Boundary layer phenomenon

Latto B, Shen CH (1970) Effect of dilute polymer solutions injection on external boundary layer phenomenon Can J Chem Eng 48 34... 

The solutions for (p y) and v y) are given by equations (5.139), (5.140), (5.141) and (5.143). In general, the integrals appearing have to be evaluated numerically for given values of the elastic constants and viscosities once the constant c has been given. Further, as in the previous example, the existence of a boundary layer phenomenon can be demonstrated and the interested reader is referred to Leslie [163] for more details. 

Some empirical equations to predict cyclone pressure drop have been proposed (165,166). One (166) rehably predicts pressure drop under clean air flow for a cyclone having the API model dimensions. Somewhat surprisingly, pressure drop decreases with increasing dust loading. One reasonable explanation for this phenomenon is that dust particles approaching the cyclone wall break up the boundary layer film (much like spoiler knobs on an airplane wing) and reduce drag forces. 

The phenomenon of concentration polarization, which is observed frequently in membrane separation processes, can be described in mathematical terms, as shown in Figure 30 (71). The usual model, which is weU founded in fluid hydrodynamics, assumes the bulk solution to be turbulent, but adjacent to the membrane surface there exists a stagnant laminar boundary layer of thickness (5) typically 50—200 p.m, in which there is no turbulent mixing. The concentration of the macromolecules in the bulk solution concentration is c,. and the concentration of macromolecules at the membrane surface is c. 

When the polar additive nonylic acid was added into hexade-cane liquid, the contact ratio becomes much smaller than that of pure hexadecane, which is shown in Fig. 39. For hexa-decane liquid, the critical speed to reach zero contact ratio is 50 mm/s, which is much higher than that of mineral oil 13604 because of its much lower viscosity. Flowever, when nonylic acid was added into the hexadecane liquid, the critical speed decreased from more than 50 mm/s to 38 mm/s. The same phenomenon can be seen in Fig. 39(h) which shows the comparison of oil 13604 and that added with 1.8 %wt. nonylic acid. The addition of polar additive reduces the contact ratio, too, but its effect is not as strong as that in hexadecane liquid because the oil 13604 has a much larger viscosity. Therefore, it can be concluded that the addition of polar additives will reduce the contact ratio because the polar additives are easy to form a thick boundary layer, which can separate asperities of the two rubbing surfaces. 

A phenomenon that is particularly important in the design of reverse osmosis units is that of concentration polarization. This occurs on the feed-side (concentrated side) of the reverse osmosis membrane. Because the solute cannot permeate through the membrane, the concentration of the solute in the liquid adjacent to the surface of the membrane is greater than that in the bulk of the fluid. This difference causes mass transfer of solute by diffusion from the membrane surface back to the bulk liquid. The rate of diffusion back into the bulk fluid depends on the mass transfer coefficient for the boundary layer on feed-side. Concentration polarization is the ratio of the solute concentration at the membrane surface to the solute concentration in the bulk stream. Concentration polarization causes the flux of solvent to decrease since the osmotic pressure increases as the boundary layer concentration increases and the overall driving force (AP - An) decreases. 

Contamination of the biosphere by stable OCPs (DDT, HCH, aldrin, dieldrin, heptachlor, and toxafene) has become a global phenomenon. With global transfer, pesticides may reach countries that never produced or used them. In the northern hemisphere, where winds tend to blow from west to east, the rate of the wind in the boundary layer between the troposphere and the atmosphere is approximately 35 m/sec. Pesticide particles may completely orbit the Earth in approximately 12 days. Over this time, the probability of the particles falling to Earth may vary, and depends on the height of their orbit at a height of 3 km above sea level, particles will remain in the atmosphere for about seven days at a height of 6 km, for 30 days at a height of 30 km, for two years [31]. 

The PAS phenomenon involves the selective absorption of modulated IR radiation by the sample. The selectively absorbed frequencies of IR radiation correspond to the fundamental vibrational frequencies of the sample of interest. Once absorbed, the IR radiation is converted to heat and subsequently escapes from the solid sample and heats a boundary layer of gas. Typically, this conversion from modulated IR radiation to heat involves a small temperature increase at the sample surface ( 10 6oC). Since the sample is placed into a closed cavity cell that is filled with a coupling gas (usually helium), the increase in temperature produces pressure changes in the surrounding gas (sound waves). Since the IR radiation is modulated, the pressure changes in the coupling gas occur at the frequency of the modulated light, and so does the acoustic wave. This acoustical wave is detected by a very sensitive microphone, and the subsequent electrical signal is Fourier processed and a spectrum produced. 

