|
|
Post by smithy1 on Jun 26, 2016 20:34:19 GMT -5
Yeah...although I'm fairly confident the mods will have done the trick, or at least be very close to being right....the upper flame tube is now very similar to the C20B item in regards the primary air at least.
Cheers,
Smithy.
|
|
|
|
Post by smithy1 on Jul 14, 2016 1:42:45 GMT -5
Hi John,
Had a chance to do a quick engine run this afternoon after work....and the mods you have suggested appear to be a success....there's no more "fluttering" like we had at Port Mac, she's much smoother, the flamesouts are "nearly" non-existent now, I did have one on a sharp deceleration but nowhere near the amount I was having previously. The combustion seems a lot happier now, not perfect but much better..!!
The temps at idle are also still good @490c...idle rpms are probably a bit low @ 22k so I'll adjust it up to ~28-30k and see what happens.. I have an event to go to this coming weekend at the Menangle Steam & Machinery Museum which should give me plenty of time and room to have a good "fiddle"....but she's looking and sounding great.
Cheers,
Smithy.
|
|
|
|
Post by racket on Jul 14, 2016 1:57:33 GMT -5
Hi Smithy
Thats good news, heading in the right direction :-)
LOL, 22K .............maybe a tad low , certainly would contribute to a flameout on a quick decel ..........probably fuel pressure dropped below P2 .
I'll look forward to hearing how your weekend goes .
Cheers
John
|
|
|
|
Post by smithy1 on Jul 14, 2016 2:15:49 GMT -5
Hi Smithy Thats good news, heading in the right direction :-) LOL, 22K .............maybe a tad low , certainly would contribute to a flameout on a quick decel ..........probably fuel pressure dropped below P2 . I'll look forward to hearing how your weekend goes . Cheers John LOL...yeah...good news indeed. I'd suggest it's highly unlikely the fuel pressure drops below the P2 during decel...fuel pressure at idle is ~105psi...!!! But there may be a very quick/sudden drop-off during a quick decel which is causing a flame-out...I may see if it's feasible to fit a "damper" to the throttle, something like one of those little doodads which stop cupboard doors from slamming shut. Max fuel pressure is up near 375-400psi @40psi P2...! I "may" have a poor spray pattern at idle, I'll pop the injector out tomorrow and check it on my test rig at work...only takes a few minutes to do... I'll definitely adjust the idle stop up a bit...can't hurt to try anyway..! Cheers, Smithy. |
|
|
|
Post by racket on Jul 14, 2016 4:50:39 GMT -5
Hi Smithy
Ah yes ,forgot , you've got a duplex nozzle , could there be some reluctance to switching over on the fast decel ??
With my TV84 engine and its simplex fuel nozzle the pressure at idle was only ~50psi whilst full power was closer to 750 psi , if we do the usual square rooting of pressures , that 7:27 , or roughly 4 times more fuel at full throttle compared to idling , your 400 psi would drop to only 25 psi idle if a simplex .
Cheers
John
|
|
|
|
Post by smithy1 on Jul 14, 2016 19:12:46 GMT -5
Hi Smithy Ah yes ,forgot , you've got a duplex nozzle , could there be some reluctance to switching over on the fast decel ?? With my TV84 engine and its simplex fuel nozzle the pressure at idle was only ~50psi whilst full power was closer to 750 psi , if we do the usual square rooting of pressures , that 7:27 , or roughly 4 times more fuel at full throttle compared to idling , your 400 psi would drop to only 25 psi idle if a simplex . Cheers John Hi John, I think with the C20 nozzle the "duplex cross-over" is quite subtle, when testing them there's no obvious or sudden change, but you could be right, it may struggle coming back down. The spray cone angle slowly changes with climbing pressure/flow, but it's not dramatic or like a light switch. I tested the fuel nozzle this morning and it's very close to standard C20B flows...and within ~3% of an overhaul condition unit.. So I can't really fault it in that regard, you could plonk it in a C20B engine and it would work nicely. At ground idle in a C20B engine, the fuel pressure is ~125psi flowing ~60-70lb-hr, at the max fuel flow stop they're ~540psi and flowing ~280lb-hr....but they do have a much higher pressure ratio across the combustion chamber, not real sure how this affects things for our purposes though. As you're well aware, the 6041 flows a bit less, (~25%) also at a much lower pressure ratio....considering this is a DIY engine and taking in all the differences...it's working pretty good. I'll try to do some fiddling over the weekend and see if things come good...I don't expect any major issues though ...and I am having fun running it. I wonder what would happen if I could get hold of a "suitably flowing" simplex fuel nozzle....something along the lines of one of those furnace nozzles for example?? I could quite easily make up a suitable adapter to make it fit in the current setup..! Food for thought..! Cheers, Smithy. |
|
|
|
Post by finiteparts on Jul 14, 2016 22:02:15 GMT -5
Hi Smithy,
I agree that your duplex probably isn't a factor. Since most duplex nozzles use a pressure controlled valve to transition between circuits, such that when you reach the "cracking" pressure, the valve lifts and the excess flow starts to flow through the larger nozzle. There should be no drastic jumps in the flow rate when the valve cracks open. Sometimes, like you said, you do see an angle change of the atomized fuel cone as the flow shifts to the secondary spin chamber, but you also often see an angle shift as the atomized fuel gains higher angular momentum with the higher fuel pressure... it all really depends on the arrangement of the duplex nozzles primary and secondary spin chambers.
