Simplest possible self sustaining gas turbine

[philip111]

I'm building a gas turbine and it only has to do one thing - self sustain (and be cheap to build). It doesn't have to produce any thrust, doesn't have to have high RPM and doesn't have to weigh little. Given these lax constraints I've redesigned the classic DIY jet engine and made it simpler. I'm curious what your guys' thoughts are about this. Going from left to right on the diagram, I've decided to mount the bearings externally for easy access and assembly. The compressor rotor will be 3d printed and similar to the FD3/64 rotor only with a smaller inlet so there's less airflow. The diffuser will be big for good pressure recovery. There is no dedicated combustion chamber in this engine, the whole outer case forms the combustion chamber, there's these pockets where gaseous fuel will be delivered and the flame can be anchored there. The idea is that the combustion chamber is big and the airflow is low so that good combustion and mixing can be achieved even like this. The shaft will be exposed to the heat though I doubt it will be an issue. The heating will cause the turbine and compressor to move in the axial direction though.The Turbine blades and the nozzle guide vanes have a low twist. There's also a diffuser after the turbine so the exit velocity is extra low which increases the work done on the turbine. Can I use circlips to fasten the rotors to the shaft? Is there some thermodynamic principle which determines the minimal temperature that the exit gases have? I'd like to run this engine cold and slow if it's possible. Thanks! 

[racket]

Hi Philip

Cold and slow , just don't happen with gas turbines :-(

The turbine temperature will need to be at least in the 300C + range and RPM >20,000

LOL, ..............you could mount it vertically and put a "chimney" on it and create an updated Da Vinci Chimney Jack that could be run slower and "colder" .

Full ceramic bearings required for minimal rotational drag

Cheers
John

[philip111]

Sept 26, 2022 19:34:02 GMT -5 racket said:

Hi Philip

Cold and slow , just don't happen with gas turbines :-(

The turbine temperature will need to be at least in the 300C + range and RPM >20,000

LOL, ..............you could mount it vertically and put a "chimney" on it and create an updated Da Vinci Chimney Jack that could be run slower and "colder" .

Full ceramic bearings required for minimal rotational drag

Cheers
John

Thanks John, I know the motor is unorthodox but I'm confident it can work. Do you know if circlips are adequate for connecting the rotors to the shaft?

[racket]

Hi Philip

Theres serious horsepower being transfered between turbine wheel and compressor wheel in a turbine engine, so secure fixing of wheels is imperative .

Even if your engine is small and barely running at self sustain levels it'll need both "fans" to be tight on the shaft , exactly the same as any domestic fan running with only a 100 watts of power , using circlips instead of nuts might be possible if the fans are secured from rotating by the use of some form of fixing, and rpm are kept low enough that centrifical forces don't stretch the circlips past their holding point .

Your extremely long and thin shaft is going to suffer all sorts of "dynamics" which will only be exacerbated by any inbalance of the fans.

With minimal rotational drag and efficient well matched comp/turb combo you might get the engine to selfsustain , but the thermal "complications" of not having a flametube will be "challenging" , you're trying to build an afterburner with a comp at one end and a turb wheel at the other :-(

All the best :-)

Cheers
John

[miuge]

So in other words you're not building a gas turbine but a gas fed fan that will spin and produce heat. I have no doubts you could achieve your goals with this design, I've seen worse on YouTube! Like John said, even a slightest amount of produced exhaust that can be directed through the exhaust wheel will probably make it spin if mounted on responsive enough bearings.

[wannabebuilderuk]

It's possible to make something incredibly simple and cheap.

youtu.be/qsvpU7G5IJg

[racket]

LOL..............that ain't simple or cheap , he might make it look that way , but that guy ain't no fool :-)

His turb temps were a tad high, life expectancy about 1 minute ...................but it did work, all credit to his perseverence .

[noshell]

Ive often pondered if a "jet engine" could be made using a vane type compressor and vane type motor(turbine).... Similar to what you would find in a die grinder.

[philip111]

Sept 28, 2022 13:24:16 GMT -5 noshell said:

Ive often pondered if a "jet engine" could be made using a vane type compressor and vane type motor(turbine).... Similar to what you would find in a die grinder.

US3844117A

They've been considered, a screw compressor or gerotor could in principle also be used. 

[enginewhisperer]

Sept 26, 2022 19:34:02 GMT -5 racket said:

LOL, ..............you could mount it vertically and put a "chimney" on it and create an updated Da Vinci Chimney Jack that could be run slower and "colder" .

I like this idea.
Either mounting the whole engine vertically, or adding a long vertical exhaust pipe should get it to "run" even if most of the power is coming from the convection.
The better the turbine and compressor work, the more fuel it will be able to burn and the more power it will make.

[philip111]

I decided I would mill the compressor wheel out of aluminum but I have a few questions concerning the shape. I'm curious about the effects the intake angle has on compressor and engine performance with things like surging, flow separation inside the compressor rotor and ease of starting the engine. My suspicion is that I'm better off using a less aggressive angle.

I would also have an easier time machining the compressor wheel so it has a disk shape and the height of the channels remains the same with increasing radii. (on most compressor wheels I've seen the channel height decreases). How could this impact compressor performance? The rotor itself will be of the enclosed type.

Thanks,

Philip

[racket]

Compressor design is a whole lot of complicated compromises , if you want to build a "Simple" engine , constructing your own comp wheel isn't going be simple .

Kurt tried it 20 years ago www.youtube.com/watch?v=HAIeEuua1hk&ab_channel=reedy93 with his wooden comp wheel

[philip111]

I've read Kurt's and Kamps' books and I'm taking some inspiration from Kurt's rotor only I plan to mill mine on a CNC. While designing the rotor a question that occurred to me is why do on commercial compressor rotors the channels that the air flows through become somewhat narrower in their cross sectional area with increasing radius. Basically if I make my compressor rotor with channels that increase in area with radius will it be bad.

Thanks,
Philip

[racket]

Hi Philip

The fact that they are made that way means they need to be , the STATIC pressure exiting the wheel is roughly half the total pressure exiting the wheel , the remaining half is dynamic/velocity that is converted into static pressure as it slows down in the diffuser, if your cross sectional area is too great exiting the angle of discharge will be very low making the diffusion less efficient , a "tangential" angle of ~20 degrees is considered roughly whats required.

You'll need to determine what rpm you'll be using , then the comp tip speed and its potential pressure rise , etc etc etc , there are whole books devoted to the subject , I'd follow the guidelines in Kurts book with regards both the comp and turb .

Cheers
John

[philip111]

Hi guys bit of an odd question, if my turbine rotor has a weight of 2.6 kg (yes I know it's a lot), what size bearing should I choose, will 608 (8mm bore) bearings be enough or should I use 6001 bearings (12 mm bore). I'm concerned about friction.