Post on 03-Apr-2018
7/28/2019 Heat Exchanger2.Ppt
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Heat Transfer with Phase Change
So far we have discussed heat transfer due to a
temperature gradient/difference
( )wbx
x
TThA
qdx
dTk
A
q
=
= conduction
convection
When a phase change takes place, the temperature on
one side is CONSTANT, but the presence of
boiling/condensing fluids affects heat transfer. Important in evaporation, distillation
LARGE h
Faith A. Morrison, Michigan Tech U.
vapor phase
liquid phase
satT
satTT>
Boilingresistance to heat transfer
At low T, few
bubbles form, and
heat transfer is by
natural convection.
As Tincreases,
more bubbles form,
increasing convection
(flow) in the liquid
phase, increasing h.
heat
Faith A. Morrison, Michigan Tech U.
7/28/2019 Heat Exchanger2.Ppt
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vapor phase
liquid phase
satT
satTT>
Film Boiling
resistance to heat transfer
As Tincreases
further, a film appears
and grows, increasing
resistance to heat
transfer
heat
Faith A. Morrison, Michigan Tech U.
Geankoplis, p260
natural
convection
nucleateboiling
transitionboiling
film
boilingRegimes of HeatTransfer during
Boiling
25.0T
radiation
becomes
important
There are
correlationsfor h for
each
regime.
Faith A. Morrison, Michigan Tech U.
7/28/2019 Heat Exchanger2.Ppt
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vapor phase
condensed phase
satTsatTT