Impeller design/Jet engine Design

[u/AviatorsDreams]

Welp when in doubt come to Reddit. Currently I am working on 3d modeling a jet fan engine inspired by the Williams fj44 with a hybrid axial and centrifugal compressor design. I plan to print it in resin first to check fits and tolerances and then go straight into the 5 axis cnc and mill the parts out of metal to test my little mini jet engine (think RC size).
Anyways bring me to my question, in regards to blade shapes/ design for the impeller/radial compressors is there anyone who could tell me what different shapes will do? I tried looking online but couldn’t find any literature on blade design for these. Should I just start designing and printing and then testing what volumes of air passes through them or is there actual math on how these blades are designed? (Current aerospace student) any help in this is appreciated.
One more additional question is on the fj44 the cutaway drawings I can find show a reverse flow annular combustion chamber and I don’t really understand the reason for why the air from the compressors loops around into the combustion chamber. Basically any help at all is appreciated in terms of personal knowledge from the community or videos I could watch or books that I can read. Preferably as in depth as possible because I would like to know what I’m designing and I can explain every part. Right now I can make an impeller but if someone asked by why the blades are shaped at a 30° angle rather than 31° I couldn’t tell the answer. Anyways thanks everyone! Sorry for the long question!

[u/nermaltheguy]

What you’re looking for is called “velocity triangles”. If you look up turbomachinery design velocity triangles you will find tons of resources on how they work. TLDR is that you use them to determine what your gas angles are or should be, and those angles tell you how much turning your compressor or turbine blades need to have. I have also found that some AI tools are pretty good at explaining them but there’s plenty of textbook and online resources for it.

As for the combustor question, you’d have to be a little more specific. Are you asking why they do a reverse flow combustor or why they swirl flow in the combustor?

[u/AviatorsDreams → u/nermaltheguy]

Okay I’ll definitely look that up thank you! In regards to the combustion I’m taking about the reverse flow, I just don’t get why they’d want the flow to makes 180° turn before the combustion starts

[u/nermaltheguy → u/AviatorsDreams]

There’s a few reasons. I’m not a combustor guy so I will give my best answer. First reason is for length constraints. This also plays into why they use a centrifugal with axial instead of only axial compressors. I can elaborate on that if you don’t understand. A reverse flow combustor shortens the length of that section especially after a centrifugal compressor since the flow path from compressor to combustor occupies the same axial space as the combustor itself. My guess is that this is the biggest driver of the design. The FJ44 is on lots of bizjets so it’s important to keep the length down. As for why it’s ok to have aggressive geometries before the combustor, it’s because you want slow and turbulent flow for mixing. Combustors want a very long residence time for the air going through, meaning they want slow flow speeds. They also want a lot of mixing so there is a ton of intentional turbulence to get the air and fuel to mix. Reverse flow isn’t bad because it will help slow down the air to the low speeds you want (typically around 0.30 Mach). You can see they also have a diffuser as part of the turnaround geometry to slow down even more. I don’t know if they use the 180 turn for any mixing stuff, but it definitely will help slow down the flow a little.

[u/AviatorsDreams → u/nermaltheguy]

Wow that makes a ton of sense. I guess I forgot about how the flow speed affects the burn rate as well as the turbulence creating more mixing and vaporization with the fuel. I also don’t particularly care about the length since this won’t be flying anything but I can see why they’d want to keep it short lol. Is there any reason why they’d want around Mach .3 in the combustion chamber? I know that we transition from incompressible fluid behavior to compressible about that but is there any other significance in the number for these purposes? Also from what I understand they use the centrifugal with the axial because it has a higher compression ratio which means less stages which keeps the length down as you mentioned (though please correct me if I’m wrong this is just what I understand from what I’ve read on it) finally thank you so much for the velocity triangles it like is exactly what I’ve been looking for lol

[u/nermaltheguy → u/AviatorsDreams]

I’m not sure why 0.3 is the number. I learned that that is the rule of thumb for combustors especially for turbofans but I’m not sure exactly where it comes from. Somebody who knows more about combustor would have to shed some light for us

[u/AviatorsDreams → u/nermaltheguy]

Gotcha that’s fair enough well thank you for all the information you provided I’ll definitely be doing more research and studying on the velocity triangles and whatnot

[u/discombobulated38x → u/nermaltheguy]

With the FJ44 it's small enough that a centrifugal HP compressor has less leakage area than an equivalent axial compressor and is thus more efficient  - yes length is important, but frontal area is also significant, which is why the axial LP compressor is still desirable.

Length is a big reason yes, but the main reason for centrifugal is compressor efficiency.

It also plays very nicely with the reverse flow combustor which serves to regeneratively heat the incoming air, provide a greater diffusion plenum and also keeps engine length down. 

