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Post by finiteparts on Nov 27, 2015 1:40:25 GMT -5
Hi John, I agree that the choking mass flow parameter is about the only thing of value on most turbine maps...turbine maps are terrible due to their "collapsing" all the constant speed curves into a single overall curve. That turbine map was probably a poor example...most likely I didn't accurately circle the choking location for the 1.06 AR curve...the general trend appears to be the smaller area housings incur higher losses, thus requiring higher choking pressure ratios...for example... The idea that the efficiency of the scroll passage decreases as its cross-sectional area decreases, falls well in line with other internal flow literature. All things being equal, as the area gets smaller, the boundary layer thickness consumes a larger portion of the flow's kinetic energy. Smaller passages would likely have tighter curvatures which increase the secondary flows and thus losses. The design of scroll housings seems to be somewhat varied in the literature, but the common theme is that the flow controlling area is the cross-sectional plane at the tongue region....though some manufacturers use the flange inlet area. This area has to be smaller than the turbine throat area in order to actually exhibit flow control...so for all scroll housings below the largest size offered, it is safe to assume that you will have a minimum area at the tongue or inlet flange. Now, that doesn't mean that you will choke at the tongue first, since the temperature and pressure drop through the turbine changes the local Mach number and thus the exducer could still choke at a condition where the scroll housing doesn't...another one of those things we have to calculate out. Irregardless, the intent of the previous post was to explain why the choking pressure ratio deviates from what is predicted by the isentropic equations and to illustrate that it may be prudent to incorporate some "loss" number into the calculations...whether it is through the incorporation of a efficiency term or an area consideration, it really doesn't matter...well...the only problem with only applying an area increase is that it does nothing to predict the choking pressure ratio and thus could lead to an incorrect mass flow assumption at full throttle conditions...especially since the mass flow rate through the turbine controls the compressor flow. I know that the calculations are a best attempt prediction at best, but by redesigning the NGVs, we are essentially doing the same thing as having a small scroll housing...we are trying to control the flow at the NGV throats...so for me, this approach makes sense. Ha! I had confidence that you were using good values for specific heats and specific heat ratios. Cohen does a good job discussing the variable gas properties in his book. I just wanted to put it out there so that new readers would "see" it and at least be aware that they can't use gamma = 1.4 and Cp =1.001 kJ/kg*K all over the engine! Ha! So lastly, I ran across these nice renderings of the Engine Alliance GP7200 engine that would make a great screen saver or garage art! They can be found here: www.enginealliance.com/media/To give you a sneak peak, here is the isometric cutaway... Enjoy! Chris
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Post by finiteparts on Feb 7, 2016 16:58:21 GMT -5
While doing some 2-D turbine sizing work I ran across an interesting point that I completely missed, till I saw it in the images. What the image shows below is a 2-D single flow passage for a radial turbine. The static temperature results show that there exists a temperature gradient in the exhaust stream of a radial turbine (the total gas inlet temperature was 1600 F...75,000 rpm). Notice that the model includes the nozzle section and a portion of the exit duct...the turbine is shown by the dark black lines. Remember that this is a 2-D CFD, so there are things that will not be fully captured, but the illustration of the phenomenon was all it took to understand the physics at play. Since one component of the rotor power generation equation involves the change in circumferential speed, we should expect that the hub streamline generates a larger change that the exducer tip streamline (much smaller change in streamline radial location). This means that there will be less temperature reduction along the tip streamline as opposed to the hub streamline. And this is exactly what happens when we see a turbine being heated due to delayed combustion or over-fueling. The above image shows a lower trim turbine wheel, but we should expect that the larger the turbine wheel trim, the more pronounced the temperature gradient should be. So a ran another case with a turbine having a 10% larger trim. It is pretty evident that there is a marked increase in the radial temperature variation...in fact, the outer 15-20% diameter shows less than 20 F temperature drop. Since the real work done by the flow on this turbine would be an integration of the local gas properties across the entire radial span, it should be quite apparent why trying to use a single radial location to represent the real geometry in a 1-D equation can lead to errors. Additionally, this pretty clearly illustrates why commercial engines use multiple thermocouples at several radial locations in the exhaust stream to estimate the "average" gas path conditions. In reality the temperature gradient will mix out as the exhaust flows away from the turbine discharge plane, but the need for several averaged measurements should still be clear. Now I still think that the use of a radiation shield on the TC is a smart move since the use of turbines with higher trims makes the "line of sight" to the hotter surfaces more of a reality and thus would drive additional errors, but using multiple probes, averaged together should make the EGT measurements more accurate. ~ Chris
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Post by racket on Feb 7, 2016 18:17:55 GMT -5
Hi Chris
Yep , a "dogs breakfast" coming out , very interesting how much variation there is with Trim change , our turbo style turbine wheels with their large Trims are only a compromise , with a very big component weighted towards moments of inertia to improve the rotor acceleration rate, and because theres always more than enough exhaust energy available to drive the comp, turbine wheel power extraction "efficiency" isn't a big priority.
