many years ago I worked for Greenwood and Batley on small impulse turbines, deLaval, Curtis and Rateau type. The major reference books are by Kearton and Stodola. The big problem with very small turbines exhausting to atmosphere is windage on the rotor, a lot of the power is absorbed just turning the wheel. The wheels should be as small and fast as possible to minimise windage, with a close fitting shroud against the wheel. Stumph type, whilst attractively simple to make, have the blades too widely spaced for efficiency. G&B's nozzle/blade design man reckoned that even with vacuum exhaust a reciprocating engine would give better efficiency at less than a few hundred horse power. It would however cost a lot more, and need a man to look after it full time, whereas a turbine would just run.. The smallest deLaval types ran at very high speed and had bulb root blades, but the Curtis type ran slower and had tee slots round the wheel into which blades were fitted, much cheaper.

When the DO stores ran out of drawing paper because they couldn't pay the bills I left and moved into gas turbines. Pity really, G&B could make anything bar a profit!

So far Mike Tilby's article is suitably pitched at my level of understanding and interest, which is certainly below yours Martin! I fancy it's both too much for those who don't care for theory or turbines and too simple (so far) for anyone with a deep interest. Suits me though, and I hope it continues to educate me to the point where I could, perhaps, have a go.

The Tesla turbine feels like a suitable target. I'm sure they work but my gut feel is they're inferior to the alternatives. It's a shame propaganda and conspiracy theories outweigh practical evidence. Same problem with some of Tesla's electrical work.

Dave

I wonder if magnetic bearings could be made/used at model sizes? They are used in industry on machinery such as centrifugal air-con chillers and in applications as small as domestic power consumption meters. Contact-free and thus friction-free, they should be good for very high rpm.

Would a model turbine produce enough power to generate enough electricity for its own magnetic bearings? Or could model-sized bearings be made with fixed magnets, using modern powerful rare earth magnets?

And there must have been at least one article in ME years ago about making a small model turbine. I remember my dad making it when I was a kid. Only ever remember it running on compressed air though. But you could blow in the inlet pipe and it would spin over so it probably would have run on steam ok. From what I remember the rotor was about 2" diameter, 3/8" thick aluminium. Blades were very simple, machined in the lathe by mounting the disc sideways on a piece of aluminium bar in the chuck with the bar machined down to half thickness minus 3/16". Indexing was via a series of holes drilled in the rotor body first so after one curved groove was machined, the blank was rotated one hole and the next groove machined, producing one blade. The tool bit was like a trepanning tool. Amazing what you remember after 50 years!

Edited By Hopper on 02/08/2018 10:40:54

Why bother with fir tree roots on an axial flow impulse turbine when pinned roots are simpler to make and have a proven track record in full size machines?

As to efficiency, Mr Reynolds and his number don't approve of small blade passages, unfortunately. Countering that however, the first stage blades on a 9,000 hp boiler HP boiler feed pump turbine can be down to about 5/16" tall. That's pretty small!

I am enjoying this series immensely. For a while I have been toying with electric power generation models, and for power generation, consistent hight speed is one of more important design criteria.

The level of engineering and science theory in these articles has been challenging but rewarding

The tolerances for blade to shroud manufacture, the work to produce large numbers of blades and the potential for failure have all been daunting. So I was delighted to see the conversation move from de Laval, to Parsons and now Rateau implementations.

It seems like the Rateau from a design and implementation perspective offer a reasonable trade-off with the potential advantage of running at lower steam pressure. I am hoping Mike will move on to provide some suggestions for builds and materials.

This thread is a few years old, replies may not be forthcoming!

I’ve just noticed there have recently been posts on this old thread.

Andre: I too am curious about what you mean by “novel designs of 'semi-turbine' that straddle the definition of 'pure' turbines and displacement type of engine”. Could you please explain? Presumably you don’t mean a turbine on the exhaust of a reciprocator?

Regarding turbogenerators on locos and road vehicles, in the past, a few designs for working miniature turbogenerators for various models have been published in M.E. I can post details if anyone wants them. (They were summarised in the article that started on p.299 of M.E. 4632, 14 Feb 2020).

Last thing is just to recommend Turbine Guy’s comprehensive thread which shows how turbine efficiency can be measured and improved by careful attention to details.

Mike

Mike and Martin,

Thanks for your kind remarks.

Byron