"Fat Boy" 12/118 thrust engine

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Hi Anders

LOL.............I just wish I had the brains/experience that these guys on the GTBA have , I'm only a backyard tinkerer .................heh heh , is it only a thousand emails ;-)

Cheers
John

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Hi Guys

Started getting inside the engine today , most things look OK , just the usual "oil burner" look with lotsa smoky residue from failed starts and oily shutdowns where some lube leaks out past the turb seal .





































Cheers
John

[racket]

Hi Guys

Rotor still looks usable , just a bit grubby on the comp blades from ingesting smoky residue , turb blades haven't been "cooked" , shaft hasn't "coloured' so lube doing its job of cooling it , the bleed air onto the back of the turb wheel and hub has contributed to things being relatively cool on the NGV mount flange as well .



Cheers
John

[turboron]

John, I believe the condition of the hardware points to your problem being combustion or aero related. Aero difficult to change at this point. I repeat my suggestion of jet tubes inserting air flow deeper into the liner to improve combustion mixing in the primary zone.

Thanks, Ron

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Hi Ron

I'm open to suggestions on the type and number of injection tubes .

My thoughts were to have 9 of then , one for every second evap tube , with their outlet discharging through the evap outlet to break it up as well as promoting swirl withing the flametube primary zone .................but I'd like your thoughts

Cheers
John

[turboron]

John, I don't have any experience with evaporators. However, I agree with the 9 jets. I would put them between evaporators to maximize swirl and make the flow more homogeneous. Most combustion liners are full of hot spots in line with the fuel nozzles. Even cans with one fuel nozzle have high temperature cells. The idea is to reduce unburnt fuel and even out the temperature distribution into the turbine.

Thanks, Ron

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Hi Ron

9 it is :-)

I have a small Jet Joe flametube that has 6 such jet tubes ( 12 evaps) , I'll shamelessly copy how they set theirs up .

Cheers
John

[smithy1]

May 9, 2017 23:00:00 GMT -5 racket said:

Hi Guys
Rotor still looks usable , just a bit grubby on the comp blades from ingesting smoky residue , turb blades haven't been "cooked" , shaft hasn't "coloured' so lube doing its job of cooling it , the bleed air onto the back of the turb wheel and hub has contributed to things being relatively cool on the NGV mount flange as well .



Cheers
John

Hi John,

Boring..! ;-) But boring is good in this case...no damage is a good thing..!

Going back to my "sit and look at it" phase.....that turbine wheel of yours just doesn't look right to me...I could be wrong but it looks to be "clipped" a bit too much. The 6041 turb wheel has much more "blade" on it to catch the impulse to produce enough power to turn the comp...but what do I know..

Cheers,
Smithy.

[finiteparts]

Hi John,

Are there distinct lines in the NGV passages at the throat? ... perhaps indicating that you had a standing shock there?

That is one short combustor...I took a stab at calculating the volume and running it through the Kinetx program that I have and it shows that you are well within the stable combustion region. For the 1.7 lbm/s I have you getting a combustor efficiency of around 98%, with over 70% margin to either blow out boundaries. I have a ton of assumptions in it though, so if you would like me to rerun it, can you give me some cycle conditions?

I assumed that your total combustor volume was around 265 in^3. The primary zone I estimated to be 7.5cm (2.95 in) and the total combustor length is 11.5cm (4.527 in). I estimated that you had around 30% of the air going into the PZ, with the rest through the dilution.

Then for the cycle conditions, I need: combustor inlet pressure and temperature, fuel flow and air flow.

The only issue with this program is that I cannot change the amount of mixing in the reactor zone. There is a defined level to meet the "well" stirred reactor condition. We can play around with adding in some recirculation elements to try to mimic the engine flow a bit better, but ultimately it will only be a sanity check...something to say that the combustor is not too small...and right now, it looks like you should have enough combustor volume. In reality, you might not have enough stirring in the combustor to fully mix the fuel air, but with vaporizer systems, this is not as big of a concern as a pressure atomizer style combustor.

I think the heat staining on the liner indicates that you are getting the reaction to kick off right up front. Remember, on vaporizer combustors, you want to keep the vaporizer discharge flows very rich to inhibit ignition in the tubes. You then want to bring in the air to start the burnout downstream of the vaporizer discharge plane. Do you know what fuel-air ratio you were targeting in the vaporizer tubes? I would think that you would set the number of vaporizer tubes based on the achievable FAR at the tube outlet.

The coke formation on the dome supports the fact that you are getting a decent majority of the fuel to vaporize...it could be a bit less, but it is not so heavy that it would suggest that you are struggling to vaporize the liquid. I also like that you don't seem to have any vaporizer tube overheating issues...no nibbled edges.

Ron, what are you referring to that suggests aero/combustion related problems?

I wonder what that white coating is?

It is also interesting to see the residual swirl on the liner internal flows...it might hurt the jet penetration a touch, but I bet you are getting plenty of jet penetration with that small of a liner.

I am sticking with my earlier theory that you just don't have enough turning in the turbine and thus have lost torque matching capability in the system. I don't think you are having combustion issues.

