Big Turbo based engine and Afterburner questions......

[mjb777]

Hi all,

New guy on here and firstly would like to say thanks to all of you who share so much great info and experience on here and other places. I work in aviation and have a background with full size turbine engine maintenance and operation and also have built a couple of smaller RC type turbine engines.

Many years ago I found a very large, (guess it's a matter of opinion...), turbo that I thought would be perfect for my long time dream to build a Turbo based jet engine. See pics.....

  
  

The thing weighs about 80KG and according to the manufacturers manual it flows about 2.5kg/ sec and PR is 3.5. Higher PR is achievable however the mass flow suffers. Max continuous RPM and TIT is 60000 and 750 deg C. Compressor inducer is 115mm and exducer is 160mm. Turbine exducer is 130mm. The compressor housing outlet and turbine housing inlet are 125mm and the turbine has an NGV and center entry housing. 

Plan is to have the engine and Afterburner controlled with an Arduino or two, compressed air starting and a simple variable AB nozzle design that I pinched from a guy I know who used to run Jet powered drag cars in the 80's and 90's.

I'm still deciding on either the conventional divergent AB inlet and flame holder, dump type or combination divergent AB inlet with dump tube as I have seen in some of the drawings posted. Maximum performance and reliability possible, and 100% RPM AB light off is important....hopefully it is achievable.

Another thing yet to be decided is Afterburner ignition. Ideally I'd like to achieve 100% RPM ignition and not have to light off the AB at lower RPM and then spool up to max RPM. Droop and overspeed during AB light off, and if AB flame out occurs, will be taken care of by the ECU. I have experience with full size torch and hot streak type AB ignitions and get the impression that the automotive type spark plug and ignition system has limitations? (I have a full size high energy two plug turbine engine ignition setup for this project...).

I'd really like to hear from some of the guys out there who have had success with these engines and Afterburners. Feedback, experience and advice appreciated!

[pitciblackscotland]

That a nice big lump of a turbo Man she a heavy thing at 80 kgs, my T30 comes in at 47 kgs be careful with you back when moving her around i use my engine crane to help out. As for AB design i can not offer any advice as i need to get my engine up and running first. Have a look at some of threads here like GT6041 powered "Green Beast"..! may give you some ideas.

Cheers,
Mark.

[racket]

Hi MJB

Checkout jetandturbineowners.proboards.com/thread/680/diy-turbines

The good old "tank turbo" .............be careful with those temperatures, the wheel material is for "cool" diesel usage , not "hot" gas turbine work .

As for mass flow , maybe work on a max of ~1.5 kgs/sec to keep efficiency high on the comp to maximise your thrust output, 2.5 kgs/sec would be the choke flow at very poor efficiency .

Your NGV throat flow area might need to be measured to make sure it isn't too big or too small for the temps and pressures you'll be running .

Before worrying about A/B design etc , get the basic gas producer running and sorted with a simple jet nozzle , once you have jetpipe "energy" numbers then you're in a position to design up the A/B knowing that any problems you run into will only be caused by the A/B and not something with the basic engine.

Cheers
John

[mjb777]

Thanks for the input so far Gents.

In my haste and excitement to finally get this project underway my recollection of what was in the manuals grew a bit.

I went back through the manuals I have for this turbo, which is an ABB RR153 and John you were right on the money and 1.5kg/sec is much more realistic. The manual does state that my model is rated to 60k rpm and 750 deg C TIT however I will definitely try to stay away from that max TIT. I figure that the 503 m/sec tip speed at 60k rpm is safe for a forged billet machined compressor wheel?

My variant also weighs in at 60kg and the variant with a different style compressor and turbine housing weighs 80kg.......

Definitely agree that the core needs to be developed before the big AB gets built and hung on the back.

John would you assist with the NGV calcs and other you have mentioned?

[racket]

Hi

Yep , I can help with the "numbers" :-)

The turb wheel exducer is pretty "open" so you'll be needing to run a choked NGV throat to get those rpm up .

You're probably better to get temps up to the 750 C whilst reducing rpm back a tad to ease the turb loading , the lower tip speeds will generally improve efficiency a bit which tends to compensate for the lower pressure , the temps need to be high to keep the thermodynamics going .............so 750 C for the T I T and say 55,000 rpm which will still be producing 1,500 ft/sec tip speed .

I have a feeling I've got some numbers for a RR153 comp wheel , I'll look through my books and get back to you .

