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Post by racket on Jun 5, 2024 19:06:50 GMT -5
Hi Guys I was having another look at the gauges video www.youtube.com/watch?v=yRET5XiYFgg&ab_channel=racketmotorman looking for any relevant info as I went up through the power setting once the A/B was operating , and I realised a very important gem had been missed , the camera shaking ............. it wasn't there :-) ............well, not until I started running out of fuel, so I guess things were "happy" , might be getting close to finally having it "tuned" :-) After cutting off the A/B nozzle I noticed some daylight entering at the inlet end and found a couple more cracks in the circumferential weld at the "step" where the outer wall meets the radial plate , probably as a result of all the pops and bangs during Mondays test whilst trying to light the A/B , so guess I'd better repair it before doing another run. Fresh drum of brew made up and ready to go :-) Cheers John
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Post by racket on Jun 6, 2024 0:27:48 GMT -5
Hi Guys Another test this arvo , it might be a while before I do another Engine still intact Cheers John
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Post by racket on Jun 6, 2024 1:01:18 GMT -5
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Post by andym on Jun 6, 2024 1:13:43 GMT -5
OH Snap...... beets my spark plug blowing out....
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Post by racket on Jun 6, 2024 1:16:48 GMT -5
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Post by racket on Jun 6, 2024 1:33:08 GMT -5
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Post by racket on Jun 6, 2024 1:45:00 GMT -5
Hi Andy
LOL...............thankfully it detached when I'd already idled down :-)
Lots to digest from the numbers , but she was running nice and smooth at 4.5 Bar P2 , P4t in the jetpipe is still high , I'm thinking maybe too many bits and pieces just downstream of the turbine wheel might be putting backpressure on the engine .
Thankfully only a bit of tin work that didn't survive .
Thrust was down from earlier in the week because of the bigger nozzle diameter , but temps still up , I'd have thought there'd be more of an improvement .
She was glowing nicely , keeping me nice and warm on a 20 deg C afternoon , I broke out in a bit of a sweat :-)
I only had one person stop to comment , a cranky old fellow who's only comment was "its a noisy bastard " before driving back where he came from , the test area is 500 mtrs from the nearest dwelling ..............heh heh , I guess his comments were correct ;-)
Cheers John
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Post by racket on Jun 6, 2024 19:46:45 GMT -5
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richardm
Senior Member
Joined: June 2022
Posts: 336
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Post by richardm on Jun 7, 2024 3:33:18 GMT -5
Watching the video I noticed that the bracket on the AB tube ( That orange glowing one on the left of the picture ) was loose. Anything to do with that incident?
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Post by racket on Jun 7, 2024 3:57:17 GMT -5
Hi Richard
Thats just a heat shield for the sparkplug/lead , its held in place by the plug's washer, but no matter how tight I tighten the plug it shakes loose and the heat shiel moves around from the vibrations being generated by the combustion within , its a violent environment.
I've been collecting data from the various videos today and nothing is making sense , the big "upside down" gauge on the side of the engine is plumbed to read the static pressure at the NGV outlets , at 1.5Bar P2 its reading 0.75 Bar , 2 Bar 1Bar, 3.5Bar 2.2 Bar and at 4.5 Bar 2.75 Bar which would indicate a very low pressure ratio across the NGV , ~80 psia P2 to 54 psias P3.5 an ~1.5:1 PR .
Also my densities and velocities out of the A/B nozzle when considered with its area aren't making sense , nor is the fuel burn rate within .............more calcs tomorrow :-(
Cheers John
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richardm
Senior Member
Joined: June 2022
Posts: 336
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Post by richardm on Jun 7, 2024 10:59:40 GMT -5
Gremlins are back....
