08 re122 g 52 2 l 13 30 re 1800 10 25 l a simpler

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Unformatted text preview: f enhancement depends on the Reynolds number. Based on his experimental studies, Kutateladze (1963, Ref. 15) recommended the following relation for the average heat transfer coefficient in wavy lamiRe 1800, nar condensate flow for l and 30 hvert, wavy Re kl 1.08 Re1.22 g 5.2 2 l 1/3 30 , Re 1800 (10-25) l A simpler alternative to the relation above proposed by Kutateladze (1963, Ref. 15) is hvert, wavy 0.8 Re0.11 hvert (smooth) (10-26) which relates the heat transfer coefficient in wavy laminar flow to that in wave-free laminar flow. McAdams (1954, Ref. 2) went even further and suggested accounting for the increase in heat transfer in the wavy region by simply increasing the heat transfer coefficient determined from Eq. 10–22 for the laminar case by 20 percent. Holman (1990) suggested using Eq. 10–22 for the wavy region also, with the understanding that this is a conservative approach that provides a safety margin in thermal design. In this book we will use Eq. 10–25. A relation for the Reynolds number in the wavy laminar region can be determined by substituting the h relation in Eq. 10–25 into the Re relation in Eq. 10–11 and simplifying. It yields Revert, wavy 4.81 3.70 Lkl (Tsat l h* fg Ts) g 1/3 0.820 , 2 l v l (10-27) Turbulent Flow on Vertical Plates At a Reynolds number of about 1800, the condensate flow becomes turbulent. Several empirical relations of varying degrees of complexity are proposed for the heat transfer coefficient for turbulent flow. Again assuming l for simplicity, Labuntsov (1957, Ref. 17) proposed the following relation for the turbulent flow of condensate on vertical plates: hvert, turbulent 8750 Re kl 58 Pr 0.5 (Re0.75 g 253) 2 l 1/3 , Re 1800 l (10-28) cen58933_ch10.qxd 9/4/2002 12:38 PM Page 539 539 CHAPTER 10 1.0 Pr = 10 Eq. 10-24 5 h (νl2/g)1/ 3 ———— — kl 3 Eq. 10-25 2 1 Eq. 10-28 Wave-free laminar 0.1 10 Wavy laminar 30 100 Turbulent 1000 1800 Re 10,000 FIGURE 10–26 Nondimensionalized heat transfer coefficients for the wave-free laminar, wavy laminar, and turbulent flow o...
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