Having an aft-fan made...

[Johansson]

While reading some of my gas turbine litterature I came upon a chapter concerning turbofan engines, one of the early versions had a combined freepower turbine/fan fitted downstream the gas producer.

Do anyone know if this has been attempted by anyone in the DIY community?

The problem is that the turbine blades nearest the hub get very hot while the fan blades further out are cooled by the air being sucked past them, but if the exhaust temps and revs were kept moderate it should be possible to make one that survives and possibly produce more thrust than the gas producer would have with an afterburner attached.

Anyone with turbine designing skills and an investment casting foundry willing to make one suitable for roughly 1kg/s mass flow? :-)

Cheers!
/Anders

[racket]

Hi Anders

LOL........now that you mention it ;-)

I've never come across anyone thats tried it , but at one time I did think about the 4th stage wheel with its labyrinth seal removed that then had a shrunk on axial comp type arrangement , the axial comp wheel needs to spin at relatively low speed otherwise the horsepower requirement gets too great , so the tensile loads on the turb blades will be reduced somewhat by the lowish rpm, whilst the shrunk on fan might have sufficient strength in itself if the mounting "band" is thick enough ............theres probably some rough drawings and numbers somewhere in my pile of papers .

Cheers
John

[Johansson]

I figured you had thought about this John. :-)

I wonder if the 3D metal printing has come far enough to pull off such a project? If the wheel was printed in good quality stainless and the hot section had a coat of ceramics to keep the metal temps down it might perhaps work.

Cheers!
/Anders

[gtbph]

Oh I haven't seen this thread yet!

Hi Anders!

Interesting idea, this could probably even be fitted to existing engines without too much change. Is that a part of the turbofan build you mentioned recently?

If I remember correctly, there are Chinese foundries that would also do prototypes/one-of inconel castings for affordable prices, but I didn't bookmark them unfortunately. If you want I would gladly try to make a 3D file of such a wheel with my program, you could then print it in wax and send it to the foundry. I have not tried to make a turbine model yet, just compressors, but I'm sure the program doesn't have to be changed much to do that. The other thing is... I have no idea if the designs I make are any good in the real world... They look OK I think, but maybe there is an error somewhere, the risk is that you end up with an expensive paper-weight.

Cheers, Alain

[racket]

Hi Alain

Yili Casting www.yilicasting.com/ does some interesting stuff.

Anders new turbofan is something different .

Cheers
John

[Johansson]

Hi Alain,

No, I am building a more conventional turbofan at the moment. Build thread will come as soon as I have had time to sit down and write something up about it.

If I had the money to spare I would be very tempted to give it a try, but my workshop budget don´t allow any new projects at the moment.

It would still be very interesting to discuss this, perhaps a joined effort that one fine day would end up in a JATO aft-fan project? If we could decide on a gas producer size that is most common for us DIY:ers and bang our heads together we should be able to come up with a design for a fan, and with the number of members here it should be possible to raise funds for having a number of fans cast and tested by willing members.

The idea of a JATO community Aft-Fan project is really cool, we´d raise the bar for DIY jet engines quite a lot with the addition of a turbofan module instead of the old afterburner.

What do the moderators say? I am not at all skilled to do any designing but I am more than willing to use my JU-01 engine as a gas producer for testing the resulting fan with once it is retired from its service in the bike.

Cheers!
/Anders

[ripp]

Hi Anders,

you mean something like that?



Cheers
Ralph

translate.google

[Johansson]

Mar 29, 2017 13:53:40 GMT -5 ripp said:

Hi Anders,

you mean something like that?



Cheers
Ralph

translate.google

Hi Ralph,

Exactly, a turbofan and power turbine combined in a single wheel. It would make a very compact and easy to construct thrust increaser, plus it would lower both noice and heat radiation from the engine which is a bit of a problem in a vehicle since things around an afterburner tends to melt...

The best part about it in my mind is that it is something totally new, turbocharger based jet engines with afterburners have been around since Jesus wore diapers so it would be very fun to come up with a decently priced aft fan wheel for the community.

