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Post by pitciblackscotland on Dec 3, 2021 2:24:55 GMT -5
Hi John, Almost hit the 3 Bar, yeah 80% humidity not really nice for man and turbine engine.
Cheers, Mark.
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ripp
Veteran Member
I'm sorry, I don't speak english, so I torment you (and myself) with a translation program,Sorry
Joined: January 2013
Posts: 236
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Post by ripp on Dec 3, 2021 2:51:06 GMT -5
Hi John, did I read that correctly? 2.5 par, 744 C, 160 lb? for all those who have trouble reading the scales like me Cheers Ralph
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Post by andym on Dec 3, 2021 10:56:33 GMT -5
Hi Guys Test this afternoon was"different??" youtu.be/ydnt9XFk83AIt was 29 deg C and 80% humidity which affected both me and the engine :-( Something weird going on , ...............several more viewings of the vid required . Cheers John Hi John A little more power..... what do you think is od, is there any way you can " T " in to the p2 line and add oil filled guage or may be move pick up.... was that jus kero... or 50/50 mix Chat Soon All The Best Andy
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Post by racket on Dec 3, 2021 17:01:44 GMT -5
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Post by racket on Dec 3, 2021 19:03:00 GMT -5
Hi Ralph
Yep , thats around what it was doing , temperatures are way too high for an engine without a jet nozzle producing backpressure ................somethings wrong .............possibly delayed combustion.
Gauges are bouncing around much more at 2.5 Bar P2 , at least jetpipe temps are fairly even between the two thermocouples . .
Interesting that T2 started at ~29 C and climbed to ~33 C during rotor acceleration under the starter before fuel was added .
Time to check some "numbers" to see whats going on , at 750C TOT and full expansion through the turbine wheel things should be different
Cheers John
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Post by racket on Dec 3, 2021 19:09:52 GMT -5
Hi Andy
Same 2:1 petrol/kero mix so keep things the same .
Yep , more thrust but more temp rise , things are "reasonable" up to 1.5 Bar P2 , but then the temps really climb .
Might do an insitu flow test of the injectors at ~45 psi pressure drop to replicate the 80 minus 35 psiP2 fuel pressure gauge reading , to find out exactly how much fuel was getting into the engine and then calculate the theoretical temp rise for 4 lbs/sec airflow .
Theres certainly more shaking going on with the extra fuel :-(
Cheers John
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ripp
Veteran Member
I'm sorry, I don't speak english, so I torment you (and myself) with a translation program,Sorry
Joined: January 2013
Posts: 236
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Post by ripp on Dec 4, 2021 3:24:33 GMT -5
Hi Ralph Yep , thats around what it was doing , temperatures are way too high for an engine without a jet nozzle producing backpressure ................somethings wrong .............possibly delayed combustion. Gauges are bouncing around much more at 2.5 Bar P2 , at least jetpipe temps are fairly even between the two thermocouples . . Interesting that T2 started at ~29 C and climbed to ~33 C during rotor acceleration under the starter before fuel was added . Time to check some "numbers" to see whats going on , at 750C TOT and full expansion through the turbine wheel things should be different Cheers John Hi John, I am very curious what your calculations will result! I ask myself where is the constructive difference between CC 102-255 HX 82 Money Pit and your combustion chamber. I can't see a big optical difference. she runs great! Cheers Ralph
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Post by racket on Dec 4, 2021 4:41:52 GMT -5
Hi Ralph
Theres not much difference between the "Money Pit" and my flametube other than my flametube is bigger in diameter to account for the larger mass flow and a bit longer which should allow more dwell time , one worked OK and one isn't for some reason :-(
Today I did an insitu test of the fuel manifold/injectors and found at 40 psi pressure drop across the injectors equating to 76 psi fuel delivery pressure with 2.5 Bar of P2 pressure, a flow of 3 lpm or ~5 lbs/min .
With a 700C degree rise in the combustor I need a F/A ratio of ~0.019 :1 , which would mean I was delivering enough fuel to heat ~4.3 lbs of air per second from ~200 C to 900 C
Density of gases at ~750C at ambient pressure in the jetpipe is ~46.4 cubic feet /lb and if 4.3 lbs/s thats ~200 CFS .
