Newby questions

[amishmafia00]

I am trying to do my homework before starting this project. I am looking to build a turbine with an HT80 turbo. Fuel delivery rates is probably my biggest unknown. I plan on running it on diesel fuel or kero. Does anyone have any ballpark numbers on fuel rates at idle and full throttle? Another thing I have been considering is the outer wall of the combustion chamber. Jet specs says to leave .5 to .75 inch air gap. My instincts would be to make it tapered larger at the primary end and feed the air in at that end also. Is this wrong thinking? Also, what would be the most cost effective option for a power turbine?

[racket]

Hi

Welcome :-)

We can generally design for a max airflow rate of ~12 lbs/min/square inch of comp wheel inducer area, and as we run an overall Air/Fuel Ratio of 60:1 , we need 12 lbs of fuel/hr/square inch of inducer , eg, a 3" dia inducer = ~7 sq ins , so ~84 lbs/min of airflow , so ~84 lbs/hour of fuel , which equates to ~12 US GPH of kero , you'll probably burn ~3-4 GPH at idle .

As for freepower turbines suitable for larger turbos theres really only the Allison C20 heli engine 3rd and 4th stage turbine wheels which can occassionally come up on Ebay , or if you can find a friendly overhaul shop that might contribute a time expired wheel .

Where in the world are you located ??

Cheers
John

[amishmafia00]

Thank you!
I am on the east coast of the US. .
Coming from the performance diesel world I couldn't help but think about a compound turbojet. Has anyone ever done that? In my head I see the turbojet equivalent of what the diesel world calls a "triple" setup. That's two turbos in parallel for the first stage feeding a single turbo for the second stage.

I saw the C20 turbines mentioned a couple times on here. Is it always just one of the wheels that gets used and not both? I may just have to contact a high school friend who was and hopefully still is a helicopter mechanic!

[racket]

Hi

East Coast of US , you shouldn't have problems sourcing a wheel .

Compounding would be possible , but a lot of complication , thrust engines use mass flow more than pressure , but for a shaft horsepower engine its a different matter , the higher overall pressure ratio should produce more energy going into the freepower , but very good intercooling required between the comp stages otherwise the outcomes are degraded .

The difference between using turbos on an IC engine and turbos as a gas turbine is that the pistons can be used to force the hot gases through the turbine stage/s at higher pressures than what the comp/s is/are producing , whereas with our turbine engines the turb stage can only swallow what it "volunteers" to swallow, we can't "force" the turb stage to do anything, if we try , the comp will probably go into surge, the highest pressure in our engine is at the comp exducer tip , the pressure goes downhill after that .

The C20 wheels are sized nicely for our larger turbos , we only need a single stage due to the limited pressure downstream of our gas producer and the gas velocities they can produce in the freepower stage , with a compounded arrangement there could be a case for using a pair of wheels to extract more energy/power, but again it creates more complexity and trying to get just the right matching would be challenging .

Cheers
John

[amishmafia00]

I figured that matching would be very difficult and intercooling would be a must. I just can't help thinking about the possibility!

[racket]

The biggest problem with a 2 into 1 would be the freepower stage , it'd need to be fed by both of the low pressure turbine stages , so pretty big freepower , there are Garrett TPE331 turboprop freepower wheels occassionally on Ebay .............stator www.ebay.com/itm/Honeywell-TPE-331-Stator/324227386607?hash=item4b7d754cef:g:~aYAAOSw7ple2oty wheel www.ebay.com/itm/Honeywell-TPE-331-Rotor/324231042489?hash=item4b7dad15b9:g:HqEAAOSwrItfD1T5 though they might not be a match for each other .

Generally speaking, higher pressure ratios have more to do with specific fuel consumption ( SFC), fuel burnt per horsepower produced, than increase in power output , and as we aren't too concerned about fuel consumption with our short duration run times, constructing compounding setups isn't high on the list of priorities , if we want more power we simply build a bigger engine and save a lot of complication .

How we differ from aircraft turbine engines is we need intercooling between comp stages , that intercooling effectively "wastes" a lot of heat energy that has to be replaced with fuel later on in the process so our SFC doesn't improve as much as it should , and generally the high pressure turbo stage runs lower turbine efficiencies which further reduce any gains from the compounding .

If you want more power , rather than compounding , simply use two "normal" engines and flow their exhausts through a slightly larger freepower than a compounded setup , often a compounded setup will produce pressures going into a single stage freepower that are higher than ideal and more energy is wasted in the freepower exhaust , they really need a twin turbine arrangement to fully maximise the advantages .

Lotsa variables with turbines :-)

Cheers
John

[dieselguy86]

John,

Is the intercooling strictly so the hp comp can survive under the heat soak and g loads? Or because it requires more power to compress the hot air? What if a titanium comp was used for the hp? Could a radial turb power 2 comps on the same shaft, similar to the tpe331 engine, but with the comps back to back? Lotsa questions, you know me lol

-Joe

[racket]

Hi Joe

Yep , the alloy second comp wouldn't survive the temps , say we used 3:1 PR in both at 75% effic , temp rise 142 C deg in the first comp , 157deg C for the temp into the second ,212 C deg rise in the second stage , 369 deg C - 696 deg F out of the second stage , kinda hot for alloy ................plus that extra 70 deg C rise in the second stage compared to the first , thats another 42 HP/lb which the turb has to supply compared to if there was a perfect intercooler between stages .

A Ti second stage comp would survive but we would still have that extra horsepower requirement , if we have an overall temp rise of 369 minus 15 = 354 C degrees , that requires 216 HP/lb of mass flow to be provided by the turb wheel ...........might be doable ............but why do it, the benefits are minimal for all the complication :-(

Cheers
John

[dieselguy86]

Hi John,

I agree with ya, my idea is for an ic driven unit similar to what we've discussed before.

[dieselguy86]

John,

Maybe you have seen this, it's a patent from honeywell for a 2 shaft turbo. Using an axial comp and turb, and the normal radial comp and turb. Variable stator and comp discharge vanes, as well as variable ngv.

I don't see it ever going into production, but makes ya wonder what they're experimenting with now.

patents.google.com/patent/US7571607B2/en

[racket]

I agree , far too complicated ( expensive) for an auto turbo

[amishmafia00]

After doing some thinking I think it would be best to start off building a gas producer to match the HP841 turbine so that I don't have to worry so much about building a power turbine. Thankfully user finiteparts has already disassembled and measured one here. Is there enough information in his measurements to figure out what size turbo would work well for a gas producer?

[racket]

Yep , theres enough info there to get a rough idea of a potential gas producer .

If I remember correctly , I did some "numbers" once for this freepower unit and felt there'd been some "fiddling" of pressure ratios to achieve the supposed power output and flows , there was a higher than normal pressure entering the unit than what we could produce from a gas turbine .

Maybe a GT4082 size turbo for the gas producer .