# How does a jet-engine compressor compress air?

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plz help me out folks
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## Answers and Replies

FredGarvin
It has various stages of rotating vanes. Air enters each stage and each stage incrementally compresses the air. The flow path the air takes gradually reduces in area as it makes its way to that latter stages. At the end of the compressor is a diffuser that slows the airstream down as well as further increases its static pressure.

Astronuc
Staff Emeritus
The rotating blades impart momentum and energy to the gas/air. Each stage adds a little more momentum/energy and gets 'pushed' into the air downstream. The opposite happens in the turbine where the heated gases impart momentum to the turbine blades. Jet engines also take advantage of the forward travel of the aircraft. In fact, ramjets simply use the high forward speed which 'rams' the air into the engine.

See NASA's site on propulsion - http://www.grc.nasa.gov/WWW/K-12/airplane/turbine.html

http://www.grc.nasa.gov/WWW/K-12/airplane/shortp.html

Keep these four things in mind:

1. Compressed air,high pressure, flows toward low pressure
2. the compressor sections are in the front of the engine and the turbine in the rear
3. the compressors are attached to the turbines via shafts
4. Between the compressor and turbine sections is the combustion chamber where fuel is mixed with the compressed air and iginited.

The compressor section is made up of alternating rows rotating blades and stationary vanes that are shaped like wings. Curved on one side and flat on the other and they function like wings too: Airflow creates low pressure on the curved side; high pressure on the flat. This imparts the direction of the airflow toward the rear of the engine: High pressure flows toward low pressure.

The leading edges of these rotating blades face the leading edge of the stationary blades and as the air cascades through each pair of rotor and stationary vanes it becomes more and more compressed.

The air enters the combustion chamber and is rapidly heated and wants to expand (but there is only so much room for expansion which is another form of compression) so it continues to the rear of the engine where the pressure is lower and it meets the turbines. The turbine are like pinwheels. The fast flowing, compressed air strikes the turbines and causes it(them) to spin.

Add more fuel for more heat , the compressed air expands faster and the turbines spin faster. Because they are attached to the compressor section via shaft, the compressor spins faster too which compresses incoming air faster and even more.

Air does not like to be compressed. It likes to be in its normal state so it roars past the turbines, out the back of the engine and blows the aircraft forward.

Danger
Gold Member
I would expand upon that last sentence just a wee bit, since it's an area of confusion for a lot of people. The term 'blows the aircraft forward' is fine in casual conversation, for those who understand the physics involved, but it might give the wrong impression to some readers. (Remember the famous editorial scoffing at the space programme; they gleefully pointed out that a rocket couldn't possibly work in space because there was nothing for it to push against.) It's the reaction thrust of the exhaust against the engine itself that causes the propulsion, rather than the exhaust working on the outside air. Good post otherwise.

FredGarvin
Air does not have a problem being compressed. It simply requires work to do it. Everything has a natural tendency to the dead state but that doesn't mean that there is a problem with doing it.

Also, there is room for another mis-interpretation of what was said. The purpose of the turbines is solely to provide work to turn the compressor section. There is no thrust developed in the turbine section. The thrust of the engine is produced via Newton's 3rd law due to the acceleration imparted to the air stream and a slight P/a contribution at the exit plane of the rear nozzle.

I'm sure you're right. But i guess the turbine should feel some sort of force when the gases are rushing out. And by problem i think Joe Mechanic meant that air has a tendency to oppose being compressed. And to state a fact, the air is in a compressed state even in the lower atmosphere. The higher atmosphere has a lower density and the air would have a tendency to rise upwards had gravity not been a factor.

FredGarvin
There is a force created by the compressor section but it is restrained either by a static structure or a balance piston. That doesn't really enter the equation for the net thrust of the engine.

Wow, you guys are great and correct! I didn't offer too much detail about thrust because the question was related to how the compressor worked. Also, I made the assumption that the question was being asked by a young student because of its brevity.

I didn't mean to imply the turbines create thrust. Just that they are used to spin the compressor section via a shaft.

wow

i finally start to understand something. and joe i got ur point. thanks to u guys i learned a lot

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Gas turbine compressors only accelerate air. The pressure rise comes when the fast moving air is dumped into the larger volume combustion section. Its all about Bernoullis principle.

FredGarvin
Gas turbine compressors only accelerate air. The pressure rise comes when the fast moving air is dumped into the larger volume combustion section. Its all about Bernoullis principle.
You only describe a diffuser or, in a way, a centrifugal compressor. Most engine applications, especially the large scale engines, use axial compressors that definitely compress the air after every stage of compression. When we develop a compressor, we measure the pressure ratio across each stage of compression. If you'll notice in any compressor cross section, the flow annulus decreases in size as you progress along the length of the axial compressor section. If your statement were true, the static pressure would actually decrease through the stages until you got to the diffuser. This is absolutely not what happens. The diffuser is a final stage of compression. It is not the only one though. On our engines we add a centrifugal compressor stage after the axial stages.

Danger
Gold Member
On our engines we add a centrifugal compressor stage after the axial stages.

I'm unfamiliar with that concept. Can you elaborate a bit? Having a bit of trouble trying to picture the blueprint in my head.

FredGarvin
It is done on smaller engine designs. It doesn't work too terribly well for larger engines. The cost/benefit just isn't there to justify their useage. Basically a centrif is piggybacked at the end of the last axial stage. From the centrif, the flow goes to the burner.

