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I understand, Al. I always appreciate your input and
viewpoint.
One reason I throw out the information and my "logic" is
so that others can examine and critique - it would be nice if I knew it all,
but, sigh! I am under no delusion that I do. You certainly bring up
food for thought and I would suggest any who may considered adopting the
"Pinched" duct give serious consideration of your
viewpoint.
External diffusion is undoubted a factor - as it
is in most forward facing ducts . I do agree that any time you
accelerate air your incur greater loses, my take was that
accelerating the boundary layer delayed separation and the adverse effects so
caused. That increased skin friction was a far lesser evil than
flow separation as indicated by the NACA study.
As to whether the boundary layer penetrates
further due to increased velocity is probably
dependent on the influence of the rate of pressure rise vs the rate of
boundary layer deceleration. Is that relationship linear or
non-linear? Given the number of variables, I suspect non-linear but
don't know.
The rapid expansion of the streamline duct configuration
would tend to indicate (to me at least) delaying separation, by keeping the
airstream energy high, until just before the core pays dividends. Now
whether my ratio of inlet, pinched and core area successfully accomplish that is
simply unknown at this point.
Using this approach, I have been able to reduce my inlet
area and make use of a very constrained space for a duct. Again, I make no
pretense that it could possibly be as effective as a full up streamline
duct for instance, but on the other hand I believe it creates less cooling drag
than a streamline duct truncated sufficiently to meet my space
constraints. It might well be that its success is largely due to external
diffusion as you suggest , but I would have expected that effect on all my
ducts.
The pinched duct does cools my installation better than my
previous approaches to a duct and has enabled me to reduce my radiator inlet
area by 33% less area than previous duct attempts. But,
your may be correct, I may have just gotten lucky on this duct shape
and it is simply "less-bad" than my previous attempts.
I do agree that all of this is conjecture from both
view points and only instrumentation, data collection and analysis might clarify
what is actually happening.
As always, appreciate your viewpoint. Thanks again,
Al
Ed
----- Original Message -----
Sent: Monday, May 07, 2007 1:30 AM
Subject: [FlyRotary] Re: Pinched
Duct
Ed;
Well, ED, the more
I look at this idea, the more trouble it gives me; so I will express a
dissenting opinion. I never argue against what works; but my conclusion
here is that - assuming it does work better in your case than a nice entrance
and a continuously expanding duct of some reasonable shape - is due to the
fact that you have external diffusion (buildup of pressure in front on the
cowl entrance) and turbulent flow at the entrance behind the prop. So
without doing some flow testing, I’d suggest caution in using this approach
for inlet scoops in front of radiators in other locations.
Accelerating the
air costs you dynamic pressure and increases friction losses, and (depending
on entrance and frictional losses) ‘pinching’ the area by more than about 30%
would cause serious choking of the flow, resulting in most of the air spilling
around the entrance. The more rapidly expanding area after the pinch and
more rapid pressure increase is at least as likely to result in flow
separation as a less rapidly expanding area, and a slower pressure
increase. And since you are now expanding from a smaller high velocity
area, I’m gonna guess that the fractional area of turbulent flow would be
larger.
The concepts you
put together make sense in themselves, but it seems to me a little like adding
apples and oranges to get more apples. So, more proof of concept is
required; well, at least to convince me.
Have you made any
measurements of the flow distribution at the core exit?
Just my opinion,
and, of course nothing personal.
Best,
Al (off to Baja for
a few days of
relaxation)
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