Question on compressors and diffusers
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Question on compressors and diffusers
Hi all, hope it's ok to ask this question here, if not I am sure I shall be moved...
When air enters the inlet, it is compressed by the compressor, increasing its mass and temperature, so pressure energy increases and volume decreases. So in this case does the static pressure increase and dynamic decrease?
In the case of a centrifugal compressor, the air is accelerated radially outwards, and my AGK notes state that velocity increases and pressure increases.
Now if you apply the mass flow is equal equation, in this case acceleration will increase dynamic but as a result, static must decrease, so total pressure essentially remains the same?
So I don't get what goes on. The vanes of the centrifugal compressor diverge, which would slow down the air and decrease dynamic and increase static, but all this does essentially nothing to the total pressure of the air?
Can someone in VERY BASIC terms tell me what is happening to the pressure? How is it compressed to begin with?
Confused, confused, confused!!!!
Would appreciate any help on this,
Thank you in advance
PM
When air enters the inlet, it is compressed by the compressor, increasing its mass and temperature, so pressure energy increases and volume decreases. So in this case does the static pressure increase and dynamic decrease?
In the case of a centrifugal compressor, the air is accelerated radially outwards, and my AGK notes state that velocity increases and pressure increases.
Now if you apply the mass flow is equal equation, in this case acceleration will increase dynamic but as a result, static must decrease, so total pressure essentially remains the same?
So I don't get what goes on. The vanes of the centrifugal compressor diverge, which would slow down the air and decrease dynamic and increase static, but all this does essentially nothing to the total pressure of the air?
Can someone in VERY BASIC terms tell me what is happening to the pressure? How is it compressed to begin with?
Confused, confused, confused!!!!
Would appreciate any help on this,
Thank you in advance
PM
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Firstly, let me apologise for what happens next as it's a while since I've flexed the muscles of my engineering degree.
To tidy up your first statement, compressing air does not increase it's mass. Also, discussing pressure change across jet engines can get confusing rather rapidly so I'll try my best until someone better steps in.
Here goes:
IIRC, total pressure is the key, and in particular, keeping as much as possible through compression, expansion, etc.
1. Air inlets are designed to present non-turbulent air at a manageable speed to the compressor. Ideally, no work is done and thus total pressure is unaffected.
2. Both axial and radial compressors are doing work on the air and thus the total pressure of the system will increase. If 100% efficiency is assumed, this air is passed across a turbine and imparts all of it's energy as work, thus leaving the system in equilibrium, hence no thrust. Combustion at high pressure is used to fix this.
All is neatly covered by a temperature-entropy (T-S) plot, which sounds horrid but is the one thing that underpins most of this and does not require degree maths to interpret.
Ultimately, if total pressure is constant, a change in dynamic pressure will drive a change in static pressure. If it is not constant, i.e. work done on/by the air, both may change.
So we have:
1. I should read my books more often
2. Mass does not increase, but total pressure changes as it does work, and is worked on.
3. TS plots are useful, try one at this link until a more meaningful answer is posted.
http://en.wikipedia.org/wiki/Jet_engines
4. You WILL get a better answer if you put this on the Tech forum.
SR
To tidy up your first statement, compressing air does not increase it's mass. Also, discussing pressure change across jet engines can get confusing rather rapidly so I'll try my best until someone better steps in.
Here goes:
IIRC, total pressure is the key, and in particular, keeping as much as possible through compression, expansion, etc.
1. Air inlets are designed to present non-turbulent air at a manageable speed to the compressor. Ideally, no work is done and thus total pressure is unaffected.
2. Both axial and radial compressors are doing work on the air and thus the total pressure of the system will increase. If 100% efficiency is assumed, this air is passed across a turbine and imparts all of it's energy as work, thus leaving the system in equilibrium, hence no thrust. Combustion at high pressure is used to fix this.
All is neatly covered by a temperature-entropy (T-S) plot, which sounds horrid but is the one thing that underpins most of this and does not require degree maths to interpret.
Ultimately, if total pressure is constant, a change in dynamic pressure will drive a change in static pressure. If it is not constant, i.e. work done on/by the air, both may change.
So we have:
1. I should read my books more often
2. Mass does not increase, but total pressure changes as it does work, and is worked on.
3. TS plots are useful, try one at this link until a more meaningful answer is posted.
http://en.wikipedia.org/wiki/Jet_engines
4. You WILL get a better answer if you put this on the Tech forum.
SR
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Hi Slow Roll
Thank you so much for that. Yes, you are right , MASS FLOW is increased, not MASS.... I read the text wrong...which almost never helps.
I understand most of what you have said, I guess what I don't understand is the two stages of the compression stage.
Accelerating the air then diffusing it. How is the total pressure increased? I don't get how work is done to the air and how work done increases its total pressure!
