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  #1  
Old 03-02-2005, 14:35
Conor Conor is offline
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Question Q2378 - Stator vanes

Hi

Can you please confirm the answer to this question.

In the stator of a turbine, the speed V and static pressure Ps vary as follows:
(a) V decreases, Ps decreases
(b) V increases, Ps decreases
(c) V decreases, Ps increases
(d) V increases, Ps increases.

Answere given: (b)

I always thought that stator vanes slow the airflow and increase its pressure, ie, convert kinetic energy to pressure energy. Should the answer not be (c)?

Thanks
Conor
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  #2  
Old 03-02-2005, 16:31
Haggis Hunter Haggis Hunter is offline
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Conor,

Stator vanes are situated in the turbine section of the engine, remember the aircraft is propelled by the velocity of the air comming out of the engine.

In the turbine section the pressure is decreased and velocity increased.

Hope it's helped. HH
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  #3  
Old 03-02-2005, 18:32
Conor Conor is offline
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Thanks HH. I think I'm getting confused with what happens in the compressor.
Everything was going fine until I decided to look back over the notes today!

Please God Monday will be the last of it!

Cheers,
C
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  #4  
Old 04-02-2005, 08:59
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yyz yyz is offline
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Quote:
Please God Monday will be the last of it!
I'm feeling what you're saying!
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  #5  
Old 04-02-2005, 10:25
DanWinterland DanWinterland is offline
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Hi all,

This is a common problem when thinking two dimensionally about a compressor. The idea of the compressor is to compress and accelerate the flow through to the combustion chamber to produce an optimised 'combustable mass' in the chamber.

If you just think about a stator blade then the increased velocity, in accordance with Bernoulli's principles, will result in a reduction of pressure. Obviously that is pretty useless for a compressor as at the end of the compressor prior to the combustion chamber you would have high velocity flow with very low pressure leading to surge.

Therefore to combat the problem the entire compressor is built in a converging duct. Hence the first row of rotors and stators are learge and the final row tiny. This has the effect of maintaining and increasing the pressure flow through the compressor and increasing the velocity.

Hence, over the Stator the velocity increases but the pressure drops, through the chamber the pressure increases.

Have a look at the piccies again and you'll see what I mean.



p.s. Forgot to say, you'll only find STATORS (Static rotors (??)) in the compressor, the term turbine is often used for the whole engine. The power turbines extract a portion of the flow velocity to drive the compressor but they have vanes not stators.

please feel free to correct me if I'm wrong Ken.

edited due to forgetfulness

Last edited by DanWinterland; 04-02-2005 at 11:21.
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  #6  
Old 07-02-2005, 21:59
Haggis Hunter Haggis Hunter is offline
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Have you done AGK Dan?

I am just simply wondering, that's all...

Conor, I just realised I owe you ?1.50! Made the bus... just, cheers mate!

HH

Last edited by Haggis Hunter; 07-02-2005 at 22:12.
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  #7  
Old 08-02-2005, 08:46
DanWinterland DanWinterland is offline
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Haggis Hunter,

Okay, slightly at crossed wires and, as usual a bit of confusion with wording. From my experience the guide vanes prior to the power turbines have been called just that, guide vanes. As the CAA uses different terminology I can only say that your post above is also correct, for that I apologise. The purpose of the guide vanes (stators) before the power turbine is exactly to reduce the pressure to a useable velocity for conversion to mechanical force. The other effect of the guide vanes (stators) is to prepare the mass flow for the correct impingement angle onto the power turbine.

As to your question on AGK, after 20 years flying gas turbine aircraft I think I'm up to speed, the only thing lacking is the CAA's usual terminology factor. But hey, if they are called stators now then I'll call them stators as well!

Cheers
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  #8  
Old 08-02-2005, 10:08
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Ken Ken is offline
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Hi Conor

The question and answer are quite correct, except strictly speaking the turbine does not have stators it has guide vanes. If you have multiple stage turbines there is a set of guide vanes between each turbine disc to redirect the flow again before the next turbine blade. They are stationary so are sometimes called stators.

With the turbine you want a high velocity, as Haggis Hunter correctly says, so turbine blades are designed to extract power from the hot expanding gasses to turn the compressor, by using the high velocity hitting the turbine blade. The guide vanes straighten the flow before the next set of turbine blades, but in effect it decreases the pressure and increases the velocity. This also prevents excessive backpressure which could work its way forward and affect the combustion process.

The compressor on the other hand is designed to increase the pressure through each of the stages, so you can deliver a good mass flow to the combustion chambers. However the increasing pressure will try to reverse the flow, so the convergent duct is designed to keep the velocity constant through the whole compressor, so that the momentum of the airflow prevents reverse flow. In effect the velocity is therefore increased in each rotor section. (see sketch 2-3-12 on page 16.18)

Velocity increases and decreases through each set of rotors and stators, and by using a convergent duct the velocity therefore stays constant, but the pressure is steadily increasing through each stage.

Hope that helps you.

Ken
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  #9  
Old 08-02-2005, 11:29
DanWinterland DanWinterland is offline
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Thanks Ken,

Thought I was getting too old and losing touch with the terminology of yoof today

HH, what was that about AGK?
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  #10  
Old 08-02-2005, 12:30
Haggis Hunter Haggis Hunter is offline
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I guess that's a no then Dan?

Quote:
Thought I was getting too old and losing touch with the terminology of yoof today
Yoof, I guess you mean youth... please explain?

Your experience will no doubt benifit guys that are going through Bristol.GS and your explaination was very detailed and well worded, I was just simply getting to the point!
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