Doubt about rc gas turbines diffuser

[andycp]

Hi, I would like to know, what´s the pourpose of the surrounded step in centrifugal diffusers, and what would happen if behind the compressor it was not this little step.

Thanks

[enginewhisperer]

that step looks to be a recess for the compressor wheel to sit in, so the face of the diffuser is in line with the "front" surface of the compressor wheel.

Baically there's a step in the metalwork to prevent a step in the air flow.

[andycp]

May 5, 2024 4:37:29 GMT -5 enginewhisperer said:

that step looks to be a recess for the compressor wheel to sit in, so the face of the diffuser is in line with the "front" surface of the compressor wheel.

Baically there's a step in the metalwork to prevent a step in the air flow.

Okay, and if my gas turbine doesn´t have this step, it will affect a lot?

[supatchai]

air out of the blade tips gets everywhere even at the back of the blade, but ideally you want most of it goes through the diffuser

[racket]

It'll certainly have a bad effect on the engine if the supersonic airflow doesn't transition from the wheel to the diffuser as smoothly as possible, theres a lot of diffusion done in that critical area

[enginewhisperer]

May 5, 2024 5:30:59 GMT -5 andycp said:

Okay, and if my gas turbine doesn´t have this step, it will affect a lot?

it doesn't need the step on the back of the diffuser, but you do need your compressor wheel outlet to be flush with the front side of the diffuser - which will generally mean having a small recess there.

[CMDR_Boom]

May 5, 2024 5:30:59 GMT -5 andycp said:

May 5, 2024 4:37:29 GMT -5 enginewhisperer said:

that step looks to be a recess for the compressor wheel to sit in, so the face of the diffuser is in line with the "front" surface of the compressor wheel.

Baically there's a step in the metalwork to prevent a step in the air flow.

Okay, and if my gas turbine doesn´t have this step, it will affect a lot?

Out of curiosity, is this a scratch-build or commercial?
Depending on a lot of things, clearances, base material and your skill level, rather than machining in the recess, you may be able to add a diffuser vane 'surround' if you will that will do two things. Namely the first is redirecting air with a small amount of additional compression into the volute chamber, and by happenstance, nets you a gradation that does what the missing recess would do but more efficiently. 

It's a bit of work and understandable if that's not in the wheelhouse of your project. There's a lot of geometry that needs to be resolved specific to your compressor size and air volume flow so you don't needlessly choke the work being done. 

[andycp]

May 6, 2024 3:39:29 GMT -5 CMDR_Boom said:

May 5, 2024 5:30:59 GMT -5 andycp said:

Okay, and if my gas turbine doesn´t have this step, it will affect a lot?

Out of curiosity, is this a scratch-build or commercial?
Depending on a lot of things, clearances, base material and your skill level, rather than machining in the recess, you may be able to add a diffuser vane 'surround' if you will that will do two things. Namely the first is redirecting air with a small amount of additional compression into the volute chamber, and by happenstance, nets you a gradation that does what the missing recess would do but more efficiently. 

It's a bit of work and understandable if that's not in the wheelhouse of your project. There's a lot of geometry that needs to be resolved specific to your compressor size and air volume flow so you don't needlessly choke the work being done. 

It´s a homemade gas turbine, you can see it in my profile, but I think I will order a new compressor with a higher tip height and avoid this additional machining

[racket]

A larger tip height will change the engine design meaning your diffuser approach angle won't be correct , we can't just make changes to a design and expect it to work

[andycp]

May 6, 2024 16:36:01 GMT -5 racket said:

A larger tip height will change the engine design meaning your diffuser approach angle won't be correct , we can't just make changes to a design and expect it to work

I mean, I won´t touch my diffuser tip height because the compressor wheel that I ordered has less tip height than my diffuser, so I will order one that have the same tip height to avoid machining 

[racket]

This is getting very confusing :-(

Please provide more info , maybe a rough drawing .

From what I can unravel from your emails , your current comp wheel is sitting too far forward due to its smaller tip height , resulting in the rear wall of the wheel not being within the recess , if this is the case then you could run the engine with this setup as the air exiting the wheel will adjust itself to the larger diffuser height , the air from the wheel will not be impacting a step at the edge of the recess, the extra clearance behind the wheel won't be a problem , the main concern is the clearance between wheel and the shroud.

