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Post by finiteparts on Mar 15, 2023 21:09:14 GMT -5
Hi John,
Yes, I was thinking about that lower TOT and I was just assuming that if your mass flow went up, then the amount of power required to compress that extra mass was a portion of the reason for the extra energy extraction in the turbine stage...and...another large portion is likely the back-pressuring due to the AB.
I agree that the turbulence losses in the dry AB are a big chunk. Those large expansion style flameholders are very lossy due to the fact that the fluid which "fills" the dump portion will loose all of it's dynamic head. Additionally, this creates a lot of shear mixing losses (which are good for mixing and combustion when in AB) that chews away at the available total pressure in the AB duct. The k-loss parameter is given by:
K = (1 - (A1/A2))^2
Then you can get the total pressure loss from another definition of k-loss:
K = (Pt1 - Pt2) / (Pt1 -Ps1) = (Pt1 - Pt2) / q
Notice that this definition is based on the inlet conditions for the dynamic head (q = Pt1 - Ps1) and assumes the inlet Mach number is less that M = 0.3
Finally, if you would rather work in discharge coefficients, Cd, then you can convert by:
Cd = 1 / SQRT(1+K)
If you send the areas, TOT and Pt4 I will do the calcs as an example.
Streamlining the spray bars (?) will help. The rule of thumb on cylinders vs airfoils is that the equivalent drag is experience by an airfoil that has a max thickness 10X the diameter of the cylinder.
That sounds very interesting on the cast axial turbine. A good source on those could really change things for the better if they are reasonable.
Keep up the good work,
- Chris
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Post by finiteparts on Mar 15, 2023 21:10:12 GMT -5
Here is an example of the drag equivalence mentioned above:
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Post by racket on Mar 15, 2023 22:16:33 GMT -5
Hi Chris
At 4.5 :1 PR the engines turb exducer is probably running choked , and with an exducer tip angle of 45 degrees , the exit velocity is very high from between the blades possibly >1200 ft/sec , exducer throat area is 0.079 sq ft , this discharges into the exhaust stub with an area of 0.142 sq ft , a 1:1.8 area ratio , with large amounts of wake behind those severely clipped blades as well as the rather large 2"dia hub , I've added a cone fairing on the front wall of the 1.5"dia A/B sprayer that goes back to within a few mms of the turb hub to help with gas flow.
I've previously considered constructing a conical "diffuser" between engine and A/B to reduce the dump step into the A/B and increase static pressure , my current dump step is rather large as it was a compromise to keep the length of the A/B to a manageable size , the extra turbulence hopefully reduced combustion times, but haven't been totally happy with the setup from the beginning , it just didn't look "right"
The cost of the unmachined axial wheel casting blank is $US720 which seems reasonable for that size , I'm still gathering more info, if you're "ïnterested" I'll pass on the supplier :-)
Cheers John
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Post by finiteparts on Mar 18, 2023 15:08:15 GMT -5
John, I was just assuming since you were comparing performance with and without the AB installed, that I would just help to quantify the total pressure loss that is experienced due to the dump style flameholder in a dry situation. This style of flameholder is not my favorite due to the relatively large pressure loss for the bluff body wake formation, but I do understand that it is easy to build and relatively straightforward to making itself successful, so I am not trying to throw stones at the design, just give a sense of how it impacts the exhaust total pressure loss. I wasn't clear on how your spraybar system is defined from the 1.5 inch statement, so I did not calculate the total pressure loss from it's presence in the high speed exhaust section, but if you give a sketch, I am sure we can work it out to help give a estimate of how it impacts the exhaust system performance. So here are the calculations for the total pressure loss: NOTE: I incorrectly put A1/A2 = 2.0, when it should be A1/A2 = 0.5, like what is done in the calcs. And here are the photos that I have of the manometer boards used for the J-69 mobile test stand. I feel like this might be a great option for the hobbyist to get a bunch of pressure measurements relatively easy and cheaply. I would be interested in the supplier of those turbine wheels. - Chris
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Post by turboron on Mar 18, 2023 15:32:41 GMT -5
Chris, your calculations are why I fell in love with gas dynamics in grad school. You can make numbers with a few basic equations such as the perfect gas laws and the continuity equation. You then can design machinery that works with help of a lot of experience.
