Back again..!... The GT6041 powered "Green Beast"..!

[racket]

Hi Smithy

Yep , severe combustion instability would be heard.

I've done some rough calcs for volume flows and theres not a lot of difference between the GT6041 and the C20 at full power , maybe 20% more from the GT6041 because of the lower PR , but that shouldn't be a big concern , just a bit more pressure drop across the FT wall if you use the entire C20 FT .

The idling volumes could be different though , I've calculated an idling mass flow for the C20 at ~2 lbs/sec given the fuel flow rate and the temp rise across the combustor to produce the interstage temp of ~450 C , but the actual PR at idling is an unknown so volumes are also a mystery .

My gut feeling is that the current flametube isn't the problem , well at least most of it , as the changes you've already made haven't cured the initial problem once the changeover to spray nozzle was made, going from the original large number of small Primary holes servicing the evaporators to a limited number of large holes one would think would have made a bigger difference to the combustion than what appears to have happened .

She's a bit of a mystery, more thinking required :-(

Cheers
John

[smithy1]

Thanks John,
You're great help....and yep...more use of my brain cell is required...as well as yours... ;-)

One thing I did think about, (with my single brain cell..!), was the distance of the 12 x 12mm holes from the injector, we calculated I think ~90-95mm-ish down the outer wall...and using the 80-90 degree fuel spray cone....on the C20 flame tube they are pretty close to that mark....but the old flame tube is considerably larger in diameter and the fuel spray would meet the walls a lot further down...closer to ~130mm....our dilution air jets at this point will be missing the mark by a considerable margin....and this is what I think/feel is happening since I'm getting fuel outside the flame tube which is probably going though the secondary holes..... just feels like not enough air in the right place to me ...your thoughts??

Some numbers off the top of my head for the C20 are as follows, and I'm pretty sure you know most of these anyway: 100% N1 rpm = ~53,000 @260lb-hr fuel flow @84psia (~69psig) compressor discharge pressure, when I do the "preset" on the FCU for the idle (I set it at 2630rpm) ~62-63%N1 ~55-58lb-hr @31 psia (~16psig) CDP....which is just shy of the nominal ground idle setting. The FCU rpm @ 100% is 4200 via the accessory drive gear train.

From these numbers we can see the idle compressor discharge pressure is not too far removed from what we'd see from the 6041 at the same %....but at higher rpms the linearity is skewed much more towards the higher side for the C20B. Of course we know the pressure ratio for the C20B is close to 7:1 where our 6041 is lucky to be 3.5-4:1....Hence the lower max fuel flow requirement for the 6041...Like you, I think we're kinda in the ball-park.

We must also remember that the C20B at idle has a lot of air loss through the compressor bleed valve, so it's idle mass flows may be even closer to the 6041 than we think...the fuel flow requirements at the lower rpm's tend to point that way at least. It's at the top end of the rpm range where we see the C20 make a lot more pressure.

Cheers,
Smithy.

[racket]

Hi Smithy

Theres one thing I haven't asked you , what is the spray pattern type of the C20 nozzle , is it hollow cone , semi solid or solid ?

At the different idling conditions , the C20 will have more "volume" which could produce better turbulence in the FT from a greater pressure drop across the FT wall.

The 12 mm dia holes should produce air jet that protrude deep enough into the FT to go through the fuel spray even if its still some distance from the wall , they'll be travelling at more than 100 ft/sec .

LOL, you might need to idle at the same P2 pressure for the C20 setup to function ;-) .................it is a fairly high idle , theres only roughly a 4:1 ratio between max and min fuel flows , very similar to our engines .

Maybe try "lowering" the primary holes closer to the theoretical fuel spray impingement point , it really is the only thing I can think to try.

I'll continue doing some more head scratching :-)

Cheers
John

[smithy1]

Hi John,

The C20B fuel nozzle is essentially a "hollow cone" in primary mode but transforms to a "semi-solid" when the secondary comes in @ ~140-150 psi...Cone angle is ~70-80 degrees in primary mode, ~85-90 degrees in secondary mode.

I popped the flame tube out today and there's no evidence of a "primary flame front"...it appears to be much lower and below the muff, the new "muff" is basically still "un-coloured".

I may do some more testing on Friday, weather permitting....As I mentioned before...she runs reasonably nicely at higher rpm/p2 so you're probably correct with the higher idle requirement. We'll get there eventually.

I'm still going to make a "full C20B" flame tube for testing purposes...cant hurt to try.

Cheers,
Smithy.

