"Fat Boy" 12/118 thrust engine

[racket]

Hi Ron

The comp is already a "60 degree" swept back unit .

Cheers
John

[CH3NO2]

Hi John,

What was P2 with the diffuser and convergent nozzle?  
P5 with convergent nozzle?
What is the diffuser expansion ratio?

Just trying to get a rough idea of the NGV and turbine delta P to see how close it could be to choking.

Tony

[racket]

Hi Tony

My dilemma with the diffuser exhaust is that it helps get the engine fired up , but once running it starts to produce sub atmospheric pressures at the exducer , quite a few psi below , this increases the pressure drop across the turb stage which is good , but the lowered density at the exducer reduces mass flow , unfortunately density drops are more "linear" than gas velocity increases which have a "square root" in the equation .

I had an ~1.8 :1 area ratio with the exhaust diffuser cone , but that ratio was probably a bit greater if I measured back from the exducer annulus , probably >2:1 area ratio , the exducer was probably choking .

LOL.............this engine is giving me grey hair , or is it simply that its taking so long its just old old creeping up ;-)

Cheers
John

[ripp]

Hello John,
I think you're focusing too much on the mass flow through the turbine.

First and foremost, the too small turbine wheel must manage to drive the large compressor wheel.
For this purpose, the NGV wings must be arranged steeper( the direction of 20 ° ). even at the risk that it comes to a compressor pumps.

Second, you have to extend the combustion chamber as far as possible. every millimeter counts.
thirdly...
I know that I repeat myself but,if you do not solve some basic problems with your turbine you will continue to step on the spot

Yours faithfully
Ralph

translate.google.com



Hallo John,
meines Erachtens konzentrierst du dich zu sehr auf den Massenfluss durch die Turbine. in erster Linie muss einmal das zu kleine Turbinenrad es schaffen das große Verdichterrad anzutreiben.
Dazu müssen die NGV-Flügel steiler angeordnet sein (in Richtung 20°, auch auf die Gefahr hin das es zu einem Verdichterpumpen kommt. ...

[racket]

Hi Ralph

Unfortunately I have to focus on mass flow , if its reduced too far, the comp will surge .

Reducing the NGV angle will increase the tangential component of the turbine inlet velocity triangle and increase gas deflection and wheel power output , but lowering the angle reduces the mass flow that can be fed into the wheels inducer , at 20 degrees the comp will probably surge

The flametube cannot be extended , its at its absolute limit , theres only a finite distance between comp and turb wheels .

I've started sourcing a turb wheel with a larger exducer that I'll experiment with it in standard form first .

I appreciate your concerns and contributions on this dilemma of mine, the engine works up to a certain point then becomes "marginal" , the fact that it works at low Pressure Ratios indicate that the combustion/flametube isn't the problem as our engines combustion is always at its most vulnerable at low PRs , as the air pressures rise, combustion gets easier as the Combustion Intensity is reduced.

I need to increase the gas deflection to increase the wheels power output , I'm hopeful the use of the larger exducer with its standard blade angles will produce that little bit extra deflection and horsepower, once I have it running I can experiment with gradually increasing the shroud clearance on the exducer to restore mass flow , its a fine balancing act thats required between mass flow and gas deflection.

There are no larger turbine wheels available to use , so I simply have to make it work , or scrap the engine .................it would have been very easy to have gone with a smaller flow compressor wheel and had a running engine by now , but I've been there and done that, I needed a bigger challenge :-)

Cheers
John

[turboron]

John, I can not wait until you try the G trim wheel. Where did you source it?

Thanks, Ron

[racket]

Hi Ron

I've got the turbo guy at my local heavy duty diesel overhaul shop looking for it , I like to shop local if I can , but if he runs into problems getting it then I'll buy off the Net.

Cheers
John

[CH3NO2]

Nov 10, 2018 19:48:34 GMT -5 racket said:

Hi Tony

My dilemma with the diffuser exhaust is that it helps get the engine fired up , but once running it starts to produce sub atmospheric pressures at the exducer , quite a few psi below , this increases the pressure drop across the turb stage which is good , but the lowered density at the exducer reduces mass flow ....

Hi John,

Can you help me with the last part you wrote here?  
The diffuser reduces static pressure at the end of the exducer.   This increases the exducer outlet velocity, decreases outlet flow density and increases Delta P across the turbine.  Easy enough.
The part I dont understand is how a reduced exducer outlet pressure would reduce the total mass flow through the exducer or any part of the system.

Thanks!
Tony

Edit: I think I can see the potential for creating a standing shock just after the exducer, if the conditions are right, but i still can't see the mechanism for reducing the mass flow by decreasing outlet static pressure.

[racket]

Hi Tony

Now where do I start, I've got some numbers scribbled in my journal from ~18 months ago but it might be easier to do a set of numbers .

