do compressor wheels "explode"?

Moderator: Mike Everman

gundamnitpete
Posts: 20
Joined: Fri Jun 02, 2006 1:13 am
Antipspambot question: 0
Location: detroit michigan

Re: re: do compressor wheels "explode"?

Post by gundamnitpete » Fri Jun 16, 2006 10:39 pm

Zippiot wrote:More like throwing a knife than a ball of paper, as the compressor blades are sharp.
true, but still very lightweight.

Zippiot
Posts: 1190
Joined: Tue Nov 15, 2005 7:55 pm
Antipspambot question: 0
Location: california
Contact:

re: do compressor wheels "explode"?

Post by Zippiot » Fri Jun 16, 2006 10:49 pm

Kevlar without resin does not usually stop a knife, and a compressor blade moving near sonic speeds is as much a knife as it is a bullet. If you need a sheild to protect yourself use steel or polycarbonate. Seriosuly polycarb is over 250 times as strong as glass, a 3/8 inch piece can stop a small handgun bullet, along with bounce an axe off of it, get a thicker piece if a large area is needed.

The case should be strong enough to stop the compressor anyways, and since air is already moving through it at a high rate most of mine that broke off got sucked into the engine causing massive damage. But mine was axial flow so it might not be able to travel back through a centrifugal flow...
Sailing Student- How do I know if my life jacket is tight enough?
Me- Can you breathe?
Sailing Student- Yes
Me- Then its too loose!

NickC
Posts: 311
Joined: Tue May 23, 2006 5:14 pm
Antipspambot question: 0
Location: Georgia

re: do compressor wheels "explode"?

Post by NickC » Sat Jun 17, 2006 4:41 am

a .45 is a subsonic gun. Most guns are subsonic especially handguns, don't you guys watch mythbusters? They shoot guns sometimes and will tell the fps and stuff. I think my turbine will pull over 63k it's got a 2.5" inducer so maybe 80-100k?

multispool
Posts: 125
Joined: Thu Jan 19, 2006 10:59 pm
Antipspambot question: 0
Location: UK

re: do compressor wheels "explode"?

Post by multispool » Sat Jun 17, 2006 10:50 am

Guys, Commercial turbocharger Compressor wheels very rarely disintegrate, in fact I've never seen or known one to do so when used in a micro turbine. Providing they are not taken supersonic and have not been machined so as to affect the hub disc integrity, just the alloy intake is adequate.
We have taken small wheels to 200k and the first thing to happen is the retrograde curve on the vanes start to straighten out under centripital force, the vanes rub the intake and the engine stops (all quite unspectacular) There is increasing use of titanium wheels that should be superior in this respect!

At the risk of going slightly off topic again, The real safety issues are the shaft and turbine wheel. The old shafts sometimes failed and the turbine would come out the back of the engine with the speed and energy of a large bullet, some wheels were never found again. The axial turbine wheel uses it's own shroud for containment and is fairly effective. A 70mm wheel for instance would use say a 1.6mm wall thickness on the shroud. There has been many instances where turbines have lost blades where the shroud will rupture but easily contain the blades. If the turbine hub fails, is another story and have witnessed one such incident where shrapnel travelled about 1 mtr but could just as easily had travelled much further depending on circumstances, engine running temperature etc.

So the moral of this is to use the right metal for the shaft EN24t steel. Do not over-rev an home made wheel (it will almost certainly have minor radius changes near the blade root plus machining marks) and always balance the rotating assembly. As a separate issue, The radial flow turbines are heavy but very strong and are much less likely to fail.

One last thing is, never look up the end of a running engine, even if it's a commercially made one, they use the same materials as we do and any engine can (and sometimes will) fail. Sorry if this comes across like a lecture but these are basic safety guide-lines. Gas-turbines are potentially very dangerous but by observing a few simple rules they can be safer than most hobbies...

NickC
Posts: 311
Joined: Tue May 23, 2006 5:14 pm
Antipspambot question: 0
Location: Georgia

re: do compressor wheels "explode"?

Post by NickC » Sat Jun 17, 2006 11:10 am

my turbine housing is like 1/2" cast steel or iron? so I guess just use a sorta thick nozzle material 18 gauge or 16 gauge? and it'll have to get through the AB too.

thanks for all the information, can they go supersonic? is it not reccomeneded?

skyfrog
Posts: 539
Joined: Fri Jul 23, 2004 11:39 am
Antipspambot question: 0
Location: Kaohsiung, Taiwan
Contact:

re: do compressor wheels "explode"?

Post by skyfrog » Sat Jun 17, 2006 12:59 pm

Hi,

Radial flow turbines or compressors are very strong in comparison with that of its axial flow counterparts, because the former have longer blade root while the later have short one. Mathematically speaking, consider the blade as a plate placed in a X-Y coordinate, we see that radial flow blades have X and Y sides clamped (or fixed), while axial ones have only X side clamped. That's a huge difference in mechanical strength. This is why contrifugal compressors rarely disintegrate. From this veiw point you could also understand why commercial turbochargers do not use axial turbine for extracting the energy from the engine outlet.

Thanks to multispool for his excellent lecture.
Last edited by skyfrog on Sat Jun 17, 2006 3:14 pm, edited 2 times in total.
Long live jet engine !
Horace
Jetbeetle

multispool
Posts: 125
Joined: Thu Jan 19, 2006 10:59 pm
Antipspambot question: 0
Location: UK

re: do compressor wheels "explode"?

Post by multispool » Sat Jun 17, 2006 3:18 pm

Going supersonic will cause a large drop off in efficiency.
The Compressor wheel tip speed can go reasonably close to the local speed of sound (which is higher than ouside the engine). The vaneless space offers a quite a bit of diffusion before entering the Diffuser so entry to the Diffuser vanes is well below MACH1.

HattoriHanzo
Posts: 69
Joined: Fri Aug 12, 2005 1:51 pm
Antipspambot question: 0
Location: Okinawa

Re: do compressor wheels "explode"?

Post by HattoriHanzo » Sat Jun 17, 2006 5:50 pm

NickC wrote:I know turbine wheels can, but will the compressor wheels? I gues they can fail, but is itas dramatic? I ask because my turbine housing is made of like damn 1/2" cast iron or steel that gets annoying to carry, that I doubt the turbine wheel would penetrate and if it did would have quite a reduced velocity, but my compressor housing, like everyone elses, is a good bit thinner. Do I need to shield anything?


You can calculate the tangential velocity and approximate the mass of the broken fan/turbine-blade, then consult Mr Newton.
I like baseball

Zippiot
Posts: 1190
Joined: Tue Nov 15, 2005 7:55 pm
Antipspambot question: 0
Location: california
Contact:

re: do compressor wheels "explode"?

Post by Zippiot » Sat Jun 17, 2006 6:00 pm

Radials also are more sturdy as the compressor blades are attatched to more than one spot (the hub of the wheel and the back of the wheel) while axials are just attatched to the hub.

Axials are much more effective BUT considerably more difficult to design and run.


O/T: when is someone gonna make a plane [r/c] that uses a ramjet over a turbine like the blackbird?
Sailing Student- How do I know if my life jacket is tight enough?
Me- Can you breathe?
Sailing Student- Yes
Me- Then its too loose!

Post Reply