II. Member Experiment: Richard Gideon
December 5, 2002
Experiments in Disc Geometry
Improvement
I would like to end this December session with an excellent
bit of experimental work performed by Richard
Gideon. Note that his
turbine is 6 inches in diameter, and the best results he obtained used a
disk spacing of 0.048 inches, with 12 blades for spacers.
Here is Richards email:
"Greetings from
Illinois. About a year ago while looking at a number of Tesla web
sites I ran across yours. As others have also noted it is fantastic.
Unlike many sites that lament, "If only someone would build Tesla’s
turbine it would solve all the worlds problems." You are actually
experimenting and building machines. I salute your efforts to design
and build a real practical working turbine and the time you have spent
developing this site.
Last spring you wrote about the improvement in
efficiency you got by using your winglet design in place of round
spacers. This immediately struck me as a tremendous improvement.
However it got me thinking about how many winglets are enough? Are 6
better than 4, is 8 better than 6, etc. How many are too many? I then
theorized that at some point when the spacing between them got small
enough or the air pressure great enough this would nullify the lift
effect of the winglet. Thus you would be left with only the reactive
force on their bottom side.
This then got me to wondering what if the
circumference of the rotor discs, rather then being smooth, had a
profile like a fine tooth circular saw blade? Also rather then the
winglets what if across the face of the rotor there were a number of
simple blades, like the paddles on an old sternwheeler steamboat.
Would this help performance? If so what would be an optimum number?
Naturally while it can be fun to sit around
thinking and theorizing about the different ways of doing something
you can only go so far with this approach. To really know if your
ideas are any good you must actually build and try them out. Therefore
that is what I did. The following is a description of the turbine I
built and the results I got.
Before starting to build a project like this you
want to take great care to determine best size and shape for the
completed device and choose just the right materials.
To this end I determined that my turbine should
have a 6-in. diameter rotor because I found a short length of 6-in.
I.D. aluminum tubing in my scrap bin that I could use as the rotor
case. Also it was a size I would be able to comfortably machine on my
10-in. lathe.
Similarly I determined that the rotor stack should be
made up of exactly 7 disc’s of .036 thick CRS. Because it was the
only suitable material that I had on hand and only enough to make 7
disc’s.
The ball bearings for the rotor shaft were chosen with equal
care having been purchased a few years back at a local junk store for
.50 cents apiece. The bearing supports and their base was made from a
length of ½ in thick by 2 ½ wide aluminum bar, that was left over
from a previous project. Finally the rotor shaft started out as a
1-in. diameter steel bar also from my scrap bin.
As you can see I obviously spared no expense in
building my turbine.
The overall design is a standard layout. The two
bearings holders are mounted to a base plate with the rotor shaft
running through them. The rotor is mounted on one end of the shaft and
there is locking collar on the other end. On the side of the locking
collar I machined a flag that runs through an optical sensor which in
turn is connected to a frequency counter so I could measure the RPM.
Also on the back of the locking collar I have a cork disc to act as a
friction material for a simple pony brake so I can measure the output
torque of the turbine. Lastly the air inlet nozzle is a rectangular
port with a convergent/divergent insert. As described in one of your
articles, this provided a definite increase in performance.
I precisely determined that the best operating
air pressure to test run my turbine was from 105 down to 80 PSI, which
coincidentally are the limits that my air compressor operates at.
Thus using the above air pressure limits, my
first series of tests was to determine the optimum spacing for the
rotor discs. I had made spacers with three different thicknesses and
started with a stack that gave a spacing of .095 in. Note these
spacers were on the rotor shaft only, there were no other spacers
between the discs.
As a result start-up torque and performance was, to
say the least, less then exciting. But from what I know about Tesla
design this was to be expected. All test runs were done with no load
and starting at 105 PSI. RPM readings were then taken when the
pressure had dropped to 80 PSI.
The following graph shows the results
I got with six different spacings. Each RPM reading is the average of
four test runs.
