Kamps Micro-Turbine Build

[Misterg]

Hello again!

Well, after a bit of a hiatus, I got back to this project and assembled the various bits and bobs I needed to try running it on liquid fuel.

The main requirement is a pump capable of providing a controllable flow of fuel that can be adjusted from a bare dribble with no delivery pressure to ~ 250cc / minute against ~1 bar back pressure from the engine (I think my 400cc/min above was a bit of an over-estimate). The proper pumps are ruinously expensive (~£300+) so I am trying a cheap RC Glow fuel transfer pump:



It is a positive displacement gear pump (I had to open it as it wouldn't run when I got it - there was a bit of plastic flash stuck in the works)



I was thinking of running it with a pressure relief valve and controlling the fuel flow with a needle valve, but as it transpires, there's so much back-leakage in the pump that it will run against a dead head without complaining too much, so I just used a cheap (and very nasty) Chinese needle valve to shut the fuel off and control the initial flow:



A portion of the fuel feed is diverted to lubricate the bearings (it has oil mixed into it for this purpose). The flow to the bearings is restricted by a fine hypodermic needle (0.4mm bore). To try and stop it blocking, I added a fine filter to the line before it.

After faffing about trying to make a neat looking arrangement, I eventually gave up and threw the lot in a old box that I had knocking about:



The fine filter is the blue thing on the right, and the needle for the lube restrictor is coiled up inside the glass tube which is there to (hopefully) provide some sort of tell-tale for the lubrication flow. (The pressure gauge is for measuring the the air pressure inside the engine - it was a convenient place to put it).

I consoled myself with making some shiny pipes to fit on the engine instead:



And a bulkhead fitting...



...for the fuel tank (made from an old battery acid container)



Which brings us to last Saturday...

Overall, very pleased with how it went - the engine ran very sweetly and was very tractable. I was reluctant to push it without knowing what speed it was doing. (I think my turbine wheel should be OK to ~98,000 RPM, but ultimately, these engines have no rev limit - short of self destruction.) As it happens, I was limited by the pump in this case, but a few more volts will overcome that (I was running a 12V pump off 8.4V).

So, if I can sort a reliable tacho, I'll stick the engine on some rails, next time, and attach a spring balance to see if it poduces anything other than noise and smoke.

Thanks

 

[Tinbasherdan]

how have i missed this? absolutely stunning work! im in awe!

 

[GRW]

Fantastic! What a cracking little jet engine!
I wonder if your neighbours like it as much mind

 

[TEO]

Just wanted to add my tuppence worth of praise and adulation;
This really is fantastic, not just the engineering which is of a level I can only dream of, but the description of the build; written in a lovely engaging style.
Thank you, I'm in awe.

 

[Misterg]

Hello folks - thank you very much indeed for the kind words.

I'm still here - I've been trying to sort out a Hall-effect RPM sensor so I can run it with a bit more confidence (I'm waiting for some diametrically magentised ring magnets to arrive from China).

I will hopefully have another run with some sort of thrust measurement before the end of Summer.

 

[Onoff]

Insane but in a good way!

Just run us through how it could explode and what would happen. Watched that thinking I'd want to be behind a screen!

 

[Lewis_RX8]

Insane but in a good way!

Just run us through how it could explode and what would happen. Watched that thinking I'd want to be behind a screen!

View attachment 353083

Obviously not this scale but heard of gas turbines overspeed and the overspeed trip not engaging they they proceed to throw themselves apart and launch turbine blades into anything around it sticking into walls or even going through them.

When sites do overspeed tests sometimes they make everyone get off site or atleast far away from the turbine , I'm sure we got put off site when they did a test even with it just being a simulated overspeed via the sensor so just being cautious and following regulations I suppose

 

[Onoff]

Obviously not this scale but heard of gas turbines overspeed and the overspeed trip not engaging they they proceed to throw themselves apart and launch turbine blades into anything around it sticking into walls or even going through them.

