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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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