84mm turbine build x2

[madrocketscientist]

I have started building a couple of 84mm turbines. They are for another project I am working on

I need around 20-25kgs of thrust so I decided to go with an 84mm turbine sized engine. From reading about the 70mm turbines that are being pushed to 22+kg, they do seems to be really highly loaded with high temps. Hopefully the 84mm size wont be breaking too much of a sweat (although the specific fuel consumption may be higher)

I started out by buying a couple of compressor wheels, in hindsight maybe I should have learned a bit more first, but there is nothing quite like making a start to get the learning curve steeper! The compressors weren't all that pricey so having to buy more wont break the bank.




And a compressor map (the closest I could find)



For shaft material, I picked up some surplus 12.9 grade bolts off Trademe (New Zealand's version of ebay), $7.50 NZD each, a good price!




I also found a couple of dead submersible pumps lying around where I work, the 125mm stainless covers will work great for the turbine outer shell.




The wall thickness is on the heavy side of 0.6mm, but I am not too worried about the turbines being heavy.

Hopefully the pictures work... and hopefully this doesn't end up another half finished turbine thread

Shannon

[racket]

Hi Shannon

63 Trim , plenty of airflow but the Pressure Ratios get a tad limited , but OK for thrust engine .

At 125 mm can size its going to be a bit "tight" getting a diffuser in there when using an 84 mm comp wheel , you really need another 10-15 mm bigger can .

With a largish ~67 mm inducer , you'll be needing ~10,600 sq mms of flametube cross section whilst still maintaining adequate clearances around the flametube for airflow ............it might be time to get pencil and paper out and do a rough drawup of the engine .

1 lb/sec at 3:1 PR is looking good :-)

Cheers
John

[madrocketscientist]

That post was a success! 

I turned he bolts down to blanks and then started making some shafts. I don't have the final bearings or the turbine wheels yet so these are mainly test pieces. I started by turning some stubs to check the fit. I used a couple of M8 bolts to test the compressor wheel fit.


The first one came out slightly undersize... While the second one was still mounted in the lathe I heat shrunk the compressor wheel onto it and turned the back face slightly to allow air and oil mist into the bearing from the front. I left a healthy radius at the corner as this does weaken the compressor wheel!. I then heated it up again and turned it around and bored a pocked to install a ring magnet for rm sensing. I hindsight I should have waited till I had the ring magnets in my hot little hands before cutting the counterbore for them .
I checked the runout using the inner bore several times during machining to make sure I hadn't bent the mounting shaft and so mucking up the concentricity ( I think that is the correct big word) of the bit I was turnng.



Very light cuts was the order of the day!

Just the shrink fit held things on tight and true.





I turned the blanks a bit closer to size

Then turned the first one to fit the bearings and compressor wheel.

I managed to get a reasonable fit but the surfaces were not concentric due to turning things in what turned out to be the wrong order. I was follow a tutorial guide from the 5bears website that now seems to be down . I said to turn the tapers on the shaft last. The engineer in me was thinking that this may not be the best idea but I went ahead and turned them last. The heat and pressure threw the shaft out of true and it is now pretty shiny scrap.

I turned up a second shaft and turned and polished all the biggest bits to size first, letting things cool in between. This one is so much better I can't get my .001mm dial gauge to pick up any variation on any of the critical surfaces. I did overcut the the thread end of the shaft and the bearing fits are slightly undersize so it's also shiny scrap too.


The bearings are just cheap Chinese ones as placeholders until I get some GRW's. I will wait until I have the GRW bearings and turbine wheels before turning another shaft. That way I can get the fits bang on.

Any comments or pointing out mistakes welcome. I am quite new to this turbine stuff

Shannon

[madrocketscientist]

Nov 24, 2017 20:21:18 GMT -5 racket said:

Hi Shannon

63 Trim , plenty of airflow but the Pressure Ratios get a tad limited , but OK for thrust engine .

At 125 mm can size its going to be a bit "tight" getting a diffuser in there when using an 84 mm comp wheel , you really need another 10-15 mm bigger can .

With a largish ~67 mm inducer , you'll be needing ~10,600 sq mms of flametube cross section whilst still maintaining adequate clearances around the flametube for airflow ............it might be time to get pencil and paper out and do a rough drawup of the engine .

1 lb/sec at 3:1 PR is looking good :-)

Cheers
John

Thanks John,

The best thing about building it myself is that I can always change it! I was looking at just buying a couple of turbines but then the manufacturers don't like me to maintain them and wouldn't supply parts if I did! If I build it myself, maintenance is easy

I will keep an eye out for some bigger cans! No big loss with these ones as they didn't cost me anything (the best price)
I have been reading up on this thread about Gerald Rutten's turbines and my impression is that he is getting away with stuffing some big wheels in a small tube!
translate.googleusercontent.com/translate_c?depth=1&hl=en&prev=search&rurl=translate.google.co.nz&sl=nl&sp=nmt4&u=http://www.modelbouwforum.nl/threads/kj66-conversies.18237/&usg=ALkJrhiZhQ6P1LjI4_JZPRbz6ypQ-NPILw

I only realized that the trim was on the high side after I had purchased the compressor wheels, all part of the learning curve!

I have started a spreadsheet with sizes for things like the flametube and evaporators and things like that, so I will have a check and see how the numbers come out. I do have a 2D CAD drawing of the turbine so far (changes daily as I learn more)

Shannon

[madrocketscientist]

A few more supplies for the turbines, I found these a while ago in some old oil column heaters I was recycling. The stainless tube is 1mm OD and 0.5mm ID, should be perfect for the needles. Thick wall with a fine internal hole

A couple of sieves for the FOD screens $3NZD each

And an assortment of metals and tube



I am not sure I would buy the K&S stainless tube again, I was hoping it was drawn seamless like the rest of their products but doesn't appear to be.

