T04 Gas Turbine - RESURRECTED!

[ashpowers]

Hi Guys,

The layout I'm using for the zone hole distribution is very similar to that of the Phoenix. They top three primary rows on the outer liner are staggered, no inner liner holes for the primary. Secondary holes have a single row on the outer liner, and 4 rows on the inner liner that place the holes between the vaporizers. The dilution zone on the phoenix has two staggered rows on the outer liner and then the larger ports on that 30-degree backwall. The one difference in the Phoenix at that 30-degree back wall is that the holes are broken up into two smaller holes between each vaporizer inlet rather than one larger one that I am using.

I have removed the outer liner from the combustor and have already started in on the vaporizer modification based on John's observation. What I have done is cut them about 1" from the inlet and am building "U" tubes to put on top of the lower 1" sections to form what looks like a "Y" but with square angles between all of the pieces. Hoping that by splitting the air and fuel flow through the vaporizer tubes will create more turbulence as well as slow the velocity of the mixture as it opens up into the upper "U" section. The increased surface area of the vaporizers should also help a good bit too and will also deliver a more homogenous distribution of the mixture into the head of the combustor - 12 vaporizer exits rather than just the original 6, evenly spaced.

I've also removed the top "cupped" front wall of the previous combustor which I will attach to the head of the existing combustor to split the airflow coming out of the diffuser and provide a good controlled flow of air into the inner liner volume. Once that has been installed I'll also incorporate some flow straighteners now that I have a better idea of how much swirl angle there is coming out of the diffuser. Really makes me want to fabricate a new diffuser using axial straightening vanes but that's a large bit to bite off right now and I think I can achieve the same result just by adding in some straightening vanes.

I'll post up pictures of the new pieces once I've got everything built. Gonna be a day or two though before I'm finished with it all.

[ashpowers]

Modified the vaporizers and the combustor was put back together. I also added in the flow separator which was made from the front wall of the previous combustor design.

All of this work is a bit tricky - lots of tiny tig welding to put everything together and probably went through 20 tungstens (had to resharpen) in the process. Literally welding at 5-6 amps can be a bit tricky and requires the steadiest of hands. It is literally like welding beer cans together - working with 0.012" thick sheet metal, LOL. Not exactly as clean as I can produce on larger pieces but it should do the trick at least for testing and then perhaps cleaning some stuff up afterwards if it is a go.

After everything was put together I gave the outer casing a glass bead shot to remove previous heat patters.





Calling it a night for now but tomorrow I'll reassemble her and give her another testrun to see how she fares. If all is finally well I'll go back in and add swirl straighteners - unfortunately I used up all of the sheet I bought in the process of making all of these parts up to now, LOL.

Just a side note: The flow separator appears to come into the head of the combustor at a level lower than the height of the combustor head. It does. This actually worked out well though - the back wall of the diffuser thins out at the same radial position that the flow area from the separator and combustor head appears to tighten up. The actual flow area remains pretty much constant. Obviously preventing the flow from having to change direction is optimal, but these pieces just came together like you see and the combined attributes of the components' geometries are complimentary to each other..

[racket]

Hi Ash

Plenty of evaporator surface there now , the "step" will promote lotsa turbulence and mixing , there not gunna be any droplet exit those evaps :-)

Cheers
John

[ashpowers]

On another note, something I've come across with the balancing rig:

Over this past weekend once tearing the engine down and checking balance again, I noticed that the group was not in balance anymore. I also have noted that there is some vibration coming from the engine at various RPMs, nothing terribly alarming, but present and concerning no less.

So I took some time to research balancing systems and knowing that I have built what is referred to as a "soft bearing" balancer I did some specific searching.

Here's a great page for anyone wanting to get the basics of the two different balancing systems (hard vs. soft):
what-when-how.com/electric-motors/types-of-balancing-machines-electric-motors/

The most important images are these two:

Soft bearing system response:



Hard bearing system response:



I mentioned this in a previous post, regarding the appearance of a double heavy spot per revolution and I summed it up to the system "bouncing" at points under and above resonance and resulting in this observation.

