Effect of Cross-Flow

Effects such as those shown in Figure 9 may also be due to a component retained by mechanisms other than adsorption with the 10,000 NMWC0 membrane but passing through the 100,000 NMWC0 membrane. This effect can be verified by evaluating the effect of cross flow on flux. If adsorption is the cause of lower output then increased cross flow will not increase filtration rate. The opposite would be true if lower fluxes were due to material retained on the membrane by size exclusion. Adsorption can also be determined by analyzing the composition of the feed and permeate to see if there is a difference. ... 

Figure 7-24 Effectiveness of cross-flow heat exchanger with unmixed fluids.

Figure 3.60 Effect of cross-flow velocity on long term flux decay.

An early correlation for the effect of cross flow in film boiling from cylinders was that of Bromley et al. [226], who suggested that, for... 

R. L. Singh, J. S. Saini, and H. K. Varma, Effect of Cross-Flow on Boiling Heat Transfer in Water, Int. J. Heat Mass Transfer (26) 1882-1885,1983. 

Spent flow from individual jets or clusters of jets must be exhausted properly or the performance of adjoining jets can fall even by a factor of 2. For slot jets and rows or arrays of round jets, experimental studies have measured the effect of induced cross-flow (i.e., cross-flow due to upstream rows of jets made to cross the downstream jets to exhaust), as well as imposed cross-flow (i.e., a flow parallel to the impingement surface that is not due to upstream jets). For ASN, the effect of cross-flow is severe [6]. No more than two jet rows should be allowed prior to exhaust. For staggered ARN arrangements, the cross-flow effect is less severe an exhaust port may be provided for every three to five rows. After five rows, the cross-flow effect drops the average Nu rapidly in the downstream direction [11]. 

Another undesirable effect of cross-flow is that, depending upon how it is directed with respect to the motion of the surface being dried, one may encounter a non-uniform moisture profile. 

Mujumdar, A.S., Huang, P.G., and Douglas, W.J.M., Prediction of heat transfer under a plan turbulent impinging jet including effects of cross-flow and wall motion. In Drying 82, Mujumdar, A.S. (Ed.), Hemisphere, New York, 1982, pp. 91-98. 

In small-diameter columns, D/Ha is usually so small that the effect of cross flow is negligible, but in large columns it may become so great that the kinetic head equivalent to Vcf may be significant in terms of liquid head. Under these conditions, the pressure in the vapor space is not constant across the cross section, and its variation is such that it forces an increase in the hydraulic gradient. In order to make Hs as large as possible, any beams or projection on the bottom of a plate should be positioned to aid the vapor cross flow. Good vapor distribution on the plate will eliminate the effect of vapor cross flow. 

Other iavestigations of cross-flow filtration iaclude the study of the coaceatratioa of bacteria (41), the coaceatratioa of fermentation cell debris (42), the coaceatratioa of electrocoatiag paiat (43), the chemical effects oa cross-flow filtratioa of primary sewage efflueat (44), and the use of tubes of different materials, dimensions, and porosity with several slurries (45). 

Calvert (R-12) has tested the correlation in cross-flow packed beds, which tend to give better drainage than countercurrent beds, and has found the effect of gas-flow orientation insignificant. However, the onset of reentrainment was somewhat lower in a bed of 2.5-cm... 

A laboratory study of surface-treatment tanks by Braconnier et al." showed the effects of cross-drafts and obstructions to airflow on capture efficiency. They found that, without obstructions, capture efficiency decreased with increasing cnrss-draft velocity but the importance of this effect depended on freeboard height. In their study, cross-draft direction was always perpendicular to the hood face and directed opposite to the hood suction flow. Follow cro.ss-draft velocities (less than 0.2 m s ), efficiency remained close to 1.0 for the three freeboard heights studied. With higher cross-draft velocities, efficiency decreased as freeboard height decreased. For example, when the crossdraft velocity was 0.55 m s , efficiency decreased from 0.90 to 0.86 to 0.67 as freeboard height decreased from 0.3 m to 0.15 m to 0.1 m, respectively. 

