The One Man Turbofan

[racket]

Hi Monty

The Garrett G Trim wheel has a 111.6mm exducer and a throat area of ~7.86 sq ins , ~20.2 mm width at the exducer tip , with 7.86sq ins - 0.0545 sq ft and gases at ~40 cu ft/lb and 1900 ft/sec flow is ~2.6 lbs/sec , but with your larger throat area its closer to 3 lbs/sec , neglecting any "flow restriction" from boundary layers which might reduce things to ~ 2.47- 2.85 with a 5% loss

My turb map for the Garret F Trim wheel ( 106 mm exducer) indicates a Corrected Mass Flow of 90 lbs/min at a 2:1 PR , it "flat lines" after that , so I imagine the 1.70 A/R scroll is choked after that , the 1.46 A/R scroll is similarly affected but chokes at 85 lbs corrected.

Now the 111.6 mm exducer , similarly clipped might flow ~12% more or say 100 lbs/min.

With a 1173 K for TIT and a 4:1 PR ( ~3.8 PR going into the NGV )Actual flow would/could be 190 lbs/min or a bit over 3 lbs/min ..............if the exducer can cope .

Your 95 mm comp inducer will be working very hard to flow even 2.75 lbs/sec and it might not be working at best efficiency

Have you checked the inducer tip "angle" , the X846 wheel has an inducer tip angle ~24-25 degrees ( 65-66 deg from axial) , similarly the X831 in the 12/118 has a shallow ~23-24 degrees angle ,

With an inlet flow of ~13 lb/min/sq in , your 11 sq in inducer will be flowing only ~2.4 lbs/sec , having a very low <40 Trim configuration your inducer tip is travelling relatively slowly so one would imagine it should have a larger tip angle to keep flows up to prevent there being any angle of attack problems

Cheers
John

[monty]

John,

Well, turns out I actually have the X856 wheel. I forgot that I went through all this a while back and decided the 95mm inducer wouldn't work if I wanted 2.8-2.9 lbm/s flow. Being the lazy dolt that I am, I didn't re-name the file, because I would have to re-build the assembly! So I actually have a 106.56mm inducer. The exducer is 156.76 (extended tip) and the tip ht is 11.8mm

Monty

[racket]

Hi Monty

OK , no problems getting your flow :-)

Might pay to check your inducer tip angle though to see what flow it'll be happiest with , also that wheel has a larger exducer tip height and smaller superback than the X846 that Anders is using , so it would appear its a more modest PR wheel which should suit you better .

Cheers
John

[monty]

John,

I couldn't find my protractor so had to trig out the angles. I got around 38 deg....(52 from axial)

My map should be leaned over a bit compared to Ander's. Which is what I was after. Hopefully not too much!

Now I've got to get deep into the NGV design. That is the last part keeping me from finishing the diffuser.

Monty

[racket]

Hi Monty

OH !!!

38 degrees is kinda huge , its certainly trying to grab a lotta air , I checked out your earlier pic of the comp (page2 of the build thread) and held up my spare wheel I have for Andy Ms engine with the 110 mm inducer and the difference in angle is appreciable.

I'll have a look at doing some numbers

Cheers
John

[monty]

Feb 1, 2019 23:02:37 GMT -5 racket said:

Hi Monty

OH !!!

38 degrees is kinda huge , its certainly trying to grab a lotta air , I checked out your earlier pic of the comp (page2 of the build thread) and held up my spare wheel I have for Andy Ms engine with the 110 mm inducer and the difference in angle is appreciable.

I'll have a look at doing some numbers

Cheers
John

John,

Yeah....bit of a difference there! The inducer blade is curved, so perhaps my numbers are larger than actual, but can't be that far off....maybe 5 degrees. 

Monty

[racket]

Hi Monty

Just been drawing up an inlet velocity triangle ,.....................:-( :-( :-( .....................not looking good , I don't think you can use that wheel , its undoubtedly a low pressure ratio wheel thats been designed to drag in as much air as possible through a limited "hole" size .

I rechecked Andy M's X858 inducer tip and its ~25 degrees , with air inlet velocities of ~550-650 ft/sec we end up with good flow triangles at the rpm we need to produce the PR required .

But with a 38 degree angle even at modest exducer tip speeds of <1,500 ft/sec maybe 2.75 PR , the inlet velocity triangle has air speeds heading towards 800 ft/sec.

