A small gas turbine with Axial Flow Compressor!

[racketmotorman]

Hi Zippiot
Very nice :D
Professional finish with "correct" blading , unlike another persons on this forum.
I'm quite aware of the requirements of axial compressor design as at one stage several years ago I was considering making a single stage axial compressor driven by the freepower turbine I eventualy used to power my bike ....

http://www.nickhaddock.co.uk/turbine_st ... ke%201.jpg

to make a lowpressure "afterburner" using air from the single axial comp stage .
I did all the design stage work but the theoretical thrust outputs and fuel burn rates , gallons/minute :( from the air mass flow rate of ~8-10 lbs/sec , finally sent me on the freepower turbine driving a gearbox and drive chain route instead.
Yeh , small overall diameter is the beauty of axial compressors , basically they're only the "normal" inducer diameter of a centrifical type , its just the low pressure ratio per stage and the need to build several stages that lets them down .
I dare say you were only running a pressure ratio of ~1.5:1 with modest efflux velocities , but with a fairly decent mass flow rate compensating and providing the thrust , how was your fuel burn rate ?
Cheers
John

[Zippiot]

It consumed over a liter in a 5-8 mintue burn, wide open throttle mind but it sucked gas like none other. Actually my calcs showed i was making 1.75:1 per stage!!! Haha doesn't sound big but every little bit helps. I found a beautiful pic a long time ago of a large hollow aluminum cone filled with 50 stages of compression!! 50 stages!! First 8 were adjustable the rest looked like a spiny pinecone. Dunno if it works, looked pro but you never know.

[racketmotorman]

Hi Zippiot
I hope you meant 1.75:1 PR overall, not per stage :D
They are thirsty devils , the bike uses ~1.5 litres/min at full bore .
In its life as a pure jet before the adition of the freepower turbine , it produced ~100-110 lbs of thrust from that !.5 l/m so your 17 lbs of thrust from the fuel you used was was pretty good .
Cheers
John

[Zippiot]

Good eh? I heard that a 777 uses a gallon every 800 feet at cruise...

Lets see 500 mph...733 feet/sec so that thing uses almost a gallon a second?!?!?

17 pounds of thrust according to the food scale we had lying around, its accurate but the thing really sounded like it was making more. I bet that if we put in a few good years of r and d we could get up to the 30 pounds of thrust made by pheonix turbines and such. Axials are very very long but much thinner, mine was 3 inches wide and almost 15 or 16 inches long (hard to tell with the tail melted away). Its efficiency was in the high 70's...

[Najm]

Good eh? I heard that a 777 uses a gallon every 800 feet at cruise...

Lets see 500 mph...733 feet/sec so that thing uses almost a gallon a second?!?!?

Um......
Do you know how much thrust those engines produce?

[Zippiot]

Was it 90,000 pounds?

I uneartyhed my notes, some of them the rest still lay on the slain computer. Total compression was 2.2:1 next to that it says compression loss in the second stage. There is almost nohing left of the stages so its hard to tell what the problem was (or even how I determined it).

I remember having problems with overheating, I was running around 1100 instead of 800-1000, that was probably the reason most of the systems failed.

[skyfrog]

I hope after John and other member's helps, Arman could finally understood that for the time being his action shouldn't be in the machineshop making parts but rather get to the library first and study hard.

I remember having problems with overheating, I was running around 1100 instead of 800-1000, that was probably the reason most of the systems failed.

Would you consider to use some inconel wheels from us ? Remember to dilute some more air into the combustor to get the TIT down to around 800 deg C first. :-)

[Zippiot]

I'll definitely call you up when I rebuild it, but I may not get a chance for another year to repair it...

Remember the best advice someone can give to to go read a book!! I paid like 60 bux for a book on rockets, and it has given me a thousand times the info I could find on the web. They are worth the hundred bux to finally udnerstand the math and science required.

[Zippiot]

For my injection I had a ring with 4 equally space injectors on it. Each injector was a piece of brass tubing, smashed in a vice with a strip of leather in it to create a flat spray. The best performance is reached when a halo of burning fuel is created inside the engine, but canister based models are much easier to work on.

For the compressor aluminum is fine to work with on large slow spinning models, but anything that revs above 50k rpm is not suitably safe [gernade]. The blades bend with the extreme forces created and shear off at the base. I used a steel wheel with many tiny dovetails on it, each blade was casted and therefore easily repalceable. The bend at the bottom is not the same as the angle at the top, so casting instead of bending proves to be beneficial, as there is no pre-existing stress on the stress points! I lost 7 blades in the 3 hours the jet ran, each one shot into the compression chamber and came out all melty in the exhaust (what is worse shrapnel flying at you at mach 0.8 or molten metal flying at you at mach 0.8?) so the only dammage done was to the fence. That would be the only percieable (check spelling) benefit of using aluminum, but don't count on that working more than once!

I did not realize that they had come out the exhaust till I examined the nozzle and took off a big piece of aluminum that had formed on it, so it is all assumption that it came from the missing 7 blades. I would get them cast of somethign stronger but I have already seen them brake off mid operation and donnot want to break a hole through another fence.


One more thing:

axial turbines are not sturdy at any size, and changing the throttle just a little bit causes extreme stress. Centrifugals are much more more sturdy to revving and general abuse.

[racketmotorman]

Hi Zippiot
Did you have a tacho on her to measure rpm or was it only guesswork ??
Its interesting how the blades were able to get thru the flametube holes and into the flames to melt .
Cheers
John

[Zippiot]

I donnot have a tach, but at wide open throttle max rpm would have been just under 63k using the tipspeed under mach 0.8 rule. I donnot know the basis of the rule it was taught to me and I never questioned it.

When a blade snapped, it took out all its friends in rows behind it. I lost 1 blade from the second compressor disk, and noticed how there was 1 or two from the compressors and stators behind it missing a blade. Sounded like gears grinding when it broke, wasn't very loud but it was rapid. My heart stopped when I heard the noise, scared the blank outta me (and no one can claim they would not crap themselves if the turbine was strapped right behind their head on a gokart!!!).

[racketmotorman]

Hi Zippiot
Mach 0.8 is about when shock losses start to increase rapidly , thats why your average jet aircraft cruises at ~M 0.8 , they could go faster but the losses just make it uneconomical.
It is a horrible sound when a turbine screeches to a halt, I had a shaft bend one time due to surge and the turbine scraped the housing until it stopped .
Without a tach you could have been doing a lot more than your calculated 63K , especially as you didn't have even a P2 gauge on her .
Very dangerous running a turbine without lotsa gauges or a well sorted fuel delivery systems that limits rpm .
I know what you mean about a turbine behind your ear :-))
http://www.nickhaddock.co.uk/turbine_me ... bicart.wmv
Cheers
John

[Jason]

Hey i am jason and iam new to axial GTs so i need to know a way to mold axial compressors. I have seen some guys molding turbines so in the same way, compressor fans and stators could also be molded.

Can any one help me in this matter......?

[celicoupe]

The main issue that I can see with your aluminum blades is that they will bend foreword. If that happens they will strike your stators. Alot of the actual thrust in a turbojet engine comes from the back sides of the compressor blades. I have been working on an axial compressor too but is has never left paper.