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.

Following 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.

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:

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.

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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