1. When you accelerate the air, you increase its kenetic energy.
This would mean a decrease in static energy right?
So how is total pressure raised.
2. At the diffuser stage, the air is slowed and is allowed to expand, therefore it looses kenetic , so static increases.
Still don't see how the pressure rises, it just seems to change from one state into another.
I am sure I am holding on to the wrong end of the stick here.
I will try the other forums, but if you have any more to add about this work done to the air I would really appreciate it.
Thank you for your previous response.
PM
Thank you so much for that. Yes, you are right , MASS FLOW is increased, not MASS.... I read the text wrong...which almost never helps.
I understand most of what you have said, I guess what I don't understand is the two stages of the compression stage.
Accelerating the air then diffusing it. How is the total pressure increased? I don't get how work is done to the air and how work done increases its total pressure!
1. When you accelerate the air, you increase its kenetic energy.
This would mean a decrease in static energy right?
So how is total pressure raised.
2. At the diffuser stage, the air is slowed and is allowed to expand, therefore it looses kenetic , so static increases.
Still don't see how the pressure rises, it just seems to change from one state into another.
I am sure I am holding on to the wrong end of the stick here.
I will try the other forums, but if you have any more to add about this work done to the air I would really appreciate it.
Thank you for your previous response.
PM
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Don't think I can help with it all. One thing though...
While you ask about compressors, looking at the engine as a whole helps clarify matters.
Compressor section hits air with a big fan, giving it additional heat, kinetic energy etc. The total energy (you cite kinetic and static) is therefore not constant and thus a change in one energy form of the air does not necessitate a change in the other as it may be supplied externally (via the compressor blades).
To tie it all up,
energy is imparted to the air via the compressor,
which is powered by the turbine,
which in turn is powered by the combustion of fuel and, importantly, air,
which is compressed by the compressor.
A bit of a 'foot-bone connected to the knee bone' but in a big circle. The magic bit is getting all this efficient enough so that there's enough energy left in the air (after turbine has turned compressor) to get thrust.
Not directly what you asked, but it hope it helps. I think the bit you're stumbling over is keeping track of what stays constant and what doesn't, which is definately the b@st@rd of it all. If it's turning something, being turned by something or having fuel burned in it the the energy state of any one thing is likely to be changing and thus 'this goes up, so that goes down' arguments can be misleading.
Anyway, I'm talking way above my pay grade but until you get owt better, hope it helps
While you ask about compressors, looking at the engine as a whole helps clarify matters.
Compressor section hits air with a big fan, giving it additional heat, kinetic energy etc. The total energy (you cite kinetic and static) is therefore not constant and thus a change in one energy form of the air does not necessitate a change in the other as it may be supplied externally (via the compressor blades).
To tie it all up,
energy is imparted to the air via the compressor,
which is powered by the turbine,
which in turn is powered by the combustion of fuel and, importantly, air,
which is compressed by the compressor.
A bit of a 'foot-bone connected to the knee bone' but in a big circle. The magic bit is getting all this efficient enough so that there's enough energy left in the air (after turbine has turned compressor) to get thrust.
Not directly what you asked, but it hope it helps. I think the bit you're stumbling over is keeping track of what stays constant and what doesn't, which is definately the b@st@rd of it all. If it's turning something, being turned by something or having fuel burned in it the the energy state of any one thing is likely to be changing and thus 'this goes up, so that goes down' arguments can be misleading.
Anyway, I'm talking way above my pay grade but until you get owt better, hope it helps
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Thanks!
YOu are right, its just the constant not so constant bit I don't get, the rest makes sense. Thank you for a VERY valliant effort!
Shouldn't you be doing something more interesting on a Saturday afternoon a week before Xmas than answering tedious questions for people like me?
Cheers and Happy Christmas
PM
YOu are right, its just the constant not so constant bit I don't get, the rest makes sense. Thank you for a VERY valliant effort!
Shouldn't you be doing something more interesting on a Saturday afternoon a week before Xmas than answering tedious questions for people like me?
Cheers and Happy Christmas
PM
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powder monkey
couple of things to poss make things make sense
1) with pressure increase you get temp increase but velocity decrease
2) conversly then with a velocity increase you get a temp and pressure decrease.
Thats how convergent and divergent ducts in compressors work but keep in mind the area that the air is traveling to is decreasing thus maintaining the overall speed of the air through the compressor.
I can take you through the compressor in more detail if you wish, ie how the air is behaving at every stage through it or the whole engine.
couple of things to poss make things make sense
1) with pressure increase you get temp increase but velocity decrease
2) conversly then with a velocity increase you get a temp and pressure decrease.
Thats how convergent and divergent ducts in compressors work but keep in mind the area that the air is traveling to is decreasing thus maintaining the overall speed of the air through the compressor.
I can take you through the compressor in more detail if you wish, ie how the air is behaving at every stage through it or the whole engine.
