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Spool systems in compressors

by anvesh111
Tags: compressors, spool, systems
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anvesh111
#1
Dec5-09, 02:35 PM
P: 32
why the spools(twin spool,triple spool) are used in compressors,why the stages in compressor must be increased
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Cyrus
#2
Dec5-09, 02:40 PM
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I think a very basic google search on jet engines would provide you with this answer.
FredGarvin
#3
Dec5-09, 08:02 PM
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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
#4
Dec16-09, 06:30 AM
P: 32
Spool systems in compressors

Quote Quote by FredGarvin View Post
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?
i mean y do we need them
minger
#5
Dec16-09, 07:40 AM
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Rather than just have say one 20:1 pressure ratio stage?
FredGarvin
#6
Dec16-09, 09:13 AM
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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
#7
Dec20-09, 03:23 PM
P: 595
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
#8
Dec21-09, 06:52 AM
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Quote Quote by mugaliens View Post
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?
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
#9
Dec26-09, 03:11 PM
P: 595
Quote Quote by FredGarvin View Post
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.
You can design for a very high stage differential pressure, if you want. It's just terribly inefficient. :)

Has anyone ever noticed that all jet engines (with axial compressors) have a much larger number of compressor stages than the turbine?
Yes. Why is that, Fred?
FredGarvin
#10
Dec26-09, 04:50 PM
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Quote Quote by mugaliens View Post
You can design for a very high stage differential pressure, if you want. It's just terribly inefficient. :)
Not in an axial machine. You can with a radial/centrif if you want.



Quote Quote by mugaliens View Post
Yes. Why is that, Fred?
One word answer: separation.
mugaliens
#11
Dec27-09, 11:10 PM
P: 595
Quote Quote by FredGarvin View Post
Not in an axial machine. You can with a radial/centrif if you want.
Yes, you can. As I said, they're simply much less efficient.

One word answer: separation.
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
#12
Dec28-09, 02:55 PM
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Quote Quote by mugaliens View Post
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.
You're right. What do I know.
mugaliens
#13
Dec29-09, 12:12 AM
P: 595
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
#14
Dec29-09, 10:43 AM
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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
#15
Dec29-09, 11:20 PM
P: 595
Quote Quote by minger View Post
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.
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
#16
Dec31-09, 06:50 PM
P: 32
Quote Quote by mugaliens View Post
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.
yeah..i agree with you...thanks
FredGarvin
#17
Jan1-10, 09:54 AM
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Quote Quote by mugaliens View Post
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.
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.
mugaliens
#18
Jan5-10, 05:32 PM
P: 595
Quote Quote by FredGarvin View Post
It is grossly apparent that you have never even seen a compressor in real life.
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!


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