The concentration of the substrate S is also uniform throughout the boundary layer. This case is often called chemical regime , because the limiting phenomenon is the chemical reaction and not the physical process of mass transfer. In that case, the gas-liquid contact area (A, o, oL...) is not a crucial parameter conversely, the liquid retention (eL) must be high to promote the reaction. 

Ary given catalytic material can be abstracted based on the same underlying similar architecture — for ease of comparison, we describe the catalytic material as a porous network with the active centers responsible for the conversion of educts to products distributed on the internal surface of the pores and the external surface area. Generally, the conversion of any given educt by the aid of the catalytic material is divided into a number of consecutive steps. Figure 11.13 illustrates these different steps. The governing transport phenomenon outside the catalyst responsible for mass transport is the convective fluid flow. This changes dramatically close to the catalyst surface from a certain boundary onwards, named the hydrodynamic boundary layer, mass transport toward and from the catalyst surface only takes place... 

Princen [57, 64, 82] and others [84] also noted the presence of wall-slip in rheological experiments on HIPEs and foams. However, instead of attempting to eliminate this phenomenon, Princen [64] employed it to examine the flow properties of the boundary layer between the bulk emulsion and the container walls, and demonstrated the existence of a wall-slip yield stress, below that of the bulk emulsion. This was attributed to roughness of the viscometer walls. Princen and Kiss [57], and others [85], have also showed that wall-slip could be eliminated, up to a certain finite stress value, by roughening the walls of the viscometer. Alternatively [82, 86], it was demonstrated that wall-slip can be corrected for and effectively removed from calculations. Thus, viscometers with smooth walls can be used. This is preferable, as the degree of roughness required to completely eradicate wall-slip is difficult to determine. 

Material transport is usually associated with thermal transport except in situations involving homogeneous phases which can be treated as ideal solutions (L4). For this reason it is necessary to consider the behavior of combined thermal and material transport in turbulent flow. The evaporation of liquids under macroscopic adiabatic conditions is a typical example of such a phenomenon. Under such circumstances the behavior in the boundary layer is similar to that found in the field of aerodynamics in a blowing boundary layer (S4). However, it is not... 

Further study is needed of the phenomenon of kinetic limitations to the neutralization of acidic aerosols. Simultaneous occurrences of acidic aerosols at gaseous [NH3] well above the equilibrium values have been reported (56, 67), and it is still unclear whether kinetic limits to microscale neutralization or boundary layer mixing (macroscale) kinetics (or both) are responsible for these limitations. An understanding of the extent of human exposure to acidic aerosols, as well as of the availability of acidic aerosols for wet scavenging... 

The principle of the model is to scan the bed surface, which is subdivided into boxes whose the width and the length are equal respectively to the spanwise and streamwise statistical periodicities of appearing of the coherent structures. In fact, some authors have shown that the phenomenon of ejection in a turbulent boundary layer could be connected with the particle s take-off. 

The second approach to concentration polarization, and the one used in this chapter, is to model the phenomenon by assuming that a thin layer of unmixed fluid, thickness S, exists between the membrane surface and the well-mixed bulk solution. The concentration gradients that control concentration polarization form in this layer. This boundary layer film model oversimplifies the fluid hydrodynamics occurring in membrane modules and still contains one adjustable parameter,... 

In a pressure-driven membrane process the molecules are generally rejected by the membrane and therefore their concentrations in the permeate are lower than those in the feed solution. However, an accumulation of excess particles can occur at the membrane surface with the creation of a boundary layer. This phenomenon, called concentration polarization, causes a different membrane performance. In particular, with low molecular weight solutes the observed rejection will be lower than the real retention or, sometimes, it could be negative. 

Now, consider flow along the surface of a membrane. The same boundary layer forms as with flow through a pipe. However, with a membrane system, because there is a net flow out through the membrane, there is convective flow to the membrane, but only dif-fusional flow away from the membrane. Since diffusion is slower than convection, solutes rejected by the membrane tend to build up on the surface and in the boundary layer. Thus, the concentration of solutes at the membrane surface is higher than in the bulk solution. This boundary layer is called "concentration polarization."2 The phenomenon is shown in Figure 3.3. 

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