jetric
Veteran Member
Joined: December 2014
Posts: 132
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Post by jetric on May 10, 2021 17:12:07 GMT -5
Hi Scott, Are you sure that your re-heat fuel pump is not running as that is quite a considerable amount of unburnt fuel out of your exhaust. Ideally you need to aim the ring of holes on your flametube dome out towards the flametube inner wall just where the dome is welded to the flametube so that this will help keep the toroidal vortex up close to your flametube dome, the ring of holes drilled as they are now are blowing jets of air through the middle of the vortex, I would of suggested inserting the shank of a drill bit into each hole and bend the hole over one by one but you have got quite a thick wall section of material on that dome, this is how I make my film cooling holes on my flametubes but they are of a thinner wall section. But as you say try your other spray nozzle first before making any changes to your flametube holes. Richard S. Well I got the new FT in and fired it up, not too great for a first off though. It started fine but smoked plenty at idle. I waited to see if it would “burn off” but it didn’t. Under throttle it was much worse so I shut it down before it attracted unwanted attention from neighbours!! I wonder if the hollowcone spray is either hitting the FT walls or is too dense for the air to penetrate. I think the first (easiest) thing to try is refitting my semi solid 80deg PLP I was using before. If that’s no good, some hole blocking maybe required. Cheers Scott
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Post by andym on May 10, 2021 17:45:46 GMT -5
Think Richard is right, but perhaps bend the holes counter to the ones around the nozzle..... there may be anothe issue ?, but see how you get on with nozzle change etc
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Post by racket on May 10, 2021 20:00:30 GMT -5
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Post by finiteparts on May 10, 2021 21:56:22 GMT -5
Hi Scott,
The first thing that jumps out to me is the row of holes that you added to the dome...it looks like a good bit of flow area that is added at a very poor location. The usual idea is to add the holes in such a way that they energize the torodial vortex in the head end and unfortunately, the row of holes in your dome appear to breakup the vortex.
What is the area that you are targetting for the primary zone? What is the estimated flow percentage that you are putting in through the swirler?
My thought here is that you may be putting in too much air into the primary zone and thus running too cold to process the amount of fuel that you are injecting. The presence of the unburned fuel vapor is telling you that fuel droplets are getting past the high temperature primary zone. If the primary zone is running cold, it could potentially allow some of the fuel to pass through unprocessed.
Another possibility is that the level of swirl produced by your swirler is too low to reach vortex breakdown and thus there is no transport of the combusted products back up to the inlet region of dome. If this was the case, there would be a vortex filament going all the way down the combustor and also produce poor processing of the fuel.
I would you suggest that you try to run a combustor test to see how your flame is anchoring before you change anything. There is so much that can be learned by just being able to see the flame itself.
Good luck!
Chris
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slittlewing
Senior Member
Joined: November 2017
Posts: 458
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Post by slittlewing on May 11, 2021 14:18:32 GMT -5
Thanks for all the replies gents and information! I didn’t realise the row of holes in the dome should point at the outer wall 😬🤦♂️ but as Rich says, it’s too thick to bend them over (2mm wall approx). They are probably breaking up the vortex and I may need a blanking ring to go over them or something. I removed the FT and all the heat markings are right in the primary zone which I think is a good sign. Internally there is evidence of fuel pooling up (some soot at bottom) but not sure if this was just as the engine was killed at the end. I went half way house to the proportions in Rolls Royce book, also adding a bit more primary due to the high TOTs before at full fuelling pointing to delayed combustion. Proportions as follows: 9% Swirler 5% dome holes 12% primary row 1 (80deg injector intersect) 12% primary row 2 (60deg injector intersect) (Total primary zone 38%) Secondary holes 18% Tertiary 44% Thanks for the paper link, my swirler is at 60 degree which by chance is their preferred option, although dimensions will obviously be different. Old injector is now back in, so fire up soon 🙏🏻 Cheers Scott
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Post by racket on May 11, 2021 17:16:05 GMT -5
Hi Scott
Flametube colour looks OK .
