The Trouble with Tesla:

Like a number of people I have gotten together a number of flat disks to try and make a working Tesla turbine (yes it was easy to build and it did work.) and then I tried hitting the web-sites for some information to improve it.

First problem I ran into, many of these sites seem to be ran by "true believers", they lack not only turbines doing real jobs, but even real numbers for the machines they have built.

Some of the claims are for more energy out than energy in, something I know is impossible! When you see that claim, you know you are dealing with a kook or a con-artist. Second problem, a lot of them seem to think that they are going to make fortune when everyone starts to using Tesla turbines instead of IC engines. Because of this they have on purpose left out some of the construction details on their web-sites. Anyone who tries to build a Tesla turbine without figuring out the fine details will have piece of junk as far as performance is concerned.

Third problem, I have NEVER, EVER seen what appears to be a reliable report on the performance you can expect from a Tesla turbine. Can it be 25% efficiencies? I believe so. Can it be a lot more? I don't know. Additionally, I do see a lot of problems trying to get it to be better. It just is not worth the money and effort to me personally to try and do all the things needed to find or create the better designs.

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Common problems promoters of Tesla turbine designs do not mentioned:

Disk Mass:

Because it is easy to stack a large number of disks in a small volume some of the web-sites about Tesla turbines like to report in Horsepower per cubic inches to show how much better than IC (internal combustion) engines Tesla designs are. Of-course in real life how small your engine is matters less than how heavy it is, and it is easy to make a heavy Tesla turbine. In terms of horsepower per kilograms weight Tesla turbines do not do that well unless the disks are very, very thin. Thin but strong disks however turn out not to be so easy to make or cheap in costs as you would think at first.

Disk Speed:

Tesla turbines can spin very fast when they are not under any loads. Very fast indeed! Because of this it is easy to surpass the strength of most common metals and plastics used to make the disks just by using input feed pressures in the 100s of PSI if you are using compressed air or a steam boiler. Once a high speed turbine starts to fail due to centrifugal effects any unbalanced forces will quick tear the disks apart to the point that the breakup of the disks acts more like an explosion than anything else. This high speeds problem add two more concerns - balanced disks & gearing down the speed.

Balanced Disks:

The high rate of rotation means any imbalance in any of the disks will generate large side forces that even if they do not interfere with the operation of the turbine, they will drastically increase the wear and tear on the bearings holding the main shaft of the turbine. Gearing Down: Tesla turbines are well know for their very high speed/low torque output. One needs a very high gear ratio to reduce the rotation rate down to something usable with most machinery, but one does also gain a high torque on the geared down output. However, the low torque of the Tesla turbine's main shaft means it is very sensitive to the friction of the main pickup gearing and bearing, plus that gearing will need to operate at a high rate of speed as well.

Disk Spacing:

The performance of the Tesla turbine depends on the boundary layer, a simple rule (not a fixed one) is the best performance is with a gap between the disks of 3-5 times the thickness of the boundary layer. However, this layer's thickness depends on what fluid you are using (i.e. air, water, steam, condensing steam, combustion gases, ....), it's speed when injected, it's temperature at all points inside the turbine, and the same for the pressure and still a number of additional factors. I noticed very, very few sites try to figure how thick this layer is, then space out the disks according to that data.

Flat Disks:

Now here is the true real killer of Tesla turbine efficiency. Imagine a simple Tesla turbine, a stack of 11 flat disks each 10 inches in diameter with a .1 gap between them and an exhaust hole in the center 1 inch in diameter. The surface area for the driving fluid entering the Tesla turbine is the (Circumference (or Diameter times Pi) * the width of the input area) = (10Pi)(10.1) = 10Pi square inches or about 31.4 square inches. But the exit hole is only one tenth the size. So any fluid entering a Tesla turbine gets slowed down while inputting energy into the Tesla turbine and at the same time must exit from the smaller area of the exhaust hole. But notice it gets worse, the area of disk gaps doing the exhausting is (1Pi)(10.1) = Pi or about 3.14 square inches but the area of the hole is .5.5Pi or about .79 square inches. Exhaust Hole Size: To solve the two above problems we have to do two things, both affect the efficiency of the turbine. First we have to make the Exhaust hole closer in size to the surface area of outer surface. So we get sqrt(10Pi/Pi) = sqrt(10) = about 3.16 in radius if we exhaust from one side only. This of-course does not take care of the surface area of the exhausting disk gaps which are still too small at 6.32Pi10.1 about 19.87 square inches. To make up for this we need to taper the thickness of the disks so as to increase the gap size between the disk's surfaces as they near the center.

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Problems resulting from trying to fix the above:

Non-Flat Disks: Making the tapered disks is no longer the simple design most people claim the Tesla turbine as being. It requires a precision lathe instead of just cutting out sheet metal.

Balance and vibration:

The metal disks need to be make of a very uniform material so that they can be balanced easily.

Boundary/Gap spacing:

With tapering disks you start to lose the ideal gap for efficient operation, if you change the taper to increase the flow the efficiency drops and the total power output drops. If you don't taper the disks to get efficient power conversion you get restricted flow and the total power output drops. Balancing how much taper is enough is again something I see missing off all the web-sites out there. Add in the fact that if you are using steam, combustion or other hot gases the nature of the fluid flow changes in different parts of the Tesla turbine as energy is extracted and gases cool because of this. Your disk design just became a major job.

Exhaust Size:

K.E. = .5MassV^2 as the fluid in a Tesla turbine tends to flow at the same rate the disks spins. If the exhaust hole is .1 the size of the outer diameter, a very rough guess is that the fluid exits with only .1^2 = .01 or 1 percent of the original K.E. This suggests that we can convert 99% of available power to useful output. That is why you see people raving about how great Tesla turbines are going to be. However, at that size of an exhaust the outflow is very restricted by the small exhaust hole and we get very little power. The redesigned exhaust is .632 the diameter of the input giving us the exhausting fluid as still containing 40 percent of the original K.E. So we have already seen the turbine drop from 99% to about 60% efficient. The restricted area exhausting from the disks into the exhaust hole 'suggests' 19.87/31.4 or another one third drop in efficiency. So already we see a big drop in possible performance and the disk gap issues will only make things worse.

Disk Surface finish:

Again something rarely looked at. It should be clear that if the disk surface was perfectly smooth there would be very little drag to transfer K.E. to the disks. However, too much drag just turns the K.E. to heat in the disks and fluid. So what is the best finish to have on the disks? I have a guess, but that is all it is.

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Conclusions:

Building a simple but low efficiency Tesla Turbine is easy, however the moment you decide to make a real power plant from one the work needed is on the same order, maybe more to develop and build a I.C. engine. Tesla turbines do have a lot going for them, but high end designs are not that easy to make otherwise lots of people would be using them today already.

Earl Colby Pottinger

Reference https://www.physicsforums.com/threads/tesla-turbine-efficien...