"Fat Boy" 12/118 thrust engine

[Johansson]

Hi John,

Larger syringe injectors might be worth a try, you might sacrifice the idle somewhat but that is a good trade if it runs better on full throttle.

Cheers!
/Anders

[racket]

Hi Anders

Yeh , now that I've got the "springs" inside the evap tubes to keep the fuel under control a change to less delivery pressure with larger bore injectors will also help with minimising the chances of blockages.

I'll start disassembling the engine today to have a look inside , check the fuel manifold and think about tightening the NGV throats a tad .

Good to see JU-02 reconstruction gathering pace :-)

Cheers
John

[racket]

Hi Anders

I've got the engine off the test stand and the exducer shroud is showing signs of a hot streak with the high temp spray paint being "scaled off" by the steel underneath over a section of the shroud upstream of the thermocouple.

Its looking like theres extra fuel burning at the bottom of the combustor , the very hot gases from which after travelling through the NGV and wheel would end up exiting ~90 degrees away, up the side of the jetpipe and inline with the thermocouple .

The test run was conducted without my start gas operating , I remembered to turn it off , which might have been a bad thing as it provides a ring of burning propane against the front wall of the flametube which should help with early combustion , there might be a need for some extra heating at that point with this engine .

I'll pull the outer can and have a look at the NGV to see if its indicating anything .

OOOPS !!!! correction , I just checked my notes from the other day and that hot spot is near the "cold" thermocouple...............ah , bummer , whats going on , the paint on the "hot" side of the engines exducer shroud is still nice and smooth showing no signs of overheating .................time for a rethink :-( ........................might need to recheck the gauge and thermos, or get a new set as they've been on the test stand for several years now...........I'll check its battery first :-)

Cheers
John

[CH3NO2]

Hi John,

If possible, swap the meters from hot side to cold side and vice/versa. To see if the anomaly follows the meter.

[Johansson]

Mar 1, 2019 19:29:43 GMT -5 racket said:

Hi Anders

I've got the engine off the test stand and the exducer shroud is showing signs of a hot streak with the high temp spray paint being "scaled off" by the steel underneath over a section of the shroud upstream of the thermocouple.

Its looking like theres extra fuel burning at the bottom of the combustor , the very hot gases from which after travelling through the NGV and wheel would end up exiting ~90 degrees away, up the side of the jetpipe and inline with the thermocouple .

The test run was conducted without my start gas operating , I remembered to turn it off , which might have been a bad thing as it provides a ring of burning propane against the front wall of the flametube which should help with early combustion , there might be a need for some extra heating at that point with this engine .

I'll pull the outer can and have a look at the NGV to see if its indicating anything .

OOOPS !!!! correction , I just checked my notes from the other day and that hot spot is near the "cold" thermocouple...............ah , bummer , whats going on , the paint on the "hot" side of the engines exducer shroud is still nice and smooth showing no signs of overheating .................time for a rethink :-( ........................might need to recheck the gauge and thermos, or get a new set as they've been on the test stand for several years now...........I'll check its battery first :-)

Cheers
John

Hi John,

Really strange with the thermocouple readings, a burned look to the shroud is a fool proof sign of temperature and if that thermocouple shows "cold" temps something isn´t adding up.

Is it possible to get a mirror and video tape the turbine exducer during a run? Might be able to spot a hot streak that way.

Even better if you can get an IR camera and check the shroud and jet pipe for the hottest/coldest zones. An awesome tool, I´ve used one for finding steam leaks on the steam turbine I worked on before.

Cheers!
/Anders

[racket]

Hi Anders

LOL ..........I think I might have found the problem :-(

After pulling the fuel manifold off and doing a propane test with all injectors flowing "equally", I thought I'd better go a tad deeper , so removed the flametube rear wall with the evap tubes , they all looked good so no problems there .

Then I had a look into the flametube to see if there were any "funny" witness marks where the injectors "splatter" against the front wall and all OK , I was getting confident that it must have been an abbberation on the therocouples ................until I turned the engine around and found a "hole" blown through the inner flametube wall , probably ~ 500 sq mms worth of hole ..................I guess there was a tad more fresh air blowing through that than the smaller Primary holes .

I'll try and get a pic of the wall damage ,.............. thankfully no damage to the turbine wheel ...............the blowout/in was between the front wall and the first row of Primary holes in an area where there normally isn't much combustion going on .

On a side note , I checked the thermocouple meters batteries and the 4 X AAA batteries were down to 1.33 V each so they were replaced , and I also found one of the sockets on the unit had a loose fit on the thermo pins , this was tightened up and the unit reassembled and fitted to the test stand .

Upon switching the unit on I noticed a temp reading of 56 degrees C on one thermo ...............the ambient temp was a bit warm , probably ~30C but not 56 , bugger I thought, this is weird , then I noticed it had a minus sign in front of the 56 , now where I live is sub tropical , we never see a minus temp , so I switched the unit off thinking I'd stuffed something up when I changed the batteries etc , just to be certain , I switched it back on and all was OK ,both temps reading correctly , maybe the unit needed to reset itself ............I don't know , but I might do a heat check on them using a bit of bar with a thermo screwed in each end, then get the propane torch and heat the centre of the bar , both theros should read the same air temp inside the bar.

