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Here are a few photos of my new radiator ducts
modeled along the Streamline Duct profile. The Right radiator duct inlet
has been reduced from 24 sq inch to 14 sq inch and the left one from 24 to 10 sq
inches. Both use a "truncated" version of the Streamline Duct profile
described in K&W Section 12.
K&W shows that the full (untruncated) version
can provide a pressure recovery of up to 84% of the dynamic pressure
potential. The Duct wall shape contribute approx 48% of this recovery
with the core resistance contributing the remaining 40%. The truncated
version looks like it will still provide approx 64% pressure recovery. Not as
good as the full Streamline duct, but, just one of the space compromises
sometimes (often?) required when putting anything in an aircraft.
In my first experiment using a truncated Streamline
duct profile, I was able to reduce my overall inlet area for my two radiators
from 48 sq inch to 33 sq inches with the left duct inlet being reduced to 9 sq
inches. This was an apporx 30% reduction in inlet area and the results was
my coolant temp increased 5 F. The right duct was still the old -very
UNstreamline duct.
So I believe that now having both ducts with a
truncated version of the streamline duct and its clearly better recovery
(than my old box ducts), I can reduce the overall inlet area even more -
down to say 50% or 24 sq inch total for the both.
Smaller duct opens have a plus and minus
side. The greater expansion ratio of entrance area to core area should
raise the pressure drop across the core and slow the air flow through the core
reducing drag. This should cause the heat transfer
coefficient to increase resulting in more heat rejected to the
airflow - the downside is there will be less airflow (less drag as a result, but
less mass flow for cooling). So long as the increased heat transfer to the
air compensates for the reduced mass flow it should work OK. This means I
should see higher air temps out of the cores than before. Now if the
expansion is too much for the duct's internal pressure to preclude flow
separation from the walls, then the cooling effect can be severely restricted,
so trying to find where those limits might be.
The flared entrances are not part of the streamline
duct. They are simply to smooth the airflow from the cowl duct opening to
the actual duct inlet. This could in effect result in more air moving into
the inlet than my previous ducts which had an approx 1/2 -3/4" blunt lip around
the entrance. Also, it may provide more inlet velocity (Bernoulli effect) which
in turn may provide more energy conversion to pressure increase inside the duct
- just speculation. But, at a minimum, the smoothed transition should
produce less drag than the previous blunt duct entrance. On the
other hand, the flares could act like an external diffuser and slow the air
before the entrance. Just too many factors to analytically account for
them and their interactions.
I have kept the old ducts and will put them
back on when I get some instrumentation hooked up so we can do a side by side
comparison.
Best Regards
Ed
Ed Anderson
RV-6A N494BW Rotary
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Matthews, NC
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