GTCP85-98C APU, with afterburner?

[tacobell]

Hello all,

Long time listener, first time poster.  I used the search function but couldn't find anyone with a similar build / question that I have.

I've had my GTCP85-98C APU from a 727 for a few years now, its mounted to a stand, runs awesome as is should, and I even built a variable geometry nozzle to add a bit of thrust to it.  Works ok, but not much of a party piece.  Even though it does run at a governed 100% RPM, I can regulate the speed somewhat with a needle valve on the fuel supply line to dump excess as needed. 

I'd like to build an afterburner for it, seems straight forward as far as fuel nozzles, ignitor, low pressure area for a flame holder, but the art of sizing is eluding me.  Not so much on sizing the ducting, as I've seem Andy Morris's videos on that, but more so figuring out how much fuel the AB should have? 

Any suggestions, things to look out for, ideas, are welcomed.  

TIA

[racket]

You'll need twice the engines fuel burn rate for the afterburner

[richardm]

There's a better way to control that engine speed. Refer to the manual and you ll find a "fuel scheduling valve" It works from the compressor discharge pressure to add or lessen fuel in response to the load. It is also connected to at least one of those probes int the exhaust flange. That probe is a thermostat that will let some of that compressor discharge pressure escape more or less to control the outlet temp.

So if you tap in this compressor discharge air and use a needle valve you can vary the comp. discharge pressure and therefore control engine speed. But dont go to low on speed and fall below self sustained speed And also monitor the outlet temp closely when experimenting with speed. Those things were not designed to be throttled.

Also be aware that Honeywell once issued a warning of non responsability to those playing with APU . Honeywell stated that since APU are designed to run at a constant speed range they were not tested for harmonic vibrations at any other speed. And of course they dont endorse any of that playing around with APU ..

Now about your afterburner project. As you know you can get quite a bit of bleed air from those engines If your not using any bleed air then your APU is kind of idling , not using a 100% of its capacity. So I would try to build an afterburner to wich that bleed air would be added . More air, more fuel, more thrust .Maybe you will end up with turbine back pressure problem but might be worth trying with an appropriately design burner .

[tacobell]

racket, very simple answer, I like it! Blending with richardm's comment about using more of its capacity, you're saying that it will produce more thrust simply by 'loading' the engine more, correct? And that loaded fuel rate is what I should look at doubling for a sustained AB burn? I do have the differential bleed air valve functioning on this unit, but is bleeding the compressor air the best route? It seems like I'm just dumping good compressed air that would otherwise be used in the combustion process? What about loading the onboard generator, or even adding a fixed propelling nozzle? When I close the variable nozzle on the unit's exhaust duct, I can see the fuel pressure increase, and of course thrust increasing as well.

I did play with using the needle valve to bleed air off of the compressor discharge line that feeds the governor, but didn't notice any different 'loading' characteristics. I found it a bit more difficult to control speed wise. I suppose that I'm not too worried about if it runs at a constant speed or not, I don't want to damage any of the internals. But it is interesting that you say that it should be loaded to produce more thrust, and then add AB on top of it. Twice the fuel burn is a good place to start. What sort of pressure's are you seeing for JetA or Diesel fuel?

I'm not sure if you fella's have seen the YouTuber Tech Ingredients? He also has a small series on turbocharger based gas turbines, as well as a segment on afterburners. Whats interesting to me, although he doesn't touch on fuel flow at all, is his design. He uses a straight duct, and just added metal strips perpendicular to the exhaust flow to create the flame holder and LP areas. Andy Morris's design uses an expanded duct to create the LP area, whats the difference?

Sorry for the long wind here, its just exciting times lol

[racket]

Hi

Please do NOT use any info from Tech Ingredients , he sounds like he knows what he's talking about but in reality he doesn't when it comes to turbines :-(

Don't worry about using the engines exhaust for the afterburner , use maximum bleed air , you'll make more noise and you'll be able to use a CD nozzle and get shock diamonds

Have a read of this Paper ntrs.nasa.gov/api/citations/19930082793/downloads/19930082793.pdf

Our dump style A/Bs use the dump annulus as the flameholder , it has pros and cons , its simple but but probably not as good as having V gutters .

Cheers
John

[richardm]

An APU is designed to run at constant speed not constant power.I would get rid of the generator hydraulic pump ( if fitted) . IMO before fitting an afetrbuner to produce more thrust one would first want his engine to run at max power. Best way to do this is to fully open the bleed valve.You now have an engine running at near its max capacity .Now this seems a waste of energy unless you reintroduce that bleed air "behind" the engine in an afterburner.

I think it's possible to design such an afterburner. You would get about 1 lb/sec of "fresh air" at about 40 psi added in the afterburner to either burn more fuel or cool it down a little. It would also be possible to modulate the bleed valve in accordance with the afterburner fuel schedule. I know It sounds complicated but it's just opening to experimentation.

Curious about what Racket think about my idea..He has been experimenting a lot he knows better than me.

[racket]

Hi Richard

Yep , simply feed the bleed air to a "combustor" , with 1lb/sec of air you'd be needing ~4 lbs/minute of fuel , a suitable sized jet nozzle on the outlet , and let the fun begin .

The actual APU exhaust can be neglected as there probably won't be much pressure energy left in it with full bleed air .