Well, I found that this presumption is not the case.

In my efforts to try and determine why I would get these "double heavy spots" at various RPMs I went through several hours of trials to change up the system's mechanical configuration to see what was causing it.

I went from running the group on the balancer with the ball bearings within the bearing sleeve to removing the bearings completely and running the group on teflon "V" blocks, to hard-mounting the piezo sensors to changing to higher durometer foam support tape. Quite a slew of trials...

I discovered that with it running on the teflon "V" blocks without the ball bearings, regardless of support structure configuration, I would always get a single heavy spot indication at each end regardless of the rotating speed. I would only see the double heavy indication when running on ball bearings.

So it occurred to me that the bearings must have something to do with it and the only explanation I came to was that there simply was not enough spring preload in the system for the bearings to keep the rotating group spinning about its true center. Sure enough, replacing the preload spring that works against the outer race of the rear bearings with a stiffer spring completely removed the double heavy spot indication. The double heavy spot was a result of the rotating group forcing the inner race of the bearing to orbit out of phase with the imbalance, thus causing the double heavy spot indication.

Adding a stiffer preload spring between the bearing tunnel and the outer race of the rear bearing also increases the preload on the backside bearing in the front bearing group. The two front bearings are angular contact bearings and they are positioned in a back-back arrangement. There is a spring washer between the rear bearing's outer race and the bearing sleeve which is there to load the two bearings' outer races against each other to sinch up the slop within the duplex. The net effect of a stiffer spring in my configuration adds more preload to the rear bearings outer races (both are deep groove radial bearings) and only to the rearward bearing in the front duplex. The design here is arranged such that when the engine comes up to operating conditions, the pressure behind the compressor wheel will create a forward axial load on the shaft, resulting in the furthest forward bearing in the front duplex coming into compression and thus adding more stability to the rotating group in the front half. The preload spring is also working against this axial load in an attempt to reduce the load that the front angular contact bearing must carry. This is the absolute best arrangement I could come up with and it takes at least three bearings to make it happen - the extra rear bearing is just an added insurance policy.

The vibration I was feeling in the engine when running and occurring at different amplitudes over various RPM ranges was a result of the harmonics associated with the imbalance acting upon the first damped system: the bearing race preload's "give" and the rotating group's imbalance amplitude. I am sure that what I was feeling was not the full extent of the imbalance either since the system has a fluid dampening system in place between the bearing sleeve and the bearing housing. The actual amplitudes were undoubtedly greater than what I was feeling at the outer casing level.

Going further with the balancing rig analysis, I "thought" the group was originally in balance but what was actually happening was that the RPM in which I was balancing at was causing a dampening of the imbalance through the bearing preload. There WAS vibration, it was just being absorbed by the bearing sleeve, bearings, and the preload spring and NOT registering as a vibration within the balancing rig. The support system was reacting to the imbalance in a negative-resonance manner. I was just inconveniently spinning her at a speed that the vibrations were being dampened, which was the maximum RPM I was getting out of the balancing rig running on a 12V system. Switching to an 18V power supply changed the maximum RPM I could spin the group to and widened the range to a point where I could go beyond the damping frequency and finally see the imbalance... Just bad luck I guess, but at least I learned something from it!

So with the double heavy spot issue and the rotating speed damping issue resolved, it brought about a consideration:
The amount of residual imbalance that exists in the rotating group will have to be overcome by the preload spring in order to maintain a true-center rotation of the group. BUT, at a certain point, increasing the preload amount is going to put more stresses on the bearing components that at sufficient levels, will lead to enough heat that will cause them to fail in short time.

SO, with that understanding, it dawned on me that there must exist a mathematical analysis that will take the forces of imbalance and apply them to the bearing geometry and resolve a preload amount that will prevent the shaft from running out of center.

