100-150 Hp turboshaft thoughts

[monty]

John,

Without a map I'm flyin' blind....Just guestimating. I did correct the flows for temp and pressure at altitude. If I increase either the fan PR or mass flow at SL, I can handle up to 200 hp with the wheel you guys are using. Around 500-600 lb of thrust for take-off depending. I need to look at the fan design to decide what is feasible. If the fan is sized for 200hp at SL, it can easily absorb 150 hp at altitude with a little PR trim. As far as operating line, I'd try to trim the fan to keep the core on a constant RPM line and let the TIT decrease. The PR would go up with decreasing corrected flow at the same rpm. I'm afraid the only way to do this is guess, build and test. I know it will work, it's just a question of how well! Testing can be accomplished on the ground, hot day, high altitude airport. Where I live, I can maybe catch a really hot humid day in the summer and test at a density altitude of around 10kft. To get much higher than that with any regularity will require a trip out to the Rocky Mountains. Of course I'll be up against TIT limits on a hot day and the engine would perform better at altitude with lower temps, but might run into comp surge...

Monty

[monty]

John,

I've been looking at the cycle, engine, and fan design....starting from high altitude and working back to SL. Everything works better with a bigger compressor wheel. Fuel consumption, thrust, everything! The Ebay seller that I ordered from dropped my wheel and bent one of the blades, so they cancelled my order and I changed it to the X846 wheel. Question is do you think this wheel will flow 3.6 corrected lb/s at SL with a a 3.5 PR. If so....exciting things! If the 13 lb/s/sqin rule of thumb holds true...2.8 lb/s or so is less exciting, but still pretty good. Engine design is progressing nicely. Though I need to drive a stake in the ground on compressor flow so I can finalize the cycle and fan design.

Monty

[racket]

Hi Monty

I've designed for a flow of 3.3 lbs/sec for Anders at 5:1 PR with the X846, 3.6 lbs/sec will be getting too far to the choke region at only a 3.5 PR , efficiency would be poor along with SFC .

3.6 lbs/sec is what an Allison 250 C20 engine flows , thats a big "hole in the front" and potentially a very powerful engine .

Some of the new GEN 2 Garrett turbos are producing huge flows per square inch of inlet , they're aiming for max flow rather than max efficiency ,............we can't have both .

This comp flows 160 lbs/min at ~14.8 lbs/sq in/min www.garrettmotion.com/wp-content/uploads/2018/05/GTX553394mm_COMP.jpg , a reasonable efficiency at 3.5 PR , if the X846 was flowed at a similar rate it'd be ~3.4 lbssec , but you'd probably have problems fully expanding it and getting it out the turb exducer.

Andy M's 10/110 engine with his X858 comp of 110 mm inducer at a lower PR than Anders has been designed for 3 lbs/sec , which just makes it through the turbine wheel, IF, theres several PSI of static pressure that will be expanded in the A/B nozzle.

Your limitation is the turbine wheel exducer flow area , gases will be pretty well fully expanded down to ambient with maybe 800 ft/sec axial jetpipe velocity , you need to use that "restriction" as the basis for flow calcs .

My 12/118 with the X831 of 118mm inducer has been designed for 3.6 lbs/sec at ~3.5 PR , but trying to get that much air/gas through the TV94 turb wheel is "challenging" , I really need a much larger turbine wheel.

If you look at the Solar T62 engine its turbine wheel has an inducer of ~6.5" dia and an exducer of ~4.625" - 117 mm with minimal hub dia., it flows ~2.2 lbs/sec and produces ~150 HP ...........so look at your turb wheel as its going to govern the outcomes.

Cheers
John

[monty]

John,

Thanks for the info. What I've been doing is trying to match the engines theta break for 16500ft and 250 kts. Theta break is the point where If I go any higher in altitude the TIT will decrease to keep rpm from rising over max. To do this I have to balance the fan/compressor work and flow rate at altitude so that I'm at max allowable TIT and the compressor and turbine are happy. I've been designing for a turbine PR of 3 and about 2.4 lb/s actual core flow @3.5PR with the nozzle expanding to around .4-.6 M behind the fan. Then I have to look at the SL performance at take-off condition. 2.4 lb/s at that flight condition corrected to SL is 3.6 lb/s. If I increase the compressor PR, it steals from the fan work, and just results in higher M# out of the jet pipe which eventually chokes and limits the design. There may be another good design point with a higher core PR, but the SL static thrust is going to suffer. A compromise I might just have to live with.