Theres a drag bike in Europe I think that has a bleed air "afterburner" for thrust power , 40 psi pressure will sound more like a rocket than an afterburner because of the extra efflux velocity :-)

Cheers
John

[richardm]

Hi John! Yes probably not much thrust left from the apu as they are designed to extract energy for other uses I would get rid of all unnecessary stuff like the generator etc Even if not loaded these implies mechanical losses just to make em turn. Loading the generator as tacobell suggest would be a pure lost.

[tacobell]

Fellas, you got me hook, line, and sinker as they say. I'm sold.

The fabrication part seems easy enough, I've already made a duct to attach to the bleed air valve for previous testing purposes, so routing that back in to the AB duct is no problem. I like the idea of regulating the bleed air output further via a manual valve, which will also help with the engine EGTs. Even though it has a functioning differential valve, through a 4" pipe the temps skyrocket.

Here's the problem though, I'm not anywhere close to as knowledgeable as you guys on this topic. Is it possible to get a rough sketch of design, duct sizes, fuel flows, pressures, etc, needed for such an epic project? Or of course references / resources that a non-engineer type can understand lol.  Measure and math, I got it, but no idea how to derive the correct formulas. I can get all of the technical engine info since I have thousands of pages on it from Boeing and Airesearch (I have lots of aviation friends 😁) if needed.

[racket]

www.youtube.com/watch?v=yUG_kDhbv54 the first few seconds show the bleed air combustor above the engines open exhaust .

Once you know the mass flow and pressure , assuming complete combustion of the air and its temperature its possible to calculate the required jet nozzle throat area , it'll run choked and will require a divergent portion downstream to fully accelerate the flow .

The combustor can be similar to our normal sore thumb ones , but with no dilution air holes as we want maximum temperature , some bleed air needs to be used as a cooling blanket along the combustor/nozzle , but otherwise its a simple sort of engineering exercise, your engine has all the controls to keep it happy at full bleed air supply .

Cheers
John

[ripp]

Aug 15, 2024 19:18:03 GMT -5 racket said:

www.youtube.com/watch?v=yUG_kDhbv54 the first few seconds show the bleed air combustor above the engines open exhaust .

Once you know the mass flow and pressure , assuming complete combustion of the air and its temperature its possible to calculate the required jet nozzle throat area , it'll run choked and will require a divergent portion downstream to fully accelerate the flow .

The combustor can be similar to our normal sore thumb ones , but with no dilution air holes as we want maximum temperature , some bleed air needs to be used as a cooling blanket along the combustor/nozzle , but otherwise its a simple sort of engineering exercise, your engine has all the controls to keep it happy at full bleed air supply .

Cheers
John

that sounds good

www.youtube.com/watch?v=4EZJkwWHQ8M

[racket]

Yep , thats the video I remember , thanks for the Link :-)

Nice shock diamonds

[tacobell]

Wow that is very cool! I would love to see a cutaway of that upper duct, how it converges / diverges, ignitor location, fuel, flameholder, etc. That looks like it packs quite the punch. Did they ever release any performance numbers on that build? The articles that I've read only mention 2:1 power to weight ratio, but I'm guessing that includes the rocket motors.

What would the advantage of a completely separate "superburner" be, compared to adding the bleed air to the main exhaust duct, and lighting it off in there? I suppose if its completely separate, you don't have to worry about heat as much?

I will retrieve some of the numbers for the GTCP85 bleed air output today, and maybe we can start putting together a basic design to start with? This isn't a max performance build, yet, just something to function and work off of as a baseline. I haven't read through that NASA article just yet, its been a busy week, but I do intend to this weekend.

Thanks again for all of your help and direction on this so far, I hadn't even considered using the bleed air for that!

[tacobell]

Here are the power output specs of the unit, not sure if those are helpful? This APU is fitted with a 40kVa AC generator, which will be removed since the AVR is mounted in the cockpit somewhere. All other accessories are required for operation - governor, fuel pump, oil pump, cooling fan, starter.  The fuel used is JetA1, if that helps.

Pneumatic Power Output Only
Bleed-air flow ..................................................... 124.2 PM
Bleed-air pressure .......................................... 94.7 IN. Hg ABS
Bleed-air total temperature ...................206° to 234°C (403° to 453°F)

Combined Mechanical and Pneumatic Power Output
Shaft power ........................................................... 50 HP
Bleed-air flow ................................................... 107.7 PM
Bleed-air pressure ........................................ 99.4 IN. Hg ABS
Bleed-air total temperature .................209° to 238°C (409° to 459°F)

Shaft power .......................................................... 100 HP
Bleed-air flow .................................................... 88.3 PM
Bleed-air pressure ........................................103.9 IN. Hg ABS
Bleed-air total temperature .................214° to 242°C (418° to 468°F)

[cebaero]

Aug 14, 2024 19:57:30 GMT -5 racket said:

Hi

Please do NOT use any info from Tech Ingredients , he sounds like he knows what he's talking about but in reality he doesn't when it comes to turbines :-(

Don't worry about using the engines exhaust for the afterburner , use maximum bleed air , you'll make more noise and you'll be able to use a CD nozzle and get shock diamonds

Have a read of this Paper ntrs.nasa.gov/api/citations/19930082793/downloads/19930082793.pdf

Our dump style A/Bs use the dump annulus as the flameholder , it has pros and cons , its simple but but probably not as good as having V gutters .

Cheers
John

Thats really sad to hear about Tech Ingredients, his videos are what got me into building my own turbine