What is the optimal blade angle for a turbine engine?

[iwant2beoz]

Hello, so I am building a small turbine engine and i need to know the best angle for the blades. Any help would be very helpful, thank you.

 

[Baluncore]

Turbine blade angle will be dependent on the profile of the blade, the dimensions of the turbine, the RPM, the mass of fluid flowing in that part of the turbine and the pressure drop available.

What fluid, liquid or gas drives the turbine ?

 

[iwant2beoz]

For now I'm useing propane as my fuel, but I'm planning on useing methane in the future. Ideally I would like to be able to use all types of fuel but I just want to get the basics worked out first.

 

[Baluncore]

We still have no idea how big the turbine is that you are considering. There is no simple answer to your question. Some web research would be a very good investment.

A turbine driven by combustion of a hydrocarbon fuel must be able to withstand very high temperatures. The material technology of the blades in a gas turbine is very advanced. At this stage the technology needed is beyond most experimenters. Blade material is fuel and temperature dependent. Air, water and steam turbines require a much lower technology.

Experimenting with gas turbines can be very dangerous. If you must experiment, I would recommend you initially follow other experimenters by modifying the turbocharger from a petrol vehicle, (avoid diesel turbochargers as they are for a lower temperature exhaust). Experimenters use the blower as the air compressor for their home built combustor, followed by the exhaust turbine, which turns the compressor on the single shaft.

Google “ diy gas turbine turbocharger †to see what others have managed to do. They do not show the destruction or the injuries they receive when, as usual, it all goes wrong. Take care.

 

[iwant2beoz]

Well my plan for now is to make the engine about 3 inches in diameter as a proof of concept for the blade and burner design. I know that this is very dangerous so I will be working behind a steel reinforced plywood wall so if explods (which I fully expect it to) no one gets hurt. I would use a car turbo but the engine design won't alow for it, so I'm running air from an air compressor directly into the burner, unlike most turbine engines this one won't have a compressor section it will need a outside compressor. That may change but for the moment that is how it is

 

[etudiant]

How are you planning to get power out of your turbine?
Presumably there is to be a geared shaft, sticking out in front of the exhaust, as there is no point to adding the turbine if you just want a jet engine when you don't need a turbine to drive the compressor.
Note that if you are not compressor limited, you can use a big turbine wheel with just short blades at the perimeter of the disk. You can lead compressed air to multiple combustors around the circumference of the turbine and still have the blades just get the flame exposure only intermittently. That should also give a slower turbine rotation speed which is easier to gear down. Of course, a bigger turbine wheel is also harder to make, offset to a degree by the benefit that the centerline and the bearings don't get the full heat load.

 

[iwant2beoz]

This is a very rough drawing but its much easier to show you then try to explain, this will be generator/turbine. I know its a long shot and I know by the looks of the drawing it won't work but I assure you that its all much clearer in my head I just can't draw.

 

[AlephZero]

Experimenting with gas turbines can be very dangerous. If you must experiment, I would recommend you initially follow other experimenters by modifying the turbocharger from a petrol vehicle

The hard part of designing a gas turbine engine is not the turbine, but the compressor. Any "rubbish" aerodynamics will produce some power from a turbine. After all, people were making useful windmills (and sailboats) for thousands of years before they knew much about aerodynamics.

But compressors are a different matter, because if a compressor doesn't "work" you tend to get no mass flow and no compression ratio. If that suddenly happens in an engine that is runnihg, the compressor turns into a turbine rotating the opposite way to the real turbine, and the result is bangs, flames coming of of both ends of the engine, and usually broken bits.

Using a professionally designed and manufactured turbocharger is a good way to "cheat" and avoid the worst of those problems, but if the OP isn't going to use a compressor, that's not an issue.

Of course for thermal efficiency you want to combustion temperature to be as high as possible (as Carnot discovered), but for a home made engine you can easily reduce the temperature to whatever your turbine material can withstand, by having "lean" combustion with an excess of air to reduce the exit gas temperature.

