Steam turbines

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Steam turbines

This topic has 7 replies, 3 voices, and was last updated 6 October 2009 at 22:23 by Windy.

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5 October 2009 at 19:43 #43986
Windy
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@windy30762
I am designing a steam turbine for my Hydroplane and hope that someone will be able to answer a few questions for me.

All the parts will be homebuilt with manual machines, no CNC.

The turbine will probably be an impulse type on the lines of a De Laval with a second stage?

It will have ceramic bearings because of the rpm.

As a matter of safety what I would like to know in laymans terms, no long formulas the bursting speed in rpm of various steels of diameters from 2 inch to 4 inch.

Also the most suitable shape for an impulse vane and ways to machine it.

Any books on the subject especialy about impulse staging on small turbines and finally other ways of reducing speed more efficiently than gears.

Thanks for a very good web site, Windy
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5 October 2009 at 19:43 #21635
Windy
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@windy30762
Help on materials.
5 October 2009 at 23:00 #43988
mgj
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@mgj
Busting load – will be the tensile stress at the root of each bade. The load which generates that stress will depend on the size and shape of the blade. Then you have to determine the thrust on the blade because you have forces in 2 directions.

The centrifugal force is solely dependent on velocity and blade mass so you can work that out in 2 seconds. But you need root area to convert that into a stress, and allowable stress varies, in old units from about 18tons tensile for cooking mild, up to to 50 tons tensile (from memory) for some of the MARAGING steels (martensitic age hardening)- depending on conditions – as does their suitability.

The aerodynamic force is a little more difficult, partly because aerodynamic equations tend to be relatively complex, and partly because the standard equations may not apply – they are for wings of relatively high aspect ratio, and you are talking in turbine blades of very short fat wings. (very low aspect ratio)

And you may well also have to make some compensation for temperature or creep – see below.

Importantly in any short fat wing, much in the way of losses is determined by spillover due to the pressure differential between the front and back of the blade – you lose effctive area. The way to stop that is to fit a plate to the wingtip (see all the F1 cars) – in a turbine that means a very close fit between blade tip and casing. You have to allow for and include blade stretch.

To work that out you have to allow for both the increase in diameter due to simple thermal expansion, and due to centrifugal loads. And all that presupposes that you know quite a bit about your materials, and the conditions inside the turbine?

So anyone who offered a one liner would be very unwise.

Edited By meyrick griffith-jones on 05/10/2009 23:01:01
6 October 2009 at 10:27 #43990
Windy
Participant
@windy30762
Thank you for your reply, I realise that there are many things to take into consideration when designing very high speed machines.

Having seen spin testing on video and the destruction when materials disintergrate, one test cell was ripped off its mountings.

The miniature gas turbine builders have inconel rotors that may be suitable but are the blades of a shape for an impulse steam turbine?
6 October 2009 at 14:37 #43993
Fred Graham 1
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@fredgraham1
Hi Windy,

If I am not mistaken are you Paul Windross as your flash steamer does look familiar? If it is Paul, Hi again, my name is Fred Graham who sometimes comes to Hull with Tom Clemment and Bob Kirtley.

You may be interested in a booklet from Camden Miniature Steam Services written by W M. J. Cairns in which he describes deatils of the TESLA turbine. There are also drawings of a small version which does not seem too difficult to make and seems to offer good power output possibilities. There are no blades invlved and a lot of the bits are made from good quality duralumin, good for keeping the woeght down on flash speedsters. I am pretty sure they still sell spoies but in case you cannot get it I have a copy and you can borrow it.

All the best and I hope you are in good health.

Fred Graham
6 October 2009 at 18:42 #43996
mgj
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@mgj
Fred, I think that building to an established design would be a good idea.

In this one I am lucky, in that my MSc was in guided weapons, so gas turbines and aerodynamics was bread and butter. I have the necessary equations available, but i also know enough to know how little I know about it when it comes to designing from scratch. (or how much one needs to find out!)

Still , one of the curses of any turbine is blade flutter (for which there are several causes) and one of the fixes for flutter is rigidity, since flutter is actually twisting under aerodynamic load. If you are going the bladed route, you might wish to think therefore in terms of titanium. While it is a nightmare to machine to good shapes, being notch sensitive, (and boy you do not want notches or poor surface finish in stressed titanium),nevertheless it offers a very high stiffness to weight ratio. From a performance point of view, that would be a better bet that steel or ali

No disrespect, but to go from scratch, when it would seem from your original post that the mathematics of aerodynamics and strengths of materials may not be your routine bedtime reading – its all getting a bit exotic, and someone else’s (proven) design might be wiser?

Edited By meyrick griffith-jones on 06/10/2009 18:43:10
6 October 2009 at 20:03 #43998
Windy
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@windy30762
Hello Fred nice to hear from you I have that book also the one by H H Harrison.

Another The Book of Modern Engines 1911 has articles about De Laval turbines and precautions in design that were took to minimize wheel bursts plus calculations for the nozzles etc.

Thanks for your message.
6 October 2009 at 22:23 #44001
Windy
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@windy30762
Thanks Meyrick for the advice and will consider all the options before finally cutting metal.
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