Forced flow

Convective heat transfer is classified as forced convection and natural (or free) convection. The former results from the forced flow of fluid caused by an external means such as a pump, fan, blower, agitator, mixer, etc. In the natural convection, flow is caused by density difference resulting from a temperature gradient within the fluid. An example of the principle of natural convection is illustrated by a heated vertical plate in quiescent air. 

Electroultrafiltration (EUF) combines forced-flow electrophoresis (see Electroseparations,electrophoresis) with ultrafiltration to control or eliminate the gel-polarization layer (45—47). Suspended colloidal particles have electrophoretic mobilities measured by a zeta potential (see Colloids Elotation). Most naturally occurring suspensoids (eg, clay, PVC latex, and biological systems), emulsions, and protein solutes are negatively charged. Placing an electric field across an ultrafiltration membrane faciUtates transport of retained species away from the membrane surface. Thus, the retention of partially rejected solutes can be dramatically improved (see Electrodialysis). 

A. Laminar, local, flat plate, forced flow... 

J. Rotating cylinder in an infinite liquid, no forced flow... 

FIG. 21-56 Typical four-tube force-flow valve-bag packer with automatic palletizing and truck- and railcar-loading facilities, Courtesy of Stone Container Corp.)... 

Generates its own forced flow with a submersible centrifugal pump. 

The most reliable recycle reactors are those with a centrifugal pump, a fixed bed of catalyst, and a well-defined and forced flow path through the catalyst bed. Some of those shown on the two bottom rows in Jankowski s papers are of this type. From these, large diameter and/or high speed blowers are needed to generate high pressure increase and only small gaps can be tolerated between catalyst basket and blower, to minimize internal back flow. 

Kellenbenz, J., and Hahne, E., Condensation of Pure Vapors and Binary Mixtures in Forced Flow, Inti. J. Heat Mass Transfer, Vol. 37, No.8, pp1269-1276, 1994. 

The two primary pumps with AC and DC pony motors flow 18,000 gpm of heavy water through the core for cooling. Following 60 MW operation, for 3 minutes, forced flow is needed to remove the decay heat. Upon loss of pony motors and primary pumps, forced flow is established through the shutdown cooling water system consisting of two pumps in parallel - one pump running and the other on standby. 

At 40 MW operation, the core damage frequency is 3.7E-04/y. The proportion of accident classes is LOCA, 50% beam tube rupture, 27% ATWS, 17% LOOP, 4% and other transients, 2 7. Three minutes of forced flow are not required and large LOCAs with break size smaller than 2.8 inches can be mitigated. 

Chromatographic plates can be connected for both capillary-controlled and forced-flow planar chromatography (FFPC), i.e. irrespective of whether capillary action or forced-flow is the driving force for the separation. The first technique is denoted as grafted planar chromatography (31), while the second is known as long distance (LD) OPLC, which uses heterolayers (32, 33). 

SERIALLY CONNECTED MULTILAYER FORCED-FLOW PLANAR CHROMATOGRAPHY... 

Figure 8.19 illustrates another example of the versatility of multidimensional OPLC, namely the use of different stationary phases and multiple development ("D) modes in combination with circular and anticircular development and both off-line and on-line detection (37). Two different stationary phases are used in this configuration. The lower plate is square (e.g. 20 cm X 20 cm), while the upper plate (grey in Figure 8.19) is circular with a diameter of, e.g. 10 cm. The sample must be applied on-line to the middle of the upper plate. In the OPLC chamber the plates are covered with a Teflon sheet and pressed together under an overpressure of 5 MPa. As the mobile phase transporting a particular compound reaches the edge of the first plate it must-because of the forced-flow technique-flow over to the second (lower) stationary phase, which is of lower polarity. 

On the basis of theory and experimental observations it can be predicted that a zone capacity of ca. 1500 could be achieved by 2-D multiple development. Because the same result can be achieved by application of 2-D forced-flow development on HPTLC plates, it can be stated that the combination of stationary phases, FFPC and "D offers a fruitful future in modem, instmmental planar chromatography. 

L. Botz, Sz. Nyiredy, E. Wehrli and O. Shelter, Applicability of Empore TLC sheets for forced-flow planar cltromatography. I. Characterization of the silica sheets , 7. Liq. Chromatogr. 13 2809-2828 (1990). 

Improve heat transfer coefficient by forcing flow past coil surfaces. 

Ivashkevich, A. A., Critical heat fluxes in the forced flows of liquids in channels, At. Energ. USSR) 8, 44 (1960) [English transl. Longo, J., Jr., KAPL-1744(1957)]. 

Two-phase flows in micro-channels with an evaporating meniscus, which separates the liquid and vapor regions, have been considered by Khrustalev and Faghri (1996) and Peles et al. (1998, 2000). In the latter a quasi-one-dimensional model was used to analyze the thermohydrodynamic characteristics of the flow in a heated capillary, with a distinct interface. This model takes into account the multi-stage character of the process, as well as the effect of capillary, friction and gravity forces on the flow development. The theoretical and experimental studies of the steady forced flow in a micro-channel with evaporating meniscus were carried out by Peles et al. (2001). These studies revealed the effect of a number of dimensionless parameters such as the Peclet and Jacob numbers, dimensionless heat transfer flux, etc., on the velocity, temperature and pressure distributions in the liquid and vapor regions. The structure of flow in heated micro-channels is determined by a number of factors the physical properties of fluid, its velocity, heat flux on... 

Reactant gases enter the reactor by forced flow. 

Thermal gradient/ forced flow Reactant gases flow from cold to hot surface... 

Figure 5.16. Forced-flow thermal-gradient chemical vapor infiltration.

The new method produces TiN powders with surface areas exceeding 200 m g that are otherwise only accessible using a forced flow reactor and a microwave plasma activator in which titanium metal is reacted with N2 in the gas phase [14]. TiN powders with considerably lower specific surface area (Sg<60m g ) were also synthesized using the nitridation of 10-15 nm-sized... 

Morita, K., Koike, S., and Aishima, T., J. Planar Chromatogr., 11, 94-99, 1998. Nyiredy, Sz., Systematic method development in classical and forced-flow planar chromatography, in Proc. Int. Symp. on Planar Separations, Planar Chromatography 2001, Nyiredy, Sz., Ed., Research Institute for Medicinal Plants, Budakalasz, 2001, pp. 137-148. 

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