vapor space (Oldshne, 1983) and 

Flow patterns of high-shear impellers, such as the bar turbine, Chemshear, and sawtooth impeller, are similar to those of radial flow impellers. The major difference is in lower pumping at higher shear. Backswept turbine and spring impeller also have similar radial flow patterns. It is important to understand the flow patterns around the impeller blades, where dispersion and attrition processes occur. Changing the blade geometry changes these flow patterns and alters the shear. 

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Effusion separator (or effusion enricher). An interface in which carrier gas is preferentially removed from the gas entering the mass spectrometer by effusive flow (e.g., through a porous tube or through a slit). This flow is usually molecular flow, such that the mean free path is much greater than the largest dimension of a traverse section of the channel. The flow characteristics are determined by collisions of the gas molecules with surfaces flow effects from molecular collisions are insignificant. 

Filtration of viscose is not a straightforward chemical engineering process. The solution of cellulose xanthate contains some easy-to-deal-with undissolved pulp fibers, but also some gel-like material which is retarded rather than removed by the filters. The viscose is unstable and tends to form more gel as it ages. Its flow characteristics make the material close to the walls of any vessel or pipe move more slowly, get older, and gel more than the mainstream viscose. So while filtration can hold back gels arising from incomplete mixing, new gels can form in the pipework after the filters. 

Small, complex-shaped glass articles such as thread guides for the textile industry and television gun mounts for the electronics industry are made by the multiform process. The dry-milled powder is mixed with an inorganic binder and a fluid vehicle, and then atomi2ed by a spray dryer into small, dried agglomerates of glass powder and binder with good flow characteristics. They are subsequently pressed to the desired shape and fired. 

Dowtherm G is a mixture of di- and triaryl compounds and has good flow characteristics at low temperatures. Dowtherm G is highly stable, and the products of decomposition consist of high molecular weight materials which remain in solution in the Hquid. Dowtherm G is intended for use in Hquid-phase systems. The fluid has a striking odor even at extremely low concentrations. 

Some amorphous copoly(ether—sulfone) fkms have been prepared (117) with Ts around 130°C with no loss in weight up to 400°C in ak or N2. Other backbones iavestigated in this class of polymers are copoly(ether—amides) (118) and copoly(ether—ketones) (119). These polymers show good mechanical properties, flow characteristics, and abrasion resistance. 

Grade XXP is better than Grade XX in electrical and moisture-resisting properties and more suitable for hot punching. It is intermediate between Grades XP and XXXP in punching and cold flow characteristics. 

Grade XXX demonstrates good electrical properties in high humidity apphcations and has minimum cold flow characteristics. 

Inasmuch as friction conditions determine the flow characteristics of a powder, coarser powder particles of spherical shape flow fastest and powder particles of identical diameter but irregular shape flow more slowly. Finer particles may start to flow, but stop after a short time. Tapping is needed in order to start the flow again. Very fine powders (fine powder particles to coarser ones may increase the apparent density, but usually decreases the flow quality. Metal powders having a thin oxide film may flow well. When the oxide film is removed and the friction between the particles therefore increases, these powders may flow poorly. 

Suspension Polymers. Methacrylate suspension polymers are characterized by thek composition and particle-size distribution. Screen analysis is the most common method for determining particle size. Melt-flow characteristics under various conditions of heat and pressure are important for polymers intended for extmsion or injection molding appHcations. Suspension polymers prepared as ion-exchange resins are characterized by thek ion-exchange capacity, density (apparent and wet), solvent sweUing, moisture holding capacity, porosity, and salt-spHtting characteristics (105). 

The solvent ratio of a semipaste remover may also be analy2ed by gas—Hquid chromatography by separating the solvents from the thickener. It is also useful to determine the viscosity and flow characteristics of the semipaste remover. A Brookfield viscometer is effective in determining the viscosity of most semipaste removers. Plow characteristics may be deterrnined by a constantometer. 

Slurry Pipelines. Finely divided soHds can be transported in pipelines as slurries, using water or another stable Hquid as the suspending medium. Flow characteristics of slurries in pipelines depend on the state of subdivision of the soHds and their distribution within the fluid system. 

Flow Regimes in Multiphase Reactors. Reactant contacting, product separations, rates of mass and heat transport, and ultimately reaction conversion and product yields are strong functions of the gas and Hquid flow patterns within the reactors. The nomenclature of commonly observed flow patterns or flow regimes reflects observed flow characteristics, ie, armular, bubbly, plug, slug, spray, stratified, and wavy. 

Fig. 13. Bubble column flow characteristics (a) data processing system for split-film probe used to determine flow characteristics, where ADC = automated data center (b) schematic representation of primary flow patterns.

Scale-Up Principles. Key factors affecting scale-up of reactor performance are nature of reaction zones, specific reaction rates, and mass- and heat-transport rates to and from reaction sites. Where considerable uncertainties exist or large quantities of products are needed for market evaluations, intermediate-sized demonstration units between pilot and industrial plants are usehil. Matching overall fluid flow characteristics within the reactor might determine the operative criteria. Ideally, the smaller reactor acts as a volume segment of the larger one. Elow distributions are not markedly influenced by... 

Another model is the Casson equation (13), which is useful in estabHshing the flow characteristics of inks, paints, and other dispersions. An early form of this expression (eq. 1) was modified (14) to give equation 2. 

The flow characteristics inside Hquid atomizers have been studied by numerous investigators (4—8). Of special interest to designers is the work reported on swid atomizers (4), fan spray atomizers (6,7), and plain jets (8). The foUowing discussion focuses on the flow characteristics of a swid atomizer. 

One proposed simplified theory (4) provides reasonably accurate predictions of the internal flow characteristics. In this analysis, conservation of mass as well as angular and total momentum of the Hquid is assumed. To determine the exit film velocity, size of the air core, and discharge coefficient, it is also necessary to assume that a maximum flow through the orifice is attained. 

The Reynolds number is sufficient as a parameter for describing the internal flow characteristics, such as discharge coefficient, air core ratio, and spray angle at the atomizer exit. 

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