"Fat Boy" 12/118 thrust engine

[turboron]

John, I run some numbers modeling the exhaust blower as a centrifugal blower. For your 3.6 pounds per sec mass flow, a suction pressure of 14.17 psia, a suction temperature of 1000 degree F, a discharge pressure of 14.7 psia ( pressure ratio 1.04), a specific heat ratio of 1.4 ( I could not find a better number) and a polytropic efficiency of .8, I calculate 8,233 actual inlet cubic foot per minute and 23.27 gas horsepower. Where am I going wrong?

Thanks, Ron

[racket]

Hi Ron

Probably nothing wrong with your numbers if using that suction pressure and temperature , but its only half a psi below ambient, which if we do the PR across the turbine stage from say a 3:1 PR going in , so 44.1 psia down to 14.17 psia only increases the PR to 3.11 :1 , so a 0.11 increase in PR , I was working on ~5 times that PR increase and at ~1,500 degs F , hence my >100 HP requirement.

With the diffusing exhaust in place , I've probably got my 0.5 extra PR across the stage due to the very high velocities ( 1500 ft/sec) exiting the turbine wheel .

The "induced draught" fans on some of the steam boilers I've worked on have had some 100 plus horsepower just to get a fraction of a psi "suction" into the fire .

When we do the calcs for the 12/118 comp wheel the numbers start getting kinda big at ~350 HP for a 3.5 PR at 75% comp efficiency ..........Heh heh , no wonder the poor old turbine wheel is struggling :-)

Cheers
John

[turboron]

John, what would happen if you injected water downstream of the turbine? It would increase the mass flow but lower the temperature of the exhaust gas. This could reduce the size of the exhaust blower. I am not sure what it would do to the horsepower. If I have some time this evening I will attempt to make some numbers. If we can get the engine running at or near power perhaps we can determine the best corrective action.

Thanks, Ron

[racket]

Hi Ron

Unfortunately downstream injection would indeed lower the temperature but also the "energy" available for making power/thrust , I've got to be able to get the engine running, but for it to also have a power output, otherwise its just a fuel burner useful as a room heater only .

I'm seriously considering temporarily refitting the diffuser exhaust as its got the immediate potential to lower my temperatures by ~100 C degrees , but it won't fix the combustion "problems" , but it will provide me with a bit more safety margin whilst doing so .

Cheers
John

[turboron]

John, I vote for this approach: "A number of the RC micro jet engines had tangential "swirl jets" fitted to their flametube primary zones to promote better combustion". Silver braze 1/2" tubes 3/4" long that protrude into the liner between evaporators to improve swirl and complete combustion. Locate them axially between the primary and secondary holes.

Thanks, Ron

[racket]

Hi Ron

Hopefully the tangential propane injection at the primary zone will do the same job , I'll be looking at using ~1 mm ID injection syringes and a flame length of 150 mm from each of the 9 injectors, this should get things rotating inside, as well as breaking up and mixing the kero "vapour/mist" coming from the evap outlets the same as the usual RC micro jets swirl jets do.

I don't want to disassemble the engine at this time, something I would have to do to fit "tubular" jets into the FT wall , but something to consider at the next full disassembly .

Cheers
John

[ripp]

Nov 16, 2016 15:53:38 GMT -5 racket said:

Hi Ralph

LOL.......yep , the comp inducer is too big for the "standard" turbine wheel , but the modifications to the exducer should allow the quantity of gases to pass .

Opening any sort of "bypass/dump" will most likely exacerbate the situation due to the reduction in mass flow passing through the turbine wheel compared to the compressor ...........let me explain a bit more on this ............we've expended energy ( HP or Kw) in compressing the air that we bypass/dump , that energy has to be produced by the turbine wheel , the turbine energy is produced by, mass flow times gas deflection, if we've reduced the mass flow then we need to increase the gas deflection , and the only way to do that with fixed NGV vanes is to increase velocity , and to do that we need a higher temperature , pressure drop being already at maximum.

An increase in temperature reduces gas density which means more bypass required which in turn creates more problems , I'm in a no win situation .

On my Garrett TV84 turbo engine back in October 1998 , I had fitted a bypass between the "sore thumb" combustor and jetpipe which consisted of a 20mm pipe and gate valve , the engine at that time was fitted with an "undersized" turbine stage , at a latter date after some expensive surge inducer failures a larger stage was fitted , the bypass was an attempt to control surge .

When the bypass was opened at a P2 pf 20 psi , TOT increased from 520 C to 760 C , P2 dropped to 12 psi , not a good outcome :-(

There is one more thing to try , and that is LPG/propane fueling , either as an "extra" to add some speed to the kero combustion or to completely replace the kero .

