All Things Nikola

This page last updated: 17 February 2020.

John at work had been talking for some time about the Tesla turbine and the Tesla valve, or more properly the Tesla valvular channel since Tesla never called it a valve and it is not a valve, it is a channel with a valve-like property.  Christmas 2019 I looked up the Tesla turbine and found that a nice gentleman had created a 3D printed version, which was very convenient, though it turned out to be only the start of my journey.  The Tesla valvular channel was more problematic; browsing the internet suggested that efficiency wasn't very good.  So I decided that I would use the pre-designed 3D printed Tesla turbine as my measurement mechanism for the evaluation of the Tesla valvular channel.

Note that there is somewhat of an evolution in the designs below as I find out what's good and what's bad so, if you intend to reproduce any of this yourself, do make sure to read through to the end before doing so.

Tesla Turbine 1

I downloaded Integza's Tesla turbine files from Thingiverse and printed them on my Prusa 3D printer in grey PLA, at 0.1 mm resolution as I wasn't in a hurry.  The only problematic part was the axis which was tall and top heavy and needed supports to stop it toppling at the stage where the bridges were being printed.

Tesla Turbine parts

I obtained two bearings with outside diameter 22 mm, inside diameter 10 mm, a brushed DC electric motor with outside diameter 22 mm (which will act as a generator) and a collar with inside diameter 3 mm to attach to the motor shaft.  I used some 8 mm long M3 hex bolts to hold the turbine together.  I managed to fit nine disks inside the turbine without any fowling, glueing the last one in place with cyanoacrylate adhesive.  I glued one half of the joint for the electric motor to the end of the axis and the other to the collar using cyanoacrylate adhesive; everything else was push-fit or bolted.  I glued a pneumatic fitting to the air inlet with Araldite (standard, not rapid) to allow me to push-fit a 5 mm inside diameter/8 mm outside diameter air hose.

Disks glued
Bearings
                mounted
Disks
                mounted
Turbine bolted
                together
Motor jointMotor Pneumatic fitting
 
I mounted the turbine on a piece of wood, leaving room to fit a lamp holder and a DC power measurement unit that I ordered from China; the wires running around the back go to a 9 Volt battery powering the DC power measurement unit.

Mounted

To make things simple to manipulate, I use quick-connect pneumatic fittings on the rest of the tubing.  There are as many types of these as there are people on the planet and they all look very similar so it is important to make a choice and remember it.  I used the "Series 21" type throughout as they work nicely with 5 mm inside-diameter plastic tubing and are relatively small and neat.

Series 21
          quick-connect pneumatic fitting

Here it is on its first test run, driven by compressed air from my little Jun-Air compressor:
 
...and here's the same but with the Watt meter in circuit.  As you can see, lots of vibration and a peak of just over 2 Watts output as the stored capacity of my little compressor is used up.
 

Further testing showed that the peak speed at that 2 Watts output was around 6800 RPM.  Remove the motor and that rose to 10500, much less than half the 24000 RPM peak achieved by Integza and nowhere near achieving sync with the incoming air flow.  Now of course I'd just thrown this together with no attempt at surface finish, vibration control, air flow, etc. but there were other issues.  My little Jun-Air compressor has a 25 litre tank versus Integza's 100 litre tank and it is just not able to keep up with the flow requirements of the model.  And you'll see below that putting my first-cut Tesla valvular channel in the air flow, with its tiny orifice, stopped the whole thing dead.

However, after posting the first video above on YouTube I was contacted by Paul Townley, AKA GravInert.  It turns out that he and iEnergySupply, who I guess is in the USA, are trying to build Tesla turbines which they believe can extract energy from atmospheric pressure in damp air.  In the comments section beneath their videos someone had posted a link to the Tesla Engine Builders Association, which, under the heading "The Open Secret" includes, such articles as "Conquest of Space - Before It Went Black" and "Flying Saucers 'Explained'".  I was either down the rabbit-hole or through the looking-glass.

