Passive flow distribution

Numbering up microchannels to large-scale capacity reactors is driven by a rigorous understanding of pressure drop in every parallel circuit Passive flow distribution permits sufficient flow to each channel. No serious evaluation of microvalves or actuators has been undertaken for high-capacity systems with thousands to tens... 

Figure 11.19 Manufacturing scale-up device with submanifolds for passive flow distribution to feed thousands of parallel microchannels. Flow is mapped using ranges to show variations from a normalized value.

Geometrical illustrations of the efficiency of thermodynamic description of the stationary flow distribution problems as applied to the analysis of closed active and open passive hydraulic circuits were already presented in Section 3.2. The geometrical interpretation of the general models for the nonstationary flow distribution in the hydraulic circuit ((23)—(28)) and chemical systems with the set redundant mechanism of reaction ((29)-(34)) is still to be carried out which will obviously require a number of nontrivial problems to be solved. 

In the analysis of behavior of passive circuits they can be considered as "fragments" of active ones. Thus, the passive scheme that includes independent loops 2342, 2352 and 24152 can be obtained by removing from the scheme of Fig. 3,a the branch 1-2 with motive pressure and setting the source at node 2 and the sink at node 1, each being equal to the flow in the removed branch. The flow distribution for such a scheme will be determined from the model ... 

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. 

In order to design such an efficient and effective device, one must understand the mechanisms by which drug is transported in the ocular interior. One issue debated in the literature for some time has been the relative importance of transport by passive diffusion versus that facilitated by the flow of fluid in the vitreous (see, e.g., Ref. 226). To predict the geometric distribution even at steady state of drug released from an implant or an intravitreal injection, one must appreciate which of these mechanisms is at work or, as appropriate, their relative balance. 

A. Turhan, K. Heller, J. S. Brenizer, and M. M. Mench. Passive control of liquid water storage and distribution in a PEFC through flow field design. Journal of Power Sources 180 (2008) 773-7 3. 

The transport mechanisms that operate in distribution and elimination processes of drugs, drug-carrier conjugates and pro-drugs include convective transport (for example, by blood flow), passive diffusion, facilitated diffusion and active transport by carrier proteins, and, in the case of macromolecules, endocytosis. The kinetics of the particular transport processes depend on the mechanism involved. For example, convective transport is governed by fluid flow and passive diffusion is governed by the concentration gradient, whereas facilitated diffusion, active transport and endocytosis obey saturable MichaeUs-Menten kinetics. 

A passive flux of water continually flows across the endothelial layer toward the stroma, which has a tendency to swell. An active pump mechanism pulls an aqueous flux in the opposite direction which controls corneal turgescence [13]. Corneal deturgescence is an ATP-dependent process of the endothelial cells and as such any disruption of the endothelium may result in corneal oedema, thereby affecting corneal transparency. The specific distribution of different proteoglycans across the cornea has recently been implicated in water gradients across the cornea. This water gradient serves to diminish dehydration of the front of the cornea, which is exposed to the atmosphere. 

P 7] The topic has only been treated theoretically so far [28], A mathematical model was set up slip boundary conditions were used and the Navier-Stokes equation was solved to obtain two-dimensional electroosmotic flows for various distributions of the C, potential. The flow field was determined analytically using a Fourier series to allow one tracking of passive tracer particles for flow visualization. It was chosen to study the asymptotic behavior of the series components to overcome the limits of Fourier series with regard to slow convergence. In this way, with only a few terms highly accurate solutions are yielded. Then, alternation between two flow fields is used to induce chaotic advection. This is achieved by periodic alteration of the electrodes potentials. 

Active stresses exerted by smooth muscle cells appear to increase the internal stresses that exist in vessel wall. The effects of passive and active muscular contraction on the residual stress in the wall have been considered. Their results suggest that basal muscle tone, which exists under physiological conditions, reduces the strain gradient in the arterial wall and yields a near uniform stress distribution. Increased muscular tone that accompanies elevated blood pressure tends to restore the distribution of circumferential strain in the arterial wall, and to maintain the flow-induced wall shear stress at normal levels. It appears that the active stresses exerted by smooth muscle cells may balance the tension within the vessel wall in a similar manner to the way that active fibroblast tension balances the stress in the dermis. 

Gas flow method. The above methods have a common characteristic. That is, they do not discriminate between the active pores and the "passive" pores. For separation applications, only the "active" pores arc most relevant. A nondestructive technique for inferring the pore size distribution of a membrane is the gas flow method. Having been developed and tested at Oak Ridge Gaseous Diffusion Plant [Fain. 1989], it departs greatly from the above methods in that the pore size distribution is based on gas flow... 

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