You could try blanking off your second row of Primary holes , 38% is getting a bit too much, that extra row could be "killing" the combustion with cold air like Chris mentioned.
Maybe tack weld a band around the FT to cover them for a quick test run to see what happens , but do your fuel nozzle change test first , one change at a time :-)
Cheers John
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Post by racket on May 12, 2021 4:15:02 GMT -5
Hi Scott My Tv84 flametube's Primary had ~10% thru the swirl vanes , 5% through the dome and 15% thru the side wall But much finer atomisation from higher fuel pressures than you are using , which helped get combustion completed quickly . High TOTs can't be fixed by increasing the Primary holes , the Secondary holes are there to "mop up" any unburnt fuel , but there shouldn't be much fuel unburnt by that time in our combustors What fuel are you using, kero , diesel , special brew ?? One other thing ..........its was a long time ago but I sorta remember thinking about the rotation of the fuel spray vs the swirler air rotation , can't remember what conclusion I came up with as I simply used a standard oil burner swirler that came with the fuel nozzle/s, I had the "squeeze" it a bit to reduce the flow area , the swirler I have on my bench here , very similar to the one I used has vanes at ~60 degrees from axial , I think mine ended up at closer to 65-70 degrees , this ones throats are 6mm whereas I closed the TV84 to 5mm Cheers John
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slittlewing
Senior Member
Joined: November 2017
Posts: 458
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Post by slittlewing on May 13, 2021 12:35:08 GMT -5
Thanks for the info John, sounds like I may have gone in a little heavy on the primary holes but the colouration on the FT leaves me thinking that it’s not too far off! I am using 80% diesel 20% petrol mix.
I do sort of wish I had gone for a higher voltage system or alternative pump that could provide higher atomisation but I don’t have the will to change it all now 😂
So I completed the test today back with the PLP injector. Before starting the test I checked in the AB and there was a decent pool of kerosene in it. This has given me some confusion, wondering on Rich’s question about whether the AB pump could have been running before. My ECU code does not provide any voltage to the AB pump without the button being pressed, and also not within 10 seconds of “recognised engine start”. Fuel can technically seep from gravity into the engine or AB with the engine off, but is stopped by non return valves on the pump exits. Additionally if fuel were to have seeped into the AB fuel ring with engine off, it would have run down into the turbine as the AB is at a 15 degree angle pointing upwards, rather then travelling up the pipe and spilling off into the dump zone. So I have no explanation as to how this fuel got there other than that it was indeed from the hollowcone injector or was sat in the turbine housing and then flushed out on the first spoolup.
I attached some rag to a length of wire and mopped it out before starting the engine.
Onto the test:
Thankfully it ran nicely 🙂 I was able to turn the idle fuelling down to reach an idle P2 of 0.5->0.6bar. I did a small burst at full throttle and saw TOTs going past 690 so backed off. Max P2 was 2.2bar vs 2.4 with old FT. At the end, the engine died when decelerating back to the low idle, but I can add more filtering to the throttle signal to slow the deceleration down. Oil pressure was once again rock solid at 4bar when running.
Seems like forward progress, I will stick with the PLP nozzle. Ahead of AB testing, I t’s not possible to say whether or not the idle is low enough for AB ignition, but it’s tempting to have a smaller 15.5gph nozzle on standby which would have better atomisation at the very low idle fuel pressures I’m running, and might allow for a tiny bit lower idle P2?
Cheers
Scott
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Post by racket on May 13, 2021 16:55:10 GMT -5
Hi Scott
Thats encouraging :-)
The hollow cone pattern might not be suitable with your radial swirler whereas with the more common axial swirler it might be fine , lotsa variables with combustion .
690 degrees is OK , add on ~150 C for the temp drop through the turb stage to power a 3.2:1 PR and you only had a T I T of 840 C , a nice buffer below your 900C max .
The fact that you were only able to achieve 2.2 Bar vs 2.4 previously might indicate your "new" combustion isn't very efficient, with a fair percentage of the fuel going out the jet nozzle unburnt , possibly because of the extra Primary holes ..................an easy test to check would be to blank off the holes in the second row .