We need to get this sorted before progressing any further

Cheers
John

[monty]

Feb 1, 2019 23:56:50 GMT -5 racket said:

Hi Monty

Just been drawing up an inlet velocity triangle ,.....................:-( :-( :-( .....................not looking good , I don't think you can use that wheel , its undoubtedly a low pressure ratio wheel thats been designed to drag in as much air as possible through a limited "hole" size .

I rechecked Andy M's X858 inducer tip and its ~25 degrees , with air inlet velocities of ~550-650 ft/sec we end up with good flow triangles at the rpm we need to produce the PR required .

But with a 38 degree angle even at modest exducer tip speeds of <1,500 ft/sec maybe 2.75 PR , the inlet velocity triangle has air speeds heading towards 800 ft/sec.

We need to get this sorted before progressing any further

Cheers
John

John,

I'll get a better measurement of inlet angle tomorrow. If I need to change the wheel, not a big deal in the scheme of things. :-)

Monty

[monty]

John,

I used a machinist protractor to get the actual inlet angle. Because of the curvature I was off a bit. It's around 30 deg.

Monty

[racket]

Hi Monty

Thanks for the update , I'll do some numbers for 2.9 lbs/sec and see how they look at 30 degrees

Cheers
John

[racket]

Hi Monty

It still seems too high an angle for a 4:1 PR sorta RPM/blade speed :-(

From some quick calcs, you'll be needing ~530 ft/sec through the ~0.08 sq feet of inducer flow area ( hub and blade area accounted for, plus a tad extra for boundary layers) to get your 2.9 lbs/sec at a static pressure of ~12.5 psia and density ~0.068 lbs/cu ft at ~275 deg K

Unfortunately theres going to be roughly 6-7 degrees difference between blade and air angles at ~ M 1.2 relative speeds :-( ....................I don't know how well things will cope , a couple of degrees one way or the other maybe OK, but 6 or 7 might be stretching the friendship .

Cheers
John

[monty]

Feb 4, 2019 22:58:53 GMT -5 racket said:

Hi Monty

It still seems too high an angle for a 4:1 PR sorta RPM/blade speed :-(

From some quick calcs, you'll be needing ~530 ft/sec through the ~0.08 sq feet of inducer flow area ( hub and blade area accounted for, plus a tad extra for boundary layers) to get your 2.9 lbs/sec at a static pressure of ~12.5 psia and density ~0.068 lbs/cu ft at ~275 deg K

Unfortunately theres going to be roughly 6-7 degrees difference between blade and air angles at ~ M 1.2 relative speeds :-( ....................I don't know how well things will cope , a couple of degrees one way or the other maybe OK, but 6 or 7 might be stretching the friendship .

Cheers
John

John,

Thanks for the numbers. Looks like I better sharpen my pencil a bit.

I was looking through my cycle spreadsheet. The fan has a 1.1 PR and the compressor stage 3.6 for an overall PR of ~ 4.

I'll add a few lines in the compressor tab to look at what the ideal inlet angle should be given local density and temps after the fan.

Monty

[racket]

Hi Monty

The fan "supercharging" will only exacerbate the situation as the increased inlet density will mean a lower axial velocity into the comp to produce the 2.9 lbs/sec flow

Cheers
John

[monty]

Feb 5, 2019 18:42:25 GMT -5 racket said:

Hi Monty

The fan "supercharging" will only exacerbate the situation as the increased inlet density will mean a lower axial velocity into the comp to produce the 2.9 lbs/sec flow

Cheers
John

John,

Yep....

What exducer tip speed and rpm were you assuming? I'm getting somewhere in the neighborhood of 18 degrees tip angle for this inducer area.

Monty

[CH3NO2]

Feb 4, 2019 22:58:53 GMT -5 racket said:

Hi Monty

It still seems too high an angle for a 4:1 PR sorta RPM/blade speed :-(

From some quick calcs, you'll be needing ~530 ft/sec through the ~0.08 sq feet of inducer flow area ( hub and blade area accounted for, plus a tad extra for boundary layers) to get your 2.9 lbs/sec at a static pressure of ~12.5 psia and density ~0.068 lbs/cu ft at ~275 deg K

Unfortunately theres going to be roughly 6-7 degrees difference between blade and air angles at ~ M 1.2 relative speeds :-( ....................I don't know how well things will cope , a couple of degrees one way or the other maybe OK, but 6 or 7 might be stretching the friendship .

Cheers
John

Hi John,



Might this be a situation where inlet guide vanes could be used to bring the air angle more in line with the blade angle?



Tony