Do you think you might just be experiencing typical turbine control system droop? Since the combustor system can respond faster to the change in fuel flow (and it has to!) than the rotor can respond to the change in burner output, you get a fuel-air ratio shift that can get close to lean blow out or rich extinction (depending on which way you go). So for example, on decel, you start to cut fuel, but the rotor hasn't responded yet and is still pumping out the same air flow. The reduction of fuel flow required to start slowing the rotor down means that with the rotor still flowing a similar amount of air, your fuel-air ratio "droops" and if it goes below the weak extinction limit, you flame out. This difference in transient response times of the different systems is why older hydro-mechanical fuel control systems used accel/decel rate limiters. Modern FADECs just program in allowable acceleration or deceleration rates. Your damper idea would be analogous to the old rate limiters and would be worth trying.
Good luck!
Chris
|
|
|
|
Post by racket on Jul 15, 2016 1:04:20 GMT -5
Hi Chris
Another thing to consider is the Actual Flow vs Corrected Flow rate of the turbine stage with a sudden reduction in fuel flow , all of a sudden the T I T has dropped to below half its previous level , but Pressure Ratio is still high due to the "flywheel momentum" of that relatively heavy radial inflow turb wheel , the comp wheel is suddenly able to flow more air further exacerbating the lean conditions in the flametube, yep , a bit slower rollback of the throttle required methinks .
Cheers
John
|
|
|
|
Post by smithy1 on Jul 15, 2016 2:20:49 GMT -5
Chris & John, thanks for your comments...they're very much appreciated and very helpful, I do take everything on-board as best I can and it's really good to get other's opinions etc.
Methinks you're both right...the 6041 is a monster of a rotor to speed up and slow down quickly....I tend to think it might be a "lean-out" issue as there's no evidence of black smoke indicating a rich mixture during the flame-out.
I think it might just have to be a case of me trying not getting off the throttle so quick, I had kinda hoped the accumulator I have in the system would help...but obviously it doesn't. I suspect with the original vaporizer system it was a lot more forgiving......but, we have to make allowances I guess, I shouldn't complain really as she's running pretty well considering all the mods I've done... and I am nit-picking a bit....
Cheers,
Smithy.
|
|
|
|
Post by racket on Jul 15, 2016 18:35:45 GMT -5
Hi Smithy
Your accumulator would need to be pretty big to cope with the fuel flow during a quick decel .
If the accumulator was 1,000 cc empty capacity , at idle with 105 psi fuel pressure the air would be compressed to ~125 cc , at 400 psi it'd be down to ~ 35cc .
On a quick decel there'd only be , 125 minus 35 = 90 cc of fuel available to cope with supplying extra fuel during those few seconds , so maybe either a big accumulator or your "doodad" ;-)
Cheers
John
|
|
|
|
Post by smithy1 on Jul 31, 2016 15:51:56 GMT -5
Hi John,
Did a car show yesterday and although the engine is running quite well at high throttle settings and also with A/B, I'm still having a few combustion issues in the low-mid rpm ranges....she still seems like there's not enough air getting to the primary zone or it's not diluting enough in the right area, I'll pull the chamber out this week to have a look and see exactly what is going on inside...
I was seeing quite a bit of fuel wetness from the combustion chamber "V" band joint area...which indicates unburnt fuel is finding it's way outside the flame tube, strange really as I haven't changed anything since I last ran the engine a couple of weeks ago and although not perfect, it was a lot happier than yesterdays run.
On a side note, I took your advice and have found a copy of Lefebvre's "Gas Turbine Combustion" paper online, so will do a bit of reading/study in the meantime.
