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Post by racket on Oct 17, 2017 3:24:02 GMT -5
Hi Guys Time for an update Having ruminated over my non working engine all winter I've finally got back into my shed and started construction of a new jetpipe/turb shroud. A pic of the old jetpipe at the exducer shroud area And the new one The old setup had ~0.030" radial clearance at the tip , the new one ~0.215" , some 0.185" greater , this works out at ~2.7 square inches of "gap" . OK OK , I know , ya gotta keep clearance to a minimum to get things working at their best , but its a case of two wrongs trying to make a "right" with this modification . Heres my reasoning , currently my exducer is the minimum flow area in the turb stage and where the max pressure drop is occurring , the reduced mass flow isn't sufficient to fillout the NGV and get its throat velocities up to max . Currently my turbines power is being mainly developed from the turb exducer gas velocity increase and deflection , but this velocity is at a more axial direction due to the severe clipping ( 55 degree) of the exducer and consequently not as much deflection power as normal . Now the NGV produces a gas stream at a shallower tangential angle ( 34 degrees) and at a greater radius(leverage) , if I can increase its velocity by unchoking the exducer by providing that 2.7 sq ins of uninhibited flow area , the NGV should choke and provide max velocity to the inducer . Also if the mass flow can increase to "design" , then hopefully the comps efficiency will also improve from its current ~75% to ~80% , this then reduces relative horsepower requirements and in the process improving the potential for the turb to have enough power to get rpm up to 100% . LOL...............lotsa unknowns , it might work , or it might not , I'll soon find out as soon as its idling , if temps are below previous numbers then its a success and hopefully things will be OK at higher PRs , if temps are higher then its a fail Time to reassemble the engine and add some mounts to the jetpipe for thermocouples and P4t, so probably a few weeks before testing begins Cheers John
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Post by turboron on Oct 17, 2017 7:01:56 GMT -5
John, good work. Your logic is correct. The flow should choke in the NGV not the exducer.
Thanks, Ron
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Post by racket on Oct 17, 2017 16:05:08 GMT -5
Hi Ron
I originally made the mistake of only doing calcs for full power where both NGV and exducer were running choked , but when getting up to full rpm/power there are problems because theres simply not enough pressure energy available for both "throats" , when I started doing the calcs for intermediate points I started getting some "unfortunate" numbers , the mass flows were too low , so I went back to my TV84 journal to try and find some "inspiration" from the numbers before and after its severe exducer clipping .
The TV84 was running a relatively larger flow turbine wheel than the 12/118 even before clipping , after clipping it was grossly over sized in the outflow area but the engine worked once I installed the "tight"choked 1.23 A/R housing to control comp flow , previously I'd been able to run up to a 1.84 A/R fat doughnut without excessive temps due to the exducer being choked and controlling comp flow as well as supplying the horsepower , inlet gas flows would have been very slow with the 1.84 A/R and probably were being bashed around by the inducer tip which would have been travelling a lot faster............it was out of tune, but still worked with good thrust levels.
Hopefully I've made some improvement, the results will be "interesting??" :-)
Cheers John
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Post by finiteparts on Oct 17, 2017 19:01:08 GMT -5
John, It will be interesting to see if you can get the engine components to match up. Here is a good reference that discusses the impact of axial and radial clearances in radial turbines... ntrs.nasa.gov/archive/nasa/casi.ntrs.nasa.gov/19700008458.pdfThe good news is that they claim as you go up in clearance it is not really linear, but sort of becomes a smaller impact relative to a linear model used in other sources. The bad news, radial clearance at the exducer has a 10X impact vs the axial clearance at the inducer...it is the big driver in clearance losses. Based on the loss models from Glassman's papers, the clearance losses for the 0.030" is around 1.5%, but at the 0.215", it grows to 12.5% (I had to assume about 120 Btu/lbm for the turbine specific work)...they might not be totally accurate numbers, but they do give the order of magnitude change. Did you keep the old housing with the 0.030 clearance or did you modify it to get too the larger clearances? The reason I ask is that I was looking at another paper the other day were they put a cone in front of the compressor inducer to reduce the mass flow through the compressor and modify the surge boundary. It appeared to work somewhat...I will have to dig around again to see if I can find it. If you are unsuccessful with the increased turbine clearance, this method might be another one worth testing. Also, with the large gap and the severe clipping, you might want to think about an averaging EGT rake. It is a tube that has multiple inlet holes at varying radial heights and one exit hole. The idea is that the multiple gas inlets mix and by the time they reach the thermocouple, they have mixed and averaged the gas temperature. If you use a single thermocouple, you will have to immerse it quite deep in the flow so as not to be measuring the outer gas flow, which did not do any work and thus will show no real temperature drop. I look forward to seeing your results. Good luck! Chris
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Post by racket on Oct 18, 2017 3:30:56 GMT -5
Hi Chris
Yep , I've kept the old jetpipe/shroud just in case , the new jetpipe needed a different Vband flange and larger diameter pipe , a 6" length of 5" dia exhaust pipe so that downstream of the turb exducer theres no contraction in area as there was with the original pipe , I've always been a bit concerned about just how long the wake is downstream of the blades and hub , I'd taper bored the pipe as much as I could , but still felt a tad uneasy about it being several millimeters smaller in diameter at the outlet than at the exducer face.
