"Fat Boy" 12/118 thrust engine

[Deleted]

Hi John

All Looking good..... :-)

[racket]

Hi Guys

I finally got the new inner and outer walls welded up in place on the front wall and new attachments made between rear wall and the rest of the flametube to hold everything in their alloted positions so that the 18mm dia holes in the front wall for the "thimbles" could be marked out opposite the outflow of each evap tube .

The thimble construction has been troublesome but finally settled on a short section of 19mm X 1.6 WT stainless tube to which was welded a 1.2 mm thick sheet and 8 X 4.3mm holes drilled in the tube wall for the airflow , the tube and endcap being turned down to 18 mm to fit through the end wall holes leaving a short step out to 19 mm at the end to prevent them falling inside during welding .

A few pics

Thimble


A view of "inside"



and "outside



Now all I've got to do is make another 17 thimbles

Cheers
John

[Deleted]

They should do the job nicely..... am sure you will soon have them knocked out.... going to be interesting to see how the effect combustion

[racket]

Hi Andy

LOL.......I hope all the extra work will be worth it :-)

Cheers
John

[racket]

Hi Andy

Well I was able to knocked out 6 thimbles today until my back cried out "enough" , ~45 minutes for each one , so a couple more days work to go , but they're looking OK , might have to source some more tubing as I need to waste a fair bit in the process.

Cheers
John

[CH3NO2]

Very cool design John. Out of the box thinking.
The design should put plenty of turbulence right at a crucial intersection point.

[racket]

Yep , hopefully a bit faster combustion.............finally got the last thimble made yesterday, time to weld them in place , which should be an easy job , they're a neat fit into their 18 mm dia holes so just slip into place against the step on the thimble and TIG , I'll have a think about orientation of the air holes , but possible a couple of the holes on a "radial" so that all thimbles are the same.

Then I need to do some testing of the flow out of the evap tube to see how much distance is required between tube and thimble to provide adequate flows , the thimbles will project 6 mm inside the flametube limiting the original flow distance between tube and front wall and possibly changing things that will require some shortening of the tubes...............one step at a time :-)

Cheers
John

[racket]

Hi Guys

A coupla pics





I ended up shortening the evap tubes by 6-7 mm to maintain the original flow spacing .

Now onto some more interesting jobs , making those 18 thimbles was both boring and frustrating at times as the welding on of the sheeting end wasn't my best workmanship , but it'll do .

Cheers
John

[turboron]

John, what was your logic on using axial inlet thimbles versus radial jets to improve mixing. My experience with radial jets protruding 2 or 3 diameters into the primary zone has been improved mixing. I believe Rolls-Royce tried these on early day engines with some success as well.

Thanks, Ron

[racket]

Hi Ron

I have an extremely short combustor , only ~65 mm long before it contracts to go over the lip into the NGV , the Length/Width Ratio is very poor at roughly 1:1, so I'm attempting to get fuel/air mixing done as soon as possible , hopefully very close to the front wall , which might also increase the wall temp to help in evaporating any last bits of fuel .

The spiral square spring in the evap tubes produces a nice swirl in the outflow and combined with air jets from the thimbles colliding with its neighbours should produce plenty of turbulence .

I looked at the RR setups which have the thimbles "between" evap outlets and decided to "improve??" on it by having the efflux from the tube cover the outflow from the thimble.

Ideally I'd like the Length/Width Ratio to be at least 2:1 but I'm limited by the short turbo shaft , the cross sectional area of the flametube needs to be ~3 inducer areas or ~ 50 sq ins , which means I end up with a fairly wide annulus which really needs larger holes to get sufficient penetration/collision , but large holes mean less holes and more "bulky" turbulence rather than the "finer" turbulence from lots smaller holes which are better for "shorter" combustion axial distances .................so I decided to move away from side wall holes and have the finer turbulence created by lotsa small holes in the thimbles , theres 144 thimble holes with each one interacting with the rich mixture exiting the 18 evap tube , I need that many tubes to get sufficient heating surface from the relatively short tubes , ................lotsa compromises trying to be reconciled .

I ended up not positioning the thimble holes on the radial or circumferential alignment , but mid way between holes on those alignments , it just looked better :-)

Cheers
John

[CH3NO2]

Apr 20, 2019 17:55:30 GMT -5 racket said:

Hi Ron

.....

I ended up not positioning the thimble holes on the radial or circumferential alignment , but mid way between holes on those alignments , it just looked better :-)

Cheers
John

Hi John,

Could you clarify the above part a bit more?

I like the rational for getting all the primary fuel and air mixed rapidly, at the top, in as short a distance as possible.

Tony

[racket]

Hi Tony

Theres 8 holes in the thimble which would allow an opposing pair to be aligned "radially" across the flametube annulus as well as having another opposing pair discharging directly towards its neighbouring thimbles .

When I positioned the thimbles like that I felt the best use of the 8 air jets wasn't being realised as only 1 jet either side of the thimble would be colliding with its neigbours jet/s, but by rotating the thimble 22.5 degrees it allowed a couple of jets either side to interact with its neighbours couple of jets ....................maybe creating more turbulence , don't know , but the "hairs on my butt" felt it looked better .

Cheers
John

[racket]

Hi Guys

I finally got the engine reassembled, only to find the comp inducer and turbine wheel exducer weren't sitting very central in their shroud/s ..................bummer :-(.................what went wrong , did I stuff up the assembly , gunna have to pull the thing apart again .....................which I did , only to find everything was put together as it should ..............so now I have to chuck up bits in the lathe and check for trueness , maybe something warped during the last run , even the outer can front edge didn't sit true to the front comp cover , so maybe look at the NGV first to see if the exhaust stub and NGV mounting face are true.

LOL....R and D is such fun .

Cheers
John

[Deleted]

Am sure you will find the issue.... but dam.... this engine making you work for the results

It's raining here this morning woke thinking.... just do it, lite it

Morrow

Chat soon

All the best
Andy

[Johansson]

Hi John,

Tell me about it, the darn things must have a life of their own. The JU-02 clearances have changed noticeably over the test run period and I´ve had to modify it a couple of times to keep it running true.

Cheers!
/Anders