Cheers,
Smithy.
|
|
|
|
Post by racket on Jul 31, 2016 17:47:28 GMT -5
Hi Smithy
Fuel outside the flametube ................sounds like a "surge??" type issue , an odd place to see fuel getting to , the pressure drop across the FT wall should keep it "inside".
Hopefully a look inside will throw some light on whats happening .
Heh heh , Lefebvre can be a bit heavy going , it takes me several reads before I think I'm getting a grasp on what he's talking about most times , certainly not written for backyard tinkerers .
Cheers
John
|
|
|
|
Post by smithy1 on Jul 31, 2016 18:02:52 GMT -5
Hi Smithy Fuel outside the flametube ................sounds like a "surge??" type issue , an odd place to see fuel getting to , the pressure drop across the FT wall should keep it "inside". Hopefully a look inside will throw some light on whats happening . Heh heh , Lefebvre can be a bit heavy going , it takes me several reads before I think I'm getting a grasp on what he's talking about most times , certainly not written for backyard tinkerers . Cheers John Hi John, I don't think it's a surge issue as the engine is "Kinda" running and sounding ok, although I stand corrected as usual. She's a bit smokey too which obviously indicates the presence of unburnt fuel....I'm suspect poor primary combustion. As mentioned, ~50-60% and above shes is much happier. I'll pop the can off this arvo and have a look-see what's going on. I checked the fuel nozzle on the test rig this morning and it's doing what it's supposed to do with a nice spray cone...so I'm a little baffled....doesn't take much to baffle my brain cell..  Cheers, Smithy. |
|
|
|
Post by racket on Jul 31, 2016 23:23:00 GMT -5
Hi Smithy
LOL ......when I mentioned surge I was thinking some sort of pulsing combustion that might be causing fuel to hit the inside wall of the FT and be forced out somehow , just thinking out loud here :-)
Looking at my data on the C20 it idles at ~65% N1 with N2 at ~ 75-100% , with interstage temps (ITT) of ~400 - 450 C , now at 65% N1 one would think it'd be shifting some air through the engine and have a bit of pressure with it as well , 65% for you is >40,000 rpm and >10 psi P2, could there be some "design" inbalance now with the small flow whilst using the C20 fuel nozzle and FT cap , I don't know :-(
Hopefully the look inside will give us some clues.
Cheers
John
|
|
|
|
Post by smithy1 on Aug 1, 2016 3:21:08 GMT -5
Hi Smithy LOL ......when I mentioned surge I was thinking some sort of pulsing combustion that might be causing fuel to hit the inside wall of the FT and be forced out somehow , just thinking out loud here :-) Looking at my data on the C20 it idles at ~65% N1 with N2 at ~ 75-100% , with interstage temps (ITT) of ~400 - 450 C , now at 65% N1 one would think it'd be shifting some air through the engine and have a bit of pressure with it as well , 65% for you is >40,000 rpm and >10 psi P2, could there be some "design" inbalance now with the small flow whilst using the C20 fuel nozzle and FT cap , I don't know :-( Hopefully the look inside will give us some clues. Cheers John Hi John, I think I'd hear it if the combustion was pulsing enough to cause fuel to hit the FT walls...but she's smooth and runs well at the higher RPM settings. Didn't get a chance to open her up today...had to work late...  The fuel nozzle I have fitted is a standard C20B Item, so maybe it is flowing substantially more than the 6041 requires....but the engine should let me know what it wants as far as fuel goes, then it's just a matter of adjusting throttle pressure to suit. I can reduce the nozzle fuel flow at any given pressure though...I might see how much fuel she's flowing at 100psi and report back. You might be right with the imbalance, I've got nowhere near the pressure ratio the C20B has at any given rpm so I might see if the Leferbvre paper has any clues...However, the compressor bleed valve is open prior to closing completely @ ~94-95% which would bring things a tad closer to our side of things. At 100% the C20B is pumping a good 84psia (~69psig) and swallowing ~260-265lb-hr of fuel @~470psi fuel pressure...quite a bit higher than my 40-42psig and ~180lb-hr requirement..! If I still have issues I might just bite the bullet and transplant a complete C20B flame tube into the beast....it will fit nicely they work over a wide range of flows and pressures...maybe I should have done this to start with..?? I dunno..!?  Cheers, Smithy. |
|
The fuel nozzle I have fitted is a standard C20B Item, so maybe it is flowing substantially more than the 6041 requires....but the engine should let me know what it wants as far as fuel goes, then it's just a matter of adjusting throttle pressure to suit. I can reduce the nozzle fuel flow at any given pressure though...I might see how much fuel she's flowing at 100psi and report back.