Cheers
John

[racket]

Hi

Found it , you're in luck :-)

Years ago my mate Andrew in Melbourne sent me up an ABB RR 153 rotor and NGV to measure up , so I'll go through the notes and calculations I did at the time to see what I came up with .

Cheers
John

[mjb777]

Wow thanks John,

your assistance would be very much appreciated. I have a compressor map for it which ABB used to have on their website but it is on an old pc that's in a box somewhere, (joys of moving countries), and ABB seem to not like to share that sort of thing anymore....guesstimations it is......

I would much prefer to not have to thrash the thing to get high performance out of it, although we all know what happens when things get exiting, so if the lower RPM works then that is great. that would give me safety room in the event the ECU takes a long time to develop and I'm the ECU for a while.

It would also be interesting to know what impact running the TIT back from max a bit would be, although I figure a TIT of 750 would result in an EGT of 600 or less which from my turbo experience is safe for even a lower end turbine material which I doubt ABB would have used given the quoted 100000 hour rotor life stated in the manual, and the price of these turbos. I have a spare rotor and bearings also, however I hope I don't have to use it!

Matt.

[racket]

Hi Matt

The NGV I measured up had 12 X 0.26" throats plus 12 X 0.39" throats for a total of 7.8 linear inches , height of vanes ~0.98", for a total area of ~7.6 sq ins , which I felt was a tad large ( ~15%) when used with my T I T of 850C , it would be even worse with a 750 C T I T .

Because the comp wheel has a rather large hub compared to a normal/smaller turbo the actual inlet annulus is "reduced" and I worked on a flow of ~3 lbs/sec which at 55,000 rpm I felt would produce a ~3.35 PR from the comp , this was based on our TV94 comp wheel map , a flow of 1.5 kgs/sec - 3.3 lbs/sec might be getting a bit high , but if the comp scroll A/R is configured for larger flows it might be OK , just another "unknown" to complicate things .

Your main concern will be the NGV , you'll need to measure the throats , hopefully you'll have 24 X 0.26" :-)

There should be roughly a 130 C degree drop required through the turb stage so a TOT of ~620C for a 750 TIT

Cheers
John

[mjb777]

Hmmm okay John, that's interesting.

I'm going to go out there and likely demonstrate my ignorance.....

I would have thought that a the larger NGV throat area would have helped mass flow and therefore thrust?

When I was a lad in TAA RR Dart and PW JT-9 assembly shop I recall the old fellas talking about how they "tuned" the NGV in the RAAF Vampire engines to get more thrust by opening them up. The downside was higher EGT and shorter hot section life but the pilots loved it. My pal who ran J34 Jet cars in the USA loved that story and promptly tore down the hot section of his engine and kept die grinding the 1st stage NGV until he'd raised EGT 100 degrees. He said it made a big difference.

I'm overseas for work at the moment, hence why I'm killing time working away on my engine plans, so wont be able to measure up my NGV for a while.

If I have the same 7.6sq" NGV casting in my RR153 then what are your thoughts regarding the sort of performance I'll achieve with it, if it even is worth converting to a jet?

Matt.

[racket]

Hi Matt

Easily explained , aircraft engines are generally configured to operate close to the surge line to maximise compressor efficiency and minimise Specific Fuel Consumption ( SFC) to increase range , endurance etc etc , by opening up the choked NGV throats the mass flow will increase, but as you've mentioned , temperatures have increased also , a 15% increase in temperature and fuel burn rate with say only a 10% increase in thrust has made the engine less efficient .

That extra 100 C degrees has created an "over temperature " , which is a big no no for an aircraft but "acceptable ??" for a land based performance vehicle with duty cycles of a few seconds at a time , including your engine :-)

Turbochargers are generally configured to have a broader operational range than a gas turbine , so there are some compromises , your comp wheel has an inlet annulus ( inducer area less hub) of ~13.6 sq ins , a mass flow of ~12.5 lbs/sq inch/min is a reasonable figure for a turbo at your Pressure Ratio , this then equates to a flow of ~2.85 lbs/sec , this could be squeezed a bit further to 3 lbs/sec without too much loss of efficiency , but 3.3 lbs/sec - 1.5 kgs/sec might be pushing things a bit far , you'd probably find your turb temps will start to spike due to the greater horsepower requirement from the comp wheel at those lower compression efficiencies, those lower efficiencies will require a greater pressure drop across the turb stage to supply the horsepower to drive the comp , leaving less pressure in the jetpipe for making thrust , you might find that thrust will be the same at both mass flow rates except you've had to burn more fuel to achieve nothing.

