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Sometimes when trying to figure out how to improve
cooling, some of us (present person excluded) actually take to measurements and
gathering hard data, such as temperature and pressure. I have notice
that at times some confusion exists about pressure measurements in
particular. There are three basic pressure measurements we are
interested in
Total or sometimes referred to as Stagnation pressure
(such as measured by a pitot tube) is the sum of the static pressure and the
dynamic pressure. Ptotat = Pstatic + Pdynamic often written
as
Pt = Po + 1/2pV^2 Some times dynamic pressure is
referred to as velocity pressure as it is due to the kinetic energy of the
moving air to the square power.
Most folks know all this, where the problem
sometimes occurs is trying to configure a pressure data collection
arrangement so as you know as to what is being measure and what
configuration do you need to measure it.
I found the attached slide which I thought was very
clear. It show the orientation of the pressure probes (looking into or
perpendicular) to the air flow and perhaps just as important - clearly shows the
other reference end of the probe. Note that the reference end of the
probe is either ambient static pressure (outside the duct) or duct static
pressure (referenced to the static pressure inside the duct).
So if your "other end" is under your cowl what are
you referencing? ambient? probably if your aircraft is not moving but then no
dynamic pressure either so its sort of boring. But, in flight it is
highly unlikely that the cowl pressure will be ambient.
Since the flow through your duct is driven by the
difference between ambient static pressure and dynamic pressure, you really are
interested in ambient static reference in most cases. But, sometimes
you might like to know the total pressure on both sides of your
core. In which case, you would need the total pressure probe configuration
with the probe pointing into the air flow on each side of your radiator.
If you just used a static probe or just a dynamic probe you would not get a true
picture of the total pressure gradient across your core.
If on the other hand you want to know how well your
diffuser is diffusing (converting air velocity into static pressure increase,
you would need a dynamic pressure probe at the exit of your diffuser with its
referenced to the duct ( not the ambient) static pressure . So where you
reference can made a significant impact on what you are measuring and how you
are interpreting the measurement.
Referring to the slide.
The left-most probe (static pressure) is
not orientated into the air flow so therefore should (theoretically) not
measure any dynamic pressure caused by moving air. Note that it is
reference to the ambient (outside) air static pressure. The far fight
sensor is - in contrast - pointed into the airflow where it can bring the moving
air to rest thereby converting the kinetic energy of the moving air into a local
pressure increase. Note that this dynamic pressure sensor is
referenced to the static pressure INSIDE the duct NOT outside the duct as was
the static pressure sensor
In the center is a probe configured to measure total
pressure. It measures the dynamic pressure by pointing into the air
flow but is referenced to the outside static pressure. So its pressure is the
algebraic sum of the pressure at each end. We know the ambient pressure is
approx. 14.7 psi standard day and depending on the air velocity we have a few
tenths of a psi dynamic pressure.
So Ptotal = Ps + P dynamic = 14.7 + 0.23 = 14.92 psi for
example
When air flows through a duct system, the duct static and
dynamic pressure components can trade amongst themselves. Kinetic energy
can be transformed into static pressure increases and static pressure
differences in the duct will result in air flow. The total pressure value
is a maxim at the duct inlet and will decrease through losses of momentum in the
duct flow. The dynamic pressure at the inlet is the fundamental energy
that drives the air flow through the duct.
Been slow on the list, so though I would throw this out
for consideration.
Ed
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