I scavenged a heating element out of a dishwasher and hopefully it is incoloy but it could just be stainless? It is 8.5mm OD.

Shannon.

[racket]

Hi Shannon

I use heating element sheathing for my evaporators , its 800 alloy www.australwright.com.au/nickel-alloys-incoloy-alloy-800-n08800/

Cheers
John

[madrocketscientist]

Thanks John,

Is there any way to tell what the type of alloy is when taking heating elements apart? From my internet search it appears that dishwasher elements do need to be quite a corrosion resistant alloy due to the detergents used in the wash. I couldn't find anything specific but it seems to me that normal stainless steel sheathing wouldn't last long as a dishwasher element.

Shannon

[racket]

Hi Shannon

Guys general use electric stove heating elements , as they're subject to similar conditions as within our engines , running red hot in air .

Cheers
John

[madrocketscientist]

I cut a couple of sample diffusers today, just to see how things line up. They are not the final design and I cut them out of signboard (a pvc foam type product) I wont cut them in alumnium just yet.





I did line the wedges at the calculated angle


Shannon.

[madrocketscientist]

And the 2D CAD so far, lots of changes to come but gives me an idea of where things are headed.


Shannon.

[jetjeff]

Hi Shannon,

I've never seen a spring putting pressure on the outer race of the front bearing, only the rear. I have seen plans where a wave washer is used on the front bearing to apply bearing preload. Are your plans correct?

Nice work so far.

Regards

Jeff

[madrocketscientist]

Nov 26, 2017 0:14:47 GMT -5 jetjeff said:

Hi Shannon,

I've never seen a spring putting pressure on the outer race of the front bearing, only the rear. I have seen plans where a wave washer is used on the front bearing to apply bearing preload. Are your plans correct?

Nice work so far.

Regards

Jeff

Yes I do intend to have the preload spring pushing the front bearing, There isn't really any difference in the force between using a spring or a wave washer, Although it is somewhat easier to fit a spring in between the shaft and housing. Wave washer tend to be a smaller inner diameter and often there isn't space. The AMT Olympus uses what looks like a pair of conical washers at the front but the shaft has to be turned down just behind the bearing to fit them in. With the spring I can also have the sliding bush in front of it to keep the bearing (hopefully) square to the housing.

There are pro's and con's as to whether the preload pushes forward or back, what they are I can't remember off the top of my head . Part of my reasoning for having it forwards is that the front runs cooler and is less likely to overheat the oil in that area, hopefully making it less likely for things to get gummed up and stick. I was considering using O-rings for bearing damping, but apparently they can also get sticky, with the consequence that the preload goes away. I am sure you can imagine what happens next.... There was an interesting discussion I was reading about how the air pressure on the backside of the compressor wheel helps offset the combustion exhaust pressure against the turbine blades and balances out the fore and aft forces on the bearings somewhat.

Shannon.


 

[lofi]

Just musing about your bearing arrangements...

If I recall correctly, the clearance between the comp blades and front cover is fairly important to efficiency. As little as possible is desirable. Making a few assumptions: 4" between the comp and turbine, shaft heat soak up to 200 degrees. With the rear bearing being captive and the front being free to move against the preload spring, you'll be seeing about 5 thou of thermal expansion making the comp move forward that much. That's a lot of extra clearance to allow for when cold! With the front bearing captive the comp would be locked in place and the turbine moves as the shaft expands. This isn't important at all on the axial turbine and easy to design for.
Another thing to consider is the front bearing is likely cooler, so better able to cope with the axial load, of which there is bound to be some. Something to think about, anyway.

Lovely project though - looking forward to more updates

/Ian

[madrocketscientist]

Thanks Ian,

Thinking along the same lines... the shaft housing must also expand somewhat, maybe it wouldn't get as hot as the shaft but it is getting some radiated heat from both the shaft and the combustion chamber. That would negate some of the clearance needed from shaft expansion alone.

(edit) Actually thinking this through further, the shaft has quite a big temperature gradient along it, from somewhere around 400+ degrees at the turbine wheel to maybe 80 degrees at the compressor end (or whatever temp the compressor wheel runs at) Hopefully the rear bearing doesn't see to close to 300 degrees as that is the rating for the bearings, (edit)

Also good point about the front bearing running cooler and more able to handle bearing loads!

I am trying to decide what material to make the shaft housing out of, some are saying that aluminium is good due to its ability to conduct heat quickly and so expand evenly, keeping the bearing alignment. I was considering making mine out of stainless, the thermal expansion is much less but so is the heat conductivity, theoretically allowing the possibility of the shaft heating unevenly and loading up the bearings unevenly.

Lots of engineering in these engines

Shannon.

[smithy1]

Hi Shannon,
Most of the RC turbines these days use a "wave" washer under the front bearing instead of a coil spring...but either way will work ok as long as the loading is correct. Compressor wheel to housing clearance seems to stay fairly constant and I don't have issues with rub at all. All of them use aluminium for the shaft tunnels, mainly for weight I suspect...but they never seem to be a cause for issues.

I have several RC turbines and my 66mm K180G turbine is good for 18kg thrust dry.....with afterburn it's pumping out a healthy 23-25kg depending on air temps and altitudes etc..! It's also only 112mm outside diameter..!

John and I still marvel at it's grunt for it's size....still brings a smile to my face every time I fire it up..!

Cheers,
Smithy.