Back to the problem at hand: in order to have a Tesla turbine that I can use to measure the performance of a Tesla valvular channel I needed one that (a) required a smaller air-flow and probably (b) employed a metal shaft.  I could have tried Integza's first turbine but since Paul Townley had been in contact I thought I'd try his first.

Tesla Turbine 2

I downloaded the STL files from the Drop Box link under GravInert's YouTube video and made two modifications:

Here are the Blender and STL files for the extras.  All the parts were printed at 0.1 mm resolution in PLA with supports everywhere aside from the gaskets from my extras (see later) which were printed in flexible PLA at the same resolution, and (added later) the base plate from my extras which only needed 0.2 mm resolution.  The parts on the right in the picture below are Paul Townley's originals, the ones on the left are my additions; Paul's file tesla_cube_single_stage_baseplate.stl is not required here.  Two each of his "nozzle" and "port" casings were required; note that I later modified these to improve bearing fit, hence they moved into my extras file and this also removed the need for the gasket once more.  From my extras five "open" rotor blades were required, one "closed" rotor blade, two of the star-shaped rotor centres and two sets of gaskets.  After printing, aside from the usual removal of support materials, the hole in diverter_block.stl through which rotary_shaft.stl fitted needed some attention with a half-round file so that the shaft fitted and turned easily.

The parts

I purchased four bearings of 9 mm outside diameter/4 mm inside diameter/2.5 mm wide, a length of 4 mm diameter metal rod, some 4 mm inside diameter collars, four 100 mm long M4 bolts and a pack of M4 brass inserts, though I realised afterwards that these latter are only required for a single-stage design; for the two stage design plain-old M4 nuts are used instead.

Bought-in parts

I began by assembling the rotors and the handle of the diverter block. 
Note the single "closed" rotor blade positioned in the middle of one of the rotors (the right-hand rotor in the top middle picture below) to make that the "male" rotor.  Cyanoacrylate adhesive was used to keep the rotor blades in place on the star-shaped rotor centres, to glue the handle parts securely to each other and to attach the handle to rotary_shaft.stl, which allowed the direction of the air flow to be changed, being careful not to accidentally glue the rotary shaft into place at the same time.

Adding a
                rotor to a rotor centre, encouraged with a soft-jawed
                vice
Male and
                female rotors half assembled
Male and
                female rotors fully assembled
Handle
                parts for assembly Peg
                inserted Knob
                on Handle
                assembled

The main body parts lined up as follows:

Bodies en
          bloc

The body is held together by the long M4 bolts but, to ensure a good air seal, I chose to attach the diverter block to its adjacent piece permanently by gripping the two together in a vice after applying cyanoacrylate adhesive around the air channels between them.  I cut two 50 mm long pieces of the 4 mm diameter metal rod and hammered them through the centres of the assembled rotors, then pushed a bearing on either end of the shaft and test fitted them in their housings.  Paul Townley had warned me to ensure that the rotor blades did not touch the side, so I was careful how I positioned the rotor on the shaft and did a little filing of the rotor blades here and there to be sure.

I found that when both halves of the casing were assembled on a rotor too much pressure was applied to the bearing, I could feel the bearing "step" as I rotated it, hence I printed myself the 1 mm thick gaskets out of "flexible PLA" to fit between the two casing halves, also acting as an air seal.

Shafting a
                rotor
Shafted rotor in bearings
With
                seal

The female rotor (i.e. the one with all "open" rotor blades) was fitted into the casing nearest the nozzle, the male rotor in the casing furthest away from the nozzle.  Here is the dual turbine fully assembled with a couple of collets attached to the shafts to which I intended to glue something for my RPM counter to bite on.

Assembled

Being impatient to see how it ran I connected it to my compressor immediately.  The first rotor span up very quickly indeed, making a very pleasing, if slightly panic-inducing, whining noise.  The second didn't start so I increased the flow; there was a "tick" noise and suddenly all motion stopped.  The first rotor had simply shattered; now we're talking!  It turned out that Paul Townley had his printed in carbon-reinforced nylon, which I could do but only with a nozzle change in the printer.  And Paul balanced his rotors.  And splitting the rotor into parts would have weakened the blades.  A redesign of my rotor blade extras was in order.

