"Fat Boy" 12/118 thrust engine

[jetjeff]

Hi John,

What is the burner can made from?

Regards

Jeff

[racket]

Hi Jeff

End walls are 2 mm 304 stainless , sidewalls 1.2 mm 304

Cheers
John

[CH3NO2]

Hi John,

What is the expectation of not using primary holes on the inner wall?

Also, the picture shows a lap joint being used on the inner wall rather than a butt joint. Is there a purpose to the lap joint in this application?

Thanks,
Tony

[racket]

Hi Tony

LOL.......there won't be any Primary holes on the outer wall either , all the Primary air will come through either the "tubes" or the front wall "thimbles" , I hoping that the combination of a spiral flow from the tubes combined with the "radial" airflow from the thimbles will have combustion happening closer to the front wall ..............full sized aero engines using vapourisers( evap tubes) use a similar setup .

The lap joint makes it easier to produce a nice tight fit between the side wall and the endwall , its an "adjustable" sliding joint initially that can then be tacked together once the wall to wall joint is welded , whereas if I do a butt weld first, the overall circumfrential length of the wall has to be spot on otherwise there'll be gaps at the weld point.

I initially cut a cardboard template and fitted it to the position and marked of the length with a 10 mm overlap , then drilled and bent the stainless wall to shape around a section of 3" pipe and trimmed the length to what the template said.

A couple of wooden rings were turned up that fitted inside the wall and a couple of 6" hose clamps tightened around the wall to bring it into a nice circular shape for fitting to the wall/s, I actually had to heat the front wall a tad to expand its bore so that the inner wall could be fitted , on cooling it produced a nice almost leakproof joint thats easy to weld, the rear inner wall fit was a firm push in job .

The overlap ended up a couple of millimeters larger than the cardboard template had , different rates of "compression/stretching" between cardboard and stainless made for the "error" .

Cheers
John

[racket]

Hi Guys

Tubes realigned, hopefully less bias towards the inner wall now ....................outer construction wall tomorrow :-)





Cheers
John

[madpatty]

Mar 20, 2019 18:46:09 GMT -5 racket said:

Hi Guys

Tubes realigned, hopefully less bias towards the inner wall now ....................outer construction wall tomorrow :-)





Cheers
John

Hi Racket,

How much ~percentage air are you diverting through tubes now?

My concern is jetspecs already gives quite fuel lean mixture through the evaps and any more air can cause approx. stoichiometric mixture within evaps thus causing fuel burning inside evaps and potential overheating.

[racket]

Hi Patty

Theres a theoretical amount based on the bore size of the tubes , but then we need to make allowances for the spiral "device" inside , the amount of fuel being evaporated , the closesness of the front wall to the tube outlet as well as any reduction in flow into the tube caused by the proximity of the fuel manifold ..........................LOL, I haven't the faintest idea what is actually getting through :-(

When you say Jetspecs gives a lean mixture , how are you deriving that ??

Cheers
John

[madpatty]

Hi Racket,

I am deriving it based on the assumption that actual mass flow from compressor distributes in separate zones(primary, secondary and tertiary) in the way jetspecs wants it to (which is 30-20-50).

Now I read your comments somewhere, where you calculated approx. fuel flow per lbs of air flow for our kind of engines.

Now according to that fuel flow and as per what jetspecs recommendation is (10% of primary area) the equivalence ratio inside evaporator tubes was about ~2.5 (which is below theoretical rich blowout limit of 3).
So you give it more air and it becomes even worse.

Now I did some experimentation with various evap designs where i was diverting much of the primary air through evaps and my evaps were burning. Though evaps were a different design than yours but this was surely a contributing factor.

Yours are straight evaps so any chances of mixture burning inside evaps is quite less because flow velocities will be high. Mine were somewhat different design were flow hit a wall and turned 180 degrees inside evaps. But any combination of flow slowing inside evaps along with stoichiometric mixture can cause burning inside evaps.

Cheers.

[CH3NO2]

The thimble concept is interesting. It will be good to see it put into action. Has me wondering what the reaction will be when the vaporizer outlet plume impinges on a "relatively" cold thimble.

Other than the flow through the vaporizers, will the remainder of the total primary air be flowing radially through the thimbles?

[racket]

Hi Patty

Tubes get ~10% of TOTAL hole area/flow, so ~ 33% of Primary air , so depending on evapouration rates etc there could be combustion if theres a localised "quiet spot" where speeds are down ..........lotsa unknowns .

Hi Tony

The front wall of our flametubes runs relatively cold , so there won't be a lotta difference with the thimble , the thimbles will pass the remainder of the Primary air , probably through 8 holes in each thimble side wall so as to get a good spread of air to mix with the tube discharge

Cheers
John

[CH3NO2]

Very cool. Your description sounds a lot like a "Pintle Injector" but in counter flow.

They are very effective at mixing.

[racket]

Hi Tony

That'd also make a nice afterburner nozzle :-)

Cheers
John

[racket]

Hi Guys

Back into the flametube again after a bit of a layup with some health issues.

The new outer wall has been drilled and bent/curved to shape and fitted to the rest of the flametube and NGV, all mounted up on the jig







Bit of a nightmare trying to get things into alignment as all of the "old bits" are no longer as "true" as they once were, having been subjected to quite a bit of abuse by a calous operator .

The front wall is "wobbly" whilst the rear with the tubes has a more of a dished appearance, that the securing screws find hard to straighten out .

I think I'll just have to weld/secure the outer wall where it lies and hope for the best , theres more volume within the flametube now as the inner and outer walls are ~7 mm further apart , so a tad more residence time for the combustion to take place .

Cheers
John

[Johansson]

Hi John,

The burner can is looking great, you´ve been spot on with the designs for my JU engines so I would be greatly surprised if there is something wrong with this one. The increased volume might be what was needed for it to work.

The JU-02 has been running a bit hot at times too, especially during the first startups when I was´nt quick enough to remove the leaf blower. Still there is no signs of heat damage to the internal bits at all, so I am a bit surprised to hear about the warpage in the Fat Boy. Hopefully the combustor modification will cure this.

Cheers!
/Anders

[racket]

Hi Anders

I think most of the damage was done when I had the blocked injectors early in development , once I fitted the high pressure fuel filter ( thanks Smithy ) the rubbish coming from the fuel pump internals stopped and those hot streaks stopped stressing the flametube bits , the "hot spots" started the warpages I feel ........................LOL, its only a tin can with lotsa holes in it , so I'm probably getting overly concerned .

Keep up the good with the JU-02 , I wanna hear it in anger again :-)

Cheers
John