Building the JU-02 gas producer

[Johansson]

Hi John,

This is great information, thanks a lot for taking your time to set up the test rig and get hard data so we won´t have to keep guessing at what has happened. There is no doubt now that the compressor has contacted the housing, I torqued the nut to specs with Loctite so my compressor should have needed just as much torque to move as yours.

It will be interesting to see what the test with dry surfaces shows, if I have to guess I think more torque is needed to spin the compressor with dry contact surfaces.

Cheers!
/Anders

[turboron]

Anders, I agree with John that the most likely cause is a compressor rub. As noted in my previous post the impeller tip probably moved forward and contacted the scroll under load. I rebuild with opened clearances is suggested. Clearances can be checked during a dummy build with Play Dough.

Note that John's test does not include the clamping effect on the impeller toe as the backplate expands and the exducer tip moves forward.

Thanks, Ron

[racket]

Hi Anders

Yep , hard data is always the way to go , even if its a bit "rubbery" , we can add or subtract 10-20% and its still well within safe limits.

The "dry" test will be interesting , I'll let you know tomorrow :-)

Cheers
John

[Chuks]

Dec 12, 2018 18:06:21 GMT -5 Johansson said:

Hi John,

This is great information, thanks a lot for taking your time to set up the test rig and get hard data so we won´t have to keep guessing at what has happened. There is no doubt now that the compressor has contacted the housing, I torqued the nut to specs with Loctite so my compressor should have needed just as much torque to move as yours.

It will be interesting to see what the test with dry surfaces shows, if I have to guess I think more torque is needed to spin the compressor with dry contact surfaces.

Cheers!
/Anders

HI Anders,
this is how the cubs looks like when they have a lion as a father.
This is the point where hoset turbine/shaft in Andy's new engine looks better than the TV94 turbine/shaft? Left hand vs right hand thread, Or should I just sit back and learn more?

[racket]

Hi Ron

The superback on the X846 wheel is rather large at 12.88 mm , its a wheel designed for high pressure ratios, as good a wheel as could be used, and was the reason I settled on it in the first place.

The wheel normally has to be a "press fit", over the last several millimetres of the quill which is at a slightly larger diameter, before contacting the seal carrier which would help with any "expansion" compared to my test which simply has a straight 7/16" drilled hole with some clearance , so no clamping loads compared to the wheel, mine is "worst case " scenario.

When I setup my wheel shroud clearances I use maybe ~8 "radial" scribed lines on the casing's wheel shroud section using a marker pen , then with 4 or 5 layers of masking tape fitted over several of the comp wheel blades to simulate the required clearance, the rotor is fitted to the engine which has a "dry" thrust bearing so that the turbine wheel can be forced forward as far as possible and then turned to show up any "tight" areas .

If everything is in order , the wheel turns with just a little bit of resistance as the "compressed" masking tape slides across the casing surface , the amount and position of witness marks on the scribed lines as well as the masking tape allows for a fairly accurate assessment of the clearances around the entire shroud area .

Its a tedious job as it requires numerous assemblies and disassemblies of the comp cover , with the cover needing to be refitted to the lathe for more "emery cloth work" to massage the shroud shape and clearances until I'm happy with the results .

But as the comp is the most "sensitive" area for losses I feel its my time well spent trying to produce the best results.

Cheers
John

[racket]

Hi Chuks

LOL............sit back and learn more ;-)

Heh heh ........It'll make two of us , as I've never seen any info on this matter before , its new for me too

Cheers
John

[monty]

Dec 12, 2018 17:54:38 GMT -5 racket said:

Hi Anders

Compressor slippage test completed .

Test rig


The oil slinger and piston ring holder were both assembled onto the shaft with lubrication on all surfaces , I used my usual synthetic lube as on the test stand , the end of the "compressor wheel" was also lubed , the comp nut was fitted using Loctite 262 and torqued up as per usual , and left for 24 hours to set.

The "torque arm" was 11 inches centre to centre, the first test resulted in 25 kgs pull and nothing happened , 25 kgs at 11 inches works out at just over 50 foot pounds .

What now me thinks , ..........a length of pipe was found that slipped over the adjustable spanner and a hole drilled in its end for the spring balance to attach at a 22inch centre/centre.

Second test obtained 16 kgs before the "comp" started to move , 16 kgs at 22 inches is ~65 foot pounds of torque .

The interesting part was that the nut didn't move, the comp mearely slipped between the nut at one end and the piston ring holder at the other , the piston ring holder and oil slinger remained stationary.

Now , assuming you had 70,000 rpm and 4.4 PR at 75% comp effic with ambient of 15 C , this would result in a ~203 C degree rise in compression , requiring ~124 HP/lb/sec of air flow.

