George, you are undoubtedly correct about necking
it down causes a restriction to air flow. Also, there is no doubt in
my mind that if you have the space to implement a full length duct
of the correct profile you will get better results. However, for
the Streamline duct that takes about 14-16" of space to run a proper pressure
recovery duct, I had less than 6". So what to
do?
My reading indicated that the
thing that causes the most severe disruption to good cooling (assume
everything else is adequate) is the separation of airflow from the interior
duct walls. This causes eddies to form which block/hinder airflow
through the part of the core they are in front of just like a piece of
cardboard would.
From what I think I understood of the information
on diffusers, it is clear that there is a "dome" of higher pressure air that
forms in front of the core due to conversion of the dynamic component of the
airstream to some pressure increase by the core and/or diffuser. It
actually forms to some extend whether you have a diffuser or not - due to
something I think they refer to as external diffusion. This is simply
the conversion of airstream energy (velocity) into pressure by molecules of
air meeting the resistance of the core and other molecules which slow down in
front of the core.
The Streamline diffuser duct describe in K&W
chapter 12 makes use of this higher pressure region to keep the airflow along
the duct walls from separating (or at least separating very late just before
the core which means if an eddy does form, it is much smaller and has less
disruptive effect). But, that is using the 16" to gradually
recovery the dynamic pressure with minimum loss. Which if done correct
will give you around 82% pressure recovery (theoretically) according to
K&W - good if it even approaches that figure in any
case.
Since I did not have that 16" to play with but
only 6" (actually even less), I was faced with "what to do?" I decided
that if I made the duct wall curve more drastically than the Streamline duct
wall due to the short distance, I was surely looking at airflow
separation. But, I decided that if I made the duct walls "pitch" in even
further into the higher pressure region that would help - but then (as you
stated, George) you start to affect the airflow which causes the higher
pressure dome in front of the core in the first place. What to
do? I decided (rightly or wrongly) that if I increased the velocity of
the air by necking it down, then even if the mass airflow were slightly less
than ideal, that the conversion of the higher velocity air to pressure
(the old 1/2pV^2 component) would compensate by keeping the pressure high
and this in turn keeping my airflow from separating from my radically curving
walls until too late to do much disruption of the airflow through the
core.
There were those who claimed that there was no
way that would work. Well, I reduced my inlet area from 48 sq inch to 28
and it works just fine thank you. Tracy Crook can vouch that I have
flown with the small openings for well over a year and he has never seen steam
or smoke coming from my engine - yet {:>).
Seriously, my analysis could be all wet, but so
long as the ducts cool, I'll stick with it.
My previous ducts were simply openings which
captured air. Photo attached. This shows the inside of two ducts,
the one on the left show the original duct shape which simply has an
inlet opening into a more or less box. The one on the right shows my
first attempt to see if my theory would work. The opening of the duct on
the right is 9 square inches and it is placed on my left radiator which is my
hot radiator (first one in the series of two). The opening of the duct
on left is original 24 square inches. After successfully flying with
this pair. I was satisfied enough to build two new ducts. The one
on the left radiator is now 18 sq inches inlet and the one on the right 10 sq
inches for a total of 28 sq inches. So a reduction of inlet area from 48
sq inches to 28 sq inches and it cools just fine as far as I am
concerned. In a hotter climate I would probably enlarge the total area
slightly. But, in any case, it proved (to me at least) that
preventing/delaying airflow separation was the crucial
factor. I am certain my ducts are not as efficient as a full up
Streamline duct, but given the space I had to work with I think my approach
has worked well.
Hope I answered your question, yes, it appears
counterintuitive, but I had a rationale for doing it. Whether the
rational was correct or there is some other factor I am not even aware of - I
certainly don't know. I didn't even sleep at the Holiday inn last
night.
Best Regards
Ed
----- Original Message -----
Sent: Monday, April 04, 2005 6:14
PM
Subject: [FlyRotary] Re: A lot to
learn
Ed,
I thought necking-down like you have done would
cause more of a restriction and would result in less pressure.
I thought that a more direct line of travel
from the opening to the core would give less restriction and more
pressure.
From what I've been told, this particular
design seems to want to follow the trumpet ( bell) shaped opening
style, whereby 1.5 times the face of the radiator, is required in the
length of the duct. I don't think 6" is enough length.
What other styles of duct have you tried which
are less successful than this design.
I must say it does look good but seems to fly
in the face of other previous recommendations.
George ( down under)
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