If that fails , then I'll redesign the engine with a large capacity bleed air system in conjunction with fitting a standard turbine wheel and a NGV with "lower" angles to produce more gas deflection from the stage so as to produce the required horsepower to power the comp , there will be full expansion through the turbine stage which will be equipped with a diffusing exhaust to maximise the pressure drop .

The bypassed air at full P2 pressure could then be fed to a separate combustor that supplies hot gases for either thrust or shaft horsepower through a "smallish" freepower wheel .

The engine isn't "dead" just yet, I'm not giving up on all the potential airflow that the 118 mm inducer promises :-)

Cheers
John

Hi John,

My suggestion to open a detour was my thoughts on this Apu



I am also of the opinion that the large compressor wheel needs all the power the turbine can deliver (new angle in ngv).

In the rc turbines I noticed the following formula

Compressor inlet area = combustion chamber inlet area (all bores and evaporator pipes) = outlet area of the axial turbine.

For a turbine engine with a radial turbine, it will probably be the same.

The constriction starts with the ngv = tip height 19.56mm? Would be an approach this to enlarge with milling 1-2 mm away ?






Chees
Ralph

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[ripp]

Nov 17, 2016 15:45:32 GMT -5 racket said:

Hi Miuge

I think I'm OK with the deflector , theres a fair bit of distance to the first holes and there is some combustion happening from the colouration of the flametube , but maybe not fast enough combustion.

A number of the RC micro jet engines had tangential "swirl jets" fitted to their flametube primary zones to promote better combustion , the proposed LPG gas injection through 9 tangential injectors should do the same job of creating swirl within the flametube as well as introducing a highly combustible fuel that should speed up the kero combustion ...............LOL , maybe :-)

Cheers
John

Please remove the air divider only for one test. This seems to block the flow from the diffuser.

To check the pressure at the end of the inside ring of the CC, you can install a brass pipe Ø 3mm.

tube by the tertiary holes.

Many modell builder of rc turbine have gone away from swirl, also I had this flame image (yellow flames)

www.youtube.com/watch?v=kG8mxsBJ7j4

and now the same as this: www.youtube.com/watch?v=tccqPOTJgqU&t=60s

or: www.youtube.com/watch?v=d4_Jc4kRP54







Cheers
Ralph

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[racket]

Hi Ralph

Thanks for the suggestions , but sorry , air divider stays in place , I don't want a repeat of my 10/98 engine experience jetandturbineowners.proboards.com/attachment/download/25 .

The model turbine flametubes are generally about as long as they are big in diameter , my flametube is only half as long as wide , there simply isn't room to have anything longer , hence I've got to use "unconventional" means to make the flametube work ...........compromises :-(

Cheers
John

[racket]

Hi Ralph ,

The APU in the pic has a 2 stage turbine to power the comp as well as a diffusing exhaust to maximise pressure drop across the turbine , the axial exhaust gas velocity out of the second turbine wheel will be relatively slow , possibly 6-800 ft/sec , I'm trying to make my wheel work with 1,500 ft/sec velocity coming out of it , this velocity is required so that the design mass flow can pass otherwise the comp will go into surge .

The NGV angles and throat areas have to be what they are for the same reason , if I change anything there'll be consequences , I'm already designing for a mass flow slightly to the lowish side of its potential , the turb wheel even with the mods made is just marginal , I'd like to have a bigger wheel , but thats not so easy to come by :-(

Cheers
John

[racket]

Hi Guys

Bit of an update .

The new LPG gas manifold made and installed .

Silver soldering the syringe needles , a fiddly job


Completed manifold



mounted onto some angle pieces welded to the front wall of the flametube, then lockwired in place through holes in the angle pieces .







I've reinstalled the diffusing exhaust to the shroud/exhaust stub and fitted a static pressure pickup port at the "throat" , as I'd like to see what sort of influence the diffuser has on exit pressure from the wheel .

Cheers
John

[jetjeff]

Hi John,

Fingers crossed. Hmmm, if the new manifold using LPG doesn't work, I'm wondering if you couldn't use this manifold as your water injection ring?

Regards

Jeff

[racket]

Hi Jeff

Maybe , but I think I'd prefer any water was injected away from the primary zone , hopefully this will help .

Cheers
John

[miuge]

I hope you get improved results with the LPG manifold. If it works, would you consider adding "swirl tubes" in those primary holes? 90deg bent tubes catching the flow coming from the comp and directing it the same way as your LPG needles.

[racket]

Hi Miuge

Something like those would be good , if the current mods don't fix things I'll probably have to do a complete disassembly which will allow some more drastic mods like that to be be made

Cheers
John