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Hi Bill,
It is my opinion, based on my limited knowledge of the
topic, that dynamic pressure in the duct is the most significant factor.
If you don't have it - you have no flow. If you do have it you will have
flow but you could have significant Major losses - that's why you may need
other types of pressure measurements to figure out the problem. In fluid
flow talk, they appear to refer to loss of energy through wall friction as
a major loss as it is not recoverable (but this is minor at our speeds) , while
trades between dynamic and static in the duct result in "minor" losses which may
or may not really be minor.
Here is my understanding, you would like to convert dynamic
energy to static pressure increase in front of the core as that slows down the
velocity reducing drag and tends to give you more even velocity distribution
across the core (assuming little or no separation of flow from the duct
walls). You would like the greatest pressure drop across the core which
results in the highest velocity through the core tubes generating
turbulence for better heat transfer.
However, there is a balancing point, more pressure drop
generally means better heat transfer from metal to air, however, it also
generally means less mass flow because of the resistance. Too much
pressure drop = too little mass flow and overheating, too little pressure drop =
great mass flow but higher duct drag and less heat transfer per unit time which
can also lead to overheating.
I like to use this example to emphasize the point.
You would get maximum pressure drop by placing a solid board across the duct -
however, the air flow would be nil and cooling likewise. On the other
hand, if you remove all obstructions in the duct (including the core) , the
pressure drop would be nil, the airflow would be maximum but cooling would
still be nil. The only significant difference is the no core
approach is cheaper and causes less drag {:>)
In any case, all the literature I have read seems to indicate
that the difference in pressure between the inlet and out let of the duct is a
(if not THE) key factor. That dynamic pressure is the only thing (assuming
no fans/blowers) that will move significant air through the duct. Since
this dynamic pressure is referenced to the dynamic pressure available in the
freestream flow as that is what it starts out as, I personally think referencing
dynamic pressure measurements to ambient air is what we are mainly
interested. This is rather than referencing it to the duct static
pressure as shown in the diagram. But, you have to
remember this is all from the guy who has not done any duct
instrumentation.
But, my reason for focusing on dynamic pressure is
that you can infer a lot from your duct dynamic pressure readings about
what is going on in the duct. If your dynamic pressure is down, then your
static pressure is up and vice versa. If you have dynamic pressure then you have
flow while static pressure does not necessarily tell you that.
However, it all really depends on what you are trying to
figure out on what measurements you take.
It would appear if you know how to interpret what you are
measuring then all provide some useful information.
That's about the extent of my limited knowledge.
Ed
----- Original Message -----
Sent: Thursday, November 08, 2007 12:28
AM
Subject: [FlyRotary] Re: Total,duct,
Ambient or Velocity????
Ed, The slide is a good way to explain the various references. I am still
confused as to what will give you the "best" data. The static in duct pressure
compared to the total, or to the velocity? It probably doesn't matter if
you use the same method all the time.
Bill Jepson
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