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Post by andym on Feb 4, 2022 2:56:24 GMT -5
Richard.... nope not had engine on test stand, but have seen from video how much more power is made when the afterburner screaches... Video Andy P at santa pod, engine test in mini, and test video i have of my engines.... as for turdine blades... out of the engines i have made, not had one damaged turbine wheel ?
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Post by finiteparts on Feb 4, 2022 19:36:00 GMT -5
Thomas,
I checked the screech frequencies with a FFT that I hand on hand and I am seeing several primary frequencies. I don't know if it's due to the mic that I am using with the FFT, but the frequencies sort of appear to move over a range sometimes. What I am seeing is:
1809 Hz (for a small blip) 2273 Hz 2602 Hz (2531 to 2648 Hz sometimes) 3609 Hz
When I do a quick hand calc (based on the cylindrical pipe resonant modes in Tim Lieuwen's "Unsteady Combustor Physics") it looks like these would be likely due to a mix of tangential and radial modes (they are roughly between 2340 and 2754 Hz depending on gas temp). A tangential mode is a circumferential back and forth pressure wave motion, often called a wash machine mode. The radial mode would be a radially in and out pressure wave motion, often called a breathing mode, because the duct wall would move in or out if it was flexible. These are pretty high frequency modes, but likely not high enough to be due to vortex shedding from a flameholder or the like.
If I use the frequencies to determine an associated length scale that would correspond, and I assume the gas temperature of: 4200 R - the length scales are roughly 10.0 inches, 7.4 inches and 5.4 inches ( 3359 Hz, 2400 Hz, 1809 Hz)
These seem to me to best align to the radial distances, but I also was guessing that your afterburner was 28 inches long(71.12 cm) and roughly 8 inches in diameter ( 20.32 cm) and I also guessed the FAR ratio in the AB...so there is a ton of assumptions in the calcs. If I had a make a guess, I would say that the center flow was too rich and you are getting coupling due to the radial variation of the fuel. Generally, without knowing how your turbine exit profile is as it enters the AB fueling plane, you would try to put similar amounts of fuel in the similar cross-sectional areas. So if you split the cross-section area into two regions, similar to how you have done with your spraybar, the center area would have a lot less area as compared to the outer area. Thus, I would have put fewer injection holes pointed into the center, as compared to the outside surface of the injector...so that I why I am saying that your central flow is over-fueled...and may be the cause of your resonant response.
It was interesting in how easily you can watch the rpms increase on the FFT, just by watching the blade passing frequencies...it appeared to me that when you got screech, your rpms may have dropped? Just curious.
As for the increased heat release when you are getting screech, it has been supposed by a few in the combustion community that the screech helps with mixing...since AB flows are usually very linear, there is sometimes limited cross-stream mixing...creating what is termed a "plug flow" in the reactor network speak. Flame spreading is one of the variables that set the overall AB lengths in older turbojets, lots of papers out there on this...some are even useful! ha! So if you got more radial or tangential mixing (i.e. more cross-flow mixing) you should get more flame spreading per inch of axial travel. When the flow in the AB gets more vigorously mixed by the acoustic beatings, it can react more quickly and thus you get a higher combustion efficiency. Not all buy this, but it seems to fit what people have seen.
Of course, nobody would design an AB to purposefully have screech due to the damage it can produce to any hardware that has a nearby resonant frequencies. I think that video is a good illustration of how there can be many active acoustic modes in the AB and it can switch around between them as conditions change. If you listen to the screech at 1:12 it switches mode at 1:14...among other places. Thus the likelihood of having a frequency crossing with this driving frequency gets more likely since the modes cover so wide a range of frequencies.
