"Fat Boy" 12/118 thrust engine

[racket]

Hi Monty

Yep , still having fun??, another testrun scheduled for tomorrow morning , I'll be up early to get the oil tank heater plugged in, and the starter's battery pack topped up with every last Volt whilst I have some breakfast.

With winter getting close in the northern hemisphere there'll be a fair few more of the turbine fraternity heading back indoors , you'll be in good company ................ I'll be looking forward to seeing more of your Build Thread activity, its going to be one beautiful engine you're building.

Heh heh , just thought I'd expand on my "contributions" with some hard earned life experience ;-)

Cheers
John

[racket]

Hi Guys

Just back from an early morning testrun , just as well as the thunder and lightening started by the time I'd finished at 7 o'clock , came home wet but happy .

A bit of video showing exhaust temps during a spoolup and higher power settings youtu.be/5jhGLn-9EfU

Cheers
John

[racket]

Another view of spoolup , it went off without any problems , the new needle valve control of the spoolup fuel worked well .

youtu.be/L0fnYKqKS_I

[racket]

And a bit more youtu.be/1zdMRrUPZ_E

Unfortunately all the video I was attempting to take of engine temps, pressures etc didn't happen , I guess my finger slipped off the button and I didn't notice in the heat of the moment :-(

[pitciblackscotland]

Hi John,
Very nice Nice glow on the free power wheel Di you get Linda out again to take videos

Cheers,
Mark.

[racket]

Hi Mark

Yep , my long suffering partner was by my side , providing moral support as always even though the engine scares the life out of her .

Cheers
John

[racket]

Hi Guys

To really blow me away , this arrived in the mail late yesterday



All I can say is ......Thank you Hurdman you're a very generous guy.

LOL...........I think I should spend the rest of the day sitting quietly and counting my blessings.

Cheers
John

[azwood]

That's awesome can't wait to see what foot-pounds you get with the valve shut.

[finiteparts]

Hi John,



Glad you got it to spool up, but that turbine looks pretty hot for a LPT, especially a shrouded disk. I would be very nervous about doing another stall test with a good wheel.

Just eyeballing it here, but it looks as though you are already near 1500F with the wastegate open.



I don't have a book on the 250C30, but the C20 manual has limits for Tt5 (turbine inter temp) under 1490 F and then only one second max to 1700F is allowed.  My guess is that the C30 isn't that far from this being that it is uncooled  and very long spans on the LPT stages.  The fourth stage wheel would see an even lower temp since the third stage would have done some energy extraction and then it would also have the benefit of only seeing the relative gas temperatures if it was spinning. It appears that you might be running that wheel a bit sporty and I would hate to see another one get damaged.  We both know that when you shut your wastgate, it will only get hotter.

I was looking at the damage to the wheel and I had a few thoughts.  The first thing is that it appears that the outer shroud of the turbine is one continuous ring.  Some of the newer disks have saw cuts in the shroud to relieve the hoop stress, but as far as I can see, your wheel did not have these cuts.  This leads me to my theory of what may have happened.



Hopefully the simple drawing will help to portray the steps.  Since the blades are immersed in the hot gas flow, they respond much quicker to the temperature changes than the shroud or the hub.  So in step 1, the blades get heated and the thermal expansion causes them to grow, but the disk is still cold so it resists their outward growth.  The shroud ring also does not respond to the gas temperatures as fast and tries to resist their outward expansion.  This puts the blades into a strong compressive stress and may have buckled a few (they look bent).

In step 2, the shroud catches up to the blade temperatures through a combination of conduction and convective heating and relieves the blades compression.

Then again in step 3, the blades respond to the now cooler gas temperatures flowing through quicker than the shroud and now the shroud is trying to keep them out at the larger diameter.  This puts the blades into a state of tensile stress and likely, since we see the blades fractured in half, this was above the ultimate tensile state of the blades.

This is a common thermal cycle that shrouded disks experience, but usually the stress levels are low enough that this builds slowly and the low cycle fatigue (LCF) of each start and stop doesn't lead to fatigue failure for several thousand cycles (well after the wheel is "timed" out).

Now there may be other ways this could have happened, this is just one proposal.  But I think it gives some good talking points.

1.  These wheels are used.  They may be timed out, which means they already have a large built in fatigue that could have been on the edge of failure anyway.