Shattered
                  rotor Rotor
                pieces

I modified my rotor blades to print whole disks which were each hammered onto the 4 mm diameter shaft, no glue required.  I printed them in the much stronger ASA (UV-safe ABS, only because I didn't have any plain-old ABS to hand), with support material on the print bed so that they could retain their slightly tapered profile since Paul had mentioned that this was a necessary feature; I hand finished the blades somewhat to make sure that shaping survived the 3D printing process.  I learned from Paul Townley that the second rotor isn't supposed to spin-up in the same air flow (I need to read more about how that is meant to work) so this time I quickly assembled the whole thing but didn't insert the second rotor and placed the solid
tesla_cube_single_stage_baseplate.stl between the two rotor-casing segments so that the second part wasn't in use at all (see later for why this was the wrong thing to do).

New
                turbine blade prints
In housing
Single stage baseplate used to close off second
                stage Assembled as single stage

Just to make sure I had got past the smashy stage, I spun this up immediately on the compressor with both exit air holes open.  You can just about hear, at around 35 seconds in, that it wants to sync and it eventually gets up to about 38,000 RPM, which is fine for an untuned first try; the little blade is rotating 633 times per second.  And no smashy, though also not the rather exciting whine of the first version.  Fiddling was required as Paul Townley had achieved more than twice that on his first run, though I'm not sure at what air pressure; mine was with an initial peak of 8.5 bar.
 
I trimmed away some gasket which appeared to be touching the shaft when the bolts were tightened and put the rotor, mounted on the shaft, into my little Unimat lathe where I abraded the sides and edges in an attempt to improve the uniformity/taper of the blades.  Paul Townley had pointed out to me a rotor balancing method that Tesla had used but it was a little too complex for me to want to set up such a thing just yet.  I also found that if I spun the rotor by hand with the bearings sitting loosely in their slots it ran for far longer than if I pushed the bearings down into their slots, so I used my Herzo hand abrading tool to open up the bearing slots somewhat and I put the rotors back into the lathe where I abraded them to reduce them in size very slightly in case they were catching on the turbine case.  I bought some 60 mm long hex bolts so that I could assemble just the first stage turbine using those M4 brass inserts hammered into the bolt holes in tesla_cube_single_stage_baseplate.stl (again, see later for why this was the wrong thing to do).  A final innovation, which Paul had mentioned but which I hadn't thought would be necessary yet given my low speeds, was to drop some 3-in-1 oil into the shaft-hole at either end before running.

Improving rotor finish Loosening bearing slots
Single turbine

Note also that the motor on my little Jun Air compressor can't keep up with "open-valve" demand so while experimenting I needed to do only timed short runs, which was fine since pulsed air flow is where the Tesla valvular channel comes into play anyway.  I added a manually-operated valve to the tubing delivering air to the Tesla turbine to give me convenient local control.

Local valve added

What was necessary, of course, was to know whether the rotor was anywhere near achieving sync with the velocity of the incoming air, a calculation which Integza talks about at this point in his video.  My compressor had a 25 litre tank and a peak pressure of 8.5 bar with the compressor switching on again when this drops to 6 bar.  The idea is to work out the volume of air this difference in pressure represents once that air has escaped compression and, given the size of the hole it has to escape through and the amount of time for which it is escaping, work out how fast it must have been travelling.

