Tesla Compressor

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Mike Everman
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Re: Tesla Compressor

Post by Mike Everman » Sat Feb 14, 2009 3:48 pm

You'll certainly have to do FEM on your disks. Stresses are highest toward the center on a spinning disk, and that is where it has been made thin and cut, and not radially. Exactly the opposite of what you would find on a conventional turbine wheel. You can analyze the annular ring alone and see what the stresses are at the inner diameter of that as a first step. If you are absolutely sure of the material you used, then you can make a determination on what percentage of ultimate you can run it at.
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Re: Tesla Compressor

Post by fastnova » Sat Feb 14, 2009 4:14 pm

I have to be honest, I am no mathematician, I wouldn’t even know were to start on working out the temperature rise, local speed of sound or much else of it for that matter.
Anyone here wana help?

I have re-done my calculations on centrifugal force, I have worked it out based on the radius at the weakest point (towards the middle where the air slots start) and the mass of the disk from that point outwards. I am hoping this is the right way to go about it? The formula I used was: (2π*RPM/60)^2*mass in kg*radius in metres

I did get it wrong first time round, it is closer too 2.5 ton! Me thinks the disks aren’t that strong!

Jonathan
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Re: Tesla Compressor

Post by Mike Everman » Sat Feb 14, 2009 5:51 pm

Methinks not as well. I'd first wrap mind around a simple tachometer. You can always spin one up until it fails, then you'll know for sure, and again, MUST know what you made it of if you are to calculate it's critical speed. Always watching the tacho will be essential later, as you want to shut down at some upper rpm or scrap a lot of work!
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Re: Tesla Compressor

Post by tufty » Sat Feb 14, 2009 7:03 pm

Even 2.5 tonnes sounds low to me, but I haven't done the math, so I'll trust you on it. I don't think it's going to be the centrifugal forces directly that cause a blowup, though, rather the static and dynamic balancing of the entire pack, or, at those speeds, bearing failure. Frankly, even destruction testing a single disk is gonna be hard to do; it's more than likely that version 1 of the turbine is going to be a "sacrificial" bounds testing device if you intend running it close to the limit.

You absolutely do want a tacho and other instrumentation connected to a datalogger when you're running it, and probably want to do teardowns and inspections on a regular basis.

But hey, what do I know.

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Re: Tesla Compressor

Post by fastnova » Sat Feb 14, 2009 7:15 pm

I have a few fail safes in mind, there is going to be a deliberate week point in the drive so should the disks start to rub on the outer casing it will simply break and disconnect the drive instantly, which is a lot quicker the I would be able to react, possibly saving many of the components.

I also plan to run the engine at 5k RPM stages, each test lasting a few mins followed by a breakdown so I can check the disks in case that have grown or warped.

I have a laser tachometer in mind, but finding one with a data logger would be handy.
Lastly I have changed my mind on the drive source, I got hold of a 1500watt electric motor that will be ideal hooked up to a speed controller. I can also then measure the amount of power the motor draws at a given RPM loaded and unloaded, thus allowing me to work out some sort of overall efficiency.

Jonathan
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Re: Tesla Compressor

Post by Mike Everman » Sat Feb 14, 2009 8:11 pm

If you're going to spin it up to useful speed, like 25-30Krpm, you'll have to use a special wound motor for many rpms per volt, like a motor for RC aircraft. A lot of the guys that do turbines use these as starter motors. A "Speed 300" is mentioned a lot; the 700 sounds better to me. Try and find one that's got the highest torque and highest no-load speed is where I'd start.
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Re: Tesla Compressor

Post by fastnova » Sat Feb 14, 2009 8:38 pm

That thought had crossed my mind, then I found this motor. It is from an old industrial hover. I’m not sure what kind of speed it is designed to work at but it must be high as it performed a similar purpose in a past life.
To disks are going to be driven via a toothed belt allowing me to gear the RPM’s up if I need to.
Also, if I cant get enough torque out of the motor to reach top speed I know where I can find another electric motor that is the same, doubling my available torque if I hook them both up.
I still need a good speed controller though.
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Re: Tesla Compressor

Post by fastnova » Sat Feb 14, 2009 10:54 pm

I have come to the conclusion that hindsight is a wonderful thing!
I just finished working out the yield strength of the disks at the (assumed) weakest point, turns out that at around 57,000RPM the centrifugal force will reach about 800Kg and the disks will deform beyond their elastic limit.
Call it 50k for a bit of safety and that is half what I had planed!
I really should have made the effort to work these things out before I started cutting metal, but on the bright side the bearings should last longer!

