Forced Convection: The most common mode of heat transfer is
forced convection. A number of
correlations have been developed for situations where fluid is flowing through
pipes and annuli. These correlations
are functions of Reynolds number (Re), Prandtl number (Pr), and
Graetz number (Gz) (see Tables 5.4 and 5.5). During computation of these numbers, the properties of the fluid
are taken at bulk temperatures.
Flow
through a tube:
Example
5.5:
Water is flowing through a tube having internal diameter 1.334" at a rate
of 10000 lb/hr. It is heated from 80 °F to 120 °F in a
counter-current arrangement. Find the
heat transfer coefficient (hi) if the following data apply:
Bulk
temperature, tb, = 100 °F
Specific heat,
c, = 0.998 Btu/(lb·°F); density,
ρ = 62.0 lb/ft3;
Absolute
viscosity, μ = 1.65 lb/(ft·hr); thermal conductivity, k = 0.363 Btu/(hr·ft·°F);
Solution: Water is flowing through a circular
tube. Heat transfer coefficient depends
upon the value of Reynolds number and Graetz number.
$
The velocity of the fluid, V, can be found once the
cross-sectional area, a, is known.
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$
Reynolds number, Re:
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$
Prandtl number, Pr:
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$
Nusselt number, Nu: Appropriate relationship when Re
is greater than 10000, so the following correlation will apply:
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