Contemplating an HX82 Turbojet

[finiteparts]

Dec 22, 2023 1:55:02 GMT -5 monty said:

Chris,

I think I understand our confusion. You are talking angles relative to "flow". I am talking static geometric angles. I "think" this is the issue.

Angles relative to flow will be the same for different trims, but static geometry will be vastly different.

Monty

Yes, I agree.  The reason that I so often call this out is that if someone inadvertently plugs in an angle relative to the tangential vector as opposed to the vector relative to the axial flow vector, there can be errors.   If they check their work, this isn't a problem.   But if they just plug and chug, then there could be errors due to a sin() needing to be a cos(), etc...

Just the engineer in me, trying to mitigate calculation errors.

- Chris

[finiteparts]

Dec 22, 2023 1:23:59 GMT -5 monty said:

Chris,

That Honeywell comp inducer appears to have around 40ish deg angle of attack....which is what I'm after. The limit is about 50. 45 would be ideal.

The turbocharger wheels are mostly all much shallower. 20-30 deg. From tangential. So 70/60 in aero speak. Everyone is looking for max flow and PR at the point they need it. The turbo world has surge slots, blow off valves, and waste gates. They don't care about matching to the turbine, so much as matching to the engine/turbo system.

I'm trying to pick parts from the bin that will work best together. The problem I'm running into is the PR/mass flow rpm or specifc speed trade the turbo world uses isn't the best for us. I'm now looking at trimming wheels to get closer, but the inlet angles are not quite right. They are for lower axial mass flow/area.

If designed properly the relative mach numbers are going to be the same for low and high trim wheels but the incidence angles will change. So the "leading edge twist" will be more for lower trim values. Lower trim increases delta H in the wheel at the same tip speed....flow coef. changes though.

Monty

Interesting side note.    The Allison A250 was one of the first applications of the compressor bleed slot to broaden the map width that was presented in:

    1. AGARD-CP-282  (https://www.sto.nato.int/publications/AGARD/AGARD-CP-28/AGARDCP28.pdf)

Another good paper on the growth of the A250 is:

    2. Stevens, E.C. and Conn, F.E., "Evolution of the Allison A250 Engine", SAE Paper no. 800603, 1980

    - Shows the C30 production compressor stage has a usable PR of 8.9 at a corrected mass flow of 5.6 lbm/s

Then I think one of the earliest publications exploring it's use in turbochargers was:

    3. Fisher F B, "Application of map width enhancement devices to turbocharger compressor stages", SAE Paper No.880794, 1988

which was then adopted by Holset before all the other turbocharger guys jumped on-board.

- Chris

[monty]

Chris,

Yes on the angles, had to correct for that in my spreadsheet.

Regarding matching the comp wheel. Simply put: The larger turbines we are running when mated to compressors sized for these frames (especially racing wheels) tend towards lower pressure ratios and higher mass flows than we want to max out the turbine. Smaller frame turbo's are used for our mass flow range, but the tip speeds are achieved with higher rpm. So I need a big diameter low trim wheel for my design point. The engine will only see that at max power SL static, or some lower RPM on colder days. Normal operation will not be choked.

Any resources you guys have are appreciated. Especially anything about estimating when the exducer throat chokes. I am doing some guessing there I'm not super confident in.

Monty

[finiteparts]

By the way, if you haven't seen this excellent lecture by Michael Casey and Hamid Hazby of PCA Engineers Limited, it is defintiely worth a watch.

youtu.be/sfzjBjCw0No?feature=shared

Also, if you haven't been to their website, I suggest going and grabbing some of their published stuff.

Enjoy!

- Chris

[finiteparts]

Monty,

There are multiple radial turbine design programs documented on the NASA TRS. Here is one by Glassman that is for off-design, which means you can specify the geometry and then solve for the flow conditions. It uses a scheme that sweeps through a critical velocity ratio as opposed to setting mass flows and pressure ratios.because at each rotor speed, there will always be a range of critical velocity ratios, whereas specifying the mass flow may be a challenge "a priori".

ntrs.nasa.gov/citations/19830024536

The equations are provided at the end of the report and you can use them as is by specifying the critical velocity ratio as 1, so you can take a similar approach and derive map like quantities.

Good luck!

Chris

[finiteparts]

Another good resource for rotor choking, etc, is:

Erain Baskharone, "Principles of Turbomachinery in Air-Breathing Engines", Cambridge University Press, 2006

[racket]

Hi Monty

Found that GT6041 turb map with the 1.70 A/R scroll

www.rbracing-rsr.com/turbo/TurboMaps/GT60turbine.jpg

Cheers
John

[monty]

I took a hard look at the turbine tip speeds and decided I was being a bit silly. Backed the RPM down to 60Krpm. That gives me about 400m/s tip speed on the turbine. I went back through all the calculations and looks like about 40 trim will work. The wheel I ordered is a 40 trim wheel. I can design for the stock inducer mass flow and see what the turbine numbers look like. It should put me in a fairly efficient part of the map. The flow coefficients on the turbocharger turbines are a bit high, and there isn't a lot data on what's possible when they are coupled with an NGV. But the load coefficient and meriodonal velocity coefficient are all good. The triangles are acceptable. I can try this smaller wheel with less tip speed and see if the numbers in Peterson are reasonable or not. Target PR is now 4.5. 200 lbs of thrust at 1.56 corrected lb/s

I can then try a design with the 856 wheel with higher tip speed trimmed to the correct mass flow.

Maybe that will answer some questions.

Monty

P.S. Turbine PR is 2.1. I can shoot for 2.5 with the larger wheel and the larger mass flow....if the turbine will take it. Or maybe go big first....and back up if it fails. ;-)

[racket]

Hi Monty

This is getting a bit "over complicated" .

