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Ed and Al,
This is all good info me, it either confirms,
clarifies or informs.
The straw concept is a timely reminder of pressure
differentials, a good example IMHO.
One thing I would really love to know is - at what
point in the inlet duct does the dynamic flow change to static pressure. I would
assume this would vary with different shaped ducts and different dynamic flow (
airflow speed).
Your opinions on this or guesstimates ie 1",
2" or 3" from the face of the rad, would be of great interest to
me.
George (down under)
Hi Al,
Not picky - some good points as always . Yes, I
agree, generalization does have its pit falls, but on the
other hand I think they can help promote a conceptual understanding
which can be refined (through study and experiments) to meet a particular
situation. As we know, cooling airflow is attempting to balance
conflicting aerodynamic and thermodynamic principles.
I also agree that much of this stuff
addresses the "Perfect theoretical duct" out of necessity as there is
only one perfect duct but many, many implementations that
fall short of perfect. So its more of a conceptual goal to be
aimed for - it may never be achieved, but provides at
least guidelines. But,this is just my opinion of
course.
Actually, I disagree, you can not "suck" air though
anything. You may create a partial pressure difference with the fan, but
it is the higher pressure air on the other end of the duct that pushes or
"blows" air through the duct into the area of lower pressure
{:>) .
But, semantics aside, yes, I agree, lower exit
pressure is what you are after and that does not always equate to larger exit
duct area. In fact, if the air heated by the core flows through a nozzle
it might even produce thrust and lower exit pressure using a smaller
exit. But, in general, I still believe that in most of our cases, we are
short of the level of duct design that would reliably permit that. What
we need is someone to invest in one of those $$$$ Computer Fluid Flow software
programs and see what they would reveal.
Ed
----- Original Message -----
Sent: Friday, November 09, 2007 1:09
AM
Subject: [FlyRotary] Re: Total,duct,
Ambient or Velocity????
It would seem "reasonable"
that a low pressure area at the exit will help flow through a duct -
no argument on that point. What the report appeared to say is that the
after a certain point opening the exit area wider does not appear to have
any additional benefit. (Exit “area” and exit “pressure” are not interchangeable
terms)
That if the duct is capable of "using up" all of the kinetic
energy in your air flow by obstructions, pressure drops and friction
losses then enlarging the exit does not necessarily add to the
flow.
Remember you can not suck air
through a duct, you can only blow it through. (Of course you can suck air through a duct – I do it
after (and sometimes before) every flight with the fan I have on the back
side of the radiator) So in effect if the straw is pinched you
can "suck" on it all you want but it won't increase flow
{:>).
If I understood the report,
it appears that enlarging the exit area beyond the frontal area of
your core provides little if any additional benefit. That does not
mean cowl flaps never work or provide benefit. In fact it appears that
the better your duct, the more benefit the cowl flaps appear to have,
the worst your duct, the lesser benefit - just the opposite of what you
might think.
Ed;
Don’t mean to be
picky, but some of these generalities are making me
nervousJ. These
things are applicable only when the duct/diffuser is operating at max
efficiency – which is rarely the case.
Lot’s of good
info.
Thanks. You’re right; it’s some kind of magic, and you don’t know for
sure until you built it and try it.
Al
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