 

[Baluncore]

"lean" combustion with an excess of air to reduce the exit gas temperature

This is when the chemistry comes into play. A lean burn is oxidising so it precludes the use of many metals in the turbine. It also requires compression of air that will not be used efficiently.

 

[etudiant]

Interesting design concept, an integrated turbine/generator.
Challenges would include keeping the magnets cool, because magnets lose it if they get hot, as well as keeping the dimensions snug enough for efficient generating. Because the turbine assembly expands when it is in combustor exit path, the setup will need to make allowances.

 

[AlephZero]

This is when the chemistry comes into play. A lean burn is oxidising so it precludes the use of many metals in the turbine. It also requires compression of air that will not be used efficiently.

For a 3 in diameter engine, I don't think the OP is going to be worried about meeting NOx emission standards on the first prototype!

And nobody has said the turbine will do more work than it takes to drive the compressor! That's one reason why decoupling the compressor and turbine makes things very much simpler. The engine will "run" even if you have to provide continuous power to drive it.

And if the OP has a source of biogas, that might already be providing a "free" supply of high pressure gas...

FWIW putting relatively heavy rotating components like magnets around the outside of the turbine is probably not a good design, if you think about the centripetal forces and stresses in the machine. Putting the generator inside the annulus of the turbine blades and the gas flow might be a better plan - or use a standard electrical generator, driven by a shaft from the turbine. A "free turbine" like this has the advantage that its speed is independent of the compressor.

 

[iwant2beoz]

As a matter of fact I do have a source of biogas that is providing free high pressure gas. its the whole reason that I started working on this project. Etudiant I'm glad you understand my picture I didn't think anyone would, I'll work on the cooling system and get back to you on that

 

[Baluncore]

A lean burn is oxidising so it precludes the use of many metals in the turbine.

For a 3 in diameter engine, I don't think the OP is going to be worried about meeting NOx emission standards on the first prototype!

You missed my point entirely, I did not consider NOx. Oxidation of the turbine material can be a real problem with the wrong turbine material / exhaust chemistry.

Petrol vehicles run a stoichiometric fuel/air mix to reduce NOx, at higher temperatures. Their exhaust is low in oxygen so corrosion of metal components of their turbines is not usually a problem. Diesel turbine materials are selected to operate in a cooler, but oxidising exhaust gas.

A petrol turbocharger fitted to a diesel engine may rust away.
A diesel turbocharger fitted to a petrol engine may melt.

 

[iwant2beoz]

Sadly I can't use a turbo as the blades face the wrong way. I will have to get the blades milled by a machine shop and figure out some way to mount them to the inside of the engine. That's my biggest challenge. The cooling system for the engine actually the easy part.

 

[Baluncore]

Further to the last paragraph of AlephZero's post #11.

Your design with the turbine at the centre and the generator outside will be a very real challenge, if not impossible to design. Bearing technology and lubrication will also be a challenge. Where are the bearings in your design? How does the exhaust gas pass the bearing supports?

When a turbine design has gas flow from the centre outwards, the gas can expand as expected but the gas also slows down. The fastest moving part of the turbine blade is then in the slowest gas flow, the slowest moving part of the blade is in the fastest gas flow. That is an inefficient design and requires a very complex blade geometry. It also requires that it operate close to a fixed optimum speed.

The limitation on turbine blades is gas velocity and the speed of sound. With a small diameter turbine it takes high RPM to efficiently extract energy. With your design, gas will happily pass down the centre of the turbine without doing any work, because the radius is insufficient to give a high blade velocity.

A practical turbine spool with relatively short external blades has evolved over time because it reduces the RPM and the variation in blade velocity from the inner part to the outer part of the blade. Effective blade velocity is proportionally less nearer the axis. Short blades have a simple design, long blades are much more complex in profile and have more modes of failure.