Thanks, Ron
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Post by racket on Mar 18, 2023 17:31:56 GMT -5
Hi Chris Thanks for the numbers :-) I'll see how they relate to my measured ones . Yep , manometers are cheap and give fine graduation at these low pressures/vacuums , a few metres of plastic tubing does the job . Video of the A/B sprayer www.youtube.com/watch?v=td13h3e7Y8o The dump style isn't the best , but not being a thrust enthusiast I went with it for simplicity . Axial wheel supplier boyuancasting.en.made-in-china.com/ I'm trying to obtain a drawing to check on throat areas to get a better idea of their potential Cheers John
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Post by racket on Mar 22, 2023 1:00:10 GMT -5
Hi Guys
Did a testrun this afternoon , but no improvements :-(
Something weird going on , last test without any jet nozzle produced ~150 lbs of thrust , this test with A/B and a lot hotter TOTs only produced 250 lbs , whilst P4t was roughly the same .
I'll get some vid on Youtube tomorrow and copy out some "numbers" for comparison .
At least the static pressure manometer for comp face worked and indicated an ~1100mm -1.6 psi pressure below ambient :-)
Cheers John
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Post by racket on Mar 22, 2023 17:08:51 GMT -5
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Post by racket on Mar 22, 2023 17:34:14 GMT -5
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Post by racket on Mar 22, 2023 18:02:20 GMT -5
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Post by madpatty on Mar 22, 2023 19:45:20 GMT -5
Hi Racket. It’s perplexing. How a bigger wheel with better efficiency and more mass flow produce less thrust? Regards
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Post by racket on Mar 22, 2023 20:51:14 GMT -5
Hi Patty
Yep , it doesn't make sense , theres something going on , I'll just have to check everything :-(
The A/B will be removed and I'll go back to basics again , that P4t number is just too low for the temps and pressures the engine is running
Cheers John
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Post by racket on Mar 23, 2023 19:22:44 GMT -5
Hi Guys
I've had a look at the numbers and found some good and some bad .
The comp efficiencies were good at ~76% when running dry and 78% when wet and when taken in conjunction with the water manometers its obvious that I'm flowing more air when dry , so more towards the choke side of the map which will hopefully mean the wet running at the higher efficiencies is pretty well in the best region and not near surge .
One strange number was at the very end of the "mobile" camera vid where I throttle back to ~0.5 Bar , the inlet vacuum ( green manometer) went "positive" by ~140 mm of water height , theres a lot of funny noises at that low power settings so maybe so "rough running" with tip surge ??
TOTs were still too high despite opening up the nozzle from 136 to 142 mm .
Jetpipe P4t didn't change much between dry and wet , or with the results of last test without any backpressure on the engine , strange , especially as the surge slot was indicating similar "vacuum" and flow when running dry, even though TOTs were 200 degrees hotter ................its not behaving as the theory says it should :-(
The A/B didn't seem to get as "ÿellow" as earlier test with the old comp wheel where I had 100 lbs more thrust , maybe this new comp is upsetting the operation of the A/B combustion .
More thinking required :-)
Cheers John
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Post by racket on Mar 24, 2023 18:04:40 GMT -5
Hi Guys
Bad news :-(
Pulled the A/B off and notice turbine wheel inducer damage , so disassembled the engine to find one of my combustor caps had failed and gone through the turbine wheel , a couple of other ones also look a tad "fragile" with heat damage , perhaps the new comp with its rough running has caused the problem .
Lots of work ahead :-(
But that expensive comp wheel is OK :-)
Cheers John
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Post by racket on Apr 1, 2023 23:52:17 GMT -5
Hi Guys
Getting a few things done .
Opened up the surge slot from 5 mm to 7.25 mm width to help at low power settings where semi surge appears to be happening, we need ~23-25% of inducer area for the slot .
Started deepening the diffuser channels from 7.5 mm to 8.5 mm to provide more diffusion across the vaneless space after the guys on the GTBA noticed a flaw in my original setup where I was trying to reconcile too many changes after having to fit a different comp wheel .
Still undecided on the flametube mods necessary to reduce future problems .
Slowly slowly I'll get there :-)
Cheers John
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