Otherwise , a successful test ..............and no red hot end cap :-)
Cheers John
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slittlewing
Senior Member
Joined: November 2017
Posts: 458
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Post by slittlewing on May 15, 2021 11:04:36 GMT -5
Thankyou for the suggestion and info John! 🙂 Yeah the 690 TOT still feels pretty high considering the AB isn’t lit though. Additionally, my 6mm TOT thermocouple sits in a stainless thermowell (maybe 8mm dia) as I was suffering them failing from the “explosive” starts back when I was getting it going on ecu. So it takes quite a while for the TC to give a stable reading, and it was still increasing slowly when it showed 690. I took the FT out today to blank the holes you suggested. Whilst I don’t have a glowing end cap anymore, instead the swirler seems to have pushed the combustion against the FT wall and I am now melting spark plug tips. There was not much left of the old tip and presumably what had melted off has gone through the turbine. After the full throttle running the FT is now blue all over. I banded the 2nd set of primary holes which puts me at exactly a 30:20:50 ratio again. I repeated the test and again got 690TOT and 2.2bar max P2, I couldn’t tell any difference. I’m not sure whether to keep it or not. Checked spark plug at the end and tip has melted off again 😬 At the moment my spark plug is a threaded fit through the FT wall. (The electrode is then bent out for a large gap so probably sits 5 or 6mm inside the wall) I have a few options, either A) widening out the hole to provide a jet of air around the plug, B) using a shorter plug where the electrode only just pokes past the FT wall and has a large air gap around it, or C) using a different type of plug with a flat end face. Previously I had issues starting the engine when there was an air gap around the plug as I used to apply impingement air before firing the plug which blocked fuel mix from getting to it. Now I fire the spark before adding air so it wouldn’t be a problem anymore. I will probably try option B) going from a 3/4” long plug to a 1/2” long which would have a nice airgap and no electrode protrusion I am pretty convinced I will change to a 15.5 nozzle, I can’t get the idle P2 any lower than 0.5->0.6bar, as if I go any lower on PWM it’s not enough fuel pressure to get through the Non return valve. Additionally the 10% drop in fuel would put me at 2bar max, and a more comfortable peak TOT. Cheers Scott
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Post by finiteparts on May 15, 2021 13:00:06 GMT -5
Hi Scott,
Did you try to try to close the row of holes in the dome? The disturbance of the PZ vortex could cause poor fuel processing such that instead of completing the majority of the reaction in the PZ region, reaction is still occurring in downstream regions. I would be more concerned about the holes in the dome than the rows downstream of the PZ. The real benefit of the swirler is that typically, if the swirl exceeds a critical value, the swirling flow experiences vortex breakdown and really compacts the region where the fuel is processed due to recirculating the hot reaction products back into the vortex bubble (inner recirculation zone) and the outer torordial vortex region (outer recirculation zone). If the dome jets "separate" the inner and outer recirc zones, you may only be getting a marginal benefit from the swirler.
That is why I suggested doing a combustor test previously. If you use a leaf blower to provide airflow and fuel at a reduced rate, you can visually see what is occurring in the primary zone. I would also suggest using a 0.25 or 0.5 gph nozzle to allow decent fuel pressure providing proper atomization for the much reduced fuel requirement...or just try using propane through your current fuel nozzle. Like the old saying, "A picture is worth a thousand words", being able to see the flame structure in the PZ will greatly aid you in understanding any problems.
I would be surprised that a solid cone fuel nozzle works better than a hollow cone when you get the airflows correct. The hollow cone fuel injector puts the fuel into the outer shear layer of the recirculation zone and thus if that shear layer is disturbed, you get poorer processing of the fuel in the PZ. In fact, I would speculate that the reduce performance of the solid cone test would say that you had lower combustion efficiency and thus a lower fired torque. The lower fired torque means that the compressor output would be reduced...thus, a lower compressor PR. Because of the lower combustor efficiency/fired torque, you have to push more fuel to get to each condition and thus higher EGTs for a given operational point.
