Compression stage

[powdermonkey]

Hi technically minded people!

I need help in AGK.
I am getting confused with the compression of air.
How does a compressor increase the PRESSURE of the air.
Accelerating the air will give you more dynamic pressure, but is that not at the cost of a drop in static, therefore total pressure remains the same?

I simply am not grasping this basic principle as I am sure I am looking at it the wrong way.
Please can someone explain to a non tech person who has never changed a sparkplug how the total pressure of the air is raised?

Sorry if I am missing something very obvious, but I can't see it!!
Any help and BASIC ( think jets for big dummies) explanation woul be great.
Thank you in advance
PM

[G-ANDY]

I'm affraid I'm not too technically minded either, but took my AGK exam last week, so I should know the "rough-ish" answer.

In the compressor the air pressure increases, the temperature increases and the speed also increases. The reason Bernoulli's theory sort of goes out the window here (speed increases, therefore temp decreases), is that the air is squashed into a smaller space - giving an increase in temp and pressure, but speed is increased by the compressors by narrowing the duct (convergent) to make sure that the ever increasing pressure stays behind the compressor blades helping to prevent a stall / surge.

Hope that helps?

Andy

[gas path]

I'll have a stab at clearing it up for you:ok:

Velocity: Decreases slightly as it passes thro' the compressor stages until it reaches combustion where there is a rapid increase. The velocity comes down again as the energy is converted across the turbine stages and increases again as it passes thro' the nozzle.

Pressure: Rises as it passes across each stage of compression and drops slightly during the combustion stage and continues to drop as it passes thro' each stage of the turbine. (JFYI it is about 436psi max for the RB211 at T.O power.)

Temperature: Again this rises as it passes across each stage of compression, rises very sharply at combustion and drops again thro' the turbine stages. (again for the RB211 at T.O. about 600 deg c at the last stage of compressor rising to aout 1400 deg during combustion and less than 580 deg at the exhaust).

[powdermonkey]

Hi G-ANDY

Kind of helps, but

A divergent duct will result in a decrease in velocity and an increase in temperature and pressure.
A convergent duct will cause an increase in velocity and a reduction in pressure and temperature.

So when the air is accelerated through either the centrifugal impeller or the rotors of the axial flow compressor, velocity increases, but my notes say pressure also increases. That is the bit I don't get! How does increasing velocity create an increase in pressure? The notes then follow on to say that the air is then slowed down, first through the diverging ducts of the impeller when dealing with a centrifugal compressor then through the diverging ducts of the diffuser, further increasing the pressure.
How can acceleration then decceleration both increase the pressure? Aslo, how is the air actually compressed. I thought the whole converging casing scenario was to ensure that the airflow went only one way, to prevent surge!
I know it's all about some change of energy but I don't get it!.............and I hate that because I won't leave it alone until I do get it, thus further delaying me in my studies!!!!

AAAAAAAAAAAAAAAAAAARGGGGGGGGGGGGGGGGGHHHHHHHHHH!

Gas Path,
Hi, and thank you but now I am even more confused seeing as the text tells me velocity is increased by the compressor.

TEXT:

Like the centrifugal compressor the axial flow compressor adds kinetic energy to the air and then converts the kinetic energy into pressure energy. To accelerate the air the axial flow compressor used a series of aerofoil section blades or rotors mounted on a rotating rotor disc and pointing outwards from the centre of the engine. :{ :{ :{ :\\

[Farmer 1]

I'll try, Powedermonkey.

The compressor sucks in air and pushes it back at high speed. The cross section of the compressor decreases towards the back, so its volume decreases proportionally, and all the while more air is being sucked in. If the volume decreases (for an increased quantity of gas) then the pressure must increase.

The compressed gas is then fed into the combustion chamber.

I don't know whether you are aware, but the sequence is the same as in the Otto cycle in your car's engine.

[Mad (Flt) Scientist]

Compression occurs by slowing the air - as noted, increasing the velocity would be classically associated with a pressure DROP.

