Sunday, 13 January 2008

Initial tests

First Test

First problem is getting the turbine up to starting speed. Schreckling says at least 3000 rpm is necessary.
Without much hope, but just to see what might happen, I borrowed the wife's hair-dryer to spin the turbine with a magnet taped to the turbine shaft

By placing a coil of wire close to the magnet I was able to generate a signal which I measured on a 'scope. It came out at 50Hz - just 3000 rpm!

So having run out of excuses, it was time to take the whole thing out into the garden and annoy the neighbours..,

Having no idea what might happen, and not intending to run the turbine beyond idle speed, even accidentally, I fitted a 36mb regulator to the propane cylinder and wedged the turbine between some high-density concrete bricks.
Oil was supplied to the bearings from a squeeze bottle.

First I spun the turbine up using the hair dryer (via a cardboard tube). Gave the oil bottle a good squeeze and opened the gas tap.

I tried to ignite the gas at the turbine exhaust but the air blast made it impossible. I was using a folded paper taper (spill) but it just blew out.

Ok. Turn off the hairdryer - as the speed of the turbine fell off I tried again.

It still refused to ignite, but as the turbine came to a halt, there was a dull 'pop' and the whole gubbins belched yellow flames out of both ends.

I hurriedly switched on the hair-dryer again (before it caught fire) and was rewarded by a lovely roaring sound and blue flame from the exhaust.

The flame held steady as the turbine speed rose. The noise wasn't excessive, no louder than a blow torch burner.

I tried removing the hair-dryer, but the turbine couldn't self-sustain and started to wind down.

Suddenly, there was another 'pop' but much sharper and high-pitched. The flame-roar went up in pitch - the sort of sound you get when a bunsen 'blows back' and starts burning at the bottom of the tube.
Obviously, the gas had been burning mainly in the exhaust region and had only just ignited properly back in the combustion chamber.

With the hair-dryer back on, there was a distinct increase in speed too, with a whining note beginning to be heard from the turbine.

I seemed to be getting some power feed-back from the turbine but it was still not self-sustaining.
I wound the gas tap fully open but got only a small increase in noise and speed. It was pretty obvious that the paltry 36mb of gas pressure wasn't pushing enough gas into the turbine - not surprising, its only a thirtieth of an atmosphere and the engine compression must be of that order too.

The turbine blades were now beginning to glow dull red at the tips.
I tried another squeeze of oil, which produced bright yellow sparks from the exhaust.

As I was standing, wondering what I could try next, I realised speed was dropping. There were no odd noises, just a gradual decline.

So I shut off the gas and let the hair-dryer run to cool the engine while I went to have a cup of tea and stop shaking..

Post Mortem

Although the casing was quite cool (about 15 minutes later) The inner parts were still very hot to handle. And it was pretty obvious what had gone wrong.

The rear bearing was glued solid with cracked oil. It was just a solid cake of carbon and tar.

I'm going to have to give a lot more thought to cooling around that rear bearing.

I cleaned up the bearing and tried another run, using a much lighter grade of oil, with the same result.

By now the bearing was in such a state, I decided to sacrifice it. I cleaned it, put it back in, and gave it another run.
This time with no lubrication.

It ran - and ran - and ran. After about five minutes, I got bored and shut it down.

It looks like this is the answer, at least temporarily. I don't expect it to survive long,
but this is only the first try and I've already learned a lot. There are lots of things wrong with the construction that I can improve. If I can get this even close to to self-sustain I will count it a major success.

I removed the gas regulator.

And I also made a couple of changes to the combustion chamber. Nothing radical - just moved the gas inlet pipe and burner ring to the inside of the chamber.

I made several attempts at starting before I accepted that I just couldn't get the flame to 'pop' back into the chamber.
It just continued to burn outside the engine

Eventually, I accepted that something was wrong and it wasn't just the extra gas pressure, so I stripped out the combustion chamber to do some tests.

I decided to make a test rig consisting of a steel tube and a nozzle for the hair dryer. With the combustion chamber mounted in that, I was able to see that there was nowhere near enough primary air entering the chamber.
The gas ring was apparently obscuring the air flow, something about it being on the inside of the air holes, rather than the outside seemed to make a big difference.

It's possible that with the gas ring mounted ouside of the chamber, the burning gasses could be passing round the outer and inner walls of the chamber
(maybe that's why the rear bearing was getting so hot?)

Anyway, drilling another set of holes in the annular plate made all the difference.

I also took a couple of days off to make an ignition circuit. I bought an ignition coil on eBay and wired it up to a 555 astable multivibrator cct. with a driver transistor (2N3055).

That saves a lot of hair-raising messing about with cigarette lighters. It also ensures that if ignition does occur, it's in the right place.

I reassembled the engine and tried again.

The combustion chamber behaved much better. But each time I got the engine spinning and ignited the fuel, the turbine began to foul the casing,

I stripped the engine and looked for rubbing marks in the turbine housing. They were all to one side, indicating the turbine was off centre.

BUT checking with a feeler gauge, it was pretty clear that the turbine was quite central, at least when cold. - So I took a 1/4mm off the diameter and tried again. And again.
And again... Eventually I had the turbine running with a clear 1mm gap all the way round but it still fouled the casing when it got hot. No way is that down to the turbine expanding.

