Back again..!... The GT6041 powered "Green Beast"..!

[finiteparts]

Yep, if you were at 21C, then this theory definitely doesn't work to help explain that! Sorry....

Just for fun, I made a quick plot of the ratio of actual rotor speed to corrected speed verses temp, based on the Holset correction values to 85F...thus you see at 85F, the speed ratio is 1.  So you can quickly look at a temp away from the 85F standard condition and get the percentage change in actual rotor speed relative to the value shown on the map.



and the same thing here with the ratio of actual mass flow to corrected mass flow as a function of pressure (height above Sea Level (SL) is shown above it) and temperature,



just for a quick visual of the response of the actual values to ambient conditions. Find the pressure in the vertical columns and the ambient temperature in the horizontal rows and you can see the relative percentage change in mass flow.  From the above table, it should be evident why some airports like Denver are terrible for pilots in the summer.

~ Chris

[racket]

Hi Chris

The corrected speed ratio graph is unfortunately incorrect :-(

At 40 deg F -500R vs 85 F - 545 the corrected speed will be ~1.044 times the actual speed , for a measured 73,000 rpm it'd be producing a PR equivilant to a rpm of ~76,200 rpm , or about an extra 0.25 PR in the case of the GT6041

At 110 deg F the corrected rpm will be lower at ~71,380 , or at ~97.7% of measured rpm , so we lose a bit of P2

Cheers
John

[finiteparts]

Hi John,

I did inadvertently have the corrected speed calculation pulling from the degrees F column and not the degrees R column...so I fixed that and changed the plot above. I checked the corrected mass flow and it should be ok since it was pulling from the degrees R columns.

The values that I am showing in the plot are the N/Ncorr and what you are stating are Ncorr/N. So we are now on the same page...thanks for catching that! I have to stop doing calculations at midnight and later! ha!

So I swapped it to show Ncorr/N, in case that is easier to use for how you are using it.




~ Chris

[racket]

Hi Chris

LOL..................I'd never be able to do any calcs at midnight, ........ unless the sun is shining , I'm hopeless :-)

Cheers
John

[smithy1]

Hi All,
Just a small update. Haven't had a lot of time lately to do much but.....

John,
I've decided to make a new tailpipe of 8" diameter, I have procured a 1m length of 316 8" pipe from my local exhaust place, it's a little on the thick/heavy side, wall thickness is 0.080" (2mm) but should last forever.
I'll keep the original mounting, spray ring and flame-holder assembly and cut the 6" pipe just downstream of the flame holder, then connect the larger diameter pipe via a 1" wide ring and make a new exit nozzle to suit. The exit will be the same size or possibly a little smaller in dia than the original so I can tune/adjust as required. One thing I'm not real sure of is how long I should make the 8" dia section??....are there any calculations I need to do to determine the optimum length of the larger diameter?? Also, I will be making up a tailpipe support bracket to help hold up the extra weight to hopefully take some stress off the turbine housing itself..

Anders,
I've finally made an RPM sensor the same as your setup, thanks heaps for the tip....all I did was open up the cheap optical rpm sensor and remove the LED and photo-diode, extend the wires as required, drill a small hole in the inlet housing and point them at the compressor nut which has been painted black with a small amount of reflective tape.....I've not had a chance to give it a full test yet though but spinning by hand gives a positive result.

Cheers,
Smithy.

[racket]

Hi Smithy ,

The AiResearch low cost turbo based jet engine they made "way back" in 1977 , that all of our subsequent "ram jet" dump combustors have been roughly modeled on, had a "body" length of roughly 30-36 inches between the dump and beginning of the jet nozzle, shorter than this and the fuel burn efficiency dropped , with an 8" pipe the burn effic was between 94-99% but only 89-93% with a 6 incher but there was a slight "extra" pressure loss with the larger pipe of ~3% , up from ~ 3% to 6% , we lose ~0.5 psi of pressure due to the dump ..............bloody trade offs again :-(

Another Paper I have, archive.org/details/nasa_techdoc_19650018859 , using a "full sized" 6,000 lb thrust engine from the 1950's found a 36 inch A/B was adequate with only small gains as it was made longer , but largish reductions in thrust as it was shortened from the 36" , with only a 20% increase in thrust with a 10 incher , it appears that we need a certain residency time within the A/B for combustion to be completed .

You should be able to shorten it to ~30 inches or a tad less as the 8 incher will have >70% bigger cross section than the current 6 incher , slowing the gas speeds considerably and allowing a lot more time for combustion ............heh heh ,or we could make it the same length as the 6" bit you cut off

As for jet nozzle size , the dump might produce a bit more turbulence in the gas flows so I'd be a bit careful about reducing the jet nozzle size to anything less than 105mm , I was having some surge problems with the original ~4 inch nozzle before cutting it back to produce the 110mm one now in place .