Looks like I've got a bit of work to do before the next test run :-)

Cheers
John

[Johansson]

Hi John,

Oops, you were lucky indeed that there was no other damage than to the inner liner. Could it be the propane that has torched the metal?

Do a test and boil the thermocouples in water before the next run, no wonder the readings were off when one of them reads almost 90°C lower than the actual temp.

Cheers!
/Anders

[azwood]

Thurmercouples are funny things it took ages to program my unit correctly and it still reads faranite.but the ambient temps are good.hope you get the ft thing worked out it will be interesting to see what did that.

[madpatty]

May 12, 2017 23:50:21 GMT -5 finiteparts said:

Hi John,

There is likely some error in my calculations. I did my best to estimate you liner holes sizes and numbers from the photos. I ran a cycle model at what I thought was your design point for your engine, 3.6 lbm/s at 3.5 PR and I assumed a T4 limit of 1600F. With my estimate of dynamic pressure in the outer annulus, and a 5% dP/p it correlated to a Cd ~ 0.8 in the plunged holes and the other chamfered holes I estimated to have Cds around 0.65. I used your 9.5mm throat diameter on your vaporizer tubes with a Cd down near 0.6 due to the rather "rough" flow path and large discharge "dump" ratio.

I estimated that your liner has a total effective area of the holes around 8.85 in^2...with the Lefebvre's equations suggesting the effective area to get 5% dP/p to be 8.1 in^2. Now I would expect that if my numbers were close, you would actually be running lower than the 5% pressure drop because of the leakages at the interfaces to the turbine and also the leakages through the gaps around the liner at the dome and vaporizer end to outer sheet. I haven't attempted to calculate their impact.

My equivalence ratio = 1.8 was based on estimated liner hole sizes from the photos you posted...there's likely some error there...also, the fact that I was assuming that the liner hole area impacts the mass flows through the liner in a linear fashion, which isn't real, but good enough for estimates. In reality as the flow comes around the liner, portions of the total mass flow turn and go through the liner. This reduces the momentum and modifies the local dynamic pressure field, manifesting its impact to each set of holes as a sensitivity to the other holes upstream and downstream of it. Sort of like a network of series and parallel orifices...

The number I quoted is the equivalence ratio, not the AFR...the AFR that I calculated for the tubes was 7.853. If your estimated AFR of 2.6 is correct, that would mean the vaporizer tube equivalence ratio is 5.64, which is well above the rich flammability limit of 3...good for keeping the vaporizer tubes alive...but I would have expected much more coking on the dome for that rich of a vaporizer flow.

I agree with you on the use of tangential flow to make the effective path through the liner longer...I was just saying that it is good to think how those jets move the internal flow.

We want to have some mechanism to recirculate some of the hot, reacting products back upstream to interact with the incoming fresh reactants. This is the primary combustion stabilization mechanism...the incoming reactants are cold and need to be heated up in order to get the reaction times fast enough to allow the combustion to complete within the short time the airflow is in the combustor. Usually this is done by setting up a vortex flow pattern of some kind...and that is why I suggested the axial jets. When you have two jets fairly close, moving in completely opposite directions, you set up a recirculation flow between them.

I hear that "combustion is a black art" comment a lot, but the people that say that don't work in combustion. Back in the 70's and 80's it may have been true, but nowadays it may be common to have to trim a combustor to meet profile or modify some cooling features to meet some durability issue, but not restructure the flowpath. Where I used to work, we made ultra-low combustion systems. We had to run really lean to keep the NOx down and that means that they were on the edge of lean blowout all the time...a much tougher combustion design problem than standard combustors. I can't think of a time when any of the combustors failed to start, anchored the flame differently than design intent, operated at grossly lower efficiencies, etc. We had pretty radically different combustors that fired off the first time and operated close to intent. The only real problem we had was combustion acoustics due to the flame instabilities at lean conditions (think of how a candle flame makes noise when you nearly blow it out). Now I agree at the homebuilder level, it may be an unknown...but I would hate for readers to think that the large gas turbine manufacturers still have such design issues. With modern design and testing tools, the problems have been reduced and the knowledge of the flow fields and the reaction kinetics are much more complete.

Good luck,
Chris

Hi racket and finiteparts,

I was going through this and had a few questions come to my mind.

If John has designed his flametube according to Jetspecs then the AFR of the evaporator tubes should be close to ~5.7-5.8 (assuming all of the fuel evaporates instantly inside the evaps or else effective AFR will be even more) giving us the equivalence ratio of about (phi=2.5-2.6) which is below the rich flammability limit.
This also assumes that all holes in the combustor have similar flowing characteristics and that mass flow through every hole is just governed only by its effective area and static and dynamic pressure distribution is sort of same in the entire combustor.

My question is shouldn't we be designing our evaps for a more fuel rich mixture (say target AFR of 2-3 or phi of 4-6) ?