The temp differences in the jetpipe profile mightn't be too pronounced with the severe clipping as theres not a huge amount of work(deflection) done in the wheel ,and I'm exhausting to atmosphere from the open pipe, so hopefully there'll be sufficient mixing by the time the gases get to the thermocouples some 5+" downstream of the exducer , though just to be on the safe side one of the thermos would benefit from having a "shroud" to nullify any temp gradient but also to provide a more realistic temperature reading from the very high speed gases exiting that highly clipped wheel
Even with a 12.5% efficiency loss at the exducer because of the large clearance I'll be more than compensating for that due to the higher entry velocities into the wheel which could be 25% higher than at present ..................LOL, this is turning into a juggling act .
Yeh thats an interesting Paper , I always like "numbers" , especially for a wheel of similar size , I'm curious as to what the outcomes would have been with higher pressures/PRs across the stage , I'll do a bit more "digesting" tomorrow when the brain is working better :-)
Thanks for the input, results will be posted , for better or for worse .
Cheers John
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turboreno
New Member
Joined: July 2014
Posts: 5
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Post by turboreno on Oct 29, 2017 8:33:15 GMT -5
John, I remember your discussion of the F trim versus the G trim TV94 turbines you posted as follows:
John Wallis note December 16, 2016 On a side note , I've been looking around the web for turbo info and came across some on our turb wheels , it would appear we could have the Garrett F Trim wheel with its ~106 mm exducer , there is a G Trim wheel, same other dimensions except for the exducer being ~111mm , the turbocharger users are giving pros and cons with regards the benefits of each , so if you're having issues with the JU-02 , it might be possible to go up to the slightly larger exducer wheel which won't be "clipped" so should have a bit more gas deflection "power" even though flow rates mightn't be much different as our wheels have that clipping to improve flow .
Doesn't this say that a G trim wheel would be a better match for your engine based on your analysis that the exducer is choking before the inducer?
Thanks, Ron
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Post by racket on Oct 29, 2017 15:26:31 GMT -5
Hi Ron
Yes, the G Trim wheel has a bigger exducer , but after I'd clipped the F Trim wheel back its exducer dia is now the same as the G Trim ................I'm not certain if the wheel we are using is a "genuine" F Trim due to the fact it has a bit of clipping to sorta make it an inbetween size ................the reason we use this particular wheel is price , its a very common wheel so relatively inexpensive .
With my new jetpipe/shroud its effectively a ~122 mm exducer dia .