Designing this engine has been quite a learning curve for me so I suppose problems like this were bound to crop up. But it puts me on good ground to build a MK2.
I’m eager to get the turbine finished as soon as possible, get some results, and move on to refining the design.
I should finish all the complicated parts tomorrow, then I will move on to mounting it.

Jonathan
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Re: Tesla Compressor

Post by larry cottrill » Mon Feb 16, 2009 12:49 pm

fastnova wrote:Funnily enough I worked it out just this morning, my calculations show that the centrifugal force will be almost 1ton on EACH disk. I am a little doubtful about these results, so will have to go over the math again. I was expecting it to be high, but not that high!
IT depends on what you mean. Total forces can seem extraordinarily high. What's important is unit stresses. You need to know how many pounds (or kg) per square inch (or sq.cm) so you can compare that to the ultimate stress capability of your material. That's what counts, i.e. that's what will tear things apart if you go too far (fast).

In other words, take a small pie section of your disk (a FEW degrees worth) and figure out the forces and the cross-sectional areas of a few zones (different radii) working from the outside in, and work out the stress as force / unit area for each zone. That, compared to the tensile strength of the material, will give you what you need to know. The worst spot (maximum stress) should be where the thin web starts, right under your thick rim.

L Cottrill

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Re: Tesla Compressor

Post by Mike Everman » Mon Feb 16, 2009 2:42 pm

What are the basic dimensions of your disks, and the material? I happen to be working on a spreadsheet now that will do this, or close enough.
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Re: Tesla Compressor

Post by fastnova » Mon Feb 16, 2009 9:11 pm

Larry, that is basically how I went about it.
First I worked out how much load (per mm2) will be applied to the disks from the (assumed) weakest point, then I worked out the exact Yield point of the material, again per mm2. Then worked out how many square mm there were, the rest is easy!

I got AutoCAD to do the time consuming work for me, I simply drew everything in 3D and it told me all the volumes and areas, HANDY!

Drawing of disks below, cheers mike!
If it helps I know the below specs are correct (I hope)

The material is 6262 T9
The specific gravity in grams mm3 is .2713
The exact volume is per disk 7674.9773mm3
Ultimate Yield point 52ksi
0.2% offset Yield point 48ksi
Attachments
Rotor Blank2dims.JPG
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Re: Tesla Compressor

Post by fastnova » Sat Feb 21, 2009 10:15 pm

I got a bit more done this weekend, was gone post some pics last week but I didn’t get round to it.
I have mostly finished the actual Tesla Pump, there is a little more milling to do on it but the hard parts are all covered.
I made a casing for the drive motor, still have to work out how to cool it down though, but this shouldn’t be to hard. I am also going to re-make the shaft, as I am not happy with the way it came out.
I should start working on the mounting next weekend so with a bit of luck I might get things finished and start testing by the end of this month.

I took a few pics of the motor casing before; during and after, hope you like them.
21022009481.jpg
The piece of metal i started with
21022009484.jpg
Using a 3” drill to remove the worst
21022009486.jpg
After drilling
21022009499.jpg
The finished product
21022009500.jpg
21022009493.jpg
The motor and new casing assembled
21022009494.jpg
21022009504.jpg
The assembled Tesla Compressor
21022009505.jpg
All my work so far
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Re: Tesla Compressor

Post by Mike Everman » Sun Feb 22, 2009 12:32 am

Looking good, man!
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Re: Tesla Compressor

Post by larry cottrill » Mon Feb 23, 2009 5:44 pm

Wow -- what a terrific, beautiful project!

Best of luck, sir -- you deserve some real success with this!

L Cottrill

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Re: Tesla Compressor

Post by Fricke » Thu Feb 26, 2009 2:45 pm

Nice machine work!

I´m following this thread with great interest.
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