The fact that you closed off an entire row of hole with no change in the EGT suggests that even with increased mixing, you didn't help the combustor efficiency...suggests that the problem is not there...which I would again suggest supports the idea of closing the dome holes. Increasing the efficiency of the PZ is really the only way to enhance the space rate heat release, since only in the PZ is there recirculation of the hot products. If you have unburned fuel in the SZ or farther aft, it becomes much harder burn it out due to steadily decreasing temperature. Reaction rates are exponentially related to temperature, so small decreases in temperature can drive large decreases in fuel conversion rate.
As for the spark plug, the high gas temperatures in the PZ are going to make it very hard to keep metal objects stuck into the PZ in solid form. If your PZ had an phi of 1.0 (ideally stoichiometric), the equilibrium gas temperature is 3883 deg F, with a SZ at around 3530 deg F and finally a combustor discharge temp of 1658 deg F (which I set as the temp limit). SO....keeping a spark plug tip alive may be a challenge. I would try to install it so that it was flush with the liner wall or slightly exposed. Maybe even weld a small side electrode that is more flush.
Finally, I just wanted to also make sure that everyone is thinking of liner hole partitioning correctly. Total liner hole area sets the pressure drop across the combustor...the smaller the total hole area, the higher the pressure drop across the liner. The higher the pressure drop, the more available stirring energy is introduced to combustor. The more stirring energy, the faster the reaction rate and thus the ability to process the fuel faster. Since the goal is to nearly complete all of the fuel processing in the PZ, this usually sets all these factors. The proportion of that area that is in the PZ set the highest temperature of the combustor, as each subsequent addition of air introduces more "cool" air that is mixed in and reduces the bulk gas temperature. So when people just add holes, they are essentially reducing the mixing ability of the combustor and this would generally be thought to reduce the combustor efficiency.
Good luck!
Chris
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Post by racket on May 15, 2021 18:28:17 GMT -5
Hi Scott
OK, spark plug might be a problem .
But did the engine run OK , did it rev up without splutters etc ??
Was your max fuel pressure at the fuel nozzle exactly the same as in all the previous tests when you achieved a higher P2 ??
Was the non return valve installed in the higher P2 test ??
I've never had a plug burn its tip off , mine generally just fit against or into the flame wall sheeting , with the earth and electrode just inside the flametube , maybe you could try a short row (40mm ) of 6 X 2mm holes "above" the plug just under the larger primary holes to provide a bit of an air blanket to shield the plug at higher power settings .
Andy and I were having a chat about your combustion issues last week and we both felt that the radial inflowing swirler air might be creating "turbulence" as it passes past the fuel nozzle hex which then could be disturbing your fuel spray, most oilburners including gas turbine combustors use axial swirlers where the swirling air is radially outboard of the nozzle , any air passing the nozzle is purely axial and unlikely to cause problems .
When I constructed my TV84 flametube the nozzle fitted into the mounting projection on the top of the cap , there were a small number of small radial holes to feed nozzle cooling air into the annulus between nozzle and mounting tube well above the nozzle so that there was time for it to enter and distribute as it turned axially along the nozzle before it was "restricted" to provide centering of the fuel spray , the fuel nozzle outlet was displaced 6 mm "above" the external swirl vanes which were some 12-15mm radial out from the nozzle exit , the swirlers had no influence on the spray until it was well inside the flametube and dispersing evenly.
A well coloured flametube isn't a problem , the radiant heat will do that , it should/might stop once past the tertiary holes
Your TOT is generally controlled by "backpressure" on the engine, unless you have flames licking out the exhaust of the turb wheel , have you increased the A/B nozzle size after the upgrading of the turbo ??
Cheers John
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slittlewing
Senior Member
Joined: November 2017
Posts: 458
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Post by slittlewing on May 16, 2021 13:30:03 GMT -5
Thanks for the input and thought given to this chaps.