Calculating the density of wet air is complicated so I chose to trust Omni Calculator.  At 60% humidity and room temperature this gave me 10 kg/m3 at 8.5 bar and 7.12 kg/m3 at 6 bar.  A 25 litre compressor tank is a volume of 0.025 m3 which gave masses of air of 0.25 kg and 0.19 kg respectively and so, between the two pressure values, 0.06 kg of air must had been emitted. Back to Omni Calculator: at "room" atmospheric pressure (1 bar) the same wet air had a density of 1.18 kg/m3 which meant that my 0.06 kg of expelled air would have a volume of 0.07 m3.  Changing to more reasonable units, 1 cubic metre contains 1,000,000 cm3 so the volume of uncompressed wet air is 70,000 cm3.  It took 20 seconds for the pressure to drop from 8.5 bar to 6 bar with the valve on the compressor completely open so the rate of air flow was 70,000 / 20 = 3,500 cm3/s.  The air entry hole in the Tesla turbine casing, as measured at the rotary shaft in the diverter block, was 4 mm in diameter giving a cross-sectional area of 3.1412 * 0.22 = 0.125 cm2.  So the average linear speed of the wet air was 3,500 / 0.125 = 28,000 cm/s.  The circumference of my 26 mm diameter rotor blade was 3.1412 * 2.6 = 8 cm, so it should on average rotate at 28,000 / 8 = 3500 times per second or 210,000 RPM.  Quite a lot, and that's the average number.

With all my improvements what did I achieve?  You can see that the rotation is still relatively poor under hand encouragement at the start of the video below and there is quite a lot of vibration when the turbine runs.
 
A peak of around 60,000 RPM (1000 times per second!), average probably around 53,000 RPM over the period, with both exit holes open.  I'm about a quarter of the way there, 25% efficiency in something that can theoretically get to 98%.  Makes me wonder if all I'm doing here is measuring the performance of some bearings.

After I published the video above Paul Townley got in contact to point out (politely) that I'd been very silly.  I knew what the air flow in a Tesla turbine was meant to be: from the outer edge of the rotor and then towards the centre of the rotor to escape from the middle.  I had closed off the two large rectangular holes in the back of the turbine with tesla_cube_single_stage_baseplate.stl, leaving the two small holes on either side of the turbine assuming that they were the intended exit holes:

Air flow through
            side holes

But that's rubbish, the air was not escaping through the middle of the rotor.  What I wanted was this:

Air flow
            through end holes

Paul had included the small side holes for some other purpose because he was sending the objects away to be 3D printed and hence needed flexibility in the result.  Those small side holes were meant to be tapped and closed off with a screw when not required, the intended exit holes being the large rectangular holes.  So I screwed an M2 hex bolt into each of the small side holes, pushed M4 inserts into tesla_cube_twin_stage_baseplate.stl (with the two rectangular holes in it) instead and reassembled the turbine.  I applied a little cyanoacrylate adhesive on top of the M2 hex bolts for additional security; I didn't want small metal bolts exiting the building and entering me at high speed.

Correct
            air exit holes

I tried again with encouraging results:
 
Rather than starting at 60,000 RPM and wilting down to 46,000 RPM, the turbine stayed at around 55,000 RPM for the period.  This suggested that it was now bearing-limited, which I needed to sort out anyway as the lilting sound is likely a result of the bearing being slightly loose in the housing due to my fettling.

Paul had suggested that GRW bearings were the best so I went in search; curiously the two UK agents for GRW were unable to supply small quantities but a Dutch seller (247industries) on Ebay could.  I also purchased some turbine oil, a thin oil used with model jet engines (from Kings Lynn model shop) and some paraffin to mix it with, 5 parts turbine oil to 95 parts paraffin.  Paul suggested that the bearings be immersed in this when not in use.  And I imported tesla_cube_nozzle_casing.stl and tesla_cube_port_casing.stl into my Blender extras file and modified them to remove the small side holes entirely and increase the width and diameter of the bearing slots by 0.2 mm so that they would come out the correct size when FDM printed, no need for the gasket.  I believe Paul's casings where resin printed which has a higher resolution and so he had no need of this tolerance.

Here are all of the parts before assembly once more: the diverter block, rotary shaft and twin stage baseplate at either end are from Paul's files, the rest are from my Blender extras.  Glue (cyanoacrylate) was applied solely to affix the handle, nowhere else, and no gasket was required.

Parts
            before assembly

And I think this time it worked; Paul agreed: possibly the fastest accelerating Tesla turbine in the world.
 