Your X856 comp wheel with its 30 degree 106mm inducer will be flowing as much as the X858 25 degree 110mm inducered wheel that Andy used , and considerably more than the X846 that Anders used with its ~24 degree inducer tip and F Trim turb wheel.

With a 106mm inducer and a 30 degree tip angle its been designed for a high flow and pressure with 8+8 blades , possibly to satisfy the 106mm Class of competition .

Also with a 60 degree outlet compared to the 45 degree of Andys wheel you'll be able to save ~6% on your RPM for the same pressure ratio , heh heh , or use the same tip speed but for more pressure :-)

Andys engine worked fine at its design point , it was a modest pressure ratio engine with that comp wheel , your X856 is more of a high pressure unit .

I think I'd be spending time getting the diffuser to comp design optimised rather than being too concerned about the comp geometry if you want to achieve your 200 lbs of thrust objective , you're going to be needing a lot of mass flow rather than very high pressure in the jetpipe , mass flow is linear, pressure isn't when it comes to producing thrust , a happy comprise is probably best :-)

Have a Merry Christmas, only one sleep to go for us , kids , grandkids , great grandkids and dogs ( forgot the food ) for me tomorrow :-)

Cheers
John

[monty]

John,

Point taken. The "shoot the engineer" moment is nigh. But there is a method to my madness. I want the numbers to make sense. I need them to if I'm going to make the fan work. I have been reading through Casey and Robinson, plus Mustapha. My numbers are lining up with the charts in their books...that gives me some confidence. I looked into getting a copy of that off design software code from NASA....that "might" work out....but there are a lot of hoops to jump through. There is a good example in Mustapha of a highly loaded turbine design. I'll be taking a similar approach. The turbine loading coef on my engine looks pretty good, though I'm not sure I can go with the completely unmolested X856. It might need a minor clip....The there are some really good guidelines in Casey/Robinson on this stuff..... I'll be spending some time with family in the next couple days. I'll be re-visiting things next week.

Merry Christmas!

Monty

P.S. Two sleeps for me ;-)

[monty]

John,

Ran a quick scenario through the spreadsheet this morning. I agree that PR isn't that important for creating thrust. However, it is helpful getting the mass flow through the turbine. If I allow the rpm to increase to 65krpm I get about 440m/s turbine tip speed. That is about the limit I would think for the temps we are running. If I set the flow for the stock X856 inducer, I get around 1.78 lb/s corrected with PR=5. The turbine loading coeff is about 1.01. That loading coeff is "just" possible. The flow coeff is off the charts at .73. Most turbines are designed for .5 or less. The sweet spot is around .3-.4. Most of the turbine maps for similar sizes to this holset max out around 1.6 lb/s corrected. That is what I was going to shoot for. At the 1.78lb/s flow level I can balance the work, but we are leaning into the impulse region of things again. If I push the NGV angle down as low as I dare ~15 deg, I can get semi-acceptable turbine inlet incidence at 19 degrees. In the 20s is where I would like to be, but 19 "might" work. I'm just not confident the turbine won't choke before I get to this mass flow. Turbine PR is around 2.3 at this design point.

The big problem is the diffuser design is completely dependent on these numbers. So it will be the "wrong" design if the turbine won't work with the mass flow.

For 1.6 lb/s corrected, the trim would bring the inducer down to about 100mm.

Monty

[racket]

Hi Monty

On a side note , E and E Turbo did/do ?? some odd HX82 turb wheels , I once asked about them but they said they were for an Order so didn't feel comfortable selling to anyone else .

Up to 134/127mm with 24mm tip height , some for ball bearings , Item # TW-0349 HX82 for 14.998mm ID ball bearings 123.8 exd 134.5 ind 24mm tip ,.............. TW-0349-1, TW- 0349-2 , TW- 0349-5 have larger 127mm exd

Merry Christmas :-)

Cheers
John

[monty]

John,

I ran a case with the X858 wheel at the same design point I tried the other day with the X856. To match the X858 inducer, the flow was actually slightly less than the X856. It was close though. Close enough I'm willing to go forward with the engine. I can't really make a lot of sense of the turbine maps if this Holset wheel will flow that much more. Seems I need to work on my choke flow calculation. Everything else seems to be working properly.

No time was wasted, I needed to put this design tool together. Now I need to test it with the engine. Crawling through all the math was instructive, and I learned a lot about how to match these components.

With the new design point, the diffuser changed a bit. I'm now using 21 channels. After reading through Patty's trials and travails with his bearings, I decided to add a squeeze film damper to the front bearing. Hopefully I can make some rapid progress and start printing casting forms soon.

Monty

[monty]

Dec 25, 2023 3:41:37 GMT -5 racket said:

Hi Monty

On a side note , E and E Turbo did/do ?? some odd HX82 turb wheels , I once asked about them but they said they were for an Order so didn't feel comfortable selling to anyone else .

Up to 134/127mm with 24mm tip height , some for ball bearings , Item # TW-0349 HX82 for 14.998mm ID ball bearings 123.8 exd 134.5 ind 24mm tip ,.............. TW-0349-1, TW- 0349-2 , TW- 0349-5 have larger 127mm exd

Merry Christmas :-)

Cheers
John

John,

The only thing I can think those larger wheels are good for would be waste gated turbos. That, or I need to REALLY work on the choke flow calculations.

Monty

[racket]

Hi Monty

I think those big HX82 wheels were for tractor pulling using ball bearings and astronomic RPM ...............LOL, a bit like my comp wheel :-(

Cheers
John