Perimeter, wetted

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

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. 

Noncircular Channels Calciilation of fric tional pressure drop in noncircular channels depends on whether the flow is laminar or tumu-lent, and on whether the channel is full or open. For turbulent flow in ducts running full, the hydraulic diameter shoiild be substituted for D in the friction factor and Reynolds number definitions, Eqs. (6-32) and (6-33). The hydraiilic diameter is defined as four times the channel cross-sectional area divided by the wetted perimeter. For example, the hydraiilic diameter for a circiilar pipe is = D, for an annulus of inner diameter d and outer diameter D, = D — d, for a rectangiilar duct of sides 7, h, Dij = ah/[2(a + h)].T ie hydraulic radius Rii is defined as one-fourth of the hydraiilic diameter. 

The hydrauhc diameter method does not work well for laminar flow because the shape affects the flow resistance in a way that cannot be expressed as a function only of the ratio of cross-sectional area to wetted perimeter. For some shapes, the Navier-Stokes equations have been integrated to yield relations between flow rate and pressure drop. These relations may be expressed in terms of equivalent diameters Dg defined to make the relations reduce to the second form of the Hagen-Poiseulle equation, Eq. (6-36) that is, Dg (l2SQ[LL/ KAPy. Equivalent diameters are not the same as hydraulie diameters. Equivalent diameters yield the correct relation between flow rate and pressure drop when substituted into Eq. (6-36), but not Eq. (6-35) because V Q/(tiDe/4). Equivalent diameter Dg is not to be used in the friction factor and Reynolds number ... 

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

X (area of opening of one fuUy open valve) wetted perimeter of opening of one fully open valve no. valves X (area of opening of one fully open valve) active (bubbhng) area... 

Wetted perimeter Perimeter of conduit in contact with moving fluid. 

It should be pointed out that the hydraulic diameter method does not work well for laminar flow because the shape affects the flow resistance in a way that cannot be expressed as a function of the ratio of cross-sectional area to wetted perimeter (Green and Maloney 1997). However, some flame arrester manufacturers use this method for noncircnlar flame arrester passages. 

Dh = hydraulic diameter, ft = 4 (flow area for the phase in question/wetted perimeter of the flow channel)... 

Cross-sectional aiea allocated to light phase, sq ft Area of particle projected on plane normal to direction of flow or motion, sq ft Cross-sectional area at top of V essel occupied by continuous hydrocarbon phase, sq ft Actual flow at conditions, cu ft/sec Constant given in table Volume fiaction solids Overall drag coefficient, dimensionless Diameter of vessel, ft See Dp, min Cyclone diameter, ft Cyclone gas exit duct diameter, ft Hy draulic diameter, ft = 4 (flow area for phase in qiiestion/wetted perimeter) also, D in decanter design represents diameter for heavy phase, ft... 

D = the hydraulic diameter, 2(flow area)/ wetted perimeter)... 

V, Exhaust velocity W Wetted perimeter of port X0 Mole fraction of oxidizing species... 

The hydraulic mean diameter dm is defined as four times the cross-sectional area divided by the wetted perimeter. For a circular pipe, for example, the hydraulic mean diameter is ... 

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