My fan work at altitude is around 150 hp, compressor is around 200 so the turbine is generating near 350hp...crazy!!

What measurements should I take from my turbine to figure out the flow capability? It has the larger 111 mm exducer and is clipped slightly. The flow area is about 13.59sqin. The inducer is approximately 130X19mm.

I really do appreciate your help, and everybody on here. Wouldn't have been inspired to tackle this insanity without what you guys are doing.

Monty

[racket]

Hi Monty

The most you'll get out through the exducer is ~3 lbs/sec , you need to measure the exducer "throat" area between the blades ,its probably ~8.6 sq ins.

Once we get past that sort of mass flow , especially at modest PRs , the inducer area gets a bit "small" requiring a larger NGV angle with greater radial inflow velocity and less tangential impact so less power produced , its a balancing act .

I think you're expecting a tad too much to be able to produce 150 HP at 16,500 ft :-( ..................there'd be more chance with a freepowered equiped engine where you could run a higher PR and have the luxury of a larger flow wheel for power production .

Have you considered "splitting " the airflow from a large comp wheel , part feeding the gas producer and part feeding another combustor and freepower wheel attached to the fan ??

Cheers
John

[monty]

John,

I have reached the same conclusion, and dialed back the expectations. I re-sized everything for 2.8-2.9 lb/s at SL. It still has pretty good performance at altitude (100-120hp) If I manage to squeeze 3 lb/s through there great! If not great! I'm designing in some wiggle room for testing and refinement.

Monty

[monty]

I've been making steady progress on the design. Finalized the gears. I'm going to have to use a custom ring gear, and probably eventually a custom pinion made of as John would say "good-shitium" Gears are module 1.5 metric gears. 15mm gear width. The gear-set is a 4.4/1 planetary hunting tooth design with 3 planets. Pinion is 10 tooth with a .5 modified addendum. Currently planning to use hardened ground 1045 steel, with the pinion being nitrided also. Planets are 1045, 17 tooth hardened and ground unmodified addendum. Ring gear is 44 tooth heat treated 1045 with a -.5 modified addendum. Mostly because I can buy the pinion and planets off the shelf. Everything has at least a FS of 2 in bending with the pinion having about a FS of 1-1.2 for surface durability at 1000hrs. The database I'm using doesn't have any data on face hardened nitrided 1045 steel, so-a bit of a guess. I'm not too worried about it because it's easy to get a FS of 1.5 or so with some material substitutions. At 66Krpm the gears hit 10million cycles really fast, so if it lasts 3 hrs, It'll probably last 1000.

I'm currently working out the details on the part interfaces and castings/machining.

I was thinking about what John said about a primer on fan design. I honestly don't know where to start. There are so many variables, and I've been looking at this problem for 15 years. The airplane and the engine must be developed together. What you would do for a ground based thrust engine is totally different than something that goes in an air-frame. My process looks more like a bowl of spaghetti than a straight line!

Monty

[monty]

John,

I did wind up going with a slightly higher PR on the fan ~1.1. This gives up some static thrust, but the additional PR is helpful in cruise. The core will be operating at around 3.9 PR overall.

My X846 compressor wheel should arrive Tuesday of this week, at which point I'm going to start looking at the diffuser and NGV designs in earnest. I'm hoping that the X846 will be in the highest efficiency island or close to it around 2.8-3 lb/s at a PR of ~3.5. I was going to take a stab a sizing the diffuser throat using the method outlined in the Casey&Rusch paper. They also talk about how to choose the correct trim by measuring the inducer angles and using inducer tip speed. I don't really want to trim the wheel, but I will if it is horribly mismatched. Feel free to tell me if you think this is the case.

I don't really know the best way to approach the NGV. In the past I used the spreadsheet by Peter Lloyd, which assumes an axial turbine.


Any advice on the best process to go about matching these components?

Monty

[racket]

Hi Monty

The Casey & Rusch Paper is a good one to use , I'll be interested to see what numbers you come up with so that I can "check" my ones I did for Anders against yours , as there were some things I didn't quite understand in the Paper, a second set of calcs will be useful to compare, and we can throw ideas back and forth.

The NGV will be an easier job as you'll be running a choked throat , I simply calculate for the theoretical area then add on 10% for "boundary" blockage , go for 3 lbs/sec flow that way if theres a bit more "blockage % " your flow won't suffer.