- Chris
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Sweetenough
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Posts: 121
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Post by Sweetenough on Feb 22, 2022 8:33:36 GMT -5
Hi Chris, Sorry for the no-response time and thanks alot for your insights! When designing the fuel ring without knowing the flow profile I just tried to make it as "even" as possible. Approxematly 2/3 of the "air" area is outside the ring so I made 10pc 1mm holes to the outside and the 5pc 1mm to the inside. So that should make a quite even distrubution over the cross-section. When igniteing the AB the rpms drop just a little bit maybe 2-3krpm but is then recovered by the speed controller. Same when I shut the AB down, I get increased rpms. Made a new "push to the limit test run" a couple of weeks ago but to my disappointment the maximum thrust was 45kg. Old design was meassured to 49kg, when looking back at test data I could see that the scale was zero before the test but.... after the test it showed 5kg so it is likely to belive that the real thrust was around the same for that test as well. Tried to do AB min/max fuel tests for 40, 50 , 60 , 70, 80 and 90krpm Film from the test www.youtube.com/watch?v=QHeobPEWF1URunned att severe screech levels and found out what components that have the same natrual frequency :-) The small "tabs" holding the supportband in position fell off, leaving 2 small holes in the AB wall at 4:58 and 7:07 in the movie. Unfortnuattly on the of camera side. When I attempted to repair this I found out that both support for the fuel injection ring was broken as well, one was totally gone, the other one broken but yammed in position. See logged data below, not so easy to follow unfortnuattly Zoomed in view of the last part of the youtube movie when running at full speed 90krpm Showing that thrust is fairly flat when reaching the "right" level, more fuel just makes more screech and other unpleasant sounds The afterburner is now repaired with "wing" support instead New test run was made to verify that the control system AB settings was ok, Seemed to work fine until.... Fuck, radiation heat melted some meassuring tubing for fuel/oil Rebuilding is ongoing, will leave for speedweekend this thursday. At least the new PWM modules worked fine and is permanently installed now at a location where nobody can see it Kind Regards Thomas
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slittlewing
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Joined: November 2017
Posts: 458
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Post by slittlewing on Feb 24, 2022 16:24:19 GMT -5
Hi Thomas,
Really interesting test video, the AB was much more stable at higher speed/P2! Quite a lot of “fluttering” of flame at the lower ones. From an outside point of view I guess the shrieking noise always coincides with almost peak fuelling almost like a warning before it blows out!
As for the fire… wow! I always did think it was brave running the AB inside your workshop 😂 but glad the repairs are not too bad and luckily the body was not fitted, especially with just few days until the big trip!!!
You are making some REALLY nice parts on the CNC machine! What type of machine is it?
Good luck and hope you have great fun on the ice 😁😁😁
Cheers
Scott
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Sweetenough
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Post by Sweetenough on Mar 1, 2022 3:34:31 GMT -5
Hi Scott, Thanks! Yes, at low speeds and low fuel it almosts sounds like a pulse jet. You are totaly right about the screech, it is a pre-waring that you approach the maximum limit. The mill is a Haas Super mini mill
Kind Regards Thomas
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Sweetenough
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Joined: April 2016
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Post by Sweetenough on Mar 1, 2022 4:04:42 GMT -5
Hi Guys, Speedweekend is over, really nice weather during the event and also that is was finally allowed after Covid. Good things that happend, managed to go over 100km/h Then a few not so good thins happened as well. The new AB seems to gett more and more hard to ignite, first run I made it was reallt easy to ignite att all speeds almost. At the last runs I had to ignite it at really low speeds and it also blowed it self out when ramping up a few times as well. But at "full load" it seed to work, I suspect that the fuel ring have cracked due to thermal shock when pumping in fuel into it. Will make a flow test with water and to check if I can see any leakage. Changed back to the old AB for the final run but then unfortnuattly I crashed, I did not get hurt but the jetoped is a little bit sad. Had 2 km/h better speed into the speed trap with the old AB so it could not be that bad after all. Unfortnuattly the mobile networks where overloaded during the event so I got almost no log data from the runs Good thing is that I now have a year to figure out how to improve handeling and maybe build a bigger engine Kind Regards Thomas
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slittlewing
Senior Member
Joined: November 2017
Posts: 458
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Post by slittlewing on Mar 3, 2022 16:08:01 GMT -5
Hi Thomas,
Firstly I think a big congratulations is in order to achieve 100kph!! 💪💪💪
Also your afterburner troubles are quite intriguing. The fact that your new AB seems to make no less thrust than the old one is maybe frustrating - but at the same time could be good news for many of us. I say this because I think your previous AB fuel injection was 3 injectors/nozzles instead of a fuel ring inside the tube. If it performs just as well then it makes AB easier to manufacture and it avoids problems with cracking, plus no obstruction to airflow.
As for the crash… if you walked away it’s all good 😁
I looked up haas mini mill on eBay and it’s out of my league hahaha.
Cheers
Scott
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Sweetenough
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Joined: April 2016
Posts: 121
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Post by Sweetenough on Mar 4, 2022 2:47:34 GMT -5
Hi Scott, Thanks! Yes it is a bit frustrating when you put in alot of work and it does not pay of. No guarantees in the DIY businees But as you say, if an easier to manufacture design is just as good it is a win for future projects Have not got around to it yet, but I need to pressurize the fuel ring with water some day to find out if it is cracked or not since the ignition/sound behavior changed between the test runs. On saturday I will go to an ice-draq racing event with my water rocket kick. Found a couple of pictures from speedweekend from both Jetoped and the kick. Kind Regards Thomas
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