GRAPH 1
| SPACING (in.) |
RPM (no load) |
| .095 |
3390 |
| .084 |
3425 |
| .070 |
3495 |
| .056 |
3615 |
| .048 |
3615 |
| .038 |
3570 |
rotor
without spacers to determine optimum disc spacing
As can be seen maximum speed was obtained with a
spacing of .056 to .048 in. I choose the .048 in. as the spacing for
all the following tests.
At this point I disassembled the rotor and
tediously milled a bunch of saw teeth, .062 deep every 5 degrees,
around the circumference of the center discs. The two end discs I left
plain to prevent air from leaking out from between the teeth. I then
reassembled and test ran the turbine.
Again I dissembled the rotor and this time
milled 36 slots .036 wide & ¼ in. deep across the face of the
rotor. Into these slots I would press and solder my blades. I started
with only two blades and then test ran the turbine. I then added two
more and ran the turbine with 4 blades. Then added 8 more and ran it
with 12 blades, and finally added 24 more and ran it with 36 blades.
Below is graph of the results:
GRAPH 2
| |
RPM (no load) |
| SAW TOOTH |
4845 |
| 2 BLADE |
5745 |
| 4 BLADE |
5835 |
| 12 BLADE |
6780 |
| 36 BLADE |
6750 |
GRAPH 3
| |
TORQUE @
1800 RPM |
TORQUE @
3600 RPM |
| SAW TOOTH |
12 |
9 |
| 2 BLADE |
15 |
12 |
| 4 BLADE |
15 |
12 |
| 12 BLADE |
18 |
15 |
| 36 BLADE |
24 |
15 |
Note: Torque readings are approximate due to
inability to accurately read a wiggly pointer on the scale.
From this it can seen that the saw tooth edge
produced about a 1200 RPM increase in speed over the plain disc’s.
With 2 blades added to the rotor the speed increased another 900 RPM.
However 4 blades had little additional effect but with 12 blades again
I had a substantial gain of about 1000 RPM. Finally it is interesting to
note that at 36 blades the speed is just starting to drop off, thus 12
blades seems to be about the optimum number. At least when running a
6-in. rotor. I believe the reason for this is that with 36 blades I
have two blades in the path of the air inlet at all times and this is
causing a turbulence that in turn reduces the efficiency. However this
was exactly what I was trying to prove one way or the other by going
to 36 blades.
Finally just for curiosity I ran the turbine
with increasing loads and produced this last graph. It shows the
obvious, that as the load is increased as the speed decreases and the
torque will increase, up to a point and then the torque will start to
drop off. Proving that these turbines really want to be run at high
speeds.
GRAPH 4
| RPM |
TORQUE |
| 6678 |
0 |
| 4998 |
6 |
| 4350 |
8.25 |
| 3930 |
10.5 |
| 2910 |
12 |
| 2040 |
15 |
| 1620 |
15.75 |
| 1440 |
17.25 |
| 1290 |
17.27 |
| 1110 |
13.5 |
| 660 |
13.5 |
| 60 |
13.5 |
Hopefully this will be of some value to you or
other experimenters, therefore if you wish please feel free to use this
on your web site.
Sincerely
Richard Gideon
Spotteddogs@iwic.net
Richard -- your project is a fantastic experiment in disc
turbine geometry improvements. The really interesting point you made
is the fact that 12 blades were ideal for maximum energy transfer.
Tesla used 12 round washers around the outer periphery of his 10 inch
design, and we used 12 winglets with our 10 inch design – 12 seems
to be the magic number.
Also notable is the experiment in spacing between the discs.
Your best results hovered around 3/64 of an inch – slightly more
than Tesla used, and considerably less than the 1/8 inch spacing we
used.
One last test perhaps all of our club members would like to see
is a comparison of your best configuration against a strictly Tesla
design – using round washers instead of blades around the outer
periphery. Please keep us posted, and thanks for your results! -- Ken
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