I'll be looking forward to hearing that A/B screech that most of the guys using a dump design seem to be able to produce :-)

Cheers
John

[smithy1]

Thanks a bunch for the info John,

30"-36" can be done quite easily, I have a 1 meter ~39" length of 8" 316.... I was actually thinking I might go with a small section of "divergent" cone from the 6" section of the original pipe, just downstream of the original flame holder, of maybe 3-4" length or so...then the straight 8" pipe for a "goodly amount", then a convergent cone back to the 105mm exit. The flame holder will still be there so a "dump" section may be a moot point, unless you think it's absolutely necessary? My 1/4 scale unit is built this way with the divergent/convergent sections...and it appears to work quite well...I think I "lucked" onto the right combination in that case.

BTW, last week I did a thrust measurement of the little car during a run at a car show at Pambula and she's pumping out just a tad under 25kgs of static thrust @ 21c and 102%...she's rated at 123k rpm and I've got her limited to 125k. There is a slight "droop" of rpm during A/B but it's only ~1800-2000rpm or so... then it slowly climbs back to 125k, the ECU is obviously seeing this droop and adjusts the pump voltage to suit....it's pretty quick to catch it too when the A/B is turned off which is good to see.

Just and hour or so ago I finished fitting up and wiring in the rpm sensor, I currently have the 5 digit readout sitting in the middle of the steering wheel until I find a better place for it. The LED and photo-diode are sitting in a hole in the inlet housing @ ~10 o'clock as viewed from the front...... and "looking" at the comp nut casting, just like Anders' setup. Progress is happening....slowly but surely.

Cheers,
Smithy.

 

[racket]

Hi Smithy

The dump isn't absolutely necessary if there is a conventional flameholder , the dump style is a "simple" and inexpensive way of producing a flameholder , but not the best way as it can increase loses compared to a normal flameholder.

Your 1/4 scaler works beautifully , so a scaled up version would be the way to go , I think I'd go for 26 inches of length between the 6" "cut off" and the jet nozzle entry , so say 4 inches of divergence then 22 inches of straight 8" pipe going into the nozzle , this should limit length as much as possible to reduce any "overhang" problems .

The 8 inch diameter is probably an inch bigger than need be , so a bit less length than the 36" "standard ??" should still produce a similar residence time ............LOL, a lot will depend on whether it "looks right" at those lengths, maybe a cardboard mockup first.

25 kgs is a very healthy thrust for such a small machine ...............how did the show go ........you have nice weather ??

It'll be interesting to see how the new tach positioning produces the rpm and if that rpm vs P2 issue is resolved, and just how much of a droop there'll be when you hit the A/B ..........lotsa interesting developments ahead ...............yep, steady progress :-)

Cheers
John

[smithy1]

Hi All,
Just a small update on proceedings...the new tailpipe for the "Beast" is nearly finished, the cones have been rolled and is now at the welders, should have it back next week sometime. I used Andy M's advice and drawings to make up an 8" OD pipe to suit the 6041. I must get myself a good welding outfit...any suggestions??

I have made and will fit some auxiliary plumbing/injectors ~ 6" short of the end of the pipe for "flame show" use..! Also have a secondary "bang-box" and igniter from a Rolls-Royce, (Allison) 250-C20B to help with A/B ignition and to compliment the hot-streak...I'll fit the ignitor just downstream of the flame holder.

Cheers,
Smithy.

[racket]

Hi Smithy

Heh heh , I'll be looking forward to seeing the improvements it makes , Andy's A/B absolutely screams :-) ................hopefully you'll get more combustion with the increased dwell time .

Cheers
John

[smithy1]

Hi All,
A bit of an update after the Xmas & New Year period, best wishes to all for the new year.

My new 8" dia exhaust pipe is finally finished and I'll be fitting the flame holder and spray ring etc to it in the next week or so. I've purposely left the exit a bit on the small side so I'll have something to play with as regards to temp tuning etc..

Hopefully I'll have it up and running shortly and making some adjustments and then some serious noise.

Cheers,
Smithy.

[racket]

Hi Smithy

Thats great news , looking forward to hearing more :-)

Cheers
John

[smithy1]

Hi All,

Finally... after several weeks of me faffing around, I've fitted the new A/B pipe and plumbing to the "Beast", unfortunately I have yet to run it , was hoping to give it a test run during the car show I was at yesterday, but it just wasn't safe to do so, too many little children running around, better safe than sorry I suspect:








Still need to fabricate a rear support for the pipe, not real happy with all the weight hanging off the turbine housing....I'll get there eventually..

There are three fuel feeds to the pipe, one is the main A/B spray ring, which can be seen underneath, the other two are supplementary feeds on either side via a 1mm nozzle in each for "flame show" requirements, these can be enabled/disabled via a separate flow control as required. There's also a RR250-C20B ignitor placed just downstream of the flame holder and in the divergent cone, essentially it's a backup ignition source which can also be enabled/disabled by a separate switch...this is to augment the ignition provided by the hot streak, which can be seen entering the outer combustion housing in the first pic. Probably a bit of overkill but....just can't help myself..  

Cheers,
Smithy.

[racket]

Hi Smithy

Now that looks "right" , beautiful :-)

Could you fit the A/B brace back up to the combustor to triangulate it back to the chassis via the other brace , the combustor "top" is pretty heavy section so should be able to cope with the A/B weight hanging from it.

Cheers
John

[Deleted]

Nice work, looking forward to seeing it roar :-)