[racket]

Hi Smithy

Still uncoloured , now thats interesting , the radiant glow from nearby combustion should have heated it enough to get some colour.

Perhaps the flame is being too centred in the flametube by the C20 nozzle which usually works with a smaller diameter flametube , the current large "gap" between flame and wall is providing sufficient "blanket" thickness to prevent colouration .

The C20 cap has both a small set of swirl vanes surrounding the fuel nozzle as well as the counter rotational end cap holes , maybe they're producing the right airflow pattern for the smaller diameter flametube, providing sufficient air next to the C20 flametube wall to blanket it from the close by flame , but in the bigger dia FT its "unbalancing" the relationships .

You probably need a 120 degree spray pattern nozzle to "cover" the bigger FT cross section.

Yep , I'd be doing the complete C20 FT to see how it works, the extra "gap" between FT and outer can won't be a problem , it should help if anything .

Cheers
John

[smithy1]

Hi John,

Yeah...she's pretty "colorless" in there.

The C20B cap only has the 54 small ~5mm holes and louvers (CW and CCW) under the holes....there's no "swirl vanes" as such like the C30/40/47 which has the small swirl vanes surrounding the fuel nozzle.

Yep...I'll be plonking a C20B flame tube in there as soon as I finish building one up to suit the 6041. Modifications...modifications... ;-)

Cheers,
Smithy.

[racket]

Hi Smithy

OK, no swirl vanes , they tend to help force air out to the wall , might be one of the contributing factors, the louvers mightn't be enough on their own ,yep , plonk a full can in there.

Modifications ............tell the truth , you love them ;-)

Cheers
John

[finiteparts]

Hey Smithy,

Just checking that you are not saying there is "no swirler"...the C20B has two rows of swirl creating devices...the inner row of louvers direct the air such that it swirls clockwise when viewed from the downstream end...and the outer row creates a swirl in the counter-clockwise sense. The louvers are the swirlers. I think if you get rid of the swirl, you will have a harder time because there is nothing to oppose the fuel injection momentum and you will shoot the fuel deep downstream.

Good luck!

Chris

[faads]

Hi guys,

My name is Luc and as my Budy Racket knows, I actually putting together a thermo generator using a GT6041 turbo. This said, I just learned that the GT6041 runs full oil flow, meaning there are no restrictor at the turbo oil inlet, and there for, requires ALOTS of oil.

Our "Standard" oil lubrication system is capable of 2 GPM @ 50 psig. and is usually capable of suplying both a GT4708R and a GTX2860R combo, but now it has to supply a GT3592R and aGT6041 combo, thus my question are:

• Does the GT6041 really runs full oil flow through?
• If not, what is the recommended restrictor size you guys are using?
• And will my 2 GPM - 50 psig. pump be sufficient?

NOTE : The GT3592 as a .040" restrictor.

Any inputs on how to lubricate that MONSTER will be appreciated.

Thanks,

Luc

[racket]

Hi Luc

Full flow , no restrictors on the GT6041 , those "brass" bushes need plenty of oil compared to the GT35's balls .

As you won't be spinning the GT60 past ~40 % of its rpm limits you probably could use a pressure reducer valve to drop supply pressure to 25-30 psi to reduce the flow , as long as you have that pressure at the turbo oil inlet port she should be OK ................don't use a restrictor to reduce flow .

Your 2 GPM might just be big enough , the main thing is keeping the oil pressure measured at the turbo supply port, at the required pressure , the flows will take care of themselves.

How is the new combo coming together ??

Cheers
John

[smithy1]

Aug 4, 2016 13:18:40 GMT -5 finiteparts said:

Hey Smithy,

Just checking that you are not saying there is "no swirler"...the C20B has two rows of swirl creating devices...the inner row of louvers direct the air such that it swirls clockwise when viewed from the downstream end...and the outer row creates a swirl in the counter-clockwise sense. The louvers are the swirlers. I think if you get rid of the swirl, you will have a harder time because there is nothing to oppose the fuel injection momentum and you will shoot the fuel deep downstream.

Good luck!

Chris

Hi Chris,
Of course you're quite correct....I think John and I were thinking more along the lines of the small C30 type swirler cap at the fuel nozzle, the C20 and C30 flame tubes are quite different in construction...which is strange really considering both the engine's similarities.

Cheers,
Smithy.

[smithy1]

Aug 3, 2016 22:30:59 GMT -5 racket said:

Hi Smithy

OK, no swirl vanes , they tend to help force air out to the wall , might be one of the contributing factors, the louvers mightn't be enough on their own ,yep , plonk a full can in there.