I am sure I can figure this out myself given the time, but I am also sure that bearing manufacturers already have these operating paramters and some sort of analysis matrix that will solve not only this single design point, but also be able to take from the RPM, ambient environment temperature and lubrication system type in order to determine the maximum absolute limits.

Balance is the first most vital parameter here I am sure. But the question is, to what kind of balance tolerance must we achieve? The more out of balance it is, the stiffer the preload spring must be, which results in more running friction within the bearing and with a given lubrication system at a specified speed, the bearing is only going to be able to take so much of the sum of variables before they stack up to the point of failure.... or more specifically, how the MTBF of the bearing is affected with a given set of these parameters at work.

Is what I am asking for here something that bearing manufacturers already have available to their consumers? Everything within the bearing manufacturing industry is so incredibly specific that it seems to me there should also be a calculable lifespan of a bearing when subjected to a particular set of operating conditions.. at least within a relatively tight +/- factor of time.

Then adding to that, the fluid dampening system: This will obviously grant us more tolerance to imbalance and thus the requirement for less spring preload, but not in a linear relationship. The dampening system only has so much "give" before the suspension bottoms out. The dampening system can absorb a given amount of amplitude which will require less bearing preload.

It seems to me that once I have the combustion issue resolved, addressing the bearing support system will be the next task at hand. I have piezo vibration sensors and I can easily incorporate one into the DAQ setup for analysis. I'm thinking forward here and envisioning a bearing lube system that does not use a mix of fuel and oil. But rather, uses P2 pressure acting into a vessel that contains the bearing lube and feeding the bearings. Varying the viscosity of the lube will open the doors to changing the dampening response of the system which can be easily datalogged and reviewed in order to optimize the rotorbearing support system's life expectancy.

Sounds like fun, eh? =)

[finiteparts]

Ash,

I am still concerned about those large holes in the 30 degree section between the vaporizers. Often times when you have multiple holes, one hole can interfere with the feed of the adjacent hole due to changes in local static pressure near the entrance.

With those dilution holes being larger than the vaporizer feeds, and now your vaporizers have two 90 degree bends, which are great for turbulence and mixing, but bad for pressure losses, I feel that you will be preferentially feeding the dilution and starving the vaporizer tubes.

When the air comes down the center of the engine towards the back cavity where the vaporizers feed, it also has those three big holes to shoot in prior to getting to the dilution holes on the 30 degree back face even. It just seems like it might actually be less pressure drop for the majority of the flow to just shoot through all of those dilution holes and out the turbine, than it would be to push some of the air all the way to the forward portion of the combustor to only turn around and go back towards those same dilution holes and then out the turbine.

Just a thought...good luck!

~ Chris

[ashpowers]

Yes, Chris - I completely see your point. At this moment I'm not entirely sure. One of the main reasons for the 30/20/50 rule is to equalize the available pressures and consequently, the massflows, at the opening of every port into the combustion chamber. All things being equal, so long as this rule is adhered to, this will be the result. But what it does not take into account is the effects of pressure drop over flow distances or the effects caused by changing directions within the flow. Additionally, this point you've made, is addressing the factor of how the localized pressures are affected between the different ports based on dynamic pressures within the flow.

Thing is, we must always try to stick to making one change at a time within a system in order to know the effects of the change. I admit, I'm a bit hasty sometimes to adhere to this standard, LOL. I've made two pretty significant changes to the system in this last go-round but it is my opinion that both of what I have done can only improve the system's performance. It may not absolutely optimize the operation. Even if she does finally operate to acceptable performance there is always that list of things you are unsure of. This is a good thing too though - if this attempt doesn't work then I am pretty certain on what change I will make next and those large dilution ports are on that next target. We'll just have to see.

One last little side note too: if she does work well in this configuration, I will still go back and add in flow straighteners to the outer liner as well as into the top flow separator "cup" so as to add some more efficiency back into the system. Up to now it just looks like I've got bigger problems.

[ashpowers]

OK, got her all back together this evening. Was having fits with the balancer and found a new balancing strategy in the process.

