Some photos of parts for 1991 microturbine reboot

[chertzyb]

Hi, 

Still sort of new here and already asked a few questions to which I received some great answers (thanks guys).  I'm in the process of planning a rebuild of a small engine I tried to build in 91 as a teenager and unfortunately never saw self sustaining operation.

As mentioned, I recently sourced a counter-clockwise turning small 50mm compressor wheel, read Kamps' book and now wanted to post some photos of the parts.

The plan is to try to


- use the original milled turbine.  According to Kamps, the blades are a bit short - 5mm should be more like 8mm.
- Cnc a wren mw44 diffuser in reverse
- Re-profile the compressor wheel to match the wren mw44 profile 
- Re-purpose the original milled ngv 
- Combustor and other dimensions based on the Wren Mw44.

The rest of the original design is scrap - it was an unusually long shaft, low profile combustion chamber and thin axle.

The main parts are shown here.  The end result should be a short shaft with a turbine wheel a bit larger than the compressor which I hope is ok considering that the rule-of-them is to roughly match the diameters of the two.  In this case the turbine has short blades and the thought is that we'd expect higher velocity gasses concentrated on a smaller surface area which would intuitively result in a faster wheel with lower torque - thus the justification for a smaller compressor wheel in relation to turbine diameter.

Would appreciate any feedback.

Thanks !

[chertzyb]

Parts -

[chertzyb]

Here's a couple of photos of one of the many compressor wheels I made in '91. I tried many different dimensions small and large. Generally speaking they'd have a slight sweep on the blades to mimic the curved sweep of a real wheel with hand curved inducer. Having just disassembled the original engine, it's pretty obvious the original diffuser is a mess (not shown for reason of embarrasment) !

The method of machining was to lathe a blank, then use a circular saw-type attachment in place of a normal lathe toolbit, then index shave out the negative spaces. Finally, bend the intake portion of each blade. The result, a 'billet'-like aluminum wheel

[racket]

Hi

I think you may need to revisit your design calculations , its imperative that the turbine stage can flow/swallow the compressors output , 5mm turbine blades seem very short and the flow area through them would be compromised .

I'm very impressed by your compressor wheel , that would have taken some work .

Cheers
John

[chertzyb]

That is my concern - we have about half the surface area of a normal turbine. That's the very reason I would using a compressor wheel that is smaller than the expected sized wheel this turbine.

As for the home made aluminum compressor it (and its many siblings) was actually quite easy to make - you just wouldn't think that it could be done on a lathe only. Machining aluminum versus the hardened steel of that turbine was like cutting through butter !

[racket]

Hi

I'm not getting a good feeling about this :-(

With very short turb blading the clearance becomes a larger percentage of the annulus , this then degrades efficiency .

Cheers
John

[chertzyb]

What should the clearance be ? Keep in nun I just want this mother to idle ...

[racket]

Hi

Probably 0.1 - 0.2 mm

Cheers
John

[chertzyb]

Current clearance is 0.05 at most

[racket]

Thats very fine , you'd need to be extremely careful about the shrouds expansion rate , a rub could easily happen .

I guess we can rule out clearances for it not starting :-)

Cheers
John

[chertzyb]

The original machining was done with plenty of care I can attest to that. In fact if the ashy is off even a split degree you'd hear rubbing - as far as thermal expansion it seems ok seeing that trying to start it in the past it did indeed see some exteme temps.

Here's my frame of thought . The current surface are of the annular space of the blades is about the same as a 44mm wren turbine wheel. It seems that by running this turbine within a frame and compressor modeled after a proven wren design that hopefully we can get ignition.

No meed to get this up to 50 or 60 newtons just want it to go that's all

[smithy1]

John is quite correct...Clearances of ~0.004"-0.006" (0.10mm-0.15mm minimum), is quite common for turbines of this size, any smaller and you'll run into "rub" territory especially if the temps get up a bit during start...which is often the case. If all the areas and clearances are in the ball park and once the engine gets over the "hump".... temps usually come down significantly.

If your engine is based on the Wren 44 areas, I'd be very surprised if she runs at self sustain....the Wren 44 rotor assy is very light and comparatively very efficient, yours appears to be quite substantial and with those small turbine blades....there may not be enough oommph at lower rpms to get up and over the hump.... if it did manage to run I suspect it will be horrendously hot.

I have several small turbine and compressor wheels here which may be of use to you but of course they all turn the wrong way for your diffuser and NGV....you would be able to make a nice vacuum cleaner though..


Cheers,
Smithy.

[chertzyb]

We are in the process of building a wren 44 as well so I have their turbine when and ngv - real nice parts. My turbine is indeed kind of chunky - what I worry about more is my ngv - this was machine with very steep angles and as a result (if you look close) you can see that the tunnel each vane creates due to the angle causes the flow to constrict. So if anything the first retro would be on the ngv. Thanks for the wheel offer - the one I have is pretty small and with a bit of shaving should fit the wren diffuser once recreated in reverse !

[chertzyb]

Here's the Wren Mw44 in progress (not including combustor and turbine wheel)

[smithy1]

Ah...the Wren 44 would be a more fruitful project methinks...all the hard work has already been done. Wren do make some very nice parts.

Cheers,
Smithy.