27-06-2012, 05:56 PM
Advanced Heat Exchangers Using Tunable Nanoscale-Molecular Assembly
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Condensation
Condensation occurs when the
temperature of the vapor is reduced below
its saturation temperature.
• The solid surface whose temperature is
below the saturation temperature of the
vapor
• Two distinct forms of condensation:
– Filmwise condensation
– Dropwise condensation
Filmwise Condensation
When liquid formed by condensation wets the
surface. It is a more common type of
condensation to occur.
• In filmwise condensation liquid condensate forms
a continuous film over the surface, this film flows
down the surface under the action of gravity,
shear force due to vapor flow, or other forces.
• The layer of liquid condensate acts as a barrier to
heat flow due to its very low thermal conductivity and
hence low heat transfer rate.
Dropwise Condensation
Dropwise condensation takes place when the
liquid condensate does not wet the solid surface.
• The condensate does not spread, but forms
separate drops.
• These drops in turn coalesce to form large drops
and sweeping clean a portion of the surface,
where again new droplets are generated.
• The average heat transfer coefficient for dropwise
condensation is much higher than filmwise
condensation.
Smooth Film Analysis
• The first attempt to analyze the film-wise
condensation problem was done by Nusselt in
1916.
• By making certain assumptions;
– The flow of condensate in the film is laminar,
– Fluid properties are constant,
– Sub cooling of the condensate may be neglected,
– Momentum changes through the film are negligible,
– The vapor is stationary and exerts no drag on the
condensate,
– Heat transfer is by conduction only, and
– Surface is isothermal.