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Post by musicislfe on May 30, 2020 13:25:49 GMT -5
Hello! I’ve been working on a Small 52mm jet. I seem to have a problem with over heating of the rear bearings. I’m using a oil/fuel mix and start with propane. I’m doing everything possible to avoid adding another input line that does the cooling/oil feed. Ideally I’d like it kero start but that’s another thread. Ive tried widening tolerances to allow for expansion. I’ve tried adding some cooling holes to cc with minor changes (better) to heat and performance. I’ve tried adding some air holes in axle hub that just ended up focusing heat directly on axle hub causing the rear aluminum bearing housing to melt. I’ve tried adding wave springs to the front of axle to allow for horizontal expansion. I’ve tried Widening the gap between the ngv and the turbine to allow fuel to Better reach the rear bearings. in fact I have no problems when cold starting and for the first minute or so runs like a champ. But the heat overtakes the rear bearing. Seems the heat transfers from the turbine to axle and bearing, as they are all touching. ive attached a video. (Fist attempt, as my camera man has abandoned me) I’ve since extended run time to about 1 minute prior to severe overheating. Could it be too much fuel? Not adequately pressured fuel pump causing too much fuel delivery? I’m left with the ultimate solution that I want to avoid. Adding a Input for a cooling system. And all the pumps and accessories takes that go with it. *sigh* i appreciate the responses and help. Thank you.    
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Post by racket on May 30, 2020 18:19:02 GMT -5
Hi I'm assuming you've built the engine to a known design.............yes/no ?? Overheating at the turbine end probably means theres no cooling air getting to that area . On my engine builds I drill holes through the NGV wall so as to "inject" air into the turbine wheel hub area , postlmg.cc/GBm3d2HH but the engines have full pressure lube systems so don't need to cool the bearings with air . On my FM-1 engine jetandturbineowners.proboards.com/thread/576/fat-lady-fm-1 I originally tried using ball bearing lubricated by some fuel/oil bleed along with compressor discharge air to cool the bearings but ran into lotsa rear bearing failures from overheating etc . I had the air bleed hole to the bearings close to the shaft tunnel in the back of the diffuser wall as per most RC micro engines , but there was an inbalance in air/gas pressures and the hot gases were feeding into the turbine bearing . To fix the problem I installed two "snorkels" from those air bleed holes out to the diffuser discharge position , the openings of the snorkels were pointed towards the diffuser passageways and received full ram air pressure exiting the diffuser , this cured the problems . There was a seal between the shaft tunnel and the back of the compressor wheel so that air could not exit forward into the low static pressure region behind the comp wheel , the staic pressure behind the comp wheel is only around half that of the combustor area so care needed to prevent cooling air going forward rather than rearwards . Your video does appear to show a bit too much fuel too soon , also gotta keep the starter power on until the engine is well past self sustain and making a few psi of pressure from the comp . Cheers John |
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Post by musicislfe on May 30, 2020 20:15:12 GMT -5
Racket I appreciate the response, Yes the engine is borrowed from a known design. Close to the shaft tunnel? Or do you mean behind The diffuser In the shaft tunnel? I’m going to try eliminating 50% of the incoming fuel by pinching off some fuel manifold line. I think I’m getting to much fuel because the pump is fighting the back pressure. Have a new one on the way. Thank you for the input.
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Post by racket on May 30, 2020 22:14:36 GMT -5
Hi
Which design are you using ??
In your video the engine is getting very close to self sustain but you pull the air start away too soon, a very small engine , especially a home made one thats having starting problems needs a lot more help from the starter and a lot more RPM , you need to be looking at 40,000 rpm startup .................have you checked your engines RPM with just the airstart , no propane ??
Where is your lube and bearing bleed air currently being supplied ??
Need more info
Cheers
John
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Post by musicislfe on May 30, 2020 22:36:41 GMT -5
Not really having starting problems. That video was one of the first attempts and have since applied more air when starting and for longer duration. Currently there is no lube or air passage in the hub. The rear of the ngv between the turbine has aprox 3mm gap For unburned fuel/oil to enter and lube bearings. But it seems that is quite inefficient.
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Post by racket on May 30, 2020 23:38:41 GMT -5
Somethings not making sense here .............the lube , fuel/oil mix bleed, is normally fed to the front of the compressor bearing along with bleed air , the mix of fuellube and air then make their way rearwards through firstly the comp bearing then the turb bearing and is then expelled against the forward face of the turbine wheel where it is centrifuged off as it cools the wheel and travels through the gap between NGV and wheel and out through the turb blades.
Could you please explain a bit more how your lubrication system works
Cheers
John
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Post by musicislfe on May 30, 2020 23:49:28 GMT -5
The cooling/oil was to be provided by unburnt fuel lead into the rear bearing area by making its way down to an easily accessible area. Like some of he model jets use. I don’t have a dedicated oil or air passage other than the gap between the ngv and turbine. I’ve considered using fuel as a coolant. So long as the excess fuel is expelled?
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Post by racket on May 31, 2020 0:01:11 GMT -5
Theres your problem :-)
Your engine's bearings haven't had any lubrication or cooling .
There hasn't been a model turbine engine lubricated by "unburnt" fuel , they ALL have some form of lubrication provided by bleeding off several milliltres/minute of fuelmix flow or having a dedicated oil tank thats pressurised by combustor pressure and supplying ~5 ml/m of thin oil to the front of the comp bearing along with bleed air.
Cheers
John
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Post by musicislfe on May 31, 2020 0:05:03 GMT -5
Thank you racket, I’m going to add a oil/fuel feed to the mix. I’ll keep you posted. I have a nozzle and diffuser ready for dedicated oil line but that is not ideal as more items are needed And that’s what I’m trying to avoid. Thanks again.
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Post by racket on May 31, 2020 0:07:39 GMT -5
Have you checked out the GTBA www.gtba.co.uk/ , they have an enormous wealth of information on small turbines |
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Post by musicislfe on May 31, 2020 1:14:15 GMT -5
I have not. I’ll take a look.
Thank you.
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Post by musicislfe on May 31, 2020 16:36:52 GMT -5
Theres your problem :-) Your engine's bearings haven't had any lubrication or cooling . There hasn't been a model turbine engine lubricated by "unburnt" fuel , they ALL have some form of lubrication provided by bleeding off several milliltres/minute of fuelmix flow or having a dedicated oil tank thats pressurised by combustor pressure and supplying ~5 ml/m of thin oil to the front of the comp bearing along with bleed air. Cheers John Thanks Racket. My other engine has a oil feed.. not sure why I opposed it so desperately. Guess the fuel burning in the hub was a concern, thanks for the input. |
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