My jetpipe is 130mm ID which is 0.1428 sq feet , 200 cubic feet per second going through the pipe would need a velocity of ~1400 ft/sec .
160 lbs of thrust if the gases are at 1400 ft/sec would only equate to a mass flow of ~3.68 lb/sec .
The actual flow might be somewhere in between , maybe 4 lbs/sec at less velocity , my P4t of ~0.55 Bar would be from ~1300 ft/sec of impact compression .
Things are sounding better :-)
Cheers John
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Post by finiteparts on Dec 4, 2021 14:57:49 GMT -5
John,
I am wondering if your fuel injectors are too "free-flowing", especially now that you have doubled the number of paths out of the manifold. I did a few quick calcs. and by doubling your injectors, you have reduced the flow resistance of the injectors to a quarter of what it was for the 9 injector manifold. This means that each injector exhibits less flow control on the fuel and small local differences will cause global flow re-balancing. The very short L/d of your injectors could also be leading to low flow balancing in each injector due to a low resistance. Generally, you want to have a controlling orifice at the inlet to the injector as opposed to having the internal friction of the tube to be the flow limiting feature, especially when the tube is in a harsh environment that could have thermal growth impacts. Recognize that for a 21g tube, a 0.001 inch change accounts for roughly 10% area difference. McMaster states that the o.d. tolerance is +/- 0.0005 inch, so the baseline variance on the i.d. could easily be 0.001 small on hypodermic tubing without any thermal growth due to local radiant heating. When we deal with very small flows, the precision effects really compound.
Two items that lead to a non-intuitive behavior are: 1) All the injectors are parallel resistances in your fluid circuit, not series resistances 2) The fuel pump is a volumetric machine, meaning that it will push the same amount of fluid for each rotation equal to the trapped volume between the gear/vane/gerotor and the case or outer rotating member. The pressure out of the pump will change, but for a fixed rotational speed, the volumetric flow rate will stay the same.
I have personal experience with a pulsating fuel manifold that I was trying to balance with Swagelok high precision valves. We used some very precise tooling to set the flow balance to each injector and when it got on test, it was a disaster. Because there was a low flow resistance in the fuel circuit, small local pressure fluctuations in the combustor (due to the backpressure of the local heat release) caused a rebalancing of each injector. Since the volumetric flow was fixed by the high precision pump, the total fuel flow out of all the injector was also fixed. But, each injector experienced a slightly different pressure drop due to the local conditions in the combustor. What we ultimately had to do was to put in a flow control orifice between the manifold and each injector, that introduced a large enough pressure drop such that the fluctuation in the local exit pressure was insignificant. Once these orifices were installed, the combustor ran beautifully, just as designed.
One other thing, I did a quick check of the tube velocity for the injector (with 18) and found roughly that the tube velocity is around 50 ft/s, which seems a bit high. I would be concerned that you would get cavitation in the flow entering the injector tube. The cavitation would reduce the fluids bulk modulus/viscosity, which would give a varying internal friction and thus varying flow through each tube. This is also a reason that the flow control orifice is used. If you look at the RB199 vaporizer injector, they have a welded in orifice that is roughly half of the exit orifice area.
I am still looking for a good suggestion on how to find a high accuracy, low cost orifice that you could use to balance your system. I think the r/c turbines use a coiled up tube to produce a larger resistance in each injector, but I am not sure about that. I might try to do some hand calcs. to check that out.
I hope this is some food for thought,
Chris
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Post by finiteparts on Dec 4, 2021 15:09:21 GMT -5
Maybe an idea...insert a short length of a smaller hypodermic tube into the injector at the upstream end. For example, if you have a 0.025 inch i.d., you could get a 0.024 or 0.025 o.d. tube with a much smaller i.d., cut a 0.10 inch section and press it into the end of the other tube. Maybe....It would be a bear to work with, but might do the trick.
Another option, silver solder over the end and micro drill a hole with the bits that you use on PCBs.
Your supply pressure will go up, but having flow control is the real goal...but I am a bit concerned that if your fuel pump is marginal on pressure capability, you might not be able to do this.
