9.2 Classification and
Criteria of Evaporation
Chapter 9: Evaporation
9.2 Classification and Criteria of Evaporation
9.2.1 Classification
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The latent heat of vaporization acts as a heat sink during phase
change.
It is supplied either by migration through the liquid, as in
heterogeneous evaporation, or directly to the interface, as in direct
contact evaporation.
In the former, the heat must migrate by conduction (and in some
cases convection) through the liquid to the interface.
Figure 9.4 illustrates the difference between heterogeneous and
direct contact evaporation.
When a beaker of water is placed on a hot plate as shown in Fig.
9.4(a), only heterogeneous evaporation can occur because heat
must migrate from the bottom of the beaker to the surface of the
water in order for evaporation to take place.
Transport Phenomena in Multiphase Systems by A. Faghri & Y. Zhang
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9.2 Classification and
Criteria of Evaporation
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Chapter 9: Evaporation
The liquid’s temperature is highest at its point of contact
with the bottom of the beaker.
Adjacent to the bottom is a layer through which heat
passes only by conduction.
Convection takes place throughout most of the depth,
causing the temperature to decrease slightly.
Just below the surface, a thin layer exists in which the
temperature drops abruptly as the interface is
approached.
The surface temperature of the liquid can be assumed to
equal the saturation temperature corresponding to the
partial pressure of the vapor at the surface.
Evaporation from a liquid film or a droplet attached to a
heated wall is heterogeneous evaporation.
Transport Phenomena in Multiphase Systems by A. Faghri & Y. Zhang
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9.2 Classification and
Criteria of Evaporation
Chapter 9: Evaporation
9.2.2 Criteria
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Evaporation from a liquid surface is a
mass transfer process where the rate at
which the molecules leave the liquid
exceeds the rate at which the molecules
condense back into liquid from the vapor.
Transport Phenomena in Multiphase Systems by A. Faghri & Y. Zhang
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9.2 Classification and
Criteria of Evaporation
Chapter 9: Evaporation
Figure 9.4
Comparison of
heterogeneous and
direct contact
evaporation
Transport Phenomena in Multiphase Systems by A. Faghri & Y. Zhang
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9.2 Classification and
Criteria of Evaporation
Chapter 9: Evaporation
Figure 9.5 Evaporation domains
for a vapor-gas mixture.
Transport Phenomena in Multiphase Systems by A. Faghri & Y. Zhang
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