Simplistically - and any engine gurus please excuse this - think of each stage of the compressor alone, with a rotor row of rotating blades and a stationary row of stator blades.

The rotor turns and imparts energy into the air, speeding it up as it does so. Because the increase in velocity is coming from energy ADDED to the air (mechanically, by the rotor) the air pressure does not drop in Bernoulli style, because that describes a closed system.

Now we have some high energy, high speed air moving towards the stator row. The stator row is shaped so as to slow down the air again, but the energy that was the velocity goes somewhere - pressure.

(In reality, the rotor row also causes a pressure increase, because the back part of the rotor gaps is divergent also. But the stators are the big pressure rise devices)

[PhoenixRising]

@powdermonkey

Firstly, you are complicating the matter for yourself by thinking in terms of acceleration and deceleration of air. You need to think velocity, and look at velocities at critical points in the system.

You have an increase in pressure across the impeller because you have flow in a divergent nozzle which you have already established gives an increase in pressure.

In the diffuser you have the velocity head (added by the impeller) converted into pressure head by slowing the flow down which further adds to the total pressure.

Any clearer?

Apply a simplified version of Bernoulli's equation to the impeller and the diffuser if you are not convinced.

[powdermonkey]

Thanks Guys
I always think too much about these things and tie myself up in knots about it until nothing makes sense!!
So the key to this is that the rotors add more energy to the air, therefore there is a rise in total pressure, unlike a venturi where no extra energy is added to the mass flow, resulting in static pressure drop for increased dynamic pressure.

Then when this exta charged air is slowed, the dynamic pressure decrease must now result in a static pressure increase, etc etc.

So its all about work done by the rotor blades, and the converting by the stator blades. Think that makes sense!!

Thank you all very much
PM

[Keith.Williams.]

Your summary is almost correct.

The spaces between the blades are divergent both in the rotors and in the stators.

If the rotors were not moving, but the air was flowing through, the velocity would decrease and the static pressure would increase both in the stators and in the rotors. But this steady decrease in velocity would eventually cause the airflow to stop.

The rotors are turning at high speed, and in doing so they add a great deal of energy to the airflow. This is primarily in the form of increased velocity. Some of this increased velocity is immediately converted into increased static pressure and increased temperature by the divergent spaces between the rotor blades. But not all of the increased velocity is used up in this way, so the overall effect is that velocity, static pressure and temperature all increase in the rotors.

The stators are not turning, so they do not add any more energy to the air. But their divergent spaces cause more of the velocity to be converted into increased static pressure and increased temperature. So as the air flows through the stators the velocity decreases and the static pressure and temperature both increase. The reduction in velocity in the stators is approximately equal to the increase in velocity in the preceding rotors.

The overall effect of these processes is that velocity increases then decrease in each stage, with the overall velocity remaining approximately constant. But both the static pressure and temperature increase in each rotor and in each stator.

This overall effect is also achieved in a centrifugal compressor.

[powdermonkey]

Hi Keith Williams

Thank you very much for that, I understand it now!!!
Next AC current.................................:{ :{ :{
But then I will have finished AGK:) :) :)

[Gary Lager]

So how do ramjets work then? :E

[rigpiggy]

different beast altogether. compression is caused by controlling the shock wave, as the air slows thru mach, it compresses/heat add the fuel to increase temp/mass flow. Ramjets have to be going faster than sound by another source to start. Jet reaction engines are firstly a heat pump. a bit simplistic perhaps, but I'm a simple man

[RatherBeFlying]

KW,

A truly excellent and succint explanation.

Now here's one I've never understood:

Why is it necessary to compress the air before the combustion bit:confused: :confused: :confused:

[Gary Lager]

Not necessary, just better.

Simplistically, higher pressure = greater combustion efficiency & higher mass flow = higher thrust for a given EGT.

Fuel still burns in the opern air (of course), but very inefficiently and with nowhere near enough 'oomph' for propusion purposes.