I guess the one positive with having the highest temperatures nearer the jetpipe wall is that our readings will be "on the safe side" , higher than the "averaged" temperature the wheel is coping with.
Looking at the two images its pretty obvious which one has the potential to extract the most power ( temperature drop) .
Thanks for sharing
Cheers John
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Post by finiteparts on Aug 3, 2016 15:32:52 GMT -5
I have been meaning to post this information up for a while, but life has a way of reshuffling your priorities sometimes! Unfortunately, I haven't been able to find one of these in my price range, but one day one of us on here might stumble on one and I would love to see one modified for a kart! So, Cummins/Holset had introduced a turbocompounding set-up on the Volvo D12 500TC and the upcoming D13TC, which is a 12 liter, 470 horse diesel, the Detroit DD15/DD16 and on the Mercedes OM473 engines. The idea isn't new, it was done on the Napier Nomad back in WWII (among other engines!). On a side note related to the Nomad, I once had lunch with David Vizard and he recounted how much of a nightmare that engine was to work on! Scania has run them for a while now on their 12 liters, but theirs use a radial turbine that doesn't seem very compact. Cummins added the axial turbine arrangement downstream of a HX55 turbocharger to increase the engine by 50 horsepower and up to 192 ft-lb of torque! Below is an excellent rendering of the arrangement as done by Scott Bondie. I put the link to his site so you could see all his other amazing work and I hope he doesn't mind that I made a copy of his image to share here. It is the best image of this equipment that I have seen anywhere, so I wanted to share it here. Check out his other artwork here: www.scottbondie.com/#!work/stackeraccordion3=9 There are some interesting videos on Youtube on these...here is just one example: www.youtube.com/watch?v=9PbxmRA9vbsWith some digging, you find out that the turbine runs at around 50,000 rpms and there it generates around 7 ft-lbf of torque, which then gets stepped down in speed and up in torque to the 192 ft-lbf torque that is provided to the crankshaft via a gear drive and a small torque convertor (allegedly from a Smart car). The torque convertor is the key to matching the turbine output speed to the crankshaft speed. These show up quite often on eBay, but for usually around $1000, which is too much for me to play with. Here is a nice image of the axial wheel, Every once in a while you see them show up with the gearbox as above, but lately, they have been showing up without it. I just thought that I would share so everyone could be aware that they are out there and be on the lookout. Enjoy! Chris
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Post by racket on Aug 3, 2016 17:54:02 GMT -5
Hi Chris
Thats a nice tidy little freepower unit and gearbox you have in those pics , and at a very reasonable price if the gearbox is OK .
The "fluid coupling" in their drive chain is also for protecting the high speed gears from being damaged by "torsional ??" issues feeding back from the crankshaft .
I've had the Volvo brochure on their FH12-500 engine for a number of years , its nice to know the bits are finally coming onto the parts market, thanks for that, I'll have to start checking out Ebay :-)
Cheers John
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Post by enginewhisperer on Aug 3, 2016 19:09:58 GMT -5
those look excellent!
