27 - 11/2/2010 Film Condensation on a Vertical Plate...

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11/2/2010 1 Condensation Chapter 10 Sections 10.8 through 10.10 Film Condensation on a Vertical Plate (laminar) Distinguishing Features ¾ Generally, the vapor is superheated ( ) , vs a t TT > ¾ Thickness ( δ ) and flow rate ( m) of condensate increase with increasing x () m ± ( ) δ T, v > T sat and may be part of a mixture that includes noncondensibles. ¾ A shear stress at the liquid/vapor interface induces a velocity gradient in the vapor, as well as the liquid. ( ) = fg v l l s sat l l h g x T T k x ρ µ 4 ) ( Can model a vertical pipe as vertical if D/2 >>> del Film Condensation on a Vertical Plate Nusselt Analysis for Laminar Flow Assumptions: ¾ A pure vapor at T sat ¾ Nl i i b lh t t Negligible shear stress at liquid/vapor interface. 0 y u y = →= 0 = = y y u Flow rate per unit width: Average Nusselt Number: l v l l g b m 3 3 = = Γ & 4 / 1 3 ) ( ' 943 . 0 = = s sat l l fg v l l l L T T k L h g k L h Nu Ja h h ) 68 0 1 ( ' + = Total heat transfer and condensation rates: fg s sat l p fg fg h T T c Ja ) ( . , fg s sat L h q m T T A h q ' ) ( = = & Properties evaluated at film temperature for liquid, Tsat for vapor
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11/2/2010 2 Effects of Turbulence : ¾ Transition may occur in the film and three flow regimes may be identified and delineated in terms of a Reynolds number defined as 4 44 Re lm ll l u m b δ ρ µµ µ Γ ≡= = ± l m l l l u b
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27 - 11/2/2010 Film Condensation on a Vertical Plate...

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