I was measuring ~4 psi below atmosphere from a static port , so 10.7 psia "ambient??" , so lets use 38.6 psia entering the NGV , giving us a 3.6 PR across the stage from a 1173 K T I T .

If we run a choked NGV as well as a choked exducer we should be getting down to "ambient" static at the exducer , a 3.6 PR from 1173 will give us a temp drop of ~319 C degrees so a temp of 854 K , density 53.2 cu ft/lb at our 10.7 psi static..................if we have say ~half our temp drop producing the gas velocity out of the exducer , lets use 160 C drop at 90% at 0.28 gives us ~1906 ft/sec , if I've got say 2.5 lbs/sec flow that equates to 133 CFS at 1906 ft/sec , I'll need an area of 0.0697sq ft or 10.04 sq ins of exducer throat

But if we use our 38.6 psia P3 and 14.7 psi out then we have a total PR of only 2.625 , a temp drop of 249 C and 924 K out at a density of 41.87 cf/lb or 104.6 CFS .

Lets again split our 2.625 PR equally for NGV and turb wheel , so 1.62 PR each and a temp drop of ~125 C degrees in the wheel , with velocity of 1685 ft/sec requiring 0.062 sq ft or 8.93 sq ins of exducer.

If all things being equal , and if the exducer was at 8.93 sq ins, then with the diffuser in place mass flow would be reduced by ~11% .

LOL..................I think thats right, they're only rough numbers , my real world ones are much more complicated due to very high radial velocities into the wheel which ten requires less pressure drop to choke the exducer .

Cheers
John

[turboron]

John, I understand you to say that if the inducer and exducer throats are not matched then the smaller throat sets the mass flow. However, once the throat reaches sonic it is only the upstream pressure that sets mass flow. Therefore, the exit pressure at the exducer can not set mass flow. Pressure ratio is not a factor is setting mass flow once the critical ratio is reached (2.4/1 for air). Once critical ratio (when the throat goes sonic) is reached to increase mass flow you must raise the upstream pressure and thereby density.

Thanks, Ron

[racket]

Hi Ron

Yep , once choked things change , that was what happened with my TV84 engine , the exducer choked when running with the grossly oversized 1.84 A/R scroll , this kept mass flow under control and prevented the comp flowing at low efficiency over in the choke side of the map , even when a smaller 1.6 A/R housing was used , the exducer was still the controlling "throat" and thrust and running parameters didn't change to any degree, it was very confusing .

After I clipped the exducer back by half an inch , the exducer was no longer the control throat and changing scroll housing A/Rs made large differences to the running parameters , the engine started behaving as the text books said it should , the 1.84 A/R was impossible to use as comp flow went into choke , the 1.60 A/R housing temp'ed out the engine at a P2 of 33 psi , the 1.39 A/R housing achieved 38 psi before overtemp but the 1.23 A/R allowed 45 psi P2 , the 1.23 A/R housing throat now kept comp flow under control as the scroll throat was probably running choked.

It can become a juggling act getting the balance right , the 12/118 has very high actual inflow and outflow speeds due to the NGV and exducer angles , this will exacerbate the situation especially the exducer flow , theres a chance I have actual exit axial velocities of >1,600 ft/sec which represent a huge amount of dynamic energy that any diffusing exhaust can turn into static pressure which only needs to be 14.7 psi at the exit , at the inlet to the diffuser the static pressure will be a lot below atmospheric .

I'm not fully up on the maths with these sorta things , and trying to measure parameters is a battle whilst running the engine , I suffer sensory overload in huge amounts .

Cheers
John

[turboron]

John, LOL. Your last sentence kills me. So true. I would hate to see our faces when things are going South.

Thanks, Ron

[racket]

Hi Ron

Yeh, I need an extra pair of eyes and a younger brain .

I'm hoping that by running the standard 112 mm exducer that it'll become the "throat" and my current NGV will be "oversized" , I've got some turb flow map data for the 106 mm exducer so should be able to get a rough idea of what the larger one will flow , then if the comp is OK with the "reduced" flow and temps are reasonable , I'll start increasing the exducer shroud clearance a couple of millimetres at a time to see what happens .

LOL..........its a learning process :-)

Cheers
John

[racket]

Hi Guys

I've dropped the 12/118 engine out of its test stand and pulled the outer can off it and things still look OK inside , so thats a relief .

I'll wait until I have the new turb wheel in my hot little hand before going any further , its really only "out with the old and in with the new" , then check exducer shroud clearances are in spec and we'll be back in business.

LOL..................am I game enough to predict the outcome , better not as it could prove embarrassing , but I'm mildly confident there'll be "improvements".

Cheers
John

[Johansson]

Fingers crossed!