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Tham khảo tài liệu 'a heat transfer textbook - third edition episode 2 part 9', kỹ thuật - công nghệ, cơ khí - chế tạo máy phục vụ nhu cầu học tập, nghiên cứu và làm việc hiệu quả | 8.5 Film condensation 439 and with h x from eqn. 8.57 f nD 2 -J- 2 1 k h - 5 nD v2 x - 0 1 4 PfiPf - Pgih fgx3 xge sin2x D 4 3 Pk Tsat - Tw sin2x D 1 3 dx 0 dx This integral can be evaulated in terms of gamma functions. The result when it is put back in the form of a Nusselt number is NuD 0.728 Pf .Pf - PgigefD3 Pk Tsat Tw 1 4 8.64 for a horizontal cylinder. Nusselt got 0.725 for the lead constant but he had to approximate the integral with a hand calculation. Some other results of this calculation include the following cases. Sphere of diameter D Nud 0.828 PftPf Pg geh fgD3 1 4 Pk Tsat Tw 8.65 This result 9 has already been compared with the experimental data in Fig. 8.10. Vertical cone with the apex on top the bottom insulated and a cone angle of a __1 4F Pf P Pg geh f x3 11 4 Nux 0.874 cos a 2 1 4 f fr f 8.66 pk Tsat Tw Rotating horizonlal disk10 In this case g U -X where x is the distance from the center and o is the speed of rotation. The Nusselt number based on L p Pfw 1 2 is Nu 0.9034 P Pf PgWfg 1 4 Pfk Tsat Tw constant 8.67 9There is an error in 8.33 the constant given there is 0.785. The value of 0.828 given here is correct. 10This problem was originally solved by Sparrow and Gregg 8.38 . 440 Natural convection in single-phase fluids and during film condensation 8.5 This result might seem strange at first glance. It says that Nu fn x or w . The reason is that Ô just happens to be independent of x in this configuration. The Nusselt solution can thus be bent to fit many complicated geometric figures. One of the most complicated ones that have been dealt with is the reflux condenser shown in Fig. 8.14. In such a configuration cooling water flows through a helically wound tube and vapor condenses on the outside running downward along the tube. As the condensate flows centripetal forces sling the liquid outward at a downward angle. This complicated flow was analyzed by Karimi 8.39 who found that - hd cos a Nu -----7 k p - Pg pfh fgg d cos a 3 11 4 z d .