What are your search terms to find these?
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Post by finiteparts on Aug 3, 2016 23:44:31 GMT -5
Try looking for Detroit DD15 or DD16 turbo stuff...
Here is one on eBay now...http://www.ebay.com/itm/401052342574?_trksid=p2060353.m1438.l2649&ssPageName=STRK%3AMEBIDX%3AIT
Good luck!
Chris
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Post by racket on Aug 4, 2016 0:58:50 GMT -5
Hi Chris
Thanks for the Link , I'll definitely file this info away as they are a ready made freepower unit :-)
LOL.............I wonder how much extra power could be extracted by it , being truck stuff it'd probably be "heavy duty" so maybe a bit of extra capacity .
Cheers John
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ripcrow
Veteran Member
Joined: December 2015
Posts: 114
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Post by ripcrow on Aug 6, 2016 4:31:08 GMT -5
Volvo has had major issues with compound turbos in the past
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Post by finiteparts on Aug 16, 2016 22:52:53 GMT -5
Can you elaborate on that? Just saying that they had major issues in the past tells us nothing...
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Post by enginewhisperer on Aug 17, 2016 0:18:12 GMT -5
Try looking for Detroit DD15 or DD16 turbo stuff... Here is one on eBay now...http://www.ebay.com/itm/401052342574?_trksid=p2060353.m1438.l2649&ssPageName=STRK%3AMEBIDX%3AIT Good luck! Chris thanks for the link someone I missed this until now, and am in the USA at the moment but not enough time.... maybe next trip
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ripcrow
Veteran Member
Joined: December 2015
Posts: 114
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Post by ripcrow on Aug 18, 2016 2:10:06 GMT -5
Turbo failures due to the compound arrangement. The actual turbo was ok until they compounded it.
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Post by finiteparts on Sept 5, 2016 12:44:11 GMT -5
Hi ripcrow,
I have been looking around to see if I could substantiate your comment on Volvo's issues with the turbocompounded arrangement and then understand what the failure mechanism is and how that might impact anyone using these for our purposes...and I have not been able to find any evidence of these failures. Can you point me to any information on this?
As for Detroit Diesels experience, I can only find that they had a situation with a turbocharger mounting bracket that didn't provide sufficient support and the resultant movement caused cracking of the turbine housings on the turbocharger.
I am curious what the downstream axial turbine could have done to make the turbocharger fail?
Thanks,
Chris
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Post by finiteparts on Nov 13, 2016 13:10:27 GMT -5
Recently I had the opportunity to buy a few used Borg Warner 6758 EFR turbochargers at a price that made the risk of buying used turbochargers acceptable. These turbochargers had been run in a Indy car back in the 2013 race season, and thus didn't have a "easy" life...but the lure of ball bearing cartridges, titanium aluminide turbine wheels and their forge milled compressors made it worthwhile. So here are a few pictures...
Notice the wonderful thin cast turbine housing...with a nice turbine inlet temperature port already to go.
The beauty of a ball bearing turbo is that they can run a tight tip clearance. And here is the rotor showing the forged miled compressor wheel. Notice how thin the blades are. This rotor (actually the whole turbocharger system) is a work of art. and here is a shot of the titanium aluminide turbine wheel. Since the rotor comes in around half of what a Inco713 wheel would weigh, they did not need to scallop the backwall of the turbine as a means to reduce the polar moment of inertia and thus could keep the backwall leakage losses down. In order to test out the ball bearing cartridges, I started to layout a single can combustor for these turbos. Once the first combustor was CAD'ed up, I ran a quick cold flow CFD model (no combustion simulated) to see how well the dome region was looking for recirculation stabilization. Unfortunately, the idea I had didn't look good at all. The single entry air pipe from the compressor wreaks havoc on the static pressure field in the annular space around the combustor liner. I had to take a few more passes to get a better feed and I will show those in a later post once I have the design more hashed out. ~ Chris
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Post by enginewhisperer on Nov 13, 2016 18:35:03 GMT -5
these also have a port for an rpm sensor in the compressor housing
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