The reason I ask, is that if forcing
more air through the core does not decrease the oil temp - could
there exist the possibility that the core is saturated and
can is simply not capable of transferring more oil heat to the air?
If that were the case, then, as you know, no amount of additional air
through the core would make any significant difference.
I don’t know what you mean by “saturated”.
More air through removes more heat – the limit is driving force
(pressure) available to push it through – until you get to the point that
tube and fin surface temps are the same as the average air temp. I think it’s a long way from
that. But apparently it’s close to the limit of the available pressure,
and the pressure drop of the core is a bit higher than expected.
You’d think that if the scoop was
even reasonable effective, it should recover about 6” out of the 9.5”
pressure available. I’m still wondering if there could be enough air
leakage around the cooler to lose a significant amount of that pressure.
Kelly wrote:
Uneducated guess but I will vote for a boundry layer problem......How
about extending
the baffle below the bottom of the wing an inch or two and
retest.......It will be dirty and
draggy but if that helps delta T it can be cleaned
up with a proper installation......IMHO
Sent: Thursday,
July 26, 2007 7:06 PM
Subject: [FlyRotary]
Re: Oil cooler inlet - what next?
Installed
sheet metal ‘baffle’ to form new upper wall of the diffuser as
shown in the photo. The idea was to assist in maintaining attached flow,
and to block leakage through the gap at the top. The baffle was done in 3
pieces in order to insert past the divider/supports in the scoop; each piece is
about 7 ½” wide. There are gaps between pieces of about 3/8 –
½” inch. The inlet pressure probe was placed at point
“D”.
Test
flight showed no noticeable difference in delta T on the oil. The
pressure measured at “D” was 3” H2O. Pressure behind
the exit fairing was again -3/4”. The pressures at C and D
(measured at different times) are at about 6” from the inboard end of the
22” long cooler. There may be some variation axially. Because
of the gaps in the baffle, and fitting around the end tanks, there is still
some air bypassing the cooler; but I don’t know how significant.
Given the 9+” H2O dynamic pressure out in front of the scoop still
indicates not good pressure recovery.
Nonetheless;
it is certainly disappointing that there was no change (within the accuracy of
the temp measurements) in the effective cooling. This suggests that the
wall shape and the air leakage are not the problem.
Calculating
back from the temp changes in oil and air suggest there is only about 1000 cfm
going through the cooler core. The extrapolation of my measured data on air
flow vs pressure drop across the core suggests that at 3” H2O there
should be about 2000 cfm through the core. Because of the centrifugal
blower I was using for flow tests I was only able to get data up to about
0.6” H2O and 700 cfm. I fit the data to Y=ax+bx2 using
regression analysis, which gave a very good fit up to that point; but
extrapolating out to 3” may be stretching it. If I assume the pressure
drop goes as the cube of the flow velocity, the extrapolation is considerable
different – about 1330 cfm at 3” H2O.
Al
-----Original Message-----
From: Rotary motors in aircraft
[mailto:flyrotary@lancaironline.net] On
Behalf Of Al Gietzen
Sent: Friday, July 20, 2007 10:42
AM
To: Rotary motors in aircraft
Subject: [FlyRotary] Re: Oil
cooler inlet
Well;
I may end up with VGs and change in upper duct wall shape. My intention yesterday
was to install VGs as a first step, test fly, measure pressure and temps; then
proceed with installing sheet metal upper duct wall change.
In
deciding where to put the VGs, I looked at things with the gear up (Photo 1).
The gear door has a bump, and there is some gap around the door.
Don’t know what all this does to BL. Ended up putting VG toward the
left side about 26” in font of scoop, and toward the right side right on
the gear door bump.
I
then spent a bunch of time trying to get the pressure measuring tube situated.
The only access is through the scoop opening, and I can’t get my
hands in there; so it is very tough. Plus the tube going in there, or
along the surface in front will affect the flow behavior, so what affect are we
going to measure. Having multiple measurements would be great; but very
difficult to achieve.
