Doubt about rc gas turbines diffuser

[andycp]

May 7, 2024 17:31:43 GMT -5 racket said:

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

Do you know any solution with the actual diffuser? 

[racket]

Scaling off your pics , the outer can is only ~33% greater than the comp exducer diameter , this is not a good number , for a homebuilt engine it pays to be a tad more generous at >60% greater

The KJ66 has a 66mm comp wheel in a 107mm outer can ID.

Is this engine your own design or is it built from an available design ??

[andycp]

May 8, 2024 4:21:52 GMT -5 racket said:

Scaling off your pics , the outer can is only ~33% greater than the comp exducer diameter , this is not a good number , for a homebuilt engine it pays to be a tad more generous at >60% greater

The KJ66 has a 66mm comp wheel in a 107mm outer can ID.

Is this engine your own design or is it built from an available design ??

It´s built from an available design but obviously I've modified a lot of parameters, the compressor wheel exducer has 48mm and the outer can ID has 61mm, but my only objective is to achieve to self-sustain my gas turbine

[CMDR_Boom]

May 7, 2024 10:05:28 GMT -5 andycp said:

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.

Just so long as you understand the turbine, shaft and compressor need to be mounted together and balanced as if they were one component. 👍 Naturally they all have different weights individually but will be rotating as one part. Hence when turbo rebuilders go to balance a new compressor, it will be mated to the turbine wheel it's going to run and 'shaved' to match. On jet engines, it's more traditional to add weighted helpers to individual blades or moving them around rather than grinding down material, but going the turbo-sourced route, grinding an assembled rotating section is industry standard practice. 

[enginewhisperer]

a lot of turbos (eg MHI TD series) have individually balanced components and it is standard practice to replace a compressor wheel without balancing the assembly.
It's probably not as good as a balanced assembly but for lower performance parts it's certainly not an issue.

I wouldn't be too worried about it for a turbine that's just aiming to self sustain - as long as the turbine wheel / shaft has been balanced and the compressor wheel has had some sort of factory balancing.

[racket]

48mm wheel in a 61 mm can , thats only a 27% increase ...............it ain't gunna work :-(

I don't know where you got the plans but without a much more complex diffuser design that wraps around the "corner" you're not getting any efficient diffusion, this will require the turbine stage to work much harder, with very high temperatures.

[racket]

I just had another look at the pic of your drawing that you posted imgbb.com/k0P9Jft , you'll notice it has a long axial section of diffuser , this is critical to maximise comp efficiency as the outer can is only ~50% greater than wheel diameter , the design has traded off radial diffusion distance for extra axial diffusion distance

[andycp]

May 8, 2024 16:53:09 GMT -5 racket said:

48mm wheel in a 61 mm can , thats only a 27% increase ...............it ain't gunna work :-(

I don't know where you got the plans but without a much more complex diffuser design that wraps around the "corner" you're not getting any efficient diffusion, this will require the turbine stage to work much harder, with very high temperatures.

So, by the moment, I will finish the new assembly with the new compressor and I will try to start up the turbine only to see what happen. But thanks to your info, I will redesign my turbine with a bigger diffuser to achieve atleast the 60%

[racket]

OK , please let us know how the testing goes .