As for the JU-02 , its much higher operating Pressure Ratio entering the turb wheel means it should cope OK , the 118mm inducer on the 12/118 is ~22% bigger in area than the 107mm in the JU-02 and was simply too big despite the severe clipping , the problem is getting up to full rpm, at full power the design "works??" with the severely clipped wheel due to higher static pressures for the gases , but at mid range theres a "hump" to get over , ...............I'm into a sorta "hung start" scenario at a 2.5 PR :-(
Cheers John
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Post by racket on Nov 4, 2017 22:07:56 GMT -5
Hi Guys The engine is reassembled and back in the test stand with its new "large bore" 5 inch OD jetpipe/shroud . A few pics Old and new pipes Fitted and ready to be used Cheers John
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CH3NO2
Senior Member
Joined: March 2017
Posts: 455
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Post by CH3NO2 on Nov 7, 2017 22:08:57 GMT -5
... The fuel "mix" exiting the tube impacts the front wall and certainly "splatters" from the tell tale witness marks The tube orientation isn't exactly axial, theres a bias towards the inner wall . Because the outer wall Primary hole "cluster" is positioned close to the front wall whereas the Primary holes in the inner wall are displaced further rearwards there could be a "radial" curling/rolling of the air/fuel which could be feeding hot gases back towards the incoming air and fuel . ... Hi John, I have a question about your picture above. If this front wall, with the evaporator splatter, was laser cut with many very fine 0.010" holes resulting in ~0.5% open flow area such that it created a boundary layer transpiring through the front wall, what do you think would be the overall operational effect on wall temperature? Does wall temperature increase or decrease? Is it advisable putting boundary layer coolant in the walls of the primary zone? Thanks, Tony
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Post by racket on Nov 8, 2017 3:13:36 GMT -5
Hi Tony
We want the front wall to get "hot" so as to help with evaporation of the fuel .
As for whether or not the metal temps would increase or decrease with the addition of a multitude of small holes , I haven't the faintest idea , there might be some localised F/A ratio that promotes combustion closer to the wall if air/gas speeds are low enough, but I guess you'll just have to try it to find out ;-)
Wall cooling of the Primary Zone is certainly a good idea , some of the large industrial turbine flametubes have millions of tiny holes over the entire FT wall , years ago a guy sent me some drawings of such a FT, the cooling air "seeps" through the wall.
Cheers John
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Post by racket on Nov 10, 2017 0:13:06 GMT -5
Hi Guys
Took the 12/118 out for a test run this morning , but couldn't get it to fire up ...............bugger I thought , stuffed up something , so went back home again :-(
On inspection of the gauges video it was plain to see what as happening , as soon as the starter was activated the oil pressure started to fall as the available voltage dropped , this also lowered the fuel pressure as its pump runs off the same 24V battery thru a PWM , so despite me setting the pump pressure at the usual pre spoolup pressure , there wasn't sufficient flow once spoolup commenced resulting in insufficient rpm for self sustain.
After about 4 start attempts the batteries were "dead" , the poor old oilpump was barely turning around once the starter activated , the severe drop in oil pressure instantly activated the Hobbs switch and cut off the fuel pump power.
Maybe the latest mods have resulted in more mass flow through the engine , this in turn results in the starter having to work harder , consuming more Amps , and starving the oil and fuel pumps of power.
The batteries have been recharging all day so I'll check their "terminal" voltage and see what we've got to work with next time .
The batteries were charged yesterday prior to the test , but I didn't check actual voltage, instead relying on the charger to indicate full charge , it might be falsely indicating full charge when they're not "really full" .
I might try a bit of leafblower and preheat propane next time to warm things up a tad before the first start attempt to make things just a bit easier on the starter.
Cheers John
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Deleted
Joined: January 1970
Posts: 0
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Post by Deleted on Nov 10, 2017 1:51:38 GMT -5
Hi John Gutted I did once have problems with a battery that showed full voltage, but once you put a load across, the voltage just dropped away. All Best Andy
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Post by Johansson on Nov 10, 2017 9:26:57 GMT -5
I feel for you John, it is very frustrating when the engine is ready to go but the supply systems are messing up...
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CH3NO2
Senior Member
Joined: March 2017
Posts: 455
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Post by CH3NO2 on Nov 10, 2017 10:50:48 GMT -5
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Post by racket on Nov 10, 2017 19:00:19 GMT -5
Hi Tony
LOL.............the level of my electronics capability sorta means I don't go anywhere near my cars electronics/battery/alternator for fear of blowing an expensive part ...........it might be safer to fit a bigger battery to the test stand .
Anders and Andy
I'll do some testing/checking of the batteries to see if I can find any faults , I had ~13.6 V in one battery and 13.8 V in the other after recharging yesterday , I'll give them 24 hours to "settle" and recheck voltage ..............then try for another test next week.
Cheers John
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