When sites do overspeed tests sometimes they make everyone get off site or atleast far away from the turbine , I'm sure we got put off site when they did a test even with it just being a simulated overspeed via the sensor so just being cautious and following regulations I suppose

There's an abandoned rocket test site on the IoW I've visited a few times, seems ideal for this! Boats going round The Needles might want to watch out...

 

[Misterg]

Just run us through how it could explode and what would happen.

The worst scenario is that the turbine is over-speeded somehow, resulting in the turbine wheel disintegrating and throwing fragments out at high speed. (The GTBA have recorded incidents of model turbine wheels disintegrating, and the bits being found embedded in fences several metres away.)

The turbine wheel could also disintegrate because of a defect in its construction without being over-speeded (it was hacked out using an angle grinder and a Dremel, remember )

The stresses on the turbine wheel are extreme: The greatest stress is right at the hole in the middle of the turbine (something like 220MPa at 100,000 RPM - above the room temperature yield strength of 316 stainless, and this is hotter than room temperature!).

At 100,000 RPM, the tips of the turbine blades are travelling at over 750 mph.

There's about 0.6 grammes of material in each blade on the turbine wheel - at 100,000 RPM the centrifugal loading makes each blade look like ~180kg.

All these stresses and loads increase with the square of the speed (double the speed = four times the stress).

There is no natural limit to the rotational speed of the turbine - if you give it more fuel, it will go faster.

Sometime before about 130,000 RPM, the average stress in the turbine *will* exceed the yield strength of the material and Bad Things *will* happen - with luck, there'll be some controlled yield and an ignominious grind to a halt... Or the whole disc could just fly apart.

(There's a particularly nasty thing called a 'wet start' where fuel builds up in the casing through repeated attempts at starting the engine. Once the engine starts, the accumuated fuel burns off in an uncontrolled manner and can accelerate the rotor into oblivion.)

The engine is designed / built with a containment ring around the turbine wheel (the outer part of the NGV and its mounting flange) which *should*, in the first approximation, at least, prevent debris being thrown outwards from the engine, restricting the danger zone to the area behind the engine, so nobody is allowed there. I try and avoid me (or anyone else) standing inline with the turbine wheel, or anywhere behind it (but I sometimes forget ).

I'm using the casing pressure as a proxy for speed in the video - 0.8 bar *should* correspond to ~~80,000 RPM, and therefore be well within the design envelope.

Other things can happen which would probably destroy the engine, but aren't as risky to spectators: The bearings could fail - probably resulting in bits of bearing being ejected and contact between the rotating and static parts; or the shaft could start whipping (critical speed is calculated as ~~130,000 RPM) which would probably also destroy the bearings in short order; There can be differential pressure effects on the rotor that overcome the effect of the bearing preload spring and result in the bearings becoming unloaded so the balls can slip and gall on the races (= bearing failure again); Uneven combustion could cause the combustion chamber or NGV to overheat and start melting. While these would be annoying, the consequences should (hopefully) be contained to the engine.

It's fun though

 

[Maker]

I wonder if the sound it makes could be an easy way to monitor the speed?

Here's the spectrum from the run at about 3 minutes into that last video:

If you assume that peak at 643Hz comes from a vibration once per revolution, that comes out to 38,500 RPM, could be about right? One of those sharp peaks near the top end is pretty close to 19x643 Hz too (As in, one vibration per turbine blade) which is reassuring.

I don't know how you chose the right peak, that one just "looks best" to me, maybe an accelerometer mounted to the body would give a cleaner reading with better defined peaks. I think this sort of harmonic analysis is used on proper industrial gas turbines to monitor their health as they wear/creep. Just an idea, something to play with if you've got an oscilloscope (With FFT functionality).


A better, and more straight forward idea would probably be to put a magnet (Or a pair to keep it balanced) in the domed nut on the front and a hall effect sensor next to it.

 

[Anguz]

That would possibly work when everything was running well, but would mislead you when things were starting to go wrong?