And I think this is what Lefebvre suggests " The AFR within the tubes varies from about 6 at idling conditions to between 2 and 3 at maximum power)
The reason for this is- When operating engine at higher power thus higher air temperatures in the combustor, the flammability limit will further widen towards the rich end and if we have mixture equivalence close already close to flammability limit then our fuel air mixture can spontaneously ignite inside the evaporators.

And further to this thought, any turbulence or flow re-circulations inside the evaporators at these conditions will further increase the chance of fuel air burning inside the evaps. 
Flow recirculation is less of a problem in straight evap tubes but is more likely in U-shaped evaps or T-shaped evaps where fuel hits the base of the horizontal arm of T.

Thanks.

[racket]

Hi Patty

We really don't know the flow rate of air through the evap tubes , I was recently looking at a "Scholary Article ??" on the KJ66 engine and it had very low percentages of air flow through its sticks compared to what would be expected from their ID sizing .

There has to be an overal pressure drop across the tubes , but as fuel evaporates and is accelerated to the air flow speed there has to be energy added to it, and taken to its extreme , if all the fuel instantly evaporated it could possibly stop air from entering the tube...................its a complex situation.

Currently my evap tubes hardly show any signs of severe heating even at their "hot" end prior to dumping into the Primary Zone , I don't know if this is a result of good evaporation rates or unforseen cooling from wall hole air injection .

As I've been having combustion problems ever since building the engine due to the extremely short length , the latest wall hole burning is going to prompt me to redesign the flametube , I need to get combustion happening faster , so intend adding the Primary Air directly at the evap tube outlets rather than through the side walls , air injection "caps" will be installed in the front wall, onto which the evap tubes discharge and spread their fuel/air mix radially , the Primary air will be injected radially from the side wall of those caps, the two mixing against the frontwall and in the "doughnut" surrounding each evap tube ............hopefully :-)

Cheers
John

[racket]

Hi Guys

Got the engine disassmbled and most things look OK except for the flametube .

Pic from shaft tunnel side



Pic from "inside" flametube , there more buildup of molten/burnt material inside



Still can't understand why its happened in this spot rather than anywhere else :-(

Cheers
John

[pitciblackscotland]

That's quite a large hole there John.

Cheers,
Mark.

[CH3NO2]

Is there, or was there, a gap between the top plate and flame can wall at the seal joint?

If there was a significant leak path at the top plate could it have created a localized recirculation zone?
In the first picture the metal at the top right of the hole (at the joint) looks like it was roasted at a higher temperature than the far left areas of picture. If there was a small leak path at the joint, it would explain a local increase of heat.

In the second picture, far to the left of the hole, the metal is jet black. Suggesting it is locally fuel rich there.
However, far to the right of the hole, the soot is scorched away. This can be interpreted a few different ways but it corresponds to the heat marks in the first picture.

The hole may have been instigated by a leak path at the top plate.

Tony

[madpatty]

Mar 6, 2019 22:46:41 GMT -5 racket said:

Hi Guys

Got the engine disassmbled and most things look OK except for the flametube .

Pic from shaft tunnel side



Pic from "inside" flametube , there more buildup of molten/burnt material inside



Still can't understand why its happened in this spot rather than anywhere else :-(

Cheers
John

Hi Racket,

This is an interesting read. Sort of explains one of the many reasons why this could be happening in your engine's flametube.

KJ-66 evaporator study

The figure 6-b is most relevant to engines of our design.



In this image you can see most of the evaporated fuel (in case of our relevance) present at that exact spot where your flametube is overheated.



And finally in this picture maximum amount of fuel air mixture is really concentrated near that inner wall spot.




You can see in the images and it's also written that as soon as the secondary air meets, that region shows maximum temperature. So i'll guess the flame was really burning close to your secondary zone hole and due to air presentation in your particular flametube design the fuel was burning really close to inner flametube.

Now they have also shown that bigger and slower moving droplets (as expected) will not form homogeneous fuel air mixtures in primary zone and burn in more distributed "local" hot spots where they achieve stoichiometry which in your case maybe close to that wall.

Cheers.

[racket]

Hi Tony

The wall was "seated" fairly tightly against the rolled edge of the front wall , so minimal chance of leakage , but there was an ~3mm "axial gutter" between the end of the sidewall and the front wall that fuel could accumulate in , and with the swirl being produced within the primary zone that fuel could move to a convenient position and possibly burn.

Yesterday I had a good long chat with Andy M about the problem and we've decided that the side walls need to be fixed to the "outside" of the endwall roll over rather than being "inside" , this will remove any chance of fuel collecting in that "gutter", it will simply flow from radial to axial smoothly and onto the side wall in a less concentrated state .

Also the evap tubes, which are currently "angled" towards the inner wall so that they exit roughly mid way between inner and outer walls , will be repositioned "axially" bringing their outlets closer to the outer wall which isn't suffering any heat issues.

The repositioning of the walls will also add a bit more to the flametube volume which won't be a bad thing as its kinda small at present .

The Secondary air holes might be displaced a tad further rearwards as well to increase the Primary Zone volume , and I might go for a single hole rather than twin holes between each evap tube to get deeper penetration and less chance of cold air contacting the evap tube .

I'll do a bit more thinking before cutting metal :-)

Cheers
John