- Chris
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Post by finiteparts on Dec 4, 2021 15:51:56 GMT -5
John,
You might think about running an atmospheric test to see if the combustion is fluctuating. Contrary to many comments, atmospheric tests are done often and are very useful to gain behavior insight and reduce risk before going to elevated pressure sector rigs or on a full engine test. When you scale the test conditions properly, they give an early look at combustor flow features and behaviors.
That is how I discovered the fueling instability before it ever got on the engine and risked damaging high dollar rotating parts.
Just a suggestion,
Chris
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Post by racket on Dec 4, 2021 17:23:18 GMT -5
Hi Chris
Thanks for the input :-)
LOL........food for thought ............theres a lot of that going around in my head at the moment :-(
Since installing the 18 X 21G injectors into the 18 X 19 G "remains" within the fuel manifold my flows at 40 psi pressure drop have reduced from 3600ml/min when I had just 9 X 19G injectors , to 3000ml/min and requiring more pressure to restore flows .
I think I might pull the engine apart and check the flametube colouration to see if things have changed .
The fact that the injectors have their outlets within the combustion itself with all its localised pressure flucturations as you mention would account for the vastly different outcomes compared with when using vapouriser tube where the injection is isolated from combustion , I'll check over my various insitu flow data with your cavitation thought in mind .
One thing that is really perplexing me is the very poor hot end performance , the comp is working at 76% effic or better , but with no "exhaust" restriction on the turbine its requiring such high temps to power the comp , its as though theres a huge amount of energy being "wasted" somewhere .
At pressures below 1.5 Bar P2 exhaust temps are very good and what would be expected with an "open" exhaust , but then it all goes bad after that ..................more thinking time required today :-)
Cheers John
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monty
Senior Member
Currently being spanked by mother nature.......
Joined: September 2018
Posts: 400
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Post by monty on Dec 5, 2021 18:14:22 GMT -5
John,
I guess I haven't been paying enough attention. So you are just injecting directly from the needles into the can with no vaporizer tubes?? That would certainly be easier to make!
Echoing what Chris was saying. Aircraft fuel injection systems for piston engines feed from the metering device into a fuel spider. Inside is a disk with V shaped notches. The notches un-port an orifice that goes to each injector. As the pressure rises the disk moves against a spring and the orifice to each injector gets larger. When fuel stops flowing the disk closes off each orifice. The key here is the main restriction is upstream of the actual injector, so any pulsing in the manifold does not effect the total flow.
I would think the vaporizer tube acts to isolate the injector from the combustion and other noise, and would tend to dampen any pressure pulses. The two phase nature of that flow and the isolated environment would tend to accomplish the same thing as the fuel spider.
In your case, it seems you are using a needle valve upstream to the manifold, and constant pump output dead headed against the needle. (I could be wrong) Have you considered a pressure regulator before the needle valve?
Monty
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Post by andym on Dec 5, 2021 18:21:36 GMT -5
John, I guess I haven't been paying enough attention. So you are just injecting directly from the needles into the can with no vaporizer tubes?? That would certainly be easier to make! Echoing what Chris was saying. Aircraft fuel injection systems for piston engines feed from the metering device into a fuel spider. Inside is a disk with V shaped notches. The notches un-port an orifice that goes to each injector. As the pressure rises the disk moves against a spring and the orifice to each injector gets larger. When fuel stops flowing the disk closes off each orifice. The key here is the main restriction is upstream of the actual injector, so any pulsing in the manifold does not effect the total flow. I would think the vaporizer tube acts to isolate the injector from the combustion and other noise, and would tend to dampen any pressure pulses. The two phase nature of that flow and the isolated environment would tend to accomplish the same thing as the fuel spider. In your case, it seems you are using a needle valve upstream to the manifold, and constant pump output dead headed against the needle. (I could be wrong) Have you considered a pressure regulator before the needle valve? Monty LOL Monty..... did you not see the injectors in my engine money pit
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monty
Senior Member
Currently being spanked by mother nature.......
Joined: September 2018
Posts: 400
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Post by monty on Dec 5, 2021 18:48:04 GMT -5
LOL Monty..... did you not see the injectors in my engine money pit Andy,
I just assumed you were injecting those into a vaporizer tube! Like I said, not paying close enough attention. Now I'm really intrigued, because I don't want to make all those vaporizer tubes.
Monty
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