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Good
Morning Nathan,
The film temperature shows up in Geankoplis on page 271 in the section
on data correlations for heat transfer with natural convection.
These
correlations include the Prandtl number, the Reynolds number, and the
Nusselt number, and thus we need to know values of density, viscosity,
thermal conductivity, etc. to use the equations. But density,
viscosity, thermal conductivity all vary with temperature. So,
which
temperature should you use look up physical properties to plug into the
correlation so that you get the right heat transfer coefficient?
The answer is given on page 271 for natural convection heat transfer
coefficients: "The fluid properties are evaluated at the film
temperature, T_f=(T_wall +T_bulk)/2..." Thus the film temperature
is the reference temperature for the fluid properties in natural
convection heat transfer correlations. See Example 4.6-1 for a
problem that uses the film temperature.
Exernal flow forced-convection heat transfer coefficient data
correlations also use the film temperature. See Geankoplis
section 4.6.
For forced-convection internal flow heat transfer coefficient
data correlations, they
do not use the film temperature. On page 261 for heat
transfer
coefficients for turbulent flow in pipes, Geankoplis gives the
correlation and then in the "fine print" below it it says "The fluid
properties except for viscosity_wall are evaluated at the mean bulk
temperature." In other words, for this forced-convection heat
transfer correlation, you average bulk-T-inlet and bulk-T-outlet and
use that mean bulk temperature as the one where you look up density,
viscosity, thermal conductivity, etc. to use in the correlation.
In class I emphasized this by saying it is essential to note the "fine
print" of all the data correlations. You must do what the
researcher
did when he/she published the data in that form or you will not get
accurate values for h.
I also wish to remind the class that I am not giving you all these
pages from Geankoplis. It was a homework assignment to summarize
all
the heat transfer correlations with their "fine print" and you are to
bring them with you as part of your two page exam sheet.s
best wishes,
Dr. Morrison
On Tue, Dec 17, 2013 at 11:06 PM, Nathan wrote:
> Good Evening Professor,
>
> I was looking at the natural convection material and was wondering
what the
> film temperature is for. I understand that it is the average of
the bulk
> temperature and the wall temperature, but I don't really
understand the
> purpose for natural convection. I know that it is important for the
> evaporators and condensers.
>
> Thank you,
>
> Nathan
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