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Well, George, I'd say your understanding of the duct air
flow essentials are as good as mine.
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. 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. 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.
But, Hey!, I'm only an electrical engineer, so all
this airflow stuff is magic as far as I am concerned.
Ed
Sent: Thursday, November 08, 2007 5:11
PM
Subject: [FlyRotary] Re: Total,duct,
Ambient or Velocity????
Ed,
If I'm understanding you correctly, it appears
that you need dynamic pressure ( flow) that turns into a high static
pressure (at the Rad face).
To maintain this high static pressure, the
dynamic flow must be free of turbulence, which is associated with flow
separation from the duct walls. Hence the need for proper divergent
angles.
There must be good pressure drop across the
Rad, not too high or you lose heat transfer, not too low as to create
excess drag. There must be some turbulence within the duct fins to enhance
heat transfer, but not too much as to create restrictions.
I still feel a low pressure area behind the rad
would be beneficial.
George ( down under)
Even in the Naca studies they often 'fess up that
theoretical considerations must give way to practical installation
considerations {:>). From what I have recently read, theoretically
if you could do your exit the best way, you might even get a small thrust
benefit - at least enough to overcome the cooling drag. However, I
think the best most can do is simply provide an unimpeded exit flow and
minimize losses.
There is some interesting information on usefulness of
cowl flaps and why they some times do not seem to make any
difference. I don't claim to fully understand it all, but it appears
that once your losses in the duct exceed a certain limit - opening up or
even creating a low pressure region at the exit does not promote more air
flow through the duct. There is only so much energy in the air
velocity to turn into dynamic pressure and if your losses in the duct total
up to your dynamic energy limit then nothing you do at the exit will improve
the flow. At least that is the way it appears to this old
brain.
But, it sure keeps an old brain from freezing up
completely trying to understand some of this. I personally believe
that all of the literature is pretty clear that the best thing you can do
with your duct work is to prevent flow separation in the
diffuser.
Cooling goes down and drag goes up - not what we
are looking for. Its now finally clear why some of the reports
quote 7-11 deg as max diffuser divergence angles (2theta) and others show
good diffuser performance up around 60 deg divergence. The reason for
the two (seemingly conflicting) different findings is two different diffuser
configurations. One with no resistance behind it and one with
resistance (radiator).
Another important basic is to set down and
figure out the air mass flow you must have to handle your critical cooling
regime (full power climb out?). That then drives your inlet size, the
size cooler you need - and as they say - is the basis from which all
else flows(pun intended). But as you say how many of us do
that.
I find that it is often similar differences that
can/do end up confusing those of us who are ignorant but trying to
understand and apparently find conflicting findings in these reports.
You reallllllyyy have to read them carefully from end to end.
Ed
----- Original Message -----
Sent: Thursday, November 08, 2007
10:28 AM
Subject: [FlyRotary] Re: Total,duct,
Ambient or Velocity????
Ed,
It seems like a cogent discription Ed. I have been studying the
problem for some time. I like your no core example, much cheaper but it
will only fly once. (And for a short time!) The question I have been
pondering is, does it really help us to consider a exit ducting to direct
our exit flows. The data you presented seems to indicate that it does. The
dynamics of the pressure drop across the core contain compromises related
to the efficiency of the heat exchanger, flow of the water in it and air
through it. Many of the designs I see lately pay very little attention to
the exit and re-merging the flow. In core-in-the-standard-inlet systems
such as yours the exit ducting may not be practical. This is a problem I
have see with the Eggenfellner Sabaru installations as well. At least the
rotary can have some exit area without the cylinders right there in the
way! The exit question tends to favor the chin scoop. The problem is that
this has always proven to be a high drag choice. Currently I'm favoring a
vertical side radiator (or radiators) ducted to the outside (cowl) blowing
into the engine area with a diversion duct to turn the air towards the
normal rear bottom exit. Possibly with a cowl flap for climb. These have
never been easy choices. Often we intend an elegant solution, only to be
rebuffed by the need for hoses, wires, and exhaust pipes and other
unimportant stuff like that. ;-)
Thanks for all your
research,
Bill Jepson
-----Original Message-----
From: Ed
Anderson <eanderson@carolina.rr.com>
To: Rotary motors in
aircraft <flyrotary@lancaironline.net>
Sent: Thu, 8 Nov 2007 5:05
am
Subject: [FlyRotary] Re: Total,duct, Ambient or
Velocity????
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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