"Fat Boy" 12/118 thrust engine

[racket]

Hi Guys

Finally got the mods finished and she's ready to be reassembled .

Witness marks amongst the black soot and gunk covering the inside of the outer can indicated there was residual swirl in the air coming from the comp diffuser ,my axial straightening vanes aren't close enough together, just a consequence of the compromises made to be able to get it cast.

The swirl angle wasn't too bad at the front of the can but as air was bled off through the flametube wall holes the swirl angle naturally increased ( think velocity triangles) until at the very rear of the can it was starting to get "rotary" , so I've added some baffles ( 6 of ) to hopefully prevent this from happening and perhaps help with the combustion process .



Time to seal her back up

Cheers
John

[jetjeff]

Hi John,

Good, hope to see it running again

Regards

Jeff

[turboron]

John, I don't understand your logic for the baffles. I can see a reduction in skin friction losses due to axial flow otherwise I don't see a benefit. How could the baffles affect combustion in the primary zone?

Thanks, Ron

[racket]

Hi Ron

Swirl at the evaporator inlets will affect their inflow , also due to a change in radius as the air moves inwards its speed increases and its static pressure drops , opposite of comp diffuser.

Better to simply reduce any risk of swirl ............LOL, gotta try everything possible to get her going, they can't do any negatives , but maybe some positives.

Cheers
John

[smithy1]

Interesting idea with the swirl baffles John...I can certainly see the logic.....and I can even hear the gears in your brain rattling around from down here in Sydney.!

My little dragster engine has the same "swirl" marks on the inside of the outer can, they're not drastic though and only do ~60dgrees of rotation from one end to the other.

Cheers,
Smithy.

[racket]

Hi Guys

All back together and ready for testing , it'll be interesting to see what the new static pressure pickup at the diffusing exhaust register





Cheers
John

[jetjeff]

Hi John,

Looks good, I hope the mods work.

Regards

Jeff

[racket]

Thanks Jeff ..........LOL, so do I , otherwise more mods until it does.

Currently doing some research on the best way to use liquid LPG , trying to find jet sizes vs flow vs pressure graphs is causing some grief , I might end up just having to do some basic experimenting using a single injector test rig with the LPG cylinder sitting on a set of bathroom scales :-)

[turboron]

John, have you considered going to a 500 psi fuel system like your original motorcycle gas turbine? I would think it would be less sensitive to swirl etc. than the evaporators? Also, I still think injecting air with protruding tubes between the evaporators may solve your combustion issue.

Thanks, Ron

[racket]

Hi Ron

I've considered high pressure injection but the complication and cost is prohibitive because of the large number of injection points required with an annular flametube , plus the injected fuel needs time to evaporate and that means some flametube length which is in short supply .

The "interesting??" thing is , the flametube is roughly the same as previous ones which have worked OK , its just that this one is wider radially( same length) and it needs to burn 40% more fuel , the actual Combustion Intensity ( BTU's/cubic foot/atmosphere/hour) is less than a previous flametube that works OK in the 9/94 engine in Andy M's drag bike.

Unfortunately, it seems that flametubes don't "scale" with reliability , I'm hopeful that the propane injection to promote swirl in the FT will also be producing a "bed" of fire that the evaporators discharge into , the injectors are positioned so that they discharge at a shallow angle against the front wall of the FT , the gas then spreads out into a sheet which burns nicely and with the capacity of greater gas flow than a jet of gas into just air.

Yep , some changes to the air entry to the primary zone may need to be considered , perhaps the wider distance in this flametube calls for less number of primary holes but larger diameter ones which will discharge deeper and collide to produce more turbulence .

I'm pretty certain that the vibrations I was feeling through the test stand and car trailer it was mounted on is combustion instability considering the "impossible" amount of fuel I was attempting to burn .

This is the first time I've had a combustion problem with any of my engines , usually they just work .

Cheers
John

[turboron]

John, I developed combustion liners for gas turbines the last few years of my career with a major gas turbine manufacturer. It is still a black art although the latest computer programs help. They are very sensitive to small changes. The stub tubes injecting air deep in the primary zone may benefit an annular combustor.

Thanks, Ron

[racket]

Hi Ron

Yep , stub tubes will be included if/when I redo the FT.

I probably needed to have gone with my original design of 27 evaporators of smaller diameter to breakup the flametube cross section into smaller "units" which would then have improved the length width ratio which is currently at a pretty poor 1.2:1 whilst combustion intensity is ~11 million BTUs/cubic foot/atmosphere/hour , 4.5 lbs/min in 0.127 cu ft at 3.5 PR , combustion has to be completed and secondary air added in 3 inches axially , it should be possible , some of the little micro engines do it in half that length ............more R and D required :-)

Cheers
John

[ripp]

Hi John,
Make a rc micro engine CC and increase the diameter. I'm sure it will work better (and without swirl)




www.youtube.com/watch?v=8E8XKVNif6g

This hole arrangement I would prefer



www.youtube.com/watch?v=gNHb2ZvhebE


Ralph

translate.google.at

[racket]

Hi Ralph

If it was only that easy :-(

My combustor already is a copy of a micro engine's with regards hole placement and division of hole areas to the various zones.

The length/width ratio plays a big part in the ability of any flametube to work , to have the same ratio as those micro flametubes, mine would need to be 250 mm long, not its current 100 mm , if it was 250 mm long , it'd work without any problems as the combustion intensity would be more than halved.

Cheers
John

[smithy1]

John,
Just as you mentioned, I also believe "scaling" is indeed an issue for our engines, remember that "tiny" little CC from the Kolibri T32 (JJ-Junior engine)?? It's barely 1-1/2" long and similar width, you had it sitting on your thumb, we looked at each other and said "it shouldn't work"...the numbers probably say it shouldn't work...but it just does, because it also "looks" right..!

I think LPG will help your engine simply because it will dramatically reduce burn time...burning real fuel probably has the flame front too far down the CC, trying to squeeze your flame tube into ~100mm length obviously doesn't help and may be an issue. She's a big engine swallowing a huge amount of air & fuel, we simply have to give it "time" to burn.

Thanks to your help, most of us now have a fair grasp of combustion requirements for our engines and I think you are generally correct when you say..."if it looks right, it probably is". Remember when I had combustion issues with the 6041 after fitting the single high pressure Allison C20B fuel nozzle? My first test start/run was horrible, it ran, very poorly, and wanted much more fuel than it should just to idle, hence the egt's were 200c higher than it should've been.

I then sat and looked at the flame tube on my workbench from all angles for several hours...and I tried to convince myself that it should work, because the numbers you and I had "crunched" said it should...but it didn't "look" right. With my knowledge of how the fuel nozzle injected the fuel into the primary zone and where it was being diluted by the primary holes, it was just all wrong....so with your suggestion of fitting a CC "Muff", I moved the primary & secondary holes around to where I thought they should be and then added some small axial cooling holes to the top/forward face until it "looked" right.... and it just worked...not perfect, but a helluva lot better than it was...! Now after a goodly amount of small changes she runs really well with good temps and the correct amount/type of "noise".

Anders is a genius at doing this sort of R&D stuff, not just with CC's, but all aspects of the engine, hence his success with his bike. You have helped just about all of us in huge ways to get our engines running properly/safely...you should be proud.

I believe we tend to "overthink" things sometimes. KISS is the best principle.

Cheers,
Smithy.