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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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