Your 0.9 Bar in the jetpipe will have produced ~1,900 ft/sec at the exhaust , your 750 C - 1023K jetpipe temp would have produced a density at the exhaust of ~40 cubic feet per pound , your 89 mm jet nozzle ( 90 mm hot) has an area of ~0,06847 sq feet , multiply by our 1,900 ft/sec gives us a maximum of 130 cu ft per sec , divide by our 40 cu ft/lb and we end up with a max flow of 3.25 lbs/sec , neglecting boundary layers and other losses , we're looking for max loads on the turb wheel here.

3.25 lbs/sec X 124 HP/lb/sec = 403 HP required by the comp , 403 HP at 70,000 rpm = 30.2 ft lbs of torque , less than half what I found in my static test this morning .

Your comp rub was caused by the wheel contacting the housing, not from slippage due to excessive power requirements.

I'll repeat the test after degreasing parts and see what difference dry parts make .

Cheers
John

John,

I got to within about 50% of the actual value! Not too bad for a quick napkin calc... . Testing is the way to go for almost anything....

Thanks for the data! It makes me much more confident in my current design. It will be interesting to see how much difference the dry fit makes. I'm going out on a limb and betting Anders a pint of beer that it won't make much difference..

I wonder how much it costs to ship a pint of beer to Anders.....

Monty

[smithy1]

So a possible transient imbalance could cause this methinks.??....I wonder what force it takes to bend the shaft enough for the impeller to hit the housing..??

Smithy.

[CH3NO2]

Brilliant testing and analysis John!

[turboron]

John, thanks for your detailed description on how you set the impeller to scroll clearance. I have always thought a sheet of paper was approximately 0.002" so your 4 or 5 strips of masking tape could be in the order of 0.008" to 0.010". This is consistent with my helicopter gas turbine centrifugal compressor experience. Perhaps Smithy has some current C30 clearances we can use as reference. We lapped the compressor impeller to the shroud to obtain maximum efficiency. Perhaps lapping would be good here to elminate any high spots.

Thanks, Ron

[racket]

Hi Ron

I just checked my journal for the 12/118 comp shroud clearances, ............the masking tape I used was ~0.003" thick, so 3 thicknesses over the exducer shroud edge gave 0.009" and the rotor turned OK when forced forward on"dry" bearings , with 4 thicknesses ....0.012", it was "tight" to turn...................Anders could probably go another thickness, so ~0.015" axially at the exducer shroud area once past "the curve" ................the Solar T62 with a similar dia comp has axial specs of 0.017- 0.023" , with 0.017-0.028 max deviation , so 17 thou minimum..............probably better to lose a tad of compression than have another touch.

Radial clearance on 12/118 inducer is 0.015" ( 0.030" diametrically) , and with dry bearings the rotor can be pushed side to side without the comp blades contacting the cover .

With the 10/98 engine , my journal says similar clearances .

Cheers
John

[Johansson]

Monty: A beer it is, if I win you can drink one for me and send a picture.

John: As soon as the compressor wheel arrives I will assemble the engine and measure the axial clearance with dry bearings, I´ll rather make the clearance a bit on the large side than risk another "touchdown". No less than 0.5mm axial since the casting might very well flex from the air loads and close the gap an unknown amount of tenths of a millimeter.

Then I´ll send the rotor for balancing and fix the NGV cracks while waiting for it to be finished, I should have the engine assembled by the end of December depending on how long I have to wait for the rotor to be balanced.

Cheers!
/Anders

[CH3NO2]

Could an ablative liner for the compressor housing be as simple as several coats of spray on varnish?

[Johansson]

Dec 13, 2018 7:37:39 GMT -5 CH3NO2 said:

Could an ablative liner for the compressor housing be as simple as several coats of spray on varnish?

I was thinking about that too, a generous 1mm clearance and then a thick layer of abrasive coating so that the compressor can sort the clearance out itself. That would be neat!

[racket]

Hi Anders

I've been having a look through your build pics and I'm not sure about the casing flexing back onto the comp wheel , the pressure will be the same rearwards as forward and I think theres probably more "area" forward , in which case the clearances would be increased .

I wouldn't go to 0.5mm clearance , thats too much , have you checked the skid marks on the casing shroud area for any variations between positions that might indicate some "misalignment" between comp and housing ??

On another note , I noticed a comment on this page jetandturbineowners.proboards.com/thread/734/building-ju-02-gas-producer?page=6 that after fitting the NGV vanes you felt the flow area might be a bit tighter , whilst you have the engine apart could you please measure all of the NGV throats and give me a total area , its a critical parameter.

Cheers
John