Good luck!

Chris

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Hi Smithy

LOL...........it does look a tad "overwhelmed" by the comp wheel .

I've been doing some numbers today comparing my TV84 engines radical clipping to this current one in relation to the comp dimensions/flows etc , I might be pushing things a bit too far .

The Precision Turbos PT118 has a similar comp inducer but its exducer doesn't have the extended tip that the X831 wheel does , I'm seriously considering removing it , this will reduce the "torque" required , effectively increasing the turbines power a tad as well as its rpm for a given pressure ratio of the comp , it also only runs a 129 mm turb wheel .

The GBTA guys have indicated I also need a larger exducer diameter , I might just make up a new larger diameter turb exducer shroud/jetpipe and run a very generous 5mm of exducer radial clearance to see what happens , the extra flow might just get the NGV to work harder with higher gas velocities and power production .

I'll spend a week or so thinking on things before committing to a course of action .

Cheers
John

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Hi Chris

Flametube is 10.25" ID of OD , and ~5.3 " OD of ID of ring for a cross sectional area of ~60 sq ins or ~3.5 inducer areas , radial width of ring ~2.5" , hopefully big enough for the 3.7 lbs/sec at full power.

Inner wall is ~2.5" long , outer 4.4 " , the main combustion volume of 60 sq ins by 2.5 long = 150 cu ins , then theres a bit across to the NGV .

The F/A ratio in the vapourisers is roughly what Lefebvre recommends,........... after a cleanup, the first ~1 inch is still "silvery" but the remaining 2.75 inches are showing plenty of heat getting into the tubes

Yep , after cleaning up the flametube and removing that light grey soot ,which I think was a result of the engine running the fuel tank dry , the heat marks on the flametube walls certainly indicates combustion starting pretty quickly .

The addition of some swirl producing "tubes" that also punch air through the vapouriser outflow won't go astray , but I do feel you're correct about the power producing limitations of the turbine wheel could be a big issue , hopefully I can "reverse" the order of choking from turb exducer to NGV throat to produce more power , currently I feel the exducer chokes and uses up most of the pressure drop leaving the NGV struggling to do its bit with limited pressure drop , I need the NGV to be running "full" and "fast" to maximise power at the "in" side of the turb wheel ran than the "out" side .

More thinking required :-)

Cheers
John

[turboron]

Chris, my suggestion is not based on any information John has provided. I have seen reports that quote significant improvements in mixing by the use of jets on fully developed combustion liners. Improved mixing can not hurt and may help overall performance.

Thanks, Ron

[finiteparts]

Ron,

I was referring to this statement

"John, I believe the condition of the hardware points to your problem being combustion or aero related. Aero difficult to change at this point."

I wasn't sure if you saw something in the hardware that I hadn't noticed.

John,

I ran your combustor with the estimate that your were putting 24% of the airflow through the PZ. I estimated the vaporizers to be at an equivalence ratio of 1.8, which sounds pretty good. I would think anything under 2 would not be too rich and thus would not expect too much coke build-up. With 24% airflow to the PZ, you should be getting around 96% combustion efficiency, which leaves only a small percent to burn out in the dilution zone. Your combustor loading is pretty high, but not near any blowout conditions. That being said, the space rate heat release is pretty high, 11 million BTU/(hr*ft^3*hr*atm). That is high...I have seen some Continental J-69s or such that have maybe 14 or 15 million BTU...but most engines are no where near that space-heat rate.

If you add some jets, try to do it in a manner that would help to create a stabilizing vortex. You really want to recirculated some of the burning products to help heat up and initiate the reaction front near the vaporizer discharge plane. I am not sure blowing across the vaporizer discharge is a good idea. The fact that Rolls Royce talks about not doing that in their public literature seems like a good reason to not do that.  A common method that they used were the little "hats" that they used to bring a jet in axially between the tubes and energize the local vortex.

Good luck!

Chris

[turboron]

John, Chris's comments does a much better job on setting forth exactly what I was suggesting by placing the jets between evaporators.

Thanks, Ron

[racket]

Hi Chris

Yep , I had combustion intensity at 11.16 , in 220 cu ins - 0.127 cu ft of flametube .

A/F ratio in the tubes was "calculated ??" at 2.6 :1

The problem I have with extra air injection in the primary zone is the flametube length, this sorta precludes any sort of axial injection , the length/width ( L/W) ratio is only ~1.2 :1 ( 3" long /2.5" wide) up to the point where the cross section narrows going into the NGV , this 1.2 :1 ratio is very small , most aero engine would have this several times bigger , even the small micro engine flametubes are >2:1 ..................and is the reasoning behind having some sort of "tangential" injection to promote swirl .

I've done a bit of rough design for the "jet tubes" positioned through existing holes using the evap outlet carbon ring witness marks on the front wall , probably use 8mm OD S/S tubing ,~ 6 mm ID with them being ~30 mm long , the discharge will hit the edges of the next two evap outlet plumes at the front wall , hopefully adding a bit of "spin" to each as well as an overall swirl in the flametube ............LOL, lets hear it for "black art " , looks good on paper ;-)

Cheers
John