As you can see, the tachometer gave up and my measurement disk flew into pieces that I mostly couldn't find afterwards, apart from the one that sliced my finger.  Very cool and a wonderful sound too.  I belatedly realised that my tachometer wouldn't go above 99,999 RPM.  Gonna need a bigger measuring device.  And a stronger reflective disk.  And some shielding.

Add Tacho

During a discussion at work one lunchtime Jonathan suggested I make my own tachometer using an optical sensor; with this I could measure acceleration as well as speed.  I purchased a QRD1114 reflective object sensor (basically an LED and a photo-sensitive transistor in a tiny plastic case) from Hobbytronics for 1.20, a Raspberry Pi Zero W with the header fitted costing 13, a power supply for the Pi costing 8 and I dug up an old 8 gigabyte SD card to use in the Pi.  I designed a 3D printable base to hold the Tesla turbine, the sensor and the Raspberry Pi.  I painted length-ways 2/3rds of the sticky-out metal shaft of the turbine with matt black paint (actually Humbrol 32 dark grey since I had that to hand) so that it had reflective and non-reflective sides.

Components for tacho
Base
Painted shaft

I followed the procedure here to set up the Raspberry Pi Zero W headless on my Wifi network, i.e. such that I could log into it from a terminal program (e.g. PuTTY) from anywhere rather than having to connect screen/keyboard etc. and use a GUI.  I followed the advice here on how to connect the sensor to the Pi, using one of the 3.3 Volt power outputs from the Pi to supply the LED and connecting the sensor output to a digital input pin on the Pi, rather than an analogue input pin (so that I could use interrupts), checking with a multimeter to make sure that it achieved the necessary 0.8 V to 2.4 V voltage swing for a digital input pin between the dark and light sides of the shaft.  Below you can see the voltage swing on a scope when the rotor was spun by hand.

Optical sensor connected up
Voltage swing
Tacho
                    and turbine assembled

I wrote a Python script to run on the Raspberry Pi which counted the number of falling edges on GPIO4 in each 100 ms interval and saved the values to a CSV file that I could load into Microsoft Excel.  For the first run I also attached a logic analyser to the optical sensor output pin to capture a live trace so that I could cross-check the accuracy of the program.

I lubed up...

Lube

And away we went.
 

Speed curve

Not too shabby.  A steady state of 185,000 RPM, ~90% of my very roughly calculated theoretical maximum of 210,000 RPM given the output of my compressor.  And that kick in the curve followed by the flatness makes me wonder if the rotor had achieved sync with the incoming air flow.  The logic analyser output agreed with the above: below are a couple of clips (upper trace digital 3.3 V logic, lower trace analogue), one of the startup and another from somewhere around the peak showing a nice clean sawtooth with the number of transitions over time to match the graph.  So the data is believable.

Logic
                    analyzer acceleration
High
                    speed zone detail

And, as independent proof, John at work had pointed out that I could probably just listen to the turbine and the audible frequency would likely be that of the rotation speed.  To test this, here is a 30 second burst of a 3.08 kHz sine wave (AKA 185,000 RPM) as an MP3 file generated using Audacity.  Try playing it at the same time as the video and you'll see that the tones match.  I tried exporting the audio from the video to an MP3 file and using Audacity to analyse the spectrum to see if it would match the speed curve but unfortunately there is far too much general hissing to pick out the wanted rotor tone; ears are more clever.

During the initial jump to 100,000 RPM in just one second the surface speed of the rotor (diameter 28 mm) went from 0 to 147 metres/second, so an acceleration of 147 metres/second/second,  Now I thought that was 15 g (147 / 9.8) but after another lunchtime conversation David, who once worked on centrifuges, pointed me at this link about converting RPM to g force which says that relative centrifugal g force (RCF) is (RPM)2 * 1.118 * 10-5 * r, where r is the radius of the rotating thing in centimetres.  That calculation makes the force 53,000 g at max RPM.

