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I’d suggest that you need some more information before you start
building a new duct/diffuser. Ed’s suggestion about getting a handle on
whether there is enough airflow but poor distribution is a good one; and Ernest
suggestion of changing shape by some simpler means is also good.
Hi
Al,
Part of me wants to dive
right into trying to fix this, but the more rational part says I should finish
my last 4 hours while it's behaving well. As a compromise, I might go
ahead and smooth out the rear of the scoop, to eliminate the flat wall back
there. It might not be the full solution, but I don't see any way it could
hurt. At the same time, I can add
some thermocouples.
Figure out some way to get an idea of flow distribution. If you are
going to measure exit air temp to determine delta T; you could get a rough idea
of flow distribution by measuring exit temp in 2 or 3 places - maybe divide the
area roughly in thirds, and measure temp in the center of each block.
Exactly what I was thinking. I
have 4 unused CHT inputs to the EM-2 that are just coiled up under the cowl for
tests just like this. I just have to find a way to suspend some of
them over the exit area of the core.
Based on my
scoop/diffuser shape, and the flow tests I did, my conclusion about where the
air is going is different than Ed’s. My guess is that there is flow
separation and lots of turbulence at the front upper part of the duct, and most
of the flow is actually going through the core near the back end. It is
also possible that the turbulence, (resulting poor pressure recovery) and the
sharp turn into the core, may result in reduced inlet flow (air spilling around
the scoop.
I have an even sharper turn that you
do, so you may be right. Putting a steering vane in the middle of the core
should help, regardless of which end has the biggest problem. Maybe I'll
do that too.
Your second version would probably result in
less flow separation, and better pressure recovery, but there could still be
issues of flow distribution and the need for some vanes.
I appreciate all the
comments.
Rusty (need to go to the range
now)
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