[RatherBeFlying]

GL

My natural gas furnace burns with an advertised 91% efficiency at close to atmospheric pressure -- the draft being provided by a fan.

Mind you there's no thrust or torque to speak of, which is all to the good as I do prefer the furnace stay put;)

[Gary Lager]

But what kind of efficiency? There are many different ways of measuring that - quantity of unburnt fuel remaining? Ratio of output to input tmeperatures? etc etc


In a furnace you want as much of the chemical energy as possible to be converted to heat - not necessarily so in jet engine. In a jet turbine engine you want mechanical work to be done on the turbine (turning the compressor or turboshaft) and the surrounding environment (producing thrust).

An ideal efficiency would have the exhaust gases at atmospheric temperature - since all energy is converted as work and none wasted as heat (or kinetic energy of exhaust). Of course, those scenarios are nonsense for many blindingly obvious reasons.

So the 91% efficiency of your furnace may not be measuring the same efficiency as would be useful in measuring the performance of an engine.

For instance, just because only 9% of fuel remains unburnt, doesn't mean that 91% of the chemical energy stored in the fuel has been released - to help achieve that, you need to tweak the circumstances of combustion somewhat by (for example) compressing things (and even then only a fraction of the true chemical energy of a fuel is ever released - see Einstein & Atomic Weapon physics for a demonstration of what happens when you make more use of that energy potential!).

[RatherBeFlying]

In the 91% efficient furnace case, we can assume just about 100% combustion. Furnace efficiency is heat delivered to the interior over heat released by combustion.

The 9% in this case is exhausted with the combustion products and as you have pointed out with other engines, exhaust heat is lost energy.

The Watt Steam engine was something like 3-8% efficient, but does combust at atmospheric pressure.

But my question is really about the necessity for compression in internal combustion engines: Otto, Diesel, turbine.

None of these engines start by simply introducing fuel and lighting it. They all have to be cranked -- why?

[Gary Lager]

Need to get my university textboks out again - I think I have already strayed beyond the bounds of my knowledge...

One guess - perhaps it is the requiremnet that expansion of gases post-combustion is necessary for operation of those self-sustaining engines you mention: by ioncreasing the pressure at combustion you can obtain a greater expansion to atmosphere. Of course, that's at the expense of some energy used to drive the compressor anyway, but that is a necessary evil for turbine/combustion engines.

I'll try and find a more definitive answer later today. I am now intrigued too!

[Slow Roll]

Posted something similar on the 'Questions' forum, but hope this may help, it's certainly the way I try and sort this stuff out...

Jet engine cycle from start to finish covered diagramatically by Temperature-Entropy (TS) plot (see pic below):

http://en.wikipedia.org/wiki/Image:TSdiagram.gif

or look at this link

http://en.wikipedia.org/wiki/Image:TSdiagram.gif

The diverging red lines show lines of constant pressure. Note, as entropy (effectively disorder) is increased by adding fuel, the gap between lines w r t temperature increases. Thus if air is compressed then combusted at high presure, upon expansion more heat will be liberated than was used to compress it. Thus have 'spare' energy (ie. thrust) as a result of the pressurised combustion.

[barit1]

To put it briefly -
The compression is necessary to make the exhaust nozzle work. In order to spin the compressor, a turbine is used.

The combustor adds heat energy. The turbine extracts some of the heat/pressure energy to do its job, leaving some excess energy to accelerate the flow out the exhaust nozzle.

Which is what it's all about!

To quote a famous jet pioneer: "We're trying to contain and harness fire - which is exactly what the Devil in Hell is all about!"

[RatherBeFlying]

sr & b1,

thanks for your comments, mind you, that TS diagram is a bit hard on the brain;)

Puttingthe Devil in Hellreminds me of a Boccacio story with the same title. Something about a hermit monk who was approached by a winsome lass for religious instruction who happened to notice that her presence roused the Devil beneath the monk's robes:E :E :E