Free perimeter

Figure 1.2 Cross section of a partly open composite microchannel the lengths stand for the wetted perimeters and for free perimeter. The areal fractions are If If ...

If we denote by the wetted perimeter, i.e. the length of the contact line between the solid and the liquid in a cross section, and the free perimeter, i.e. the length of the front not in contact with the walls in the same cross section, (1.8) yields the condition [21]... 

D, Equivalent diameter of a cross section, usually 4 times free area divided by wetted perimeter D, for equivalent diameter of window m ft... 

Laminar Flow Normally, laminar flow occurs in closed ducts when Nrc < 2100 (based on equivalent diameter = 4 X free area -i-perimeter). Laminar-flow heat transfer has been subjected to extensive theoretical study. The energy equation has been solved for a variety of boundaiy conditions and geometrical configurations. However, true laminar-flow heat transfer veiy rarely occurs. Natural-convecdion effects are almost always present, so that the assumption that molecular conduction alone occurs is not vahd. Therefore, empirically derived equations are most rehable. 

For rectangular ducts Kays and Clark (Stanford Univ, Dept. Mech. Eng. Tech. Rep. 14, Aug. 6, 1953) published relationships for headng and cooling of air in rectangular ducts of various aspect rados. For most noncircular ducts Eqs. (5-39) and (5-40) may be used if the equivalent diameter (= 4 X free area/wetted perimeter) is used as the characteristic length. See also Kays and London, Compact Heat Exchangers, 3d ed., McGraw-Hill, New York, 1984. 

Noncircular Ducts Equations (5-50 ) and (5-50/ ) may be employed for noncircular ducts by using the equivalent diameter D = 4 X free area per wetted perimeter. Kays and London (Compact Heat Exchangers, 3rd ed., McGraw-HiU, New York, 1984) give charts for various noncircular duels encountered in compact heat exchangers. 

The hydraulic radius is the cross-sec tional area divided by the wetted perimeter, where the wetted perimeter does not include the free. sur-... 

The classic signature of erosion-corrosion is the formation of horseshoeshaped depressions, comet tads, grooves, or sand dunelike surface contours oriented along the direction of fluid flow (Figs. 11.1,11.2,11.3,11.5, and 11.8). Occasionally, erosion-corrosion will produce smooth, almost featureless, surface contours (Fig. 11.15), although even in this case oriented metal loss often exists around the perimeter of the affected region. If erosion-corrosion has been recently active, affected surfaces will be free of accumulated deposits and corrosion products. 

A packed bed of particles of diameter dp and fractional free volume c is modelled as a group of parallel capilaries with a perimeter... 

Free-standing sensors—These sensors, which include active infrared, passive infrared, bistatic microwave, monostatic microwave, dual-technology, and video motion detection (VMD) sensors, consist of individual sensor units or components that can be set up in a variety of configurations to meet a user s needs. They are installed aboveground, and depending on how they are oriented relative to each other, they can be used to establish a protected perimeter or a protected space. More details on each of these sensor types are provided below. 

Fig. 12. The kinetics of contraction of full-thickness guinea pig skin wounds separate collagen-GAG matrices into three classes. The wound half-life t,/2 is the number of days necessary to reduce the original wound area to 50%. An inactive matrix does not delay wound contraction significantly relative to the ungrafted control and eventually allows formation of a linear scar. An active, cell-free matrix delays wound contraction by about 20 days but eventually allows lull contraction to occur. An active matrix, which has been seeded with a minimal number of skin cells, delays contraction significantly, later arrests it, and eventually induces synthesis of a new dermis and epidermis within an expanding wound perimeter...

More fundamental objections to Young s equation center on the issue of whether the surface is in a true state of thermodynamic equilibrium. In short, it may be argued that the liquid surface exerts a force perpendicular to the solid surface, yLV sin 6. On deformable solids a ridge is produced at the perimeter of a drop on harder solids the stress is not sufficient to cause deformation of the surface. This is the heart of the objection. Is it correct to assume that a surface under this stress is thermodynamically the same as the idealized surface that is free from stress Clearly, the troublesome stress component is absent only when = 0, in which case the liquid spreads freely over the surface, and Figure 6.6 becomes meaningless. 

Both singlet and triplet states are generated by the orbital promotion of an electron, n- -it transitions are totally allowed. These energy values can also be calculated from HQckel molecular orbital (HMO) method. For benzene, the free electron perimeter model has been found to be useful. The energy levels and nodal properties of benzene molecule are given in Figure 2.19. 

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