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

[smithy1]

Aug 7, 2016 23:31:03 GMT -5 racket said:

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

Hi John,
There's something in the back of my mind telling me to "really avoid" your butt hairs..  

With the C20B cap, there are indeed two rows of CW and CCW louvers....but on the very outer edge of the cap there is also a set of longitudinal air holes running lengthwise down the outside of the tube, and again another set of the same type of holes between the primary and secondary holes... where the lower portion of the flame tube is attached. I believe these are to help keep the tube walls cool via a boundary layer.

See a cut-away of one here:

www.google.com.au/imgres?imgurl=https%3A%2F%2Fjetstuff.files.wordpress.com%2F2013%2F12%2Fcombustion_chamber.jpg&imgrefurl=https%3A%2F%2Fjetstuff.wordpress.com%2Ftag%2Fcombustion-chamber%2F&docid=f6EyeOT3rqMGjM&tbnid=m5dAIun-2WnqIM%3A&w=1008&h=756&bih=993&biw=1920&ved=0ahUKEwiOj4Txv7HOAhXGHJQKHbBOBDIQMwgfKAIwAg&iact=mrc&uact=8


Here's a better shot of the boundary layer cooling vents:

www.google.com.au/search?q=Rolls-Royce+250+C20B+combustion+chamber&source=lnms&tbm=isch&sa=X&ved=0ahUKEwiorPfrv7HOAhWFupQKHRAADAcQ_AUICCgB&biw=1920&bih=993#tbm=isch&tbs=rimg%3ACXI2fJ7O7YkJIjgyYIJgRuJ1yEwpBBPSUlAbcD0xHcaZmNv874xF2RTT3rZDed_1Ia2xVA7oapTcO5j-22kD3wmzZyCoSCTJggmBG4nXIEVWhw7jyLyRXKhIJTCkEE9JSUBsRq6ERyO47NykqEglwPTEdxpmY2xEMBjbVyQav2ioSCfzvjEXZFNPeEY79WX3us9hYKhIJtkN538hrbFURh_14KPviaUFUqEgkDuhqlNw7mPxFxkIV4q2oYryoSCbbaQPfCbNnIEWJleXwPpj20&q=Rolls-Royce%20250%20C20B%20combustion%20chamber&imgrc=_onyW_-2M-nBjM%3A

Sorry for the long link addresses.

Cheers,
Smithy.




 

[racket]

Hi Smithy

It still amazes me that they get away with so few main dilution holes in such a short flametube .

Yep , the wall cooling "blanket" slots are like I used in my TV84 flametube

LOL, my butt hairs are to be avoided at all cost

Cheers
John

[finiteparts]

Hi Smithy,

"...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."

The original 250 combustors were not super efficient as far as combustors go and they had some relative high emissions. With a 28% increase in the airflow from the C20B to the C28, there was even more burden to make the engines meet emission standards pushed out by the EPA in the early 1970s. The C30 combustor is a growth version of the C28 combustor, with a increase of 62% airflow over the C20B, it carries on the pre-chamber arrangement of the C28, which allowed them to reduce emissions over the typical mission to 50% of the baseline C20 style can....this included unburned hydrocarbons, oxides of nitrogen and carbon monoxide emissions.

If I recall, they also improve the altitude relight capability of the engine, which means the low flow, low pressure capability of the combustor was improved.

I have those papers around here somewhere. I will dig them up and see if there is a link to them online. As I recall, they were really detailed and quite interesting.

Chris

[smithy1]

Hi Chris,

Yeah...the RR reps always tell us of the "poor" C20B emissions and the need to make sure the CC is in good condition during our regular maintenance inspections...they frequently change/tighten the service limits to try and accommodate this....We have an "in-house service bulletin/technical variance" which gives details on "adjusting" the upper CC louver angles to help with combustion efficiency...I don't know whether it actually has a positive effect or not...they do light-off a tad better though.

Cheers,
Smithy.

[racket]

Hi Smithy

Thats interesting , it could account for our trouble getting the combustion to behave once in the larger FT body, it sounds like the C20 design requires everything to be just right .

Cheers
John

[smithy1]

Aug 10, 2016 19:52:05 GMT -5 racket said:

Hi Smithy

Thats interesting , it could account for our trouble getting the combustion to behave once in the larger FT body, it sounds like the C20 design requires everything to be just right .

Cheers
John

I suspect you're right John, the C20B engine seems to suffer when the compressor is worn and/or is nearing the end of it's overhaul life, some of them suffer "hung starts" due to the lower PC pressure at a nominal rpm compared to a "fresh" compressor. The FCU needs to see a goodly amount of PC pressure to it's acceleration bellows to allow acceleration to progress at a decent rate.....if the PC pressure is down just slightly, she'll hang and/or be very slow to accelerate over the hump to ground idle. Disabling the generator once off the starter helps in this situation too.

When I get a customer who has this issue and they're stuck out in the middle of nowhere or at altitude, I get them to force the bleed valve to the nearly closed position during start to help get them out of trouble, once started they are usually good to go and the bleed valve can then be reset back to normal.

I'm hoping the C20B FT will be ok with the mass flow of the 6041 at the lower end of the rpm range....I may have to "re-adjust" the fuel flows vs fuel pressure via the fuel nozzle.... but it should be reasonably close. TBH, there's not a helluva lot between them at the bottom end....especially since the C20B has the compressor bleed valve "bleeding off" until ~94-95%..!

I'm sure we'll find out eventually.

Cheers,
Smithy.

[smithy1]

Hi John,
I had a chance on Wednesday to run the 6041 engine with a "Full C20B" combustion liner, it runs, poorly, but the EGT was much higher than before, approx 190c-200c higher at idle!! I was seeing 490c-500c previously, but is now close to 700c at idle, not what I was expecting at all.