Then I remembered that one of my brother-in-laws, Toby, works in the design of fans and he confirmed that, though the RCF number is interesting, the number I should be looking for was hoop stress, the tangential force, which is: (density * r2 * w2) / 3, where r is the radius of the thing in metres and w is the speed in radians/second.  The density of ASA is virtually one, the speed in radians/second is (2 * 3.1412 * rotations per second) so for my 28 mm rotor this becomes (0.0142 * (2 * 3.1412 * (185000 / 60))2) / 3 = 24,500 Newtons.  For a rotor with disks of thickness 2 mm the surface area over which that force is applied would be 3.1412 * 28 * 2 = 175 mm2.  24,500 Newtons applied across 175 mm2 is 24,500 / 175 = 140 Newtons/mm2.  Checking on the web, ASA seems to have a tensile strength of 30ish megaPascals, AKA Newtons/mm2.

So with these maximum speeds I'm running at more than four times the strength of my material.  Toby also mentioned that they put concrete walls between them and their turbine fans.

Anyway, the rotor survived without flying apart, thankfully, as I hadn't installed a shield.  The surface speed of the rotor at max RPM was 271 metres/second.  This article suggests that a .38 calibre pistol, apparently a popular small hand gun, has a muzzle velocity of 286 metres/second and, interestingly, fires a bullet weighing 6 gm, just twice the weight of the rotor, though it is only able to inflict as much damage as a hockey puck at full pelt. But I'd better make that shield.

To reinforce that point, I thought I'd try a second run and, two seconds in, there was a bang as the disks of the rotor suffered rapid unplanned disassembly; 192,000 RPM and rising this time.  Maybe the dip and then rise in the acceleration on the first run was fortuitous? I will open the air inlet valve more slowly in future.

Bang!
192,000 RPM

While I had been sorting the tachometer out, Paul had suggested a number of additional experiments: increase the number of disks and try using an air bearing.  I thought it was worthwhile seeing if I could make the rotor edge go supersonic; 340 metres/second (232,000 RPM for my rotor diameter) looked like it could be feasible.

To be continued...

Tesla Valvular Channel

I took the picture of the Tesla valvular channel from the patent:

Tesla valve
              drawing

...and followed the procedure described in this YouTube tutorial to import it into Blender as a usable shape; basically, from a fixed camera viewpoint, trace around the salient parts of one section of the shape with Bezier curves and then duplicate/join that three times to make a set of eight.

Outline
                      of the valve in Blender
Valve full length

Finally I extruded the 2D form vertically and made it into a solid that could be printed, 115 mm long.  I did try a version smaller than this (90 mm long), as I had the impression that the pressure exerted on the valvular channel is a factor and the smaller the geometry the less external pressure would be required, however 115 mm long is about as small as my 3D printer could print the features of the valvular channel distinctly.  Here are the Blender and STL files, which need to be printed with supports on the print bed only (see below for why).

Solid
                    shape
Closed
                    shape
Inside the
                    valve

Here's a cross-section of what it looked like in my printer's slicer program and during printing at my 3D printer's highest possible resolution, 0.05 mm, in PLA:

Sliced
Printing the
                    valve
Half way
                    through printing

I wanted to be able to push on the 5 mm inside-diameter plastic tubing so I modified the outside of the valvular channel to be round, like a pencil, and tapered the ends.  Here's a quick tour, going through it the "wrong" way, courtesy of a couple of days rendering from Blender.
 
This had to be printed with supports (on the print bed only).  Even then I had some trouble stopping leaks from the connection to the tubing so cyanoacrylate adhesive was applied to the tapers as the tubing was pushed on.

Rounded body
Connected

I placed my Tesla valvular channel in the air-flow to the turbine and... nothing happened.  Either way around, the tiny aperture of my valvular channel constricted the air flow so much that the turbine would not rotate at all.  Either I needed to make the valvular channel bigger or I needed a turbine which could work with a reduced air flow; possibly both. To be continued once I've bottomed-out the Tesla turbine part...


Back to Meades Family Homepage