Using this approximation and substituting eqs 1014 kl

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Unformatted text preview: liquid, W/m · °C height of the vertical plate, m surface temperature of the plate, °C saturation temperature of the condensing fluid, °C At a given temperature, l and thus l l except near the critical point of the substance. Using this approximation and substituting Eqs. 10–14 kl and and 10–18 at x L into Eq. 10–8 by noting that x L hx L 4 h (Eqs. 10–19 and 10–21) give hvert 3xL Re 4g l ( ) l 3 3 2 l 4g 3 2 l 2 l 3 kl hx kl 4g 2 3h 3l vert/4 L 3 (10-23) Then the heat transfer coefficient hvert in terms of Re becomes hvert 1.47kl Re 1/3 g 2 l 1/3 , 0 Re 30 (10-24) l The results obtained from the theoretical relations above are in excellent agreement with the experimental results. It can be shown easily that using property values in Eqs. 10–22 and 10–24 in the specified units gives the condensation heat transfer coefficient in W/m2 · °C, thus saving one from having cen58933_ch10.qxd 9/4/2002 12:38 PM Page 538 538 HEAT TRANSFER hvert W m kg kg J s2 m3 m3 kg m · °C kg m · s · °C · m 3 1/4 J W3 m1 s m6 m3 · °C3 °C 1/4 W4 m8 · °C4 W/m2 · °C FIGURE 10–25 Equation 10–22 gives the condensation heat transfer coefficient in W/m2 · °C when the quantities are expressed in the units specified in their descriptions. to go through tedious unit manipulations each time (Fig. 10–25). This is also true for the equations below. All properties of the liquid are to be evaluated at the film temperature Tf (Tsat Ts)/2. The hfg and are to be evaluated at the saturation temperature Tsat. Wavy Laminar Flow on Vertical Plates At Reynolds numbers greater than about 30, it is observed that waves form at the liquid–vapor interface although the flow in liquid film remains laminar. The flow in this case is said to be wavy laminar. The waves at the liquid– vapor interface tend to increase heat transfer. But the waves also complicate the analysis and make it very difficult to obtain analytical solutions. Therefore, we have to rely on experimental studies. The increase in heat transfer due to the wave effect is, on average, about 20 percent, but it can exceed 50 percent. The exact amount o...
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This note was uploaded on 01/28/2010 for the course HEAT ENG taught by Professor Ghaz during the Spring '10 term at University of Guelph.

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