The humble beginnings of something BIG; JU-03!

[andym]

nice start :-)

[Johansson]

Thanks, so far I haven´t made a single drawing so for starters I focus on making the parts that are non critical.

John is providing me with some good figures to aim at regarding combustor size and stuff like that, but in general I guess this will be an engine based on napkin drawings just like the JU-02...

[andym]

Most wonderful things start as a sketch on a napkin...... my bike start life as drawing Pizza Hut napkin..... am sure we are all going to be withess to something spcial 

[Johansson]

It is special for sure, we´ll see if it turns out to be a runner as well...

I spent most of the afternoon welding up the second hub and turning them down to size.



It actually looks like the cone clamps are keeping the hubs at a consistent 90.0 degree angle to the shaft even after loosening and retorquing, I was a bit worried that they were one fit wonders but the hubs run true even after several refittings.

Cheers!
/Anders

[andym]

Dam good start that......

[Johansson]

With the solid 50mm rotor and these hubs I have a very rigid engine core, that is a must since without one the rotors might end up looking like vinyls left out in the sun for too long...

This evening I made a cad drawing of the engine based mostly on guesswork and what looks right, it is enough to give me a hint of the final dimensions.



A 500mm long shaft tunnel should give me plenty of room for the combustor, I will fit air deflectors to get the air eveny distributed so a fair bit of clearance around it is needed.

Next up is making the compressor wheel, I think it is better to make it before the diffusor plate since clearances and such is easier to figure out with the comp in place.



Anyone with experience with vaneless diffusors in low pressure engines?

I will probably do some testing with different backsweep in the comps during the R&D, so it would be nice not to have to redesign the diffusor plate every time.

Cheers!
/Anders

[richardm]

Don't know much about vaneless diffusers but I've found this. Could be of some interest to you..

www.researchgate.net/figure/Vaneless-and-vaned-diffusers_fig1_34199536

[andym]

Hi Anders

boy its going to be big.... will you be mounting the engine on a chassis.... would love to see an afterburner on it.... with or without , will be interesting, the sound it makes 

[Johansson]

Jan 2, 2024 17:56:43 GMT -5 richardm said:

Don't know much about vaneless diffusers but I've found this. Could be of some interest to you..

www.researchgate.net/figure/Vaneless-and-vaned-diffusers_fig1_34199536

Thanks for the link Rickard, it was interesting that a vaneless diffuser can be pretty efficient and more tolerant of off-design flow figures than a vaned one.

I think that I will try the vaneless route for now, it will allow me to make the engine diameter smaller and a small loss in efficiency shouldn´t cause much of a temp rise with the very low P2 this compressor will produce.

I have not gotten my head around the whole diffuser thing yet.

I mean, after channeling the discharge air into perfectly angled ducts that opens up and lets the air expand, it gets dumped around a corner into the combustor area where all hell breaks loose.

I would understand the importance if every individual diffuser duct were connected directly to a flow-through combustor where the air has to be at its highest pressure and swirl free for good mixing with fuel, but in the case with a vaneless diffuser and an annular combustor wouldn´t the sought after expansion occur anyway when it flows around and into the combustor?

[Johansson]

Jan 3, 2024 1:52:39 GMT -5 andym said:

Hi Anders

boy its going to be big.... will you be mounting the engine on a chassis.... would love to see an afterburner on it.... with or without , will be interesting, the sound it makes 

Hi Andy,

With a 230v motor starter and a jet duct attached it will be a bit over a meter long, I will build a cradle for it with a load cell and some wheels to roll her around.

If and when it runs and is producing its theoretical thrust I might be tempted to add a power stage to it, with over 400lbs of calculated thrust it should send a small kit car flying down the track.

/Anders

[richardm]

Look at this engine en.wikipedia.org/wiki/Pratt_%26_Whitney_Canada_PW100

See the numerous tubes connecting the compressor to the combustor? A bit complicated but could be simplified for a DIY job. I know those engines for I flew em. The combustion is tangential in the annular chamber. the air is swirling and the fuel nozzles are at an angle so the flame is swirling around inside the can. Makes for a shorter chamber. I guess one could make away with two or three of those tubes..

[Johansson]

That is one nice engine, both a LP and HP centrifugal comp with separate shafts! A bit beyond the DIY horizon though...

[richardm]

Quite beyond indeed but bits and pieces of inspiration from here and there ..Then let our imagination do its trick. We can't imagine something we dont know

[Johansson]

Yup, thousands of hours and millions of tax dollars spent on research. Might as well nip in and "borrow" some good ideas, and then simplify them down to DIY levels.

I altered the drawing for a smaller overall diameter with a vaneless diffuser, this looks proportionally right and I will still have room for a big ass combustor under the lid. Click the image for the full view, in the post it shows up cropped for some reason.



Much easier now without having to plan for the oil entry and drain for journal bearings, all I need to get in there is a fuel line for the fuel spray injectors and wiring for the ignitor.

For a fuel pump a 230v pressure washer pump might work, 150bar pressure and 15L/min capacity will give a nice fuel mist and an AC motor controller will be the throttle.

Cheers!
/Anders

[finiteparts]

I have not gotten my head around the whole diffuser thing yet.

I mean, after channeling the discharge air into perfectly angled ducts that opens up and lets the air expand, it gets dumped around a corner into the combustor area where all hell breaks loose.

I would understand the importance if every individual diffuser duct were connected directly to a flow-through combustor where the air has to be at its highest pressure and swirl free for good mixing with fuel, but in the case with a vaneless diffuser and an annular combustor wouldn´t the sought after expansion occur anyway when it flows around and into the combustor?

Anders,

The dump loss downstream of any bluff body (i.e., the exit of the vane-island diffuser) is a strong function of the flow Mach number. Since the discharge flows out of the compressor impeller are usually in the high-subsonic velocity range, the need to follow it with a diffuser is required. With a well defined diffuser, you can get the diffuser discharge down to Mn < 0.2 or so. This means that even if you just "dump" the flow at the exit of the diffuser as opposed to trying to efficiently expand it further, you will have a very small efficiency loss. It is all about balancing the demand for low engine weight (smaller diameter diffusers with higher losses) verses high diffuser efficiency, which would need a larger area ratio arrangement that is a lot more material and weight.

There is also the issue of how much you can diffuse the flow before it becomes unstable and "stalls". This could cause all kinds of heartache on the combustion liners heat transfer and also individual injection holes can have varying static pressure ratios driving them. Rolls Royce talks about something similar to this that happened on their Spey combustors due to igniter or crossfire tube wakes. The had to add flow straighteners and an aft-wall to force the air into the holes.

There are a lot of trades and that is why the vane island diffusers are often left to just dump prior to the outer turn.

Hope that helps,

Chris