spool systems in compressors

mugaliens

Gee, Fred, I must have missed 'em sitting out there on the wings as I tootled along through the sky. I always wondered what the throttle body was connected to... Thanks for clearing that up!

I'm a retired USAF officer with 2,400+ flight hours and an aero engineering degree from Virginia Tech.

You have yourself a nice day!

Cyrus

During those 2,400+ hours, were you designing jet engines? That is not to say we don't appreciate your service or your piloting knowledge, but please do not think that means you outclass Fred in jet engine design - it's a bit rude, to say the least.

mugaliens

Elsewhere I had actually acknowledged Fred's expertise. However, I also pointed out his entering assumption was limited to modern jet engines, designed for their overall efficiency, rather than expanded to consider the possibility (not the practicality) of the OP's question.

Regardless of Fred's credentials, he continues to dodge the issue while resting on laurels. To top it off, his comment, here, was rude and demeaning.

Fred can fend for himself, Cyrus. Whether he chooses to admit his error on the possibility vs practicality issue or not is up to him.

minger

To bring another discipline in here, I'm not so sure a single spool would be possible mechanically. Problems arise as it is seeing pressure ratios in the teen range. That's a lot of force to put on the blade roots...

mugaliens

Again, it's possible, not practical (efficient). Obviously the blades of such an engine would have to be far more sturdy than on typical multi-stage compressors.

As for pressure ratio concerns, compare water, with it's density 1000 kg/m^3 density, and air, with it's sea-level, 20 deg C density of 1.2 kg/m^3 (more than 800 times greater). Yet we use water turbines all the time which handle stresses far exceeding that of a hypothetical single-stage air compressor.

Turbine blade strength isn't an issue, as they can be designed to be as strong as would be required.

minger

From what I understand, reaction water turbines are typically only used in low head appliactions. I doubt they rotate anywhere near the speed that gas turbines do either.

I could fly by flapping my arms if I really wanted to. It would be very practical or efficient, but it could work.

mugaliens

Due to the momentum and viscosity of water, they do not need to.

Well this is a rather inane remark. Since you chose not to address the science/physics/engineering issue directly, I'll respond in kind: Good luck, but please don't overtrust your theory launch yourself off a cliff!

I'm flummoxed by your, Fred's, and others' errant belief that I am in any way proposing single-stage designs, particularly given my oft-repeated "possible, but not practical" mantra.

However, I am equally opposed to the hip-shooting "nope, can't be done" engineering nonsense that's been shoveled into this thread. What part of "possible, but not practical" are you and others failing to understand? Are you errantly believing I'm proposing single-stage designs?

Fess up!

Astronuc

From what I read, no one is asserting that one is proposing a single-stage compressure design with a high compression ratio. However, I believe one asserted that it is possible. Please provide an example of such an axial compressor, or failing that, please provide the equation for such a compressor.

Aircraft compressor stage compressor ratios might go to 4 or 5:1, but in the example cited earlier (http://en.wikipedia.org/wiki/Axial_compressor#Spools), the spool engines still have multiple stages, each with a compression ratio ~2:1. The wikipedia article is a bit misleading, because the RB.211 series is a three spool design (first run in Aug 1968). The RB.211-22 has a Single-shaft fan, 7-stage i-p compressor, 6-state h-p compressor, single-stage h-p and i-p, three-stage l-p turbine. Later designs in the series have a 6-stage i-p compressor. The total compression ratio is ~27:1. This is the point that FredGarvin was making.
http://en.wikipedia.org/wiki/Rolls-R...11-22_series_2
http://www.flightglobal.com/pdfarchi...0-%201205.html
http://www.flightglobal.com/pdfarchi...0-%200493.html

I'm not so sure it is "easy". Please provide an example or the equations for an axial compressor stage.

There are numerous design constraints such at the blade tip speed and the speed of sound in the compressed air. The back pressure on a single stage with a compression ratio of 20:1 would be enormous and the rotation of the flow would seem to be problematic.

minger

At which point, it is no longer a reaction turbine, it's an impulse turbine. They can use high head application, but like you said, you're using water momentum, not pressure to drive the blades. Because the wheel is spinning at 1/2 speed of the water jet, the actual force on the blades is not astonomical.


Simply saying "It can be done" doesn't either.


There's a big difference between not practical and not doable. There is reason behind one. It's not practical to drive a Hummer. However, if I have the money to do so and feel the need to look cool, why the hell not?

Could there be reason for a single-stage axial machine like you're describing? Sure, perhaps there are packaging concerns, maybe a single-stage can cost less than multi for a limited life application. However, if one of our customers came to us and asked for such a machine, we would respecfully decline, and not because it's not practical.

Well, in all honesty, we'd probably take the cabbage and then just not deliver (either on time or meeting goals). You know, the usual.

mugaliens

At 280 psi, it would have to be driven beyond mach. I never claimed one was ever designed, much less built. I merely stated it was possible.

Astronuc

Please demonstrate that it is possible, or at least provide the equation for the compression ratio of a single compressor stage for an axial flow.

mugaliens

(Sigh), it's beyond my dusty, 25-year-old degree. But it wasn't beyond the talents of my propulsion instructor who used it as an example of what's possible, but not practical.