Superposition of Flows Potential flow solutions are also useful to illustrate the effect of cross-drafts on the efficiency of local exhaust hoods. In this way, an idealized uniform velocity field is superpositioned on the flow field of the exhaust opening. This is possible because Laplace s equation is a linear homogeneous differential equation. If a flow field is known to be the sum of two separate flow fields, one can combine the harmonic functions for each to describe the combined flow field. Therefore, if d)) and are each solutions to Laplace s equation, A2, where A and B are constants, is also a solution. For a two-dimensional or axisymmetric three-dimensional flow, the flow field can also be expressed in terms of the stream function. 

The velocity, viscosity, density, and channel-height values are all similar to UF, but the diffusivity of large particles (MF) is orders-of-magnitude lower than the diffusivity of macromolecules (UF). It is thus quite surprising to find the fluxes of cross-flow MF processes to be similar to, and often higher than, UF fluxes. Two primary theories for the enhanced diffusion of particles in a shear field, the inertial-lift theory and the shear-induced theory, are explained by Davis [in Ho and Sirkar (eds.), op. cit., pp. 480-505], and Belfort, Davis, and Zydney [/. Membrane. Sci., 96, 1-58 (1994)]. While not clear-cut, shear-induced diffusion is quite large compared to Brownian diffusion except for those cases with very small particles or very low cross-flow velocity. The enhancement of mass transfer in turbulent-flow microfiltration, a major effect, remains completely empirical. 

Effect of Added 1-Hexadecene on the Performance of Cross-Flow Filtration with Poly wax 500/655-Activated Catalyst... 

A continuous cross-flow filtration process has been utilized to investigate the effectiveness in the separation of nano sized (3-5 nm) iron-based catalyst particles from simulated Fischer-Tropsch (FT) catalyst/wax slurry in a pilot-scale slurry bubble column reactor (SBCR). A prototype stainless steel cross-flow filtration module (nominal pore opening of 0.1 pm) was used. A series of cross-flow filtration experiments were initiated to study the effect of mono-olefins and aliphatic alcohol on the filtration flux and membrane performance. 1-hexadecene and 1-dodecanol were doped into activated iron catalyst slurry (with Polywax 500 and 655 as simulated FT wax) to evaluate the effect of their presence on filtration performance. The 1-hexadecene concentrations were varied from 5 to 25 wt% and 1-dodecanol concentrations were varied from 6 to 17 wt% to simulate a range of FT reactor slurries reported in literature. The addition of 1-dodecanol was found to decrease the permeation rate, while the addition of 1-hexadecene was found to have an insignificant or no effect on the permeation rate. 

Holdich, R. G., Cumming, I. W. and Ismail, B. Chem. Eng. Res. Des. 73 (1995) 20. The variation of cross-flow filtration rate with wall shear stress and the effect of deposit thickness. 

In Eq. (38) the hydrodynamic velocity is that used to evaluate the momentum of the phase. In viscous flow it is the term used in establishing the shear from a knowledge of the viscosity when effects of cross linking of fluxes (01, 02) are neglected. The hydrodynamic velocity and the diffusional velocity are related by... 

Others may have concerns that are more scientific. There has been much concern about the spread of pollen from GE plants and how this movement could affect non-GE crops and native plants. If organic crops cross-pollinate with GE crops, it is possible that consumers would reject the crops. There is also the concern that GE traits could be transferred and persist in wild plants in such a way as to disrupt the natural ecology. I can imagine that Rachel Carson would have been concerned about the potential disruptive effects of pollen flow. At the same time, she may have thought GE plants could be beneficial if they could help reduce pesticide use. 

In hot-wire probe designs, the cooling effect of the flowing process gas stream is detected, as was already discussed in connection with anemometers (see Figure 3.50). A major limitation of the hot-wire-type mass flowmeters is that they do not detect the mass flow across the full cross section of the pipe, but do so only at the tip of the sensor. Therefore, if the sensor is installed in a nonrepresentative location, the resulting reading will be in error. 

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