Seems to me that there's not enough air getting into where it needs to be, I'm guessing the pressure drop across the C20 liner is not as it should be. I'll do some number crunching and see what I come up with. The pressure ratio of the C20 compressor is near 7:1 and the 6041 is only ~3.8:1-ish....need to figure out what needs changing etc...

I still have the old flame tube so I can plonk it back in to get me going....I've made them "interchangeable" which helps.

Cheers,
Smithy.

[racket]

Hi Smithy

My rough calcs give the 6041 flowing ~15.4 cubic feet per second ( CFS) to the flametube , the C20 flowing ~13.4 CFS , or ~15% difference , this shouldn't be enough to make such a huge difference in those temps , IF , the C20 flametube hole area is big enough in the Tertiary Zone , I sorta suspect it mightn't be anywhere near what the 6041 flametube has .

I'll be interested to hear what you find :-)

Cheers
John

[smithy1]

Sept 9, 2016 16:31:13 GMT -5 racket said:

Hi Smithy

My rough calcs give the 6041 flowing ~15.4 cubic feet per second ( CFS) to the flametube , the C20 flowing ~13.4 CFS , or ~15% difference , this shouldn't be enough to make such a huge difference in those temps , IF , the C20 flametube hole area is big enough in the Tertiary Zone , I sorta suspect it mightn't be anywhere near what the 6041 flametube has .

I'll be interested to hear what you find :-)

Cheers
John

Yeah...that makes sense.....I suspect it's close but not close enough. I might have a bit of a think and another look at the C20 Flame tube to see if there's something I can do to it to make it work.


Cheers,
Smithy.
 

[racket]

Hi Smithy

Have you measured the various hole areas in the C20 flametube ??

Cheers
John

[smithy1]

Sept 10, 2016 22:58:06 GMT -5 racket said:

Hi Smithy

Have you measured the various hole areas in the C20 flametube ??

Cheers
John

I will tomorrow John, I have a couple of them at work for easy access. I'll give you the numbers then. 

Cheers,
Smithy.

[smithy1]

Hi John,

6041 inducer @ 106mm = ~8824 sq-mm.

Here's some C20B flame tube numbers for you:

Top cap has 54 x 5mm holes for an area of: 1060.3 sq-mm

There are 22 x (3x6mm) wall cooling slots on the periphery of the top cap with an area of: 396 sq-mm

Below that there are (6 x 14mm holes and 6 x 12mm) primary holes with a total area of: 3512 sq-mm

Then another row of 22 3x6mm wall cooling slots: 396 sq-mm

Then on the bottom section there are 14 x 9mm tertiary holes for an area of: 572.6 sq-mm

And finally 2 x 31.5mm dilution holes with an area of: 1558.2 sq-mm

Total area = ~7492 sq-mm.....As you mentioned previously....it's ~15% less than the 8824 sq-mm of the 6041.

I think we might need to open her up a tad...Your thoughts..??

Cheers,
Smithy.

[racket]

Hi Smithy

Gotta query the area of those 14 mm and 12 mm dia primary holes , I only get 924 sq mm for the 6 X 14 mm and 679 sq mms for the 6 X 12 mm holes for a total of 1603 sq mms , not your 3512 sq mms .

I ended up with the primary holes at 35% of the 8824 sq mm 6041 inducer area , secondary zone 15% , so ~50% of total area which is spot on for the "combustion" section .

When we get to the Tertiary cooling holes , those couple of 31.5 mm holes produce ~1558 sq mms which is only ~18% of the inducer area , theres another 32% missing ..........LOL, might account for the higher temps .

Cheers
John

[smithy1]

Sept 11, 2016 16:47:21 GMT -5 racket said:

Hi Smithy

Gotta query the area of those 14 mm and 12 mm dia primary holes , I only get 924 sq mm for the 6 X 14 mm and 679 sq mms for the 6 X 12 mm holes for a total of 1603 sq mms , not your 3512 sq mms .

I ended up with the primary holes at 35% of the 8824 sq mm 6041 inducer area , secondary zone 15% , so ~50% of total area which is spot on for the "combustion" section .

When we get to the Tertiary cooling holes , those couple of 31.5 mm holes produce ~1558 sq mms which is only ~18% of the inducer area , theres another 32% missing ..........LOL, might account for the higher temps .

Cheers
John

Hi John,

Thanks for your input as usual....really appreciate it.

LOL...yes...you're quite right about those 12mm & 14mm holes...(dumb fingers syndrome ) I now get a tad over 1600sq-mm also.....after doing it again..! Not thinking straight on an early Monday morning..!

Methinks there's a few more holes to go in the Tertiary/Dilution zone then...I'll have a good look at the CC and see if I can come up with a decent solution. When it was running the other day I felt there was a lack of air getting in, as the combustion, although sorta-kinda working, was rather poor.

With the smaller diameter of the C20 CC, I'll have to rethink my "side mounted" igniter and propane injector....they'll fall a bit short...not by too much though, ~10-12mm or so, need to get them inside the CC rather than on the outside..

I'll do some calcs and see how many holes and what size I'll need to add.

Cheers,
Smithy.

[smithy1]

Hi John,

I've done some quick calcs and it seems I'll need another ~2924sq-mm to bring the C20 Tertiary zone up to specs, that's quite a large area to add....There's several ways to accomplish this:

12mm x 26 holes = ~2938sq-mm
13mm x 22 holes = ~2926sq-mm
14mm x 19 holes = ~2926sq-mm
15mm x 16 holes = ~2832sq-mm
16mm x 14 holes = ~2814sq-mm

Take your pick..! What would you do??

Cheers,
Smithy.