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----- Original Message -----
Sent: Friday, March 10, 2006 5:36 PM
Subject: Fw: Duct Flow Separation - the cooling killer
----- Original Message -----
Sent: Friday, March 10, 2006 5:03 PM
Subject: Duct Flow Separation - the cooling killer
One thing that I believe I have learned about ducts and
diffusers relative to cooling is summed up in this extract from a NACA
study.
The bottom line is flow separation is the
cooling killer in a duct. The eddy of turbulent air acts to disrupt
airflow, reduce pressure recovery and block the smooth flow of air to the
core. What is interesting is the very thing you want (i.e. pressure
recovery) for maximum cooling effectiveness and minimum cooling drag is the very
thing that works to force flow separation of the boundary layer near the wall
inside the duct.
Several duct designs (The Streamline Duct and the
Exponential Horn duct, for example) are designed to delay boundary layer
separation by keeping the air velocity in the duct high (boundary layer
energized) throughout the duct and also to delay pressure recovery
(expansion of the duct/diffuser area) until the last possible moment in front of
the core. Its been show that with this approach using the streamline duct,
that flow separation might be delay until the vicinity of 60 deg divergent of
the duct wall. This separation would happen so close to and so
far up into a corner of the diffuser immediately before the core
face that a minimum area of the core was affected. The streamline duct
has been reported in K&W to recover up to 84% of the dynamic
pressure. I have personally had excellent results using a truncated
version the streamline duct with a tractor installation.
If you have sufficient fin/core area and sufficient
air flow - you will cool! However, you may encounter a lot of unnecessary
cooling drag. Not so important in a bi-plane but I would think very
important in the canard style. Note while the summary acknowledged the
significant of skin friction in reducing pressure recovery, it points out that
flow separation is a much more significant factor (see attachment for extract of
report).
This was extracted from a Naca study designed to try to
make sense of a number of reports (sometimes with conflicting results) on
cooling. For those of you interest the report number
is;
NACA-WR-L-208 which is the redesignation of the original
report number War Time Report - Advance Restricted Report L4F26 by John
Henry. Good reading in that it attempts to separate the cooling wheat from
the chaff.
Ed
---- Original Message -----
Sent: Friday, March 10, 2006 4:25 PM
Subject: [FlyRotary] Re: NACA's, Cooling and Sport
Aviation Mag..
> Ernest, I did not
see that specific paper. Do you know an easy to find
> it?
Thanks.
>
> Regards; James Freeman
>
> Ernest
Christley wrote:
>
>> James wrote:
>>
>>>
Second point: I believe that a reversed direction NACA scoop would
>>> make an excellent low drag exit port for the cooling air
stream.
>>> If anyone tries it, please let us know your
findings.
>>
>>
>> According to the article, NACA
tried it. It didn't work very well. A
>> straight sided
'boxy' exit works much better.
>>
>
>
> --
>
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