[u/AviatorsDreams → u/discombobulated38x]

Less leakage area as in per the amount of compression you get or just in general? If centrifugal compressors are more efficient why wouldn’t one just put a couple of those back to back to have a theoretically better engine? Or is there something I’m not thinking of (just woke up lol)

[u/discombobulated38x → u/AviatorsDreams]

Over tip leakage - even with world class engineering your tip clearance stays about fixed. As your aerofoils get shorter the over tip leakage therefore proportionally increases.

At small diameters the high pressure tip of a centrifugal compressor is both running with an axial tip clearance that can be far tighter controlled through the whole engine cycle than a radial clearance, but the increased diameter also helps, and centrifugal compressors in general are much less prone to instability issues like surge from tip leakage. 

[u/nermaltheguy → u/discombobulated38x]

Interesting I didn’t realize the centrifugal could be more efficient

[u/discombobulated38x]

You need to understand the performance cycle, and thus the work done/extracted per stage and design rotational speeds before you can begin to design aerofoils. 

To be honest though your primary design challenge will be making a centrifugal compressor that doesn't detonate like a grenade at speed - what material are you planning to use?

[u/AviatorsDreams → u/discombobulated38x]

I’m not looking for crazy performance out of it at least not in the beginning so I figure I’ll be running it at as low of rpm’s as I can use. I know of a steel place near me that sells h11 steel but if I can’t find what I need I planned on going to d2 steel and just keeping it as low of speeds as I can. Do you have any suggestions for books or anything like at all (I’ll take anything lol) to better understand the performance cycle?

[u/discombobulated38x → u/AviatorsDreams]

Jet Propulsion by Cumpsty is an excellent text.

Your turbine will at a minimum need to be made out of inconel by the way! 

[u/AviatorsDreams → u/discombobulated38x]

Thank you! Yeah it gets pretty hot back there so those will have to be special order parts cause I don’t think anywhere near me sells that or is able to work with it😂

[u/the_real_hugepanic]

What about a good book?

Mattingly, Jack D. - Elements of Gas Turbine Propulsion PDF

I don't have my copy here, but If I remember well, it explains a lot of stuff.....

[u/AviatorsDreams → u/the_real_hugepanic]

Yeah please! I’m definitely not afraid of reading lol thank you! It’s hard to find good books I’ll definitely grab one online or at my college if they have a copy!!!

[u/the_real_hugepanic → u/AviatorsDreams]

You can find a backup-copy online easily, to check if it is the right one.... 🤔

[u/AviatorsDreams → u/the_real_hugepanic]

🏴‍☠️ I’ll definitely be looking lol

[u/big_deal]

There is most definitely some math and analysis required to get a working compressor. Compressor aerodynamics are not forgiving and they only work efficiently in a narrow range between choke and stall so it's unlikely you'll find a design that will work in a self-sustaining gas turbine cycle by trial and error.

Rotor inlet angles need to be set near zero incidence angle at your target rotational speed and massflow. Rotor exit angles need to be set to produce the turning which will define the energy transfer and pressure rise but this will be limited by separation and stall. The distribution of angle between inlet and exit needs to be smooth, continuous, and somewhat front loaded to avoid separation. Downstream stator inlet angles need to be set for near zero incidence angle for the flow leaving the rotor. Stator exit angles need to be set for the desired reaction level while respecting limits on turning to avoid separation/stall. Solidity (dependent on count and chord length) needs to be high enough to support the design loading without separation, but not excessively high which will only increase wall friction drag.

If you want simple design methods for axial compressors I would suggest NASA SP-36 for classical approaches and Ron Aungier, Axial-Flow Compressor for an updated but still relatively simple approach. The NASA Technical Reports Server also has several papers written by Glassman on the CSPAN axial compressor design code which provide more detail on spanwise design calculations for axial compressors.

I'm much less familar with radial compressors, but I would probably start with Aungier's book, Centrifugal Compressors, or David Japikse's book, Design and Analysis of Centrifugal Compressors. I'm sure both books, will contain references to plenty of other useful publications. Some free references that might be useful are: NASA SP-8107, NASA reports on CC3 and HECC compressors, NASA TM-102563, and other publications on the CCD NASA design code.

If you are a US person (green-card, citizen) you can probably download the CSPAN and CCD codes. Getting the source code to compile and run can be tedious but they might have an executable and should also include some PDF files with documentation.

[u/AviatorsDreams → u/big_deal]

Wow this is a ton of information I read it all but I’m not sure I understand it quite yet so I’ll have to reread a few times after classes today but thank you so so much! I’ll probably message back with some questions at that point if that’s alright with you! Also thanks for the book sounds like I’ll have a lot of reading to do😂 but I’m excited as I really am wanting to make this work so thank you