Modifications ............tell the truth , you love them ;-)

Cheers
John

LOL... ;-) Yeah..I do like to fiddle...!

A complete C20B/B17 flame tube is in the works.... I might need some help regarding the making of the lower cone section...I've done some research regarding the rolling of the cone etc....but I think my numbers don't make sense...I made a paper template around the original but would like a little confirmation.

I need to make a cone to fit the underside of the C20 flame tube to match the old setup, the C20 flame tube has a short "skirt section" where it fits over the #1 nozzle shield in the C20 engine, I wish to utilize this skirt for the lower cone section for the 6041... The cone section has an upper diameter of 159mm where it  has a sliding fit into the bottom skirt of the C20 unit, and a height of 205mm, no problem there, but the bottom of the cone is an 88mm x 60mm rectangle where it fits/feeds into the turbine housing...this gives a basic diameter of ~148mm. I did a quick and dirty drawing and the cone section appears to be very shallow compared to my "paper clone". I'll be adding a ~5 or 10mm overlap around the cone for welding purposes, or should I just butt weld??. Just want to confirm with you before I start cutting metal. Maybe I'm overthinking things again. I'll be using some nice 0.020" 304 stainless sheet, I have a 80cm x 60cm piece to play with.

Cheers,
Smithy.

[smithy1]

Hi John,

During last week when I was scratching my head and trying to utilize my brain cell ;-)... I sat in my little shed and looked at the flame tube I have in the "Beast"....the 12 x 12mm holes I had drilled into the new muff just didn't sit right with me, I know the math makes sense and the combined areas were all correct but my spidey senses said it just didn't "look" right.

I had it in my head that the air was forced into the primary combustion zone a bit too high and was missing the fuel spray cone by a goodly margin...so just as an experiment I plugged "every other" of these 12mm holes effectively halving the area in this zone and then drilled 12 x 6mm holes ~40mm lower down to see if it would hit the fuel spray a bit better....I know this doesn't add up correctly area wise but...it just looked right to me....anywho..

I had a small show to do with a few friends at Colo High School for their "Science Week" on Friday, so I used this as an opportunity to do a couple of "test sessions". I did a few starts during the day....To my amazement the engine now starts really easily and runs up to the normal low 22-23k idle as it used to....as you may remember while we were in Port Mac, the engine was fluttering quite badly at idle and flaming out randomly, indicating a poor combustion stability. This is now all-but gone and the idle is smoother than it's been in a long time...not as it was with the old original vaporizer system, but pretty good nonetheless, acceleration is also excellent and I ran her up to full noise at ~40-41psi P2....I still have the random flame-outs on deceleration but she soon re-ignites as the rpm/P2 drops...figuring out how to deal with these flame-outs is my next major challenge.

I'll still be utilizing/fitting the C20 flame tube but will have the old unit as a back-up...and since they're easily interchangeable, there won't be any "fitting" issues, which is a good thing.

Cheers,
Smithy.

[racket]

Hi Smithy

If I remember correctly the cone was made a tad longer than necessary so that it poked into the slipjoint at the bottom of the combustor , I then fiddled with the end, slowly shortening and beating it into the rectangular shape , it wasn't the easiest of job for me to do , I got the cone rolled at the sheet metalworks as it was too strong for me to do .

As for getting the cone template made ..............LOL, I can't remember how its done , ..............this might help though craig-russell.co.uk/demos/cone_calculator/

I'd make the cone a tad longer at both ends to start with , the "big" end can be split down with a thin side grinder cutoff blade and the "ears" folded in a tad for the C20 FT to slip over/into .

Cheers
John

[racket]

Hi Smithy

Thats excellent news about the FT mods you made :-)

I'll always go along with the notion that if it looks right, it often is, and if the hairs on ones butt are uncomfortable then things need changing , theres usually several remakes of the hole layout with my micro engines flametube walls , I draw them up to scale on cardboard ...........then start changing things until those butt hairs tell me to stop .

I really have no idea how the C20's counter rotational cap mods influence the airflow further down the FT , theres not much info around on that method of producing combustion turbulence , the swirl vane around the fuel nozzle is the more common way of doing things ............its all a learning process for me .

I hope the students were suitably impressed by your "demonstrations" , my youngest son went through high school there ............he would have loved it if it had happened whilst he was still there, .........LOL , it might have sent him into engineering instead of med science , then he could be helping me now ;-)

Cheers
John