Obviously, the whole rear end bearing mount is moving sideways by at least 1mm when it gets hot.

Back to the drawing board. But hey! - Who said it was going to be easy?

Mark 11

I completely remade the engine. This time, I made the central rear bearing support a single unit with the turbine stator.

The old turbine wheel was a bit undersized by now so I made a new one to the same pattern.

This time, I was able to get the engine to spin without fouling, even with the turbine and casing glowing red-hot.
But I still couldn't get it to self sustain.

I made a simple device to measure rotational speed using a reflective photodiode looking at a locknut on the shaft (It pokes out of the front of the engine)
and using the hairdryer to spin the turbine, I took a series of speed measurements.

With the compressor removed, cold speed was 4000 rpm. Igniting the fuel took that up to 8000 rpm at minimum throttle setting but increasing the gas supply made only a slight difference to speed with unburnt fuel coming out the back and burning in a big yellow flare.

Refitting the compressor and repeating the run resulted in a cold speed of 3000rpm and a 'hot' speed of 6000 rpm.

There was a very small throttle effect in that increasing the gas supply off the minimum resulted in a small increase in speed ~200-300rpm before the yellow flare appeared.

The conclusion I draw from those tests is that the compressor has no observable effect in increasing the airflow. In fact, if anything it's reduced. It seems that the turbine isn't supplying enough power, Or conversely, the compressor loading is too great.

It's interesting that the compressor restricts the airflow from the hairdryer and results in a lower 'cold' speed. I had expected it to act to some degree as a turbine in the airflow, obviously it doesn't.

In a way, this is encouraging - it shows I am getting real drive from the turbine, not only from the cold airflow but also from the burning fuel. So the turbine is behaving reasonably well and the combustion seems to deliver useful energy to the turbine (It isn't being completely wasted) .

In fact, the speed doubling effect of the burning gas suggests that the volumetric flow must be doubling so the heat is causing the gas to expand by roughly a factor of two.

That would correspond to a doubling of temperature (pressure changes are relatively small and can be ignored). So from (say) 290 degrees K, at ambient the temperature would be going up to 600 or so, which is 300 degrees C. Not unreasonable for an average temperature of the gas? The turbine is not glowing at all when running, so I think that's fair.

Sadly, I have to admit, I simply don't know enough about jet turbines to understand what is wrong.

There's only one answer to ignorance, I have to learn. I need a crash course on jet turbine theory. I will have to buy some books.

Later (much later)

As an aside, it looks as if all the gas turbine theory books have been written by Germans.
That's unfortunate. While they have a well-deserved reputation for painstaking accuracy, that same virtue makes them blind to approximation.
There is no German word for 'simple explanation'. Every text goes into minute detail over turbine/compressor efficiency and the effect of blade thickness/angle with the result
that the real 'nub' of the narrative gets pushed into marginal notes. The basic formulae are often stated as an afterthought, often in obscure, non-standard notation with no attempt to identify the parameters correctly.

After several days and several dollars spent on books, I finally have the answer. It's really simple.

The shaft power of a turbine or a compressor (same calculations but in reverse) depends primarily on the peripheral speed of the rotor.

It turns out that the impeller doesn't actually compress the gas in any significant way. What it does is to speed it up. Compression actually occurs in the diffuser when the gas is slowed again.

In simple terms, the K.E. of the gas as it exits the compressor is just m . vc2 /2 .
where vc is the compressor 'tip speed' and m is the mass of gas involved.

(This is a simplification because the real formula involves the scalar product of two velocity vectors, the wheel itself and the gas flow - but by happy coincidence, efficiency is maximised when these match, so v squared is a fair approximation)

The KE given to the turbine is a similar calculation, resulting in m . vt2 /2 .

Assume that all the power(energy) supplied by the turbine goes to the compressor. (at steady speed)
and we get: -

m . vc2 /2 = m . vt2 /2

Since the mass of gas entering the engine per second is the same as the mass leaving (conservation of mass) the factor m/2 cancels.

so vc = vt

The tip speed of the turbine MUST be equal to the tip speed of the compressor.

Furthermore, because they are mounted on the same shaft, they rotate at the same speed.

So they must have the same diameter

That's where I've been going wrong! My compressor diameter is 85mm compared to the turbine's 60mm

That means the compressor is demanding twice as much power as the turbine can supply at any given rotational speed (852 = 7225 : 602 = 3600 )

That's probably why the speed of the turbine drops so dramatically when the compressor is installed.

I hasten to add, before someone blasts me, because what I have said is based on one possible configuration for the gas flow, the resulting conclusion is not the only solution. It is possible to match compressor turbine of different diameters, but it isn't likely that the solution will hold for different flow rates. But this IS the reason why most designs for a model jet engine (or a turbocharger) have similar sized compressor and turbine wheels

I have two options. Make a new engine and turbine, or make a new compressor.

If this is going to be truly a home-made jet engine, I have to go for the second choice.

Having thought about the construction options: wood, metal, composites, It seems the easiest option is the most exotic.

I've never worked in carbon-fibre before but I know enough about the alternatives to doubt the feasibility of making a metal or wooden turbine.
(Yes, I know Schreckling used wood. But it was an exotic laminate and heavily reinforced with CF)

So I've bought a carbon-fibre student's kit



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