Jeff Spitzer wrote:

Tracy Crook wrote:

BTW, that cooling system chart that Neil sent looked odd to me.  I thought
outlet area was always supposed to be larger than the inlet.  Mine is.
Have I got it all wrong?

Maybe and maybe not.  It all depends on the system.  If the system uses
mostly internal diffusion (as opposed to external) then the outlet will
usually be bigger than the inlet.  This is because mass flow in equals mass
flow out.  At the inlet the free stream velocity and density enters
(remember, no external diffusion) and since the air is heated the exit area
for the same velocity will be larger.  OTOH, most cooling systems use some
degree (usually a majority) of external diffusion.  In this case the
velocity entering the inlet is much less than free stream.  If the density
change is small then the air will exit out of a smaller hole to acheive
free stream velocity at the exit.  This assumes the system has adequate
pressure left over to achieve the free stream velocity at the exit (not
usually).  If not then the exit grows to accomodate the proper exit
velocity.  Either answer can be right depending on the overall design and
efficiency.

Note that the pressure required to reaccelerate the air to free stream
velocity is less than the pitot pressure of the free stream that the inlet
sees.  This is because the air is less dense after being heated.  This is
the essence of how thrust can theoretically be produced.

I will scan a sketch of internal vs external diffusion unless Paul has a
better one from Horner or something.  Are you all following this?

Jeff Spitzer

Your message and Fred's message arrived 16 minutes apart on external
diffusion.
Fred won.

Hoerner did not mention external diffusion in the chapter on Drag Due
to Internal-Flow Systems. If he did I missed it. Did you see it Neil?
Scan away Jeff I have nothing so far.

Just when I thought I had heard it all on this news letter.....:-)

Paul Lamar
 
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