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Post by finiteparts on Jan 4, 2024 10:26:03 GMT -5
As for the vaneless diffuser, certainly there is some chance to get more expansion when it enters the combustor outer annulus space, but you would want that to be at a low speed, again to minimize any sudden expansion losses that could occur. The vaneless diffuser passage creates a very "controlled" area expansion that hopefully allows the flow to diffuse in a smooth fashion. The adverse pressure gradient of the diffuser (low static to high static pressure) is like rolling a rock uphill. Any sudden trips and it will "stall" and destabilize. So trying to do more expansion in the combustor annulus could be destabilizing if there are sudden flowpath area changes. Flow trips such as dirt or machining marks can trip diffusers and it is really important to control the area expansion of the diffuser to avoid instabilities.
I think it should be stated that vane-island diffusers are really a tool to be used on a high pressure ratio compressor stage. Vaneless and low-solidity diffusers are really good options for compressor stages with PR < 4.0. Vane-island diffusers are more capable of accepting high-subsonic, transonic or even super-sonic entrance flowspeeds and thus why they are good choices for high PR compressors. They usually have relatively "long" passages to keep the expansion ratio low and thus require relatively large outer diameters.
- Chris
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Post by Johansson on Jan 4, 2024 12:56:08 GMT -5
Vaneless and low-solidity diffusers are really good options for compressor stages with PR < 4.0. Thanks for explaining this Chris. The quoted section was exactly what I wanted to hear, I cannot hope to get more than PR 1.5 out of the JU-03 compressor so the vaneless route makes sense then. |
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jetric
Veteran Member
Joined: December 2014
Posts: 147 
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Post by jetric on Jan 4, 2024 13:52:41 GMT -5
Why don't you make a scaled up exact copy of a Kurt Schreckling FD 3/64? Then at least you have got a proven design and it would be impressive to use a wooden comp wheel and flat sheet twisted blades turbine wheel!! Richard S. Vaneless and low-solidity diffusers are really good options for compressor stages with PR < 4.0. Thanks for explaining this Chris. The quoted section was exactly what I wanted to hear, I cannot hope to get more than PR 1.5 out of the JU-03 compressor so the vaneless route makes sense then. |
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Post by Johansson on Jan 4, 2024 14:44:51 GMT -5
Why don't you make a scaled up exact copy of a Kurt Schreckling FD 3/64? Then at least you have got a proven design and it would be impressive to use a wooden comp wheel and flat sheet twisted blades turbine wheel!! Richard S. Already done, Axel Borg made a 2:1 scale FD3. And I really don´t like following other peoples drawings.  |
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jetric
Veteran Member
Joined: December 2014
Posts: 147
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Post by jetric on Jan 4, 2024 16:21:42 GMT -5
Yep I know of that one but it's not exactly large scale at twice the size of the original! I was on about making one with a casing diameter of one meter. How are you making your axial flow turbine? Richard S. Why don't you make a scaled up exact copy of a Kurt Schreckling FD 3/64? Then at least you have got a proven design and it would be impressive to use a wooden comp wheel and flat sheet twisted blades turbine wheel!! Richard S. Already done, Axel Borg made a 2:1 scale FD3. And I really don´t like following other peoples drawings.
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Post by Johansson on Jan 4, 2024 16:31:44 GMT -5
The idea is to make segments with a number of blades cut from a single sheet of metal, and bolt them to the steel hub I just made. Then 3D-print a press jig to make the curve on each blade.
After that they can be twisted to the right angle.
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Post by Johansson on Jan 5, 2024 1:59:50 GMT -5
I was on about making one with a casing diameter of one meter. Go for it! It would be really fun to start a new trend in the DIY community. With Johns latest engines it feels like we´re reaching the end of the thrust road with "automotive" rotor parts, by making our own rotors doubling or tripling the thrust potential would be possible. My balls are tingling from the thought of 500+HP turboshaft engines made from sheet metal by happy DIY:ers in sheds. |
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Post by Johansson on Jan 5, 2024 12:18:53 GMT -5
The jigsaw did a good job today. First off the compressor back plate was cut out from 2.5mm mild steel sheet.  The hub got a 1.5mm recess and 12 holes threaded for m6 screws.  Here is the compressor hub and backplate fitted together, quite a bit of difference in size compared to the 3D printed model.  After a bit more cutting I had the diffuser plate cut out as well from 5mm aluminum, here is the comp parts on top of it. It will be interesting to see how the minimal vaneless design will work.  A closeup on the ring that surrounds the compressor and makes up the rear clearance, I will lay a weld seam around the outer edge and grind it to a nice radius. This is the "floor"of the diffuser section where the vanes would be fitted if there were any.  Cheers! /Anders |
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richardm
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Joined: June 2022
Posts: 411
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Post by richardm on Jan 5, 2024 12:43:00 GMT -5
Question: Are you going to weld the vanes? If so how are you going to prevent the back plate from warping, I built a centrifugal fan years ago, welding blades to a kind of similar back plate. Material was 1/4" thick. It warped quite a bit. Made another one with rivets instead...
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Post by Johansson on Jan 5, 2024 14:30:43 GMT -5
Question: Are you going to weld the vanes? If so how are you going to prevent the back plate from warping, I built a centrifugal fan years ago, welding blades to a kind of similar back plate. Material was 1/4" thick. It warped quite a bit. Made another one with rivets instead... I bought me a spot welder a while ago, really neat tool for jobs like this. I will make the compressor blades with a small bent edge on both top and bottom for welding, shouldn´t warp at all. |
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richardm
Senior Member
Joined: June 2022
Posts: 411
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Post by richardm on Jan 5, 2024 15:57:07 GMT -5
Well in my case it was the back plate disk that warped... Good luck with it. Got a spot welder too Great tool for sheet metal. Does neat job with a stainless Mine has a max combined thickness of 3/16"
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Post by Johansson on Jan 5, 2024 17:05:33 GMT -5
Well in my case it was the back plate disk that warped... Good luck with it. Got a spot welder too Great tool for sheet metal. Does neat job with a stainless Mine has a max combined thickness of 3/16" Thanks, if spot welding won´t work I will probably go with rivets like you did. |
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richardm
Senior Member
Joined: June 2022
Posts: 411
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Post by richardm on Jan 5, 2024 18:08:47 GMT -5
Well if you can afford some trials on a scrap piece mimicking you build first and see how it goes Better than ruining all your work. Or may be seek advice e from professional welders. They might have some ideas to co control the warping.
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Post by Johansson on Jan 8, 2024 15:14:20 GMT -5
Spent a couple of hours on the shaft tunnel tonight, I got halfway through making the two bearing houses. I considered taking a photo or two but I guess you´ve seen a chunk of metal in a lathe before. |
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Post by racket on Jan 8, 2024 15:41:29 GMT -5
LOL.....Photos please ;-)
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