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determined in Chapter 14). Other examples of evaporation are the drying of
clothes, fruits, and vegetables; the evaporation of sweat to cool the human
body; and the rejection of waste heat in wet cooling towers. Note that evaporation involves no bubble formation or bubble motion (Fig. 10–1).
Boiling, on the other hand, occurs at the solid–liquid interface when a liquid is brought into contact with a surface maintained at a temperature Ts sufficiently above the saturation temperature Tsat of the liquid (Fig. 10–2). At 1
atm, for example, liquid water in contact with a solid surface at 110°C will
boil since the saturation temperature of water at 1 atm is 100°C. The boiling
process is characterized by the rapid formation of vapor bubbles at the
solid–liquid interface that detach from the surface when they reach a certain
size and attempt to rise to the free surface of the liquid. When cooking, we do
not say water is boiling until we see the bubbles rising to the top. Boiling is a
complicated phenomenon because of the large number of variables involved
in the process and the complex fluid motion patterns caused by the bubble formation and growth.
As a form of convection heat transfer, the boiling heat flux from a solid
surface to the fluid is expressed from Newton’s law of cooling as
q·boiling h(Ts Tsat) h Texcess (W/m2) (10-1) where Texcess Ts Tsat is called the excess temperature, which represents
the excess of the surface above the saturation temperature of the fluid.
In the preceding chapters we considered forced and free convection heat
transfer involving a single phase of a fluid. The analysis of such convection
processes involves the thermophysical properties , , k, and Cp of the fluid.
The analysis of boiling heat transfer involves these properties of the liquid
(indicated by the subscript l) or vapor (indicated by the subscript v) as well as
the properties hfg (the latent heat of vaporization) and (the surface tension).
The hfg represents the energy absorbed as a unit mass of liquid vaporizes
at a specified temperature or pressure and is...
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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.
- Spring '10