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OK, Al, let me restate in a more comprehensive manner and
see if that helps.
We know that "dynamic pressure" is actually measured by
the increase it causes in localized static pressure. So
the term "dynamic pressure" is actually just referring to the energy
potential (Kinetic) of the moving air to cause a localized increase in static
pressure - if that air movement were brought to a stop.
In other words, if we had a flow of air with a
specific velocity and specific density, that air would have a ambient static
pressure (say at sea level of 29.92" HG). The moving air would
also have a static pressure potential (Dynamic pressure) based on its
velocity and density. So that if a tube were used to measure this "Dynamic
Pressure" it must first bring that part being measured to a stop the action
of which converts the dynamic pressure potential of the moving air to a
localized static pressure increase in the tube.
So the total static pressure at the measuring point
would be the static pressure of the ambient air (29.92"HG) plus whatever
increase was caused by stopping the moving air or converting its dynamic
potential to static pressure. So Pt = Pa + Pd with Dynamic Pressure
component, Pd = p*1/2V^2.
So in case of a duct there is, of course, only ambient
static pressure in the duct if there is no air flow through the duct. Once
there is airflow then you also have potential pressure in the form of the
kinetic energy of the moving air. So that Pt = Pa + p1/2V^2. p being
air density, V being the velocity.
The streamline duct (theoretically) can convert 84%
of the moving air potential dynamic pressure to static pressure increase.
So that at the widest part of the duct just before the core you would have a
total static pressure Pt = Pa + 0.84*p1/2V^2.
But, using differential pressure gauges with tubes pointed
into the moving air, we are not measuring total pressure, but the pressure
increase due solely to the moving air. In other words, if you were
measuring 5" H20 and then the air stopped moving , the gauge would read
zero.
So with the manometer you are measuring the pressure
above ambient pressure or that resulting solely from the dynamic pressure
potential of the moving air being converted from kinetic energy to static
pressure. Yes, the ambient pressure is present but you are not measuring
it. With no moving air the water levels in you manometer would all be
exactly at the same level..
The fact is that you are measuring static pressure at both
locations - the 9.5" before the duct was a static pressure increase in your
measuring tube - cause by stopping the moving air so its refer to as
dynamic pressure. The fact is that you were also measuring static pressure
3.25" at the location in the duct - but both resulted from the transformation of
the air's kinetic energy into a local static pressure increase. Therefore,
the fact that you were measuring considerably more pressure before the duct than
inside it indicates that the air stream's velocity is not being efficiently
transformed into static pressure in the duct.
This implies that perhaps there is less air velocity
entering the duct than your measurement a couple inches in front suggests OR
there is sufficient eddies and adverse flow situation inside the duct that
precludes the efficient transformation into a static pressure increase.
I do not have an aerodynamic or gas dynamics background,
so I could certainly be wrong. But, that is my understanding based on the
somewhat extensive reading I have done.
Ed
----- Original Message -----
Sent: Monday, July 16, 2007 5:48 PM
Subject: [FlyRotary] Re: FW: Oil cooler
air flow
if the free air velocity (160)
converts to 12"H20 and you had a streamline duct inlet actual had that coming
in then theoretically you could get approx 12 * .84 = 10.8" inside the
duct. Since you measured 3.25" static in front of the core, that would
indicate a significant lack of pressure recovery inside your duct (what ever
the reason). There are several reasons this might be
happening.
I think the
confusion here is whether we’re talking “dynamic” pressure or “static”
pressure. Are you saying that the maximum static pressure in the duct is
0.84 of the dynamic pressure at the entrance to the duct? If that is true, I
have been under a misconception. I measured 9.5” dynamic pressure out in
front of the scoop; and 3.25” static pressure near the face of the core – just
below the midpoint.
1. The air flow and velocity
is considerably reduced from what you are expecting (too small opening/exit -
which I don't believe to be the case)
2. The boundary layer is a
significant part of your duct total air flow and as a consequence
its lesser velocity has less dynamic pressure
potential.
3. A significant part of
your duct flow is chaotic with eddies which does not provide recoverable
pressure - or it is much reduced. (The boundary layer could be
contributing to this)
4. Some combination of the
above.
Right, now I
would suspect that the boundary layer could be the culprit in that it can
contribute to 2 and 3 above. But, as you know, this is speculation on
my part
I’m sure you’re
right; a combination of 2 and 3. Yesterday I measured the static pressure
near the upper surface of the duct; an inch or so in front of the core –
less than 0.25” H2O. That confirmed to me that the
“flow is
chaotic with eddies”, as you
say. I think the addition of a
vane is worth a try.
Al
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