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Post by finiteparts on Jun 7, 2024 12:23:06 GMT -5
John, I think your gonna need a more capable P2 gauge soon! Looking good. Static pressure measurements in regions of strong streamline curvature can give readings that are very different to what would be calculated in a one-dimensional type equation. There is a really illustrative video from the NSF that shows this very clearly... youtu.be/BnQQkrWyFjM?feature=sharedJump to 21 mins in to see what I am referring to. When I run your P2, assuming it is a total pressure, against your NGV exit Ps (and assume a 5% pressure loss through the combustor) I calculate that your NGV exit Mach number for the 3.5 and 4.5 Bar cases is roughly Mn ~ 0.81 and then 0.84 respectively. But, depending on where your static pressure tap is located, there can be significant measurement errors due to flow swirl, wakes, etc., but let's just ignore that and for now assume the readings apply across the entire flow section. A 5% error in the static pressure measurement for the 4.5 Bar case would swing the NGV exit Mn =/- 0.05 from the nominal 0.84, so it is sensitive. I know you changed the NGV throat to look into this effect, but it should be stated that when you are near the upper portion of the flow curve (thus near choked flow rates) small changes in effective area cause pronounced Mn changes. Judging from your Pt/Ps ratios your current throat effective area is roughly 103.5% of the critical flow area. So you can see that any area "tuning" that you would need to do should be relatively small. Likely, when you went from 10 to 9 mm, if you are still at the 23.5 mm height, you changed the throat area by just over 11%. Thus you reached the critical throat effective area and then went past it to reduce the swallowing capacity of the NGVs by something over 7%. Really great work there...that is really something to be proud of. - Chris
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Post by racket on Jun 8, 2024 1:54:27 GMT -5
Hi Chris
Thanks for the Link :-)
Thanks to the info contained I might reposition my inlet static pressure probe position , the current IGV angle is now positioning a vane where it could corrupt the reading.
Its nice to have confirmation that I'm getting closer :-)
After collecting the A/B pressure and temp numbers I've got more unknowns , one would imagine a velocity out of the A/B at the sort of temps I'm seeing would be ~2500 ft/sec when having close to 1 Bar of P4t to get things moving , with a density at the nozzle at ~70 cu ft/lb , and a 155mm - 6.1 inch dia nozzle of 0.2 sq ft ............2500 X 0.2 =500 cfs , 500/70 = 7.1 lbs/sec ................wishful thinking as that would produce 550 lbs of thrust :-( ..............so maybe lotsa turbulence filling out the nozzle or my P4t pitot reading is way out ??
Lots to think about, maybe once my head clears , its still hurting after yesterday , too much noise isn't appreciated by this old body :-)
Cheers John
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Post by finiteparts on Jun 8, 2024 18:17:14 GMT -5
John, here are a few thoughts for your calculations.
1. The sudden area expansion style AB's are quite loss prone.
I took a swag on you diameter ratio and visually estimated it to be roughly a 2 to 1 for the outer duct to the inlet duct sizing. With this area ratio and using an incompressible correlation, the estimated loss in total pressure is roughly 15.4%...so, you don't have the total pressure that you are measuring to drive the flow out of the nozzle, you only have roughly 85% of that.
This is why these style flame stabilizers are not desirable to use on full scale applications. Using a turbine exit diffuser and standard flameholders can get similar stability without such a severe total pressure loss.
The pressure loss due to the heat addition (Rayleigh Loss) would add to this, but I am not sure of the flows and thus can't make a good duct Mach number estimation to see if it is significant.
2. Just a reminder that you should be including the fuel mass flow through the exhaust nozzle because the mass added in the AB can be more significant than the combustor fuel flow, which is usually ignored.
Do you have an estimate of the total fuel flow rate?
I am trying to use the thrust equation and substitute in the mass conservation equation. This allows the mass flow rate to be taken out as a variable and then it just becomes a function of the measured values to back out the exhaust jet velocity. The trick is to be able to go from the total properties to get the local static properties that set the velocities through the nozzle throat.
Currently, I am estimating that if you had a full 2 Bar of total pressure at the entrance of the AB, then take a 15.3% total pressure loss at the sudden expansion and then flow out to the 1 Bar ambient, I get roughly that you are flowing 5.43 lbm/s through the nozzle at 6.1 inch diameter. I am assuming a realized gas temperature across the AB of 2960 F and basing the gas properties on that (i.e., gamma ~ 1.26, Mn, etc.).
Now, I didn't estimate the thermal growth of the nozzle as it is glowing like the sun and obviously not still 155mm in diameter, so that might be another set of calcs to sharpen the pencil.
Hope these ideas help in some way,
Chris
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Post by racket on Jun 8, 2024 19:51:53 GMT -5
Hi Chris
Thanks for your thoughts, much appreciated :-)
Yep , losses are higher due to the extra large step , it was a compromise to keep A/B length under control and also help with combustion from the extra turbulence .
Theres ~8 lpm of fuel going into the A/B , 6 kgs- 13.3 lbs/min , so ~0.2 lbs/sec , maybe add on 5 lbs/min - 0.08 lbs/sec for the engine
LOL......the nozzle is probably closer to 160 mm when hot , ~6.5% growth in area .
I'm going to take your advice from last week and do a dry run to gather some data to compare with , I think the A/B if combusting OK so no need to pursue that any further .
Thanks again
Cheers John
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