spool systems in compressors

anvesh111

why the spools(twin spool,triple spool) are used in compressors,why the stages in compressor must be increased

Cyrus

I think a very basic google search on jet engines would provide you with this answer.

FredGarvin

Your question doesn't make a whole lot of sense to me in the current form Can you restate it? It appears that you are asking why if you have multiple spool engines must the number of compressor stages increase. Is this correct?

anvesh111

i mean y do we need them

minger

Rather than just have say one 20:1 pressure ratio stage?

FredGarvin

Compressors, especially axial compressors are very limited as to the pressure ratio across them allowed. That means more stages required for a higher required pressure ratio.

mugaliens

By "allowed," I take it to mean you're referring to what the laws of physics allow? Or is it simply a case design optimization, whereby obtaining the same, final compression ratio via multi-stage axial compression does not require planetary gearing or substantially greater weight or strength of materials than would a design with fewer stages?

FredGarvin

I mean allowed in that compressors are constantly working against an adverse pressure gradient which places a lot of limitations on what kind of delta P across a stage you can have. Has anyone ever noticed that all jet engines (with axial compressors) have a much larger number of compressor stages than the turbine?

mugaliens

You can design for a very high stage differential pressure, if you want. It's just terribly inefficient. :)

Yes. Why is that, Fred?

FredGarvin

Not in an axial machine. You can with a radial/centrif if you want.



One word answer: separation.

mugaliens

Yes, you can. As I said, they're simply much less efficient.

Separation occurs when turbines designed for a one pressure differential encounter a significantly greater pressure differential. It's a consequence of the design, as efficiency is highest when operated closest to the stall line.

Turbines with fewer stages and specifically designed for higher per stage pressure differentials do not experience separation within their design parameters. However, as I've mentioned, they're significantly less efficient than turbines designed for optimal efficiency, and which, as a consequence of their design, have more stages.

In short, it's entirely doable. It's simply not practical.

FredGarvin

You're right. What do I know.

mugaliens

As an engine R&D guy, you know a great deal, Fred. This was simply a matter of extrapolating from existing design, in which case you were correct, vs considering a possible, but far less efficient, and therefore impractical design, for the concept mentioned by anvesh111 and minger.

minger

Whoa whoa, apparently your sarcasto-meter didn't pick me up properly.

We do centrifugal compressors, but I know enough about axial systems to know that a 20:1 pressure ratio (for example) on a 5' fan blade is going to cause some issues.

mugaliens

Absolutely. As I said, it's not impossible. Merely grossly inefficient, and therefore impractical.

A similar, and familiar approach would involve a ramjet. Below Mach 0.5, they have almost zero thrust due to poor compression ratios, and below 600 kts, they're grossly inefficient. Between Mach 2 and 4, however, they'll outperform any turbojet.

The issue is one of compression. In a turbojet, we obtain efficiency by using multiple compression stages. A ramjet achieves efficiency by means of high mach to achieve that compression.

In a one-stage design, achieving a 20:1 compression ratio is "easy": just increase the velocity. When we do so, however, we find that achieving that high of a compression ratio requires a mach turbine (airflow over the turbines is mach), which introduces all kinds of wonderful problems.

Grossly inefficient. Very impractical.

But possible.

Again, we do not use multiple stages because achieving high compression with one stage is impossible. We do so because using multiple stages is much more efficient.

anvesh111

yeah..i agree with you...thanks

FredGarvin

It is grossly apparent that you have never even seen a compressor in real life. I would suggest you read Hill and Peterson. You have linked to it. Try reading it especially section on axial compressors.