With 100lbs of thrust from the gas producer and moderate ITT temps around 700°C, what kind of performance is reasonable to expect from such a fan properly designed?

Cheers!
/Anders

[pitciblackscotland]

Hi Anders,
Here is a couple of vids of diy turbofan engines. Not what you have in mind but worth a look.

Cheers,
Mark.

www.youtube.com/watch?v=NV0YU5KCy6M

www.youtube.com/watch?v=ESaK6ti5BM8

[racket]

Hi Anders

One problem I see is the tip speeds , the "freepower" wheel is large due to the lowish density gases passing through it at modest velocities , it might have a tip speed of say 500 ft/sec minimum even if running at 50% N2 , to get a reasonable airflow we'd be needing fan blades of say half the radius of the turb blades , so fan tip speeds ~750 ft/sec, when we then take the inlet air speed into the velocity triangles we start getting up into speeds approaching M 0.8 which is starting to get kinda fast and will require a fair bit of horsepower.

A 100 lb thrust engine should give 100 HP , so big enough to produce a decent flow of air , but a 100 hp freepower is roughly the size of a 3rd stage C20 turb wheel at ~180 mm , so the fan at 270 mm , thats a big casting which would be rather expensive , >$US1,000 and probably closer to $2,000 :-(

Now an alternative could be an industrial fan like the Multiwing au.multi-wing.com/Products/BrowseByProfile/PressureMAX but max tip speeds are down in the 500 ft/sec range and they're a bit big ..........they're used in hover craft with IC engines spinning at a fraction of our turbine wheels , we really need a gearbox .

Or , lets think a tad laterally here, if the "freepower" was fed high pressure bleed air that had been heated in another combustor , the freepower could be smaller , maybe a turbo turbine , but then it'd spin faster needing a smaller/cheaper fan , but small would probably be a complicated transonic design , lets rule that out :-( ..........unless we could find a C20 first stage axial comp wheel .

I might need to think a bit more on this in light of newer developments since I last visited it more than a decade ago .

Cheers
John

[racket]

Hi Mark

I remember that first one with the twin gas producers , it was a Mike Heward engine if I remember correctly , it used aircraft gas turbine engine parts for the freepower and fan , 300 lbs of thrust sorta comes to mind .

Cheers
John

[pitciblackscotland]

Mar 29, 2017 18:59:08 GMT -5 racket said:

Hi Mark

I remember that first one with the twin gas producers , it was a Mike Heward engine if I remember correctly , it used aircraft gas turbine engine parts for the freepower and fan , 300 lbs of thrust sorta comes to mind .

Cheers
John

Hi John,
Yeah we did have a discussion about this engine years ago, have not seen any updates on the engine project.

Cheers,
Mark.

[racket]

Hi Mark

I think once Phill ( don't know where I got Mike :-( ) went to work for Bladon Jets www.bladonjets.com/about/the-team/ all his previous engine work was stopped and others are producing his designs.

Cheers
John

[gtbph]

Hi all,

OK, I'll try to extend my program and make a few pictures of a blisk based on the numbers John provided, just for fun and to see what it looks like. I want to be able to model turbines anyway because something with a 2-stage axial turbine would be nice to try to build, some day.
A 2-stage axial turbine would be a cool community project too, something a bit smaller than the allison turbines, say ~120 mm... I wonder if it could be made "adjustable" for different flows, if we make the wheels big, and grind them back to the needed size. But then the NGVs would have do be done individually as it's probably not possible to make them adjustable too. Anyway, this is just a dream at the moment.

John, thanks for the YiLi link, this seems like a big, high quality foundry. Have you contacted them in the past, do you know if they are willing to sell very small batches or single pieces? I had seen a website of a smaller foundry that explicitly stated they do prototypes, but if the prices are similar this big foundry would be the better choice I think.

Cheers, Alain

[racket]

Hi Alain

I have been in contact with Yili regarding their 180 mm NGV and turbine wheel castings , both for myself and another person , the cost was $US750 for each as rough castings , unfortunately I couldn't source a suitable compressor wheel so didn't take it any further :-(

Yili have been getting better over the last few years , I'd have no worries about their products being up to the task .

Cheers
John