To Both Ed and Al.
Thanks!
I guessed it was a gradual change subject to
length and shape of the inlet duct/ diffuser.
I had hoped that with a decent design (we are
all aiming for) that you might expect to achieve max static pressure well
prior to the Radiator.
However it may be that the optimum static
pressure might be design dependent and happen just before the rad. If figure
if it was within the rad it would be restrictive.
Thinking on it further, the more further forward
(of the Rad face) the optimum static air pressure is, it may suggest that the
rad is too restrictive.
I'm not sorry I asked, a little less confused
and more things to think about.
George ( down under)
George, here you are getting into something we have not
discussed in depth.
Two equations/laws of fluid dynamics are involved.
Bernoulli's equation and an equation called the law of continuity.
This equation relates to the fact that you don't create or lose mass in
the duct, so the mass flow is a constant everywhere in the duct. The
mass flow is frequently shown as the product of air density*cross section
area*air velocity = mass flow or simply p*A*V
The equation goes something like this, the
p1A1V1 (mass flow at point 1) = p2A2V2
(mass flow at point 2). Since the air is normally considered to act like
it is incompressible at the lower speeds we are talking about, that
means the density p1= p2, so we can drop them from the
equation for this explanation.
That leaves us with A1V1 = A2V2 or the
product of the area and velocity at point 1 is equal to the area and velocity
at point 2 in the duct. Now if A1 = A2 then V1
has to equal V2 for the two sides of the
equation to be equal. But, what if A2 = 2*
A1 or the cross section area of point 2 is made twice the cross
section area of point 1. Then if A2 = 2*A1, we can
substitute 2*A1 for A2 in the equation and
we have the following.
Taking A1V1 = A2V2 and substituting we
have A1*V1 = (2*A1)*V2. So what does that tell us
about the air velocity at point 2 now that we have doubled the cross section
area there?
Well solving the equation for the new V2, We can call
the new velocity at point 2 V2n (for V2 new)
with V2o being the old velocity at point 2.
So we have V2n =
A1V2o/(2*A1) Now we can cancelled the
A1 in the numerator and denominator on right side of
the equation leaving
V2n =
V2o/2 This shows us that the new
velocity at point 2, V2n is 1/2 the old velocity
(V2o) at point 2 or V2n =
0.5V2o
So what this says is the velocity starts changing
(slowing in this case and the pressure increasing ) as
soon as the cross section area A2 starts to increase from A1.
The process continues until the area stops expanding (or the kinetic energy of
the moving air has all been converted to a static pressure increase) and
that is where the process is finished as the duct/diffuser has expanded to its
maximum area. Actually, this process happens with both nozzles and
diffusers just the opposite way. Its derived from the Bernoulli equation
and the continuity law.
So if you had a duct whose cross section area continued
to expand for a distance of 2" or 20" or 200" then theoretically
the pressure would continue to build and the velocity to decrease until all of
the kinetic energy of the moving air has been converted to pressure
increase. This is all theoretical, there are losses and turbulence and
etc, that makes a difference, but you get the ideal. It depends on your
specific diffuser dimensions.
Think of it this way, George, some wind tunnels have
diffuser which expand over 10's of feet while some microscopic cooling systems
have diffusers measured in 10th's of an inch.
Now aren't you sorry you asked
{:>)?
Ed
----- Original Message -----
Sent: Friday, November 09, 2007 4:52
PM
Subject: [FlyRotary] Re: Total,duct,
Ambient or Velocity????
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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