Your numbers won't be a lot different to those I did for Anders , his engine sounded pretty good at a 4:1 PR on his last testrun :-)

Cheers
John

[monty]

Guessing at some stuff.....slip especially:

[monty]

Oct 21, 2018 15:46:44 GMT -5 racket said:

Hi Monty

The Casey & Rusch Paper is a good one to use , I'll be interested to see what numbers you come up with so that I can "check" my ones I did for Anders against yours , as there were some things I didn't quite understand in the Paper, a second set of calcs will be useful to compare, and we can throw ideas back and forth.

The NGV will be an easier job as you'll be running a choked throat , I simply calculate for the theoretical area then add on 10% for "boundary" blockage , go for 3 lbs/sec flow that way if theres a bit more "blockage % " your flow won't suffer.

Your numbers won't be a lot different to those I did for Anders , his engine sounded pretty good at a 4:1 PR on his last testrun :-)

Cheers
John

John,

Yes...compare notes we must... I can calculate the choked area on the NGV easily enough, but i have no idea what angles to use!!

Anders' engine is my inspiration at the moment!

Monty

[racket]

Hi Monty

Yes slip , now that gets a bit tricky with these billet wheels , the "advertised" outlet angle is 60 degrees , but when you look at the wheels exducer from the front , it appears as though the raked blade root is 60 but the tip at the shroud is a different angle , somewhat more laid back , so I went for a 50 degree outflow angle which should be sufficient "slip" plus any other changes from the blade angle change , an outflow air angle ~19-20 degrees seems to be OK , with a radial component of ~500 ft/sec .

With the NGV , I workout the theoretical choke flow area , add 10 % , divide into my 18 throats ,...... then its necessary to actually draw it up using the vane thicknesses intended , the thinner the vanes , the lower the angle ( more tangential ) , but theres generally a couple of millimeter of vane thickness to be taken into account , this then increases the angle , but you should end up at ~22-24 degrees , vane height of 19 or 20 mm depending on the turb inducer height.

Cheers
John

[monty]

John,

I've been crawling through the paper and calcs for the diffuser. The problem is the area ratio is based on the assumed value for the polytropic efficiency. I assumed about .8. Some of the numbers don't jive. I'm in the ballpark with the values in the paper, but the numbers don't match fig 2 and 4. My values for work coeff, and inlet blockage are all about the same as the paper, but the calculated area ratio doesn't match??? The assumed efficiency in the paper is more like .86

My calculated Ad*/Ai* value is .6. Fig 2 and 4 show closer to .36 for Mu2 of about 1.6. I need to look at the effect of changing the assumed efficiency. Still working on it.

Outflow angle is around 17 deg with about M 1.6 tip speed. My best guestimate for outlet flow is ~ M 1.3 with slip.

Monty

[monty]

John,

Numbers are making a lot more sense now. I found a typo in one of the equations. The Ad*/Ai* number is highly dependent on efficiency and stage PR. If I run numbers closer to Ander's engine I get Ad*/Ai* around .36. If I pick an aspect ratio of about 1 for the channel and 8 degrees for the diffuser wall (about optimum based on what I've learned) I get 15 channels just like you have. With my lower pressure ratio I need a larger diffuser throat, since the density exiting the compressor is lower. My Ad*/Ai* is closer to .45 and I need 17 channels. I might want to look at slowing down the impeller a bit to increase efficiency. My redline might need to be more in the 64Krpm range or so with this wheel. I'm going to play around with the design a bit now that I've got the spreadsheet behaving properly.

What did you measure for the impeller throat area?

Monty

[racket]

Hi Monty

I measured ~5500-5600 sq mms for the inducer throat area .

At what radius is your diffuser tip and throat ?? .............the greater it is the smaller the area ratio , Anders is at ~0.30

I originally calculated a RPM of ~62,500 rpm for a 4:1 PR which gives an "averaged" tip speed of ~1650 ft/sec at 154 mm dia average exducer , but after examining the exducer angles more closely it probably needs your ~64,000 as part of the exducer appears to have an angle lower than 60 degrees.

With regards the Ad/Ai ratio we can go a bit more to the lower side than theoretical because we don't need a wide flow range as required when the comp is part of a turbocharger on an IC engine.

The diffuser total included angle can go out to 12 degrees without much penalties , but I tend to find a 15 degree wedge and ~10 degree divergence comes up a lot of times.

With the wheels tip height , its advertised at 8.8 mm , but at the extended tip its lost ~1 mm , so an 8 mm diffuser tip height is about max allowing for 0.3 mm shroud clearance at the exducer.

Cheers
John