Hi Guys
I'm about to construct the diffuser for the water exchanger.
Can some of you guys with operating setups critique the design please.
So it consists of a K&W diffuser followed by a wedge.
And a happy Christmas to you all.
Steve Izett
Perth Western Australia
Change the 5.75" dim to about 2 inches.
The idea is to keep a constant cross sectional area duct up to
the K&L wedge diffuser. That minimizes the tendency to get wall
separation in the duct due to a drop in pressure.
Also raise the core in the cowl as far as you can.
It would be less risky if the core was 19.5" instead of 17.5 long.
It might work at 500 cubic inches but 600 cubic inches would
be better. Check out pick up truck cores.
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Let me expand on this. It is very difficult and tedious at best to
design and build a K&W diffuser when you are dealing with offset
openings. Particularly if the openings are a different shape.
So keep it simple and use a constant area duct in conjunction with
a K&L wedge diffuser.
Paul Lamar
Thanks Paul
Will rethink design.
Steve
I did not put that in the book. However I did put a 3d in the book that
gives the impression that a K&W diffuser can be used with
a K&L diffuser. Sorry about that. On further thought it is a bad
idea for reasons outlined above.
I was thinking about it some more today and I think I have it figured
out how to draw and build a S shaped constant area duct using
a Rhino built in function. Straight walls are not the best. That
S shaped duct will give a smother transition from a typical AC cheek
scoop to a side mounted rad.
Here it is. Rhino is an amazing program to be able to do this in
15 minutes. It will take you a lot longer to build it :)
This is your Christmas present :)
Paul Lamar
Merry Christmas.
Thanks for putting your head around this. My first thought was exactly
that. Rotate the duct to match the K&L. I thought I could make it out of
1/2 foam pieces then smooth, seal layup etc. See diagram attached.
I was basing the design in K&W, specifically: In K&W Ch12 p.277 we read
"The use of streamline diffusers is not limited to the symmetrical case.
Any
other streamline of the free flow about the conducted cooler block may be
used as a wall. Tests where the straight centre line was taken as the
second diffuser wall (this has the highest pressure gradient) gave results
only slightly inferior to the sometrical diffuser." And on the next page
p.278. "It was found that the losses were again about the same as those
for the symmetrical streamline diffuser."
Steve
That is great that you have the Kuchemann and Weber book
"Aerodynamics of Propulsion". That is a collector's item.
Here is the illustration you refer to. Did you find it in Oz?
That is a great idea for making the form. I'll have to remember that.
Pay attention folks. This is important. Just be sure the inlet
area is the same as the outlet area as in the 3D's.
I admire you Steve for your stoicism in the face of great adversity :)
Your a saint. Keep that chin up as you always do.
Paul Lamar Merry Christmas
Thanks Paul
No I don't have the book, just a copy of chap 12 that someone posted ages
ago. Can you explain why we wouldn't diffuse, even a little to reduce the
velocity of the airflow prior to entering the K&L diffuser? Hope you have
a great Christmas.
Steve
I theory that should work. It should reduce the velocity of the air and
the skin friction drag in the duct between the scoop and the K&L diffuser.
The problem is designing a complex shape 3D K&W duct that will work.
The area must gradually increase according to K&W theory.
It would be interesting to try. One would want to instrument it
with a zillion pressure taps and record the wall data pressure while
testing it in a wind tunnel.
One would first use the K&W coordinates to make a symmetrical 3D dwg.
Then plot the area along the straight center line. Then construct a
non straight center line between the two non similar and non axial
openings. Then assemble area disks at right angles to the new center
line every few inches. Fill in the gaps and hopefully your done.
Now the problem is; air has a range of characteristics like inertia,
adhesion and viscosity. Think of watery molasses.
Can one really expect this to work under those circumstances ?
My gut reaction is no :)
If air tends to stack up in one part of a constant area duct the pressure
increases in that area forcing the air to conform to the duct shape. It
is self stabilizing intuitively I would think.
Paul Lamar
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