Your turbo is certainly a candidate for making a turbojet from regardless of the NGV throat area , it can always be "modified" , the main requirement for a thrust engine is a large hole at the front ,which you have , Pressure Ratio isn't as critical as long as its past a 3:1 ratio and even "lowish" T I Ts are more "forgiving" if an A/B is fitted as percentage thrust increase is greater if starting from a lowish TOT than a higher one as the percentage increase in thrust is based on difference in square root of absolute temperatures without or with A/B operating , normally its ~40% increase with a hotish TOT but gets closer to 50% with a cooler TOT, A/B max temps will be roughly the same regardless .

Cheers
John

[mjb777]

Thank you John,

Yes that makes total sense and it will be very interesting to see what thrust performance will be achievable with it. I would really love to build that magic 300lb thrust engine one day, but that would require one hell of a turbo or rotor!

Going on what you have said, I'm guessing that I can expect to make about 150lbs with a decent afterburner?

Plan is to build the combustor as per the legendary RCDON layout and calcs, for low pressure vaporisation fuel delivery and liquid fuel starting. And the more I think about it I'm keen to build the afterburner, yes yes I know that's a long way down the road, as per the type in the drawing below my engine layout sketch I hacked up on my Surface Tablet.......




Matt.

[racket]

Hi Matt

You should be able to produce ~140 lbs "dry" and ~ 200 lbs "wet" without much trouble.

Please don't use RC Dons combustor setup , it has some serious flaws , he does some beautiful construction work but the performance of the engine is below what it should be .

Having built a large vapouriser engine jetandturbineowners.proboards.com/thread/78/garrett-gt6041-powered-kart , the amount of work involved with the flametube would have been less if I'd used a single high pressure fuel nozzle, vapourisers are needed in our "micro" engines , but for a DIY turbo based engine I don't think I'd go to the trouble , my TV84 based bike engine with its single spray nozzle was a simpler and better setup

For afterburners , Andy M is the man to talk to , this is his "product" in action www.youtube.com/watch?v=oGV23aUU1bA&feature=youtu.be

Cheers
John

[mjb777]

Hmm okay, interesting.

I'm very interested to know your thoughts as to why that style limits performance? I'd rather avoid a complex and current hungry high pressure fuel system.....I figure with the amount of fuel I'll be moving with the whole thing at full noise a high pressure fuel system will require some serious pumps and power, as opposed to a low pressure vaporiser arrangement?

Is it also the bypass gap arrangement, over the conventional style of combustor where all primary and cooling air delivery happens via holes in the combustor that you think causes high losses?

If I'm to go down the high pressure fuel and conventional combustor road then I will be needing a pile of your wisdom! (however i just remembered i do have a can from a full size can annular type engine in my pile of jet treasure however it was a lot shorter than the RCDON formula required.....). perhaps I can stretch that in the right places to get enough combustion space prior to feeding it to the turbine housing? Just when you think the design idea is reaching maturity........

Are there details and pics of your TV84 combustor in here somewhere?

200lbs thrust and associated fire and noise......... oh yeah! 8)

Matt.

[racket]

Hi Matt

Yep , RC Dons combustor is "old design" , all air through the flametube wall for good control/distribution/metering .

Also his "truck and branch" vapouriser doesn't work as well as individual units where fuel distribution is better controlled .

The right angle bend going into the turb scroll also can cause problems , a straight in approach is less problematic with less losses.

The TV84 flametube was typical aero design , centre spray nozzle surrounded by air swirl vanes in a domed flametube head , louvered vents for wall cooling , it was built pre digital camera days so there might be some prints somewhere in my stuff here at home .

Cheers
John

[mjb777]

Oct 3, 2017 1:38:27 GMT -5 racket said:

Hi Matt

Yep , RC Dons combustor is "old design" , all air through the flametube wall for good control/distribution/metering .

Also his "truck and branch" vapouriser doesn't work as well as individual units where fuel distribution is better controlled .

The right angle bend going into the turb scroll also can cause problems , a straight in approach is less problematic with less losses.

The TV84 flametube was typical aero design , centre spray nozzle surrounded by air swirl vanes in a domed flametube head , louvered vents for wall cooling , it was built pre digital camera days so there might be some prints somewhere in my stuff here at home .

Cheers
John

I'd very much appreciate any pics or drawings you can share John. How does a "JetSpecs" derived non bypass combustor with a low pressure vaporisation setup sound as a compromise? ........

I'd like to stick with the parallel combustor as I'd really like to keep the thing "compact"....... Only power station turbines and APU's have combustors sticking out all over the place..... hehe.

Matt.