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Heat Transfer Handbook part 74. The Heat Transfer Handbook provides succinct hard data, formulas, and specifications for the critical aspects of heat transfer, offering a reliable, hands-on resource for solving day-to-day issues across a variety of applications. | FILM CONDENSATION ON LOW FINS 725 the interface induces a pressure decrease within the film. The expression for this pressure variation known as the surface tension pressure gradient is obtained by differentiating eq. 10.13 with respect to the fin arc length s dP _ d 1 r1 _ dK ds ds ds 10.14 where 1 r2 0 for a two-dimension surface. Figure 10.2 shows the coordinate system for a condensate film on a convex fin surface profile. The coordinate measured along the liquid-vapor interface is 5 . The coordinate measured along the fin surface is 5. The location 5 0 is the point of symmetry and is referred to the fin tip. The film has thickness 8. The condensate surface turns through a maximum angle of 0m and a maximum arc length Sm. The coordinate measured perpendicular to the base-metal surface is y. The curvature of the liquid-vapor interface shown in Fig. 10.2 decreases for increasing values of the coordinate 5 . In general decreasing pressure gradients can be achieved with fin tips of small curvature. A general function for the liquid-vapor interface curvature k can be represented as a function of 5 K C1 - C-2S Z 10.15 where C1 C2 and Z are arbitrary constants. As illustrated in the following section specification of the interface curvature allows the fin designer to investigate the influence of the fin shape on the condensation performance. 10.3.3 Specified Interfaces Gregorig 1954 proposed to increase Nusselt condensation by shaping a convex condensate surface such that surface tension forces alone would produce a film of constant thickness Tw sm ii 4 kl l T sat 8 nXpi om 10.16 By using h ki 8 the local heat transfer coefficient for Gregorig s surface becomes O K mki 1 4 h bl Tsat Tw Sm J 10.17 Zener and Lavi 1974 proposed a convex shape that gives a constant-pressure gradient along the convex surface. The local heat transfer coefficient for the Zener and Lavi profile is h oPQmki 1 4 2vi T sat Tw Sm s 10.18 726 CONDENSATION The average heat transfer coefficient for .
