On 11/21/07, Ron Springer <ron2369@sbcglobal.net> wrote:
Mass flow rate = density x area x velocity

Consider an example with the inlet and exit areas the
same.

Then, density times velocity is the same at the inlet
and the outlet. The density at the outlet will be
lower than at the inlet because the air has been
heated. So, the velocity must be higher at the outlet
than at the inlet. This is true for this case
regardless of the drag created by the radiator, or the
efficiency of the system, as long as inlet amd outlet
areas match.

(Actually, if drag is increased too much, the inlet
will spill air and the mass flow rate will be lowered
to below that of a full flowing inlet. A smaller
streamtube of air would be captured, there would be
external diffusion prior to the lip of the inlet, and
the effective inlet area (or capture area) would need
to be used instead, which is less than the physical
area of the inlet.)

More sophisticated examples can be done for non-equal
areas, but then it becomes more involved. The point is
that the air exits with significant velocity as long
as there is significant mass flow and the effects
can't be ignored.

Ron

Your logic is correct but the fact is that inlet & outlet sizes are
not equal.   It is a rare installation that has outlet size equal (or
smaller) than the inlet.  I have never seen an example of this in a
home built but I have read of a couple (in  *extreamly*  refined
cooling systems).  Most are at least 2 times the inlet size.  I
typically orient my approach toward real world conditions, not
theoretical possibilities.  Optomizing outlets with equal or even
smaller area than inlets is the way to go *IF* your inlet diffuser is
already optomized.  EVERYTHING depends on getting the diffuser right
regardless of whether you are a thick or thin rad fan.

  Estimates of  "correct" inlet & outlet area ratios are all over the
map with Paul L's paper designs being the largest on the outlet.  He
likes zero back pressure on the back side of the rad with big louvers
to the airstream which are about the most draggy kind I can imagine.

Tracy