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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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