Boundary layer, resistance

Sweating, the other powerful heat loss mechanism actively regulated by the thermoregulatory center, is most developed in humans. With about 2,6 million sweat glands distributed over the skin and neurally controlled, sweat secretion can vary from 0 to 1 I7(h m ). The other, lesser, passive evaporative process of the skin is from the diffusion of water. The primary resistance to this flow is the stratum corneum or outermost 15 pm of the skin. The diffusion resistance of the skin is high in comparison to that of clothing and the boundary layer resistance and as a result makes water loss by diffusion fairly stable at about 500 grams/day. 

Means of reducing radiation load that include small leaves, which may also be reflective and/or hairy, and leaf rolling. Hairy leaves also increase the boundary layer resistance and so augment the stomata as means of restricting water loss. 

SLM Simple flux equation based on film theory considering aqueous boundary layer resistance and membrane diffusion Optimal condition determined with respect to pH of the aqueous phase [57,58]... 

By analogy to electrical systems, the resistance r can be thought of as consisting of several components. For convenience, three such components are often defined, a surface resistance (rsurl), which depends on the affinity of the surface for the species, a boundary layer resistance (rh(ulll), which depends on the molecular diffusiv-ity of the gas in air, and a gas-phase resistance (rgas), which depends on the micrometeorology that transports the gas to the surface ... 

FIGURE 3 Illustration of the concept of an external mass transfer boundary layer resistance and associated concentrations of the rejected species, B. 

All of the many biological transfer processes combine to determine a net surface resistance to transfer. Empirical relationships can be used to infer stomatal resistance from data on photosynthetically active radiation, water stress, temperature, atmospheric humidity and carbon dioxide levels. The resulting net surface resistance has been coupled with mathematical descriptions of aerodynamic and boundary-layer resistances in a "big leaf" model derived on the basis of agricultural and forest meteorology literature (4). At present, the big-leaf model is relatively coarse, permitting application only to areas dominated by maize, soybeans, grass, deciduous trees, and conifers. 

For the thick boundary layer, is 8 mm s 1 and rbJ, is 130 s m 1 (Table 8-1).1 Boundary layer conductances usually are larger and boundary layer resistances are smaller than their respective values for diffusion along the stomatal pores, which we will examine next. 

In industrial apphcation, a thin PV membrane on a UF porous support membrane is generally employed. Thus, the concentration gradient on the feed side cannot be ignored. By assuming boundary layer resistance in series with membrane resistance the following equation [29,45] may be derived ... 

The product D 5 is the intrinsic permeability that can be obtained from experimentally determined values of D and S or through experiments at very high levels of mrbulence such that the boundary layer resistance is eliminated K = L/D S). A number of correlations are available in the literature for reliable estimates of for various geometries of the modules [29,45]. 

Raghunath B and Hwang ST. General treatment of liquid-phase boundary layer resistance in the pervaporation of dilute aqueous organics through tubular membranes. J. Memb. Sci. 1992 75(l-2) 29-46. 

Discrepancies between experimentally obtained and theoretically calculated data for cadmium concentration in the strip phase are 10-150 times at feed or strip flow rate variations. These differences between the experimental and simulated data have the following explanation. According to the model, mass transfer of cadmium from the feed through the carrier to the strip solutions is dependent on the diffusion resistances boundary layer resistances on the feed and strip sides, resistances of the free carrier and cadmium-carrier complex through the carrier solution boundary layers, including those in the pores of the membrane, and resistances due to interfacial reactions at the feed- and strip-side interfaces. In the model equations we took into consideration only mass-transfer relations, motivated by internal driving force (forward... 

These differences between the experimental and the simulated data have the following explanation. According to the model, mass transfer of titanium from the feed through the carrier to the strip solutions is dependent on the resistances boundary layer resistances on the feed and strip sides, resistances... 

Comparing the simulated results with the experimental data, it appears that at higher flow rates, where boundary layers resistance becomes less... 

For two-phase systems, mixing promotes faster mass transfer by creating higher interfacial area due to smaller bubbles or drops. Turbulence also helps reduce the boundary-layer resistance around drop or bubble surfaces, leading to faster mass transfer. 

Perhaps the most realistic model is the random pore model of Bhatia and Perlmutter (1980 1981a, b 1983), which assumes that the actual reaction surface of the reacting solid B is the result of the random overlapping of a set of cylindrical pores. Surface development as envisaged in this model is illustrated in Figure 11.12. The first step in model development is therefore the calculation of the actual reaction surface, based on which the conversion-time relationship is established in terms of the intrinsic structural properties of the solid. In the absence of intraparticle and boundary layer resistances, the following relationship is obtained ... 

Pressure and concentration gradients for an asymmetric membrane with boundary-layer resistance. 

Dialysis is being considered to recover a product A with molecular weight 150 from a dilute aqueous stream. The principal contaminant is a polymer B with molecular weight 15,000. Predict the initial fluxes of A and B if the membrane has a porosity of 45 i rcent, a mean pore size of 0.05 pm, and a thickness of 30 pm, and the feed solution contains 1 percent A and 1 percent B. Neglect boundary-layer resistances and assume pure water on the product side. 

Meyers, T. P. (1987) The sensitivity of modelled S02 fluxes and profiles to stomatal and boundary layer resistance, Water Air Soil Pollut. 35, 261-278. 

The Cake Filtration Model describes the filtration of particles which are much larger than the pores and will be retained, without entering the pores. The particles deposit on the membrane surface contributing to the boundary layer resistance. Included in this model is deposition due to concentration polarisation. 

The effect of linear velocity on the observed reaction rate (Fig. 1) shows, for this case, the presence of a significant boundary layer resistance below a flow rate of 235 ml/min. The existence of non-negligible pore diffusional resistance is de-ducible from Fig. 2, in which the dependence of observed reaction rate on film thickness is depicted. Overall the immobilized cells exhibited about 50% of the specific activity of the free cells (in fermentation) toward the production of citric acid. 

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