[CMDR_Boom]

Slightly off-topic to your question, but I did go through your other post to get more intimate with your build. Did you ever get your balance issue resolved?
While it's extremely important to have proper balance of your rotating section, it's usually done (at the factory with repurposed components) with a laser-based apparatus to be high precision specific to the compressor and/or turbine set. Naturally that's rather cost-prohibitive, but if you live somewhere within reach of a turbo rebuilder or machine shop with such capability, it's worth getting it 'micron perfect'. My first few efforts, I picked up dubro balancer for small props and went to work over several days with stones and files. It's a low-buck option, but you really have to be tantamount to achieving perfection when your compressor falls into the hobby size with rpms in the 150-210k range. Even getting them up to idle speed is a disaster waiting to happen in short order with just a few grams in the wrong place.

Back to your recess issue, again it would depend on a few things, but another alternative that's probably most achievable at home without specialized tools would be to fabricate or pick up a lapping tool adequate for your compressor size. Going in stages of aggressiveness, it would take a fair amount of effort and time, but you could get an adequate recess cut down fairly easily with this route. Silicon carbide or even aluminum oxide (SiC wears better, Al2O3 is easier and cheaper to procure but is 'softer') in a few different grit sizes and polishing compound paste makes a very decent and low-cost alternative to the commercial stuff, and you can tailor it to your application and material. Aluminum for instance cuts fairly rapid with a hardened steel mandrel lapping tool. Slow and steady, get your micrometer set or an accurate depth gauge on a caliper, take your time and it'll cut down to perfection.

[andycp]

May 7, 2024 4:02:30 GMT -5 racket said:

This is getting very confusing :-(

Please provide more info , maybe a rough drawing .

From what I can unravel from your emails , your current comp wheel is sitting too far forward due to its smaller tip height , resulting in the rear wall of the wheel not being within the recess , if this is the case then you could run the engine with this setup as the air exiting the wheel will adjust itself to the larger diffuser height , the air from the wheel will not be impacting a step at the edge of the recess, the extra clearance behind the wheel won't be a problem , the main concern is the clearance between wheel and the shroud.

If there´s no problem with the rear space of compressor wheel, it will be okay for me. There´s no problem with the clearance between compressor wheel and the shroud with my new design.

I will upload new pictures of my turbine for you, because I want that you see the difference between my old and new diffuser design (with curved edges to have a higher divergent space, and Im thinking about lower a little bit more the edges to have even more space, but firstly I want to know if it a good idea  )

[andycp]

May 7, 2024 4:16:24 GMT -5 CMDR_Boom said:

Slightly off-topic to your question, but I did go through your other post to get more intimate with your build. Did you ever get your balance issue resolved?
While it's extremely important to have proper balance of your rotating section, it's usually done (at the factory with repurposed components) with a laser-based apparatus to be high precision specific to the compressor and/or turbine set. Naturally that's rather cost-prohibitive, but if you live somewhere within reach of a turbo rebuilder or machine shop with such capability, it's worth getting it 'micron perfect'. My first few efforts, I picked up dubro balancer for small props and went to work over several days with stones and files. It's a low-buck option, but you really have to be tantamount to achieving perfection when your compressor falls into the hobby size with rpms in the 150-210k range. Even getting them up to idle speed is a disaster waiting to happen in short order with just a few grams in the wrong place.

Back to your recess issue, again it would depend on a few things, but another alternative that's probably most achievable at home without specialized tools would be to fabricate or pick up a lapping tool adequate for your compressor size. Going in stages of aggressiveness, it would take a fair amount of effort and time, but you could get an adequate recess cut down fairly easily with this route. Silicon carbide or even aluminum oxide (SiC wears better, Al2O3 is easier and cheaper to procure but is 'softer') in a few different grit sizes and polishing compound paste makes a very decent and low-cost alternative to the commercial stuff, and you can tailor it to your application and material. Aluminum for instance cuts fairly rapid with a hardened steel mandrel lapping tool. Slow and steady, get your micrometer set or an accurate depth gauge on a caliper, take your time and it'll cut down to perfection.

So I will solve my balance problem by buying a new compressor wheel and because it will arrive already balance, my only task to do is to machine the shaft with CNC to seat the new compressor. And for the turbine, I will do a new balancing with rocking method.

[racket]

Thats an extremely small diameter diffuser, I doubt that theres actually much diffusion happening at all other than inefficiently at the dump .