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Only for those reallllllyyyy fascinated by airflow in
ducts/diffusers {:>).
For a time I had been puzzled by
reference in literature to an "ideal" or at least maximum expansion ratio
for the inlet/outlet area of a diffuser. Much of the literature point to a
maxim divergence angle from the inlet to exit of a diffuser of
7-11degrees. On the other hand there is literature that points to
such diffusers as the Streamline diffuser which has a divergence angle
approaching 65 degrees and with 84% efficiency as being great. So how can
it be both ways??.
It became apparent after a lot of reading that the
difference appeared to be whether there was a "resistance" to the airflow right
at the exit of the diffuser such as a radiator/heat
exchanger.
For conditions (such as a wind tunnel) where there is no
resistance (radiator) at the exit of the diffuser, separation will occur if
the divergence angle is greater than aprox 11 deg (depending on type of diffuser
and some other factors. But when a resistance (radiator) is
present things are different. I think I puzzled out why that happens,
but that is just my speculation so I won't bore you folks with that.
Instead I recently found some documentation that gives some hard facts about the
two diffuser conditions.
I recently found an Naca study that
specifically made this point even with charts which showed the divergence angles
of diffuser with and without resistance and their efficiency. In fact, it
even states that with a resistance (radiator) SHORTER inlets to a diffuser is
apparently better than longer.
Also there is reference to the fact that if the boundary
layer is thinner (rather than thicker) that separation caused by the adverse
pressure gradient of a diffuser is delayed. A thinner boundary layer is
equated to a boundary layer with a higher velocity and one that does not travel
far (and therefore slow down and grow into a thick boundary layer).
These two findings of short inlet and higher velocity
boundary layer having some benefits (if in front of a radiator) offers some
encouragement that my "Pinched" ducts may in fact have some merit.
But, regardless of that point, I though some of you might
find this interesting. I extracted two sections which hopefully is good
enough to read. If you want a copy of the report (high speed only), send
me an e mail
Second jpg with diffuser with resistance coming in next e
mail.
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