Here's a pretty good cut away of an example:
http://www.ent.ohiou.edu/~me321/quiz.info/GasTurbine/compressor.jpg [Broken]

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I forgot about axial flows getting a small rise at each stage. Its common to find centrifugal compressors on helicopter engines and APUs. The popular Allison 250 has a few stages of axial feeding a centrifugal. Centrifugal compressors require less power for starting.

Danger
Gold Member
Cool picture, Fred. I'm still a bit confused, though. Dare I assume that this is something done with turboshaft engines as opposed to 'thrust' engines? I'm asking because it appears to me that the burner, and thus the exhaust, are at a right angle to the inlet flow. Is that the burner 'above' the centrifugal stage? (The shiny thing looks like it might be a fuel valve.)

Search for a Rolls Royce "Nene". This is a typical thrust engine with a centrifugal compressor.

hmmmmmm

what are the other types of compressor and plz explain abut them buddys and tell me which one has the highest efficiency

Thought Iwould add that large engines ejecting big,slow moving fuel-air mixtures are more efficient than smaller engines ejecting small,fast moving volumes of fuel-air.

Danger
Gold Member
Thanks for the tip, Nicky. It makes sense now. I didn't realize that the airflow was realigned after compression.

FredGarvin
More efficient in terms of what?

AlephZero
Homework Helper
Cool picture, Fred. I'm still a bit confused, though. Dare I assume that this is something done with turboshaft engines as opposed to 'thrust' engines? I'm asking because it appears to me that the burner, and thus the exhaust, are at a right angle to the inlet flow. Is that the burner 'above' the centrifugal stage? (The shiny thing looks like it might be a fuel valve.)

Radial compressors tend to be larger diameter than axial, so they don't fit too well in a high bypass ratio engine simply because they take too much space radially. Which is a shame, because the short axial length gives a "stiff" engine design and reduces shaft whirl problems etc.

That's not so much of an issue with turboshaft engines, because the "bypass air flow" is the large diameter propellor. The last pure radial compressor Rolls Royce engine was the Dart turboshaft - engine desgn started in 1945, and still in commercial use 60 years later.
http://www.vickersviscount.net/Pages_Technical/TechnicalRollsRoyce1.aspx.

A bit of history on some early jet engine configurations might be interesting -
http://www2.eng.cam.ac.uk/~bcb/whittle/whitt-r.htm

FredGarvin
Cool picture, Fred. I'm still a bit confused, though. Dare I assume that this is something done with turboshaft engines as opposed to 'thrust' engines? I'm asking because it appears to me that the burner, and thus the exhaust, are at a right angle to the inlet flow. Is that the burner 'above' the centrifugal stage? (The shiny thing looks like it might be a fuel valve.)
Sorry for not getting back with you Danger. The use of a radial or centrif. compressor does not depend on engine. We make thrust producing engines with a tandem axial/centrif set up. Avco-Lycoming (Now owned by Textron) produces my beloved T-55 power turbine which does not have a centrif. So you can see that it really depends on the cycle we are working with and the engine envelope as well.

There are engineering tradeoffs for either type. Some things the centrif does better than the axial and visa versa. Aleph hit it on the head that the bigger boys prefer the axial compressors. The big benefit is that you get a higher efficiency from an axial package. However, you need a lot more length to accommodate the axial than you do the centrif. The centrif can also provide a higher pressure ratio than you can get away with in a single stage of an axial compressor which again leads to the longer axial length which drives the length of shaft and required bearings and shaft dynamics issues...

You are correct that the outlet of the centrif going to the burner is at a right angle. It's not a big deal. The diffuser takes care of that and directs the flow appropriately. I like Aleph's link on Whittle. Good stuff there. I found it interesting that during WWII, the Germans went pretty much with axial compressors. Whittle started right off with centrifs because, as I am told, we had a lot of experience with aircraft engine superchargers.

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Danger
Gold Member
Not to worry about the delay, Fred; we all have real lives too.

Nicky's and Aleph's links pointed out my mistake right away. I, also, enjoyed them very much and have bookmarked them. The fact of being able to redirect the compressed flow also helped clear up a problem that I had with how people managed to make thrust jets out of old turbochargers without ridiculous efficiency losses. Of course, now I'm going to have to try it...

efficient in terms of compression.
the compressor compresses right?

FredGarvin
There is more than one definition for efficiency.

hmmm... i din know that......
so other than compression what does a compressor do?

fred i know my questions are stupid but plz help meeeeeeeeeeeeeeeeeeeeeeeeeeeeeeeee

i wanna be like u guys, knowing everything.u rok like hell
plz help me out

Danger
Gold Member
hmmm... i din know that......
so other than compression what does a compressor do?

That's all that it does, but there are side-effects to compression such as thermal changes to the air.
And with the exception of a couple of guys who survived about 10 minutes here, no one will profess to know more than a tiny fraction of human knowledge.

so is there any expression for compression and thrust?

Danger
Gold Member
That's squarely in Fred's arena, but I don't think so. There are a lot of variables that contribute to creating thrust, such as type of fuel, combustion temperature, nozzle design, exhaust temperature...

FredGarvin
Theoretically, the most basic equation for thrust is

$$F_t= \dot{m}(V_{OUT}-V_{IN})$$

where
$$\dot{m}$$ = air mass flow rate
$$V_{OUT}$$ = air exit velocity
$$V_{IN}$$ = air inlet velocity

1.ooohhhh i thought thrust= exhaust velocity * rate of combustion
is this wrong?

2.which part of the aircraft experiences this reaction force(thrust) first? (is this question stupid?)
3.is there a difference between thrust and propulsion?
4.and for compression?
there's no theoretical equation for compression?

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