What occurs during the initial trial run to see where the balance is, there is a phase lag of the heavy side indication... i.e. where it shows the heavy side is actually some number of degrees after where the heavy spot really is. In a soft bearing balancer you are supposed to bring the group up a good bit above the first resonance of the the supports where the lag will run 180 degrees out of phase. Problem with this is the current setup requires really high RPMs to get to that point and at those speeds, the 2048S/s of the DAQ setup starts narrowing the information window.

So what I found to do is I would run her up to a speed approaching resonance with a nice large piece of 3M double sided adhesive foam tape at the zero degree mark of the group. The phase lag, knowing well off where the heavy spot is, is easily identifiable. The weight is removed and the group spun up to the same identical speed. From there I would simply add back the phase lag and put a trial weight 180 degrees out of phase of that point (since the "point" is the heavy side and that needs to be counterweighted). Then another trial run is performed to see if the correction weight was too much, too little, and if it was indeed at the correct position.

This process actually was very repeatable and given that it was near resonance, any little bit out of balance would show up quite dramatically. So I managed to get the balance down to a point where I am literally attaching a piece of this 3M foam tape that is about 1mmX1mmX1mm in size and it would make a change of about 0.5V at ~6000RPM. I was able to get the residual balance down into the 0.4V range. I Dont know exactly how much that little tiny piece of foam tape weighs but it would have to be in the single digit milligrams I am sure. So, the balancing went well and the engine was fully assembled with the modified combustor.

One change to note - I did find that in the balancing process several days ago that the rear preload spring just wasn't enough to keep the inner race centered so I went to a larger spring that is providing about 3xmore preload force. Still less than a pound of preload on it but it did make a good difference on the balancer.

So I'm going to fire her up tomorrow morning around 9 or so once all the neighbors have gone off to work. Wish me luck!

-Ash

[stoffe64]

Good luck with the testrun Ash !

[ashpowers]

Well, despite the rather significant changes I have made to the combustor, the engine is still having the same exact problem. It is astonishing that even with so much changed absolutely nothing has changed in its operation.

I switched the propane over to the main injector ring feed and tried starting her up. What I am seeing is she wants to run a little better, however, there is a TON of heat at the turbine inlet and the rear case wall and jet nozzle flange started glowing.

I suspect that what is going on goes back to my original thoughts that those large dilution ports are wicking a good bit of fuel back out of the vaporizers and it is getting blown out of those large dilution ports and burning off through the NGV and turbine resulting in flames out of the jetpipe. With the dilution ports so large that puts a big flow demand on the available air coming down through the inner section of the engine. At the base of the combustor, there is a hard 180-degree turn the air has to make to enter into the vaporizers but all around the vaporizer ports there are those large dilution ports. I just dont think there is enough left over available pressure to force enough air (and consequently, fuel) through the vaporizers.... some of which is being carried off with the air passing through the dilution ports..

I'll pull her down and see about modifying that rear inner wall. Probably will cut out some discs with a smaller hole to put over the existing holes and add the flow area back into the outer liner.

This is beginning to drive me absolutely nuts!

On a happy note, the bearings are still running smooth as silk. =)

[racket]

Hi Ash

I was just having a look at your turbine wheels exducer .............heh heh , you've been doing a bit of grinding on her , she's very "open" with quite a large angle on the exducer , was it this way when you had it running before ??

Good to hear the bearings are fine :-)

Cheers
John

[ashpowers]

Hi John,

Yeah, the turbine was nice and clipped like you see in the current one - I ground the replacement to the same angle as my original.

I opened the engine up and made a few mods to the combustor. I'll post some pics of what I did tomorrow once I have a chance to test run her to see the results in the morning.

-Ash

[ashpowers]

OK, going to have to take a step back here and go through this piece by piece. Yet another failed attempt to start the engine this morning. Got a fresh tank of propane too as she was a bit low previously. Tried starting up on propane and bringing in the jet-a: hung start. She just doesn't want to get up over the hump.