A large diameter alternator will need many poles. The same electrical power could be generated from less poles with a smaller diameter alternator that is both easier to build and has a lower cost. High RPM with many poles will generate very high frequency current flow in the copper coils. The stator laminations for use at those frequencies must be very thin, better described as foil. You will find it very difficult to build the stator from iron foil laminations and so will need to use an iron or ferrite powder similar to that selected for use in high frequency switching power supplies. Unfortunately, stator geometry will require you cast your own magnetic components.

This all pushes the design towards the slower RPM traditional turbine having short blades on a larger diameter spool, driving a three phase alternator as is typically used in a car.

I'm sorry about being the Devil's advocate, but you are jumping in at the deep end here and I don't want you to drown.

 

[iwant2beoz]

I appreciate the critique, I am not an engineer, I'm a bio major in college with an over active imagination. But I think that with time and can overcome the flaws in my design. I was thinking of putting a stationary center shaft with fixed blades to forse the gas against the outer blades but I'm unsure if that will work.

 

[Baluncore]

Static blades working with blades on the rotating spool deliver improved performance, but the combination is more complex to design and generates more noise.

One very important design consideration is to keep the diameter of the bearings as small as you possibly can. That will give a longer bearing life. The minimum bearing diameter is usually decided by the diameter of the shaft being supported. It will probably be necessary to continuously lubricate the bearings so there will also need to be some form of oil seal at each end of the shaft.

When operating, the input gas pressure will place an axial force on the turbine spool. That will require a thrust bearing. Steam turbines are usually designed to be symmetrical so as to eliminate the axial thrust. Single shaft turbochargers are similarly arranged so the pressure of the exhaust gas on the turbine tends to cancel the compressed air pressure on the blower.

A diesel generator can be easily adapted to run on gas. They are used locally to co-generate electricity from landfill and sewage gas. They do not require a compressor or high pressure gas storage. They have automatic start and stop. Maybe that is an easier way to use your gas.

 

[iwant2beoz]

Will the exhaust gas continue to expand after combustion? If so I could use the same principal as a fish gill and use the increase in volume of the gas in conjunction with an increase in the diameter of the turbine. Ie. Make it sort of cone shaped.

 

[iwant2beoz]

It would look something like this

 

[SteamKing]

Will the exhaust gas continue to expand after combustion? If so I could use the same principal as a fish gill and use the increase in volume of the gas in conjunction with an increase in the diameter of the turbine. Ie. Make it sort of cone shaped.

Yes, turbines with many stages are roughly cone-shaped to accommodate the expansion of the gas as it flows through the turbine. This is most noticeable in steam turbines.

 

[Baluncore]

For a conical turbine see the picture ; http://en.wikipedia.org/wiki/Compound_turbine
Only the moving vanes are shown. The static vanes would be part of the casing.

 

[iwant2beoz]

Baluncore I didn't quite understand what you ment when you said that the " a large diameter alternator will need many poles" why will I need many poles?

 

[Baluncore]

The cross sectional area of the magnetic core of each of your coils is not useful unless it is linked to it's adjacent cores by a similar magnetic sectional area outside the windings. So a few wide poles would require a wide magnetic path outside the coils. All that unnecessary external magnetic material can be reduced by increasing the number of poles, or by reducing the diameter of the alternator. If you do not reduce the pole size or the diameter of your alternator then your coils will also need significantly more copper wire which will have more resistance and so higher losses.

High turbine RPM with a large diameter external alternator results in excessive alternator material and higher losses. Your present design is inside-out. As AlephZero suggested in post #11; you are better using a smaller diameter alternator with a larger diameter turbine.

 

[iwant2beoz]

Perhaps you are right. Thank you for your help :)

 

[rcgldr]

Although they are expensive, why not just buy a radio control model turbine meant to drive a propeller (radio control turbo-prop)? Some examples:

http://www.rc-airplanes-simplified.com/model-jet-engines.html

 

[iwant2beoz]

Although that little engine is really cool, I'm not so much interested in finding a way to burn the biogas as I am in finding a more efficient turbine engine. But thank you for the link