Major flaws in my design...

[doctorklein]

Hi,

A group of students and I are making a jet engine at Upenn. Below I have included pictures of our assembly on solidworks as well as engineering drawings for each part we actually plan on manufacturing (with the exceptions of spacers). I have already been told that the radial diffuser needs thinner fins so it doesn't act like a nozzle. If you can spot any other critical design problems my team and I would greatly appreciate it as we are beginners!

The red is steel, blue is aluminum, green is brass, yellow is inconel, and the lighter blue is just the glow plug. Additionally, the bearings are actually made of silicon nitride.

[doctorklein]

Also to be clear, I used thomas kamps engine as the basis for this, so I use "my design" very loosely.

[racket]

Hi

Have you considered checking out the GTBA www.gtba.co.uk/ they have considerably more experience with engines of this size , and their "Library" is immense

Cheers
John

[turbotom]

Did you do a proper flow area calculation of the engine? In your drawing, the NGV doesn't appear to have any curvature, i.e. it just directs the flow onto the turbine blades parallel to the axis. Moreover, the flow area of the turbine wheel appears to be way too small compared to the comressor. The hub of the turbine close to the shaft could also be beefier. For a typical stress distribution of a similar, slightly bigger turbine wheel that we had cast and that performs well up to approx. 60lbs of thrust @115krpm, see attached screen shot (elastic deformation shown exaggerated). The highest stress is usually around the bore. If there's too little material there, the bore will plastically deform ad get bigger until possibly a wheel fracture occurs (though usually this won't actually happen since the increasing imbalance and decreasing blade tip clearance causes the rotor to come to a grinding stop before). The position of the vaporizer tubes vs. the dilution air holes is quite important. You don't want to direct too much cold air diretly onto the vaporizers. It's better to place them between the rows of holes in the outer liner. In engine of this type, the net rotor force is usually pointing to the front. So having the axial preload spring located behind the compressor bearing is the standard.

[doctorklein]

turbotom
Thank you for all the feedback. You're right, the NGV blades have no curvature as shown in the photo, however my team and I plan on bending the fins after initial manufacturing because we have very somewhat limited access to reliable CNC mills and lathes. I did not know the turbine area was too small (I will have a look at that for sure as well as the turbine hub). I'll also have a look at the vaporizor tube position in relation to the combustion chamber holes as well as the spring placement.

[smithy1]

Sept 14, 2021 23:24:50 GMT -5 doctorklein said:

turbotom
Thank you for all the feedback. You're right, the NGV blades have no curvature as shown in the photo, however my team and I plan on bending the fins after initial manufacturing because we have very somewhat limited access to reliable CNC mills and lathes. I did not know the turbine area was too small (I will have a look at that for sure as well as the turbine hub). I'll also have a look at the vaporizor tube position in relation to the combustion chamber holes as well as the spring placement.

Just remember the "Kamps" design is very old, possibly getting towards 30 years, their thrust output was minimal and rpm's were quite limited due to many factors ....these miniature turbines have changed immensely over the last 10 years or so and designs of this type are now quite powerful for their size with rpm's approaching 130k+.

I have a highly modified Kingtech K180G, it's rated to a nominal 18kg thrust and is only 112mm in diameter, turbine wheel is 66mm, (not much more than in your design). With afterburner it is putting out  ~24kg-f ....the bare engine itself weighs quite a bit less than 1kg.

www.youtube.com/watch?v=1XqoYhSxKCg

Smithy.

[finiteparts]

Hi,

I just thought that I would point out that your fuel injectors are sticking too far into the vaporizer tubes.  The idea is to get the fuel in contact with the "hot" wall of the vaporizer tube as early as possible and to keep the fuel wetting the inner surface as much as possible to allow heat to transfer to the liquid fuel sheet and possibly get the fuel to boil off.

I would suggest that you do the stack-up calculations to determine where the entrance of the vaporizer tube  is during operation and then place the fuel injector exit at approximately 2 x diameter of vaporizer tube from the vaporizer entrance.  This puts the fuel injector downstream of any vena contracta that may form at the vaporizer inlet and thus give as uniform an airflow as possible at the fuel injection plane.  Additionally, I would bend the end of the fuel injector tube at a slight angle to give the fuel a helical trajectory inside the vaporizer tube.  This gives the fuel a longer dwell time(due to the longer flow path) on the hot vaporizer surface and increases the chance of getting liquid fuel up to temperature where you get a good conversion to vapor.

If you read the literature, Rolls Royce and Snecma both found that mitered corners in the vaporizer help to keep the end of the vaporizer tubes from locally overheating.  The classic walking stick can suffer from local overheating of the inner part of the tube bend because the secondary flow pulls the fuel film off the surface and throw it out to the outer part of the bend.  The mitered corners create enough turbulence that all the surfaces get some of the liquid fuel splashing on the walls and thus it keeps the walls from overheating and oxidizing away.

In case you haven't done very much research on vaporizer design, it should be mentioned that you do not want all the fuel to vaporize before it exits the end of the tube.  The reason is that you are using the latent heat of vaporization of the liquid fuel to keep the vaporizer tube wall below the melting temperature and this is how you make sure the tubes survive.  You can do a quick hand calculation to figure out how much tube length you can cool with just the air and you will quickly see that trying to cool it with air is capacity limited and not a good idea.  Of course you will have to assume a hot side temperature, but if you run a range of temps you can see how small a lever air cooling is.

Good luck!

Chris

[finiteparts]

Are you and the other students doing this for a design project or just for fun? I did a turbocharger based engine for my senior design project when I was working on my BSME. PM me if you would like any help on any other specific problems.

- Chris

[jetjeff]

Doctorklein,

I think the hardest part of homebuilding a small self contained turbine, is balancing the rotor (with a homemade axial turbine). Static balancing is not nearly accurate enough. The talk of rotor balancing in both the Schreckling book and Kamps book didn't work for me. You need to have a turbocharger rebuilder balance the rotor. I can't speak for PA, but here in Michigan no local rebuilder would balance my rotor, I'm assuming liability issues.

As somebody already mentioned, the thin Inconel turbine wouldn't survive the heat and high speed rotation. Better to machine an axial turbine wheel from thicker material, 1/4".

Regards

Jeff Pittel