A leaf blower style visual combustion test is pretty much impossible to do without taking the engine out of the bike, which is a very involved job and not something I would repeat unless absolutely necessary. Or I would need to roll a new liner and recreate the combustor which also seems to be a big job for potentially very marginal gain.
The engine is basically starting and running nicely and smoothly as per previous video, the glowing end cap issue is solved, albeit at the current death of spark plugs instead 😂 I think that can be sorted though.
In terms of blanking off the dome holes, I will give it some thought as to how this might be possible. They are on a reasonably steep angle of the dome and I can’t fit my Tig torch inside, so it would need some kind of ring fitting over them on the outside. Presumably then I would need to open the second set of primary holes back up or drill new, to Re gain the lost area.
The aim of all this would purely be to try and reduce the TOT at max fuel burn, but I now have a feeling after seeing this unchanged with a couple of configs (and two FTs) that this might just be a function of the amount of energy being released by the fuel and not actually due to any delayed combustion (There is no smoke or sign of Unburnt fuel at full throttle).
The bike systems (NRVs etc) have remained unchanged throughout, the FT is a tiny bit undersized (4.5” dia) for the new 69mm comp turbo, and the AB exit hole I opened out to 74mm to reduce TOTs during the previous failed AB attempts (with the loose orifice plate and high idle). The current FT hole area with the blanked second row are matching the original turbo 63.5mm inducer, for a bit more air jet penetration.
Other than possibly blanking the dome holes, I am still very tempted to just fit a 15.5GPH nozzle allowing a lower idle P2, which will both have a better chance of reliable AB ignition, and also not push the TOTs so high at full throttle for a (hopefully) reliable setup. It seems more sensible than trying to eek every last bit out of it to get ~0.2bar higher P2. This is of course also partly driven by a want to “finish” this project and divert full attention to the kart going forwards!
Cheers
Scott
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Post by racket on May 16, 2021 17:20:05 GMT -5
Hi Scott
TOTs are a function of "backpressure" they've got nothing to do with your fuel nozzle size .
If you removed your afterburner , your temperatures should drop and your P2 should go up with your current fueling system .
I can understand your desire to "finish" the project and go on with the kart .............I can only ever work on one project at a time , they need 100% of my focus.
The 15.5 gph nozzle will get your idle down and the full throttle setting should have better atomisation as well ................yep , time to finish the project and move on :-)
Cheers John
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slittlewing
Senior Member
Joined: November 2017
Posts: 458
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Post by slittlewing on May 26, 2021 17:18:26 GMT -5
So I have been doing some more testing with the Jetbike. Firstly, I tried a 1/2" long spark plug instead of 3/4". The electrode would sit just around the FT wall and get air coming past it. When I tried to start the bike, I got the old EMI issues with the ECU, maybe the electromagnetic interference bouncing around in the gap between engine casing and FT. Then I tried a different type of plug (3/4") without a protruding electrode but with a normal gap instead of 3mm like I have been setting the plugs. The ignition unit provides sparks over a much faster rate with a small gap. This also fubared the ECU with EMI. The only other point to note was that it was raining at the time, so the air was quite damp but I have no idea if that had anything to do with it! So back to "Plan B"... cooling the standard plug. I have added a triangle of 2mm cooling holes below the spark plug: I also did more tacks on the blanking ring (on second set PZ holes) as I am keeping it. The 15.5GPH 80deg PLP nozzle arrived and is fitted too. I am hoping this will be the last time I have to remove the FT, as I have probably done it about 8 times now with all the experimenting!! Another item I needed to finish was the air filter. On the previous turbo I 3d printed an adapter and bonded the bellmouth to the turbo using polyurethane sealant. This time I want it to be removable so I added a spacer to a new bellmouth, and some tabs which are clamped by the same U-bolt which holds the front of the turbo. Lastly I added an aluminium grounded plate (as an EMI shield) to the front of the ECU case which faces the engine. The ECU case previously had air holes on this face, and the fans at the back pull air through to cool the MOSFETs. I flipped one fan to push air in from the back, and the other one will extract it. It may be a waste of time but thought it couldn't hurt!! Cheers Scott
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