I tried running her on straight propane through the main injectors and still a hung start.

So I've pulled her apart to take a closer look at things and found a couple of sore spots I didn't previously see.

1) The turbine blade-housing gap. This turbine had previously been used in the engine and suffered a bearing failure which put the blades into the housing. This ate away the blades a bit and has opened up the gap to about 0.045" or so.
2) I had to make a new NGV outer housing for this engine and when I machined the "cup" that sits onto the vanes, I didn't quite cut enough out radially and this created a small gap between the top of the vanes and the ngv outer "cup" of about 0.025".

Perhaps the combination of excessive tip clearance as well as an increase the the NGV flow area has put this engine into an arrangement that there just isn't enough velocity in the flow because of the increased flow area and combined with the excessive tip clearance, the turbine just isn't generating the kind of power it needs to run the comp. Adding to that, the larger cooling air bleed port for the bearings is also reducing the efficiency a bit as well.

I am right at the 20% comp inducer to turbine exducer ratio with the bits I'm using which I know puts the system almost to the edge of being able to run well. I think the problem I'm having is more to do with how all of these smaller artifacts are stacking up in the end.

I can address all of the mechanical issues - already dismantled the NGV, bead blasted everything and ready to machine the ngv outer housing so it will sit properly onto the ng vanes. I can get another turbine and lathe out another shaft - I've got that process down to a "T" now, LOL.

I've gone back through the design with my turbine calculator - just wanted to run through the numbers here to get a confirmation on my calcs.

At 115KRPM, 0.86# massflow, PR of 2.65, TIT of 850C, comp work req of 69.93HP, with CC loss of 5%, NGV eff of 90%, turb eff of 65%, I've got an NGV flow area of 2.09 in^2. Right now my NGV is right at 1.998 in^2, so I'm right around 5% undersized which I wouldn't expect to be a problem.. If anything, the slightly reduced flow is picking up a little bit more velocity going into the turbine.


What I am considering to do though is to change out the compressor wheel to a 60 trim T04E which is the next step down from the 60-1. This would increase my comp/turb area ratio to 24%. I dont know the blade tip heights on these wheels right at the moment though and that would be a pretty important thing to know - if the 60 trim has the same tip height then it will only require making a new inducer housing and then a slight mod to the NGV to bring the flow area down to around 1.6 in^2.

Because I know this engine will run with the parts I'm using I'm going to address the issues I know it has and then retry again. I'm just hoping that she wont be so borderline that when I put a freepower on her, it pushes the system to the same point I'm at now. Of course, once she gets over the hump things should start working in favor of producing excess but.... will just have to see.

Starting to think: with all the time I've put into this engine I probably could have finished my TV94 engine, LOL. *sigh*

[rythmnbls]

Ash,

.045 is a pretty big gap. I don't know much about radial turbines, but I doubt an axial of similar size would run with such a gap.

That's where I would start.

Regards.

Steve

[ripp]

Hi Ash,

1. the combustion chamber of your Phoenix is out of date. You lose too much pressure air beause of the large holes next to the evaporator tubes for a good start. The evaporator tubes may also be made ​​of thin stainless steel (0.3 mm). These tubes are available for example in a model shop in all sizes. You can swap later this tubes by Inconel. I advise you strongly a combustion chamber with the design of GR130 or GR180 depending on the compressor inlet surface of your compressor wheel.
All components of a perfect combustion chamber are well described on the website of Gerald Rutten and with illustrations.

www.youtube.com/watch?v=-5HTCzm34OA

and here are some pictures from my own components







regards

Ralph

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[ripp]

Sept 12, 2014 15:18:33 GMT -5 rythmnbls said:

Ash,

.045 is a pretty big gap. I don't know much about radial turbines, but I doubt an axial of similar size would run with such a gap.

That's where I would start.

Regards.

Steve

2. I am of the same opinion!
    It should be 0.2-0.25 mm.

regards

Ralph

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