2.  Thermal transients a VERY hard on component life.  We should make every effort to move the throttles slowly and allow the parts to get into thermal equilibrium as much as possible.  Very fast temperature changes lead to very high internal stresses and very quick fatigue failures.

3. We should do our best to make sure the components that we are using are not being used at conditions that are more extreme than the original design intent.

4. Stationary parts receive higher heat loads than rotation parts.  The relative temperatures that the blades see while spinning around are reduced from the total temperatures that we see in the stationary frame of reference by the kinetic energy required to get the gas onboard  the rotating frame of reference.  I think the stall test would be ok if you had similar gas temperatures to the original design intent (which I would guess are more like 1200-1300 F), but stalling the blades in a overheated condition is asking for trouble.

Good luck!

Chris

[racket]

Hi Chris

Yep , you're spot on with your diagnosis , the hung starts that occured during the previous testing were grossly over temp and quite unexpected , the damage was done in an instant due to the very thin blade section on the Allison wheel .

The one bright spot with having a dead wheel installed is that I don't need to worry about killing it , it allows me to "experiment" .

I'm seriously looking at water injection into the interstage duct at higher power setting above a 3:1 PR , the range where I'm currently getting "colour" from the wheel , the gas producer turb wheel is still OK at the temps being run , but the 4th stage wheel needs considerably less as you mentioned ,..... years ago I'd calculated a temp near 700C for the C20 4th stage design temp if exhaust was at the desired 627C and interstage 793C .

The spoolup this morning was "text book" , reasonably quick without any visible flames and a lowish idling temp in the 400s C at ~1.7 :1 PR , if that can be replicated in future it should negate some of the potential problems .

Then as the power comes up the wastegate can be closed as long at interstage temps don't spike , and the water injection can keep the freepower from colouring up, but with full mass flow , I don't like wasting all that energy.

Heres an interesting article on fast spoolup and power production for Allison engines
www.researchgate.net/publication/283673973_Engine_quick_start_in_case_of_emergency_-_A_requirement_for_saving_fuel_by_means_of_engine_shutdown

More experiments to follow :-)

Cheers
John

[azwood]

Nov 5, 2019 3:37:58 GMT -5 racket said:

Hi Chris

Yep , you're spot on with your diagnosis , the hung starts that occured during the previous testing were grossly over temp and quite unexpected , the damage was done in an instant due to the very thin blade section on the Allison wheel .

The one bright spot with having a dead wheel installed is that I don't need to worry about killing it , it allows me to "experiment" .

I'm seriously looking at water injection into the interstage duct at higher power setting above a 3:1 PR , the range where I'm currently getting "colour" from the wheel , the gas producer turb wheel is still OK at the temps being run , but the 4th stage wheel needs considerably less as you mentioned ,..... years ago I'd calculated a temp near 700C for the C20 4th stage design temp if exhaust was at the desired 627C and interstage 793C .

The spoolup this morning was "text book" , reasonably quick without any visible flames and a lowish idling temp in the 400s C at ~1.7 :1 PR , if that can be replicated in future it should negate some of the potential problems .

Then as the power comes up the wastegate can be closed as long at interstage temps don't spike , and the water injection can keep the freepower from colouring up, but with full mass flow , I don't like wasting all that energy.

Heres an interesting article on fast spoolup and power production for Allison engines
www.researchgate.net/publication/283673973_Engine_quick_start_in_case_of_emergency_-_A_requirement_for_saving_fuel_by_means_of_engine_shutdown

More experiments to follow :-)

Cheers
John

Guess that's a plus for my turbo based wheel it's dam solid it probably weights 5kg+

[racket]

Yep , your freepower is a lot more "heavy duty" , the Allison wheels are a tad "fragile" by comparison

[azwood]

Nov 6, 2019 22:32:50 GMT -5 racket said:

Yep , your freepower is a lot more "heavy duty" , the Allison wheels are a tad "fragile" by comparison

I just hope it wants to slow down when I back off there's a lot of rotating mass thair

[racket]

Hi Guys

Wastegate valve has its own "throttle" now .



Finally found a use for that old bike twistgrip I've been carting around for 30 years , thats the biggest throttle valve its ever operated :-)

Now for the interstage water injection

Cheers
John

[turboron]

John, we never threw anything that might be useful away on the farm when I was growing up. You never know when your going to need the bit of baling wire. I still have the habit today. Drives my wife crazy.

Thanks, Ron