While
doing that, I spent some time looking in there with a small mirror. What
I noted was that initial gaps above and below the cooler (required to slide the
unit in and out) had changed a bit. The cooler is supported on pads of
‘Cool-Mat’ insulation. Those have compressed just a little,
so now there is very little gap at the bottom, and 1/8”+ along the
top. That is a fairly substantial leak, and the loss of pressure at the
top likely exacerbates the flow separation. I decided it wasn’t
worth going to test the VGs as long as that leakage gap was there.
Taking
the wing off (mostly getting it back on because of next to impossible access to
nuts), and removing the cooler looks a bit much right now. I realized
then; that by putting in a sheet metal ‘false’ upper duct wall, I
could extend it up into the gap at the top (photo 2), thereby changing the
shape, and (mostly) closing the gap at the top.
The
false wall has to be in three parts for the three openings, and there will be
gaps between because of the supporting dividers; but it could make a
substantial difference. I made the piece for the center, and considered
testing just that; but the upper gap concerns me enough that I think I’ll
try to get all three fit in.
Then
go take a flight test. Unfortunately this combines three changes, VGs, closing
gap, and changing duct wall. I had hoped to test these one at a
time. If there is a substantial change; it will be easy to remove the VGs
to see what that effect was.
Of
course I’ll let you know when I get some results.
Oh,
the price of innovationJ.
Al
-----Original Message-----
From: Rotary motors in aircraft
[mailto:flyrotary@lancaironline.net] On
Behalf Of Thomas Jakits
Sent: Thursday, July 19, 2007
10:31 AM
To: Rotary motors in aircraft
Subject: [FlyRotary] Re: Oil
cooler inlet
Monty thinks the emphasis is on the BL.
I believe (don't know), the main-problem
is the upper ductwall shape. Even if you have perfect BL flow, the upper wall
shape is still not good and will stall the flow.
At the end of the game you want good flow at all
speeds and be able to close any ducts to limit excess cooling (when you
hopefully get there).
Obviously BL will play a role in your installation as
the intake is rather narrow.
However BL or not - BL does not mean there is no flow,
just slower and more turbulent, but still generally going towards the cooler.
Aerodynamics in the duct should be much the same for
laminar, turbulent, any flow, as long as there is flow.
When things stall is when flow pretty much ceases (in
the stalled area ....), no matter how well things where at the entrance.
The stall in this case is rather "easy" to
get, as the speed seems rather low already. Still may be good enough if you can
do away with the stall.
So I suggest to work on the duct wall first and
optimize it.
As suggested, with some kind of sheet, alu,
fiberglass, etc. You can curve it more and more until you peak.
Maybe pinched ducts (copyright Ed!!) are not working
here, but it may as well - if they work a Ed's theory explains (energizes the
flow...)
If this works, modify according to the best shape
found.
Then try to improve with VGs or sanding or turbolator
tape.
Then go for the exit - after all it is a differential
pressure game....
On 7/18/07, M Roberts <montyr2157@alltel.net> wrote:
I think you need to do something to
energize the boundary layer. If you can't divert it you need to put some energy
into it. It is probably getting slow and separating from the face of the duct.
That is what your data seems to indicate to me.
I like the shape that Thomas
proposes better than what you have now, however, I still think you will need
some VG's in front of the inlet.
I know it may seem counter
intuitive, but turbulence may actually help in this case. You will not get very
efficient internal diffusion, but it will be a lot better than what you have
now. I don't think that putting a turning vane will help too much without doing
something to energize the boundary layer first. You'll just have a slow thick
low energy layer, and a high energy layer separated by a turning vane.
It is really easy to duct tape some
aluminum VG's in front of the inlet and see what it does.
You may need a combination of
Thomas' contour, VG's and a turning vane. Go with the easy fix and work your
way up in complexity.
--
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