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Why do turbine engines require a compressor section

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Why do turbine engines require a compressor section

Old 18th Nov 2011, 17:11
  #41 (permalink)  
 
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Folks when you get all done with this piston engine stuff (no compressors or turbines), could you do a google-idiot style translation into turbine engines for the casual thread reader.

Maybe a distinction could then be made between TIT (Turbine Inlet Temperature), EGT (Turbine Exhaust Gas temperature) out the tailpipe and pressure out of the compressor,

I doubt that timing enters into this for a gas turbine since the flowrate is constant and so is the fuel and ignition.

The casual reader would appreciate simple comparisons with everyday effects e.g. welding torches, bunsen burners, baloons, home oil heaters etc.
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Old 18th Nov 2011, 21:46
  #42 (permalink)  
 
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Hi Oggers.

I've already explained why I used the words crank, piston and cylinder in my first post - because I thought it would be easier for people like you to understand the thermodynamic concept ask by the OP if I talked about the Otto Cycle. I used those words to try and explain the concept to most readers on this forum, but I never said anything other than the fact that the main reason for thermodynamic efficiency increase in high compression is reduced heat waste to the fluid. All of the false arguments were introduced by you.

I'd really like to hear Oggers explain how ignition timing, or valve timing, or mixtures, or flame fronts, or pumping losses, or any other of his false arguments explain why a high compression turbine engine is more efficient than a low compression turbine.
I'm listening.

It's about the total fluid heat change from start to finish (once the fluid is returned to atmospheric pressure). It is lower in a high compression engine.
FOR THE FOURTH TIME, I'm still listening. You really are choosing to avoid this.

Luc,

Why don't you read all my posts, rather than glaze over the last one?

The point regarding adding heat to the fluid is the total heat added to the fluid over the entire cycle (ie, once the fluid is returned to atmospheric pressure).

I'll say it again, for either a piston or turbine pressure, then higher the compression ratio or pressure ratio, the less overall energy will have been absorbed by the fluid by the time it has returned to atmospheric pressure.

Higher compression/pressure ratio means for a given fuel flow, less heat exchange has occurred to the exhaust gas by the end of the cycle. It allows more energy to be extracted as useful work.
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Old 18th Nov 2011, 23:52
  #43 (permalink)  
 
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ok, when we come to turbines i will give here some impressions on the turboprop GA aircraft i currently fly- a cheyenne III with the pt6a-41 engines. the pt6a-41 is a turboprop two shaft engine which incorporates a three stage axial compressor followed by a 1 stage centrifugal compressor driven by a 1 stage compressor turbine. after this we have a 2 stage power turbine which drives the prop via a gearbox. the both shafts are not mechanically coupled and the low pressure turbine drives only the prop - so we call the pt6a is a free turbine.

the engine is like common nowaday design flat rated - so the rated power output is a mechanical limit and the engine is able to keep rated power above ISA or keep rated power in thinner air when you climb until its thermodynamical limit ( ITT or compressor speed) is reached. in other words- the engine could develop on ground more power that the gearbox is approved for.

basicly on this engine you give with the power levers an input to the FCU ( fuel control unit ) to set a target compressor speed. in regard to air density and outside temperature a given amount of fuel is needed to keep this speed. this will result in a given force to the power turbine and a given torque - so power output.

at take off you are mostly torque ( so power output) limited and the turbine is at its mechanical limit . the ITT and compressor speed are below its limit. when you climb out and do not touch the power levers the compressor speed stays the same. the ITT also but torque and fuel flow decreases. this is due the fact the FCU ( fuel control unit) keeps like said a target compressor speed . in a climb out the air gets thinner and the "resistance" on the compressor stages also. so the compressors try to spin faster and the FCU has to decrease fuel flow to keep the same speed. due to less fuel and gas driving the power turbine the torque ans power output also decreases.

when you want to keep the same power output in a thinner air in a climb you will have to push the power levers more and more forward. this will result in a faster and faster spinning compressors and a higher and higher ITT until at a given altitude you match the maximum ITT or compressor speed. here the turbine reaches its thermodynamical limit.

sooo... when the air gets thinner and the compressors deal with a lower pressure ratio ( in pistons compression) the ITT rises .thats a fact. i found and attached a pic at our top of climb in FL 280 with a cheyenne III with pt6a-41 engines so you can have a look what the torque, ITT, compressor speed amd fuel flow is here. at this altitude the engine is at its thermodynamical limit - so the compressor speed is at company limits resulting in a given ITT and torque far below its redline ( mechanical limit)

now we can talk why it is so a a turbine engine.
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Old 19th Nov 2011, 11:10
  #44 (permalink)  
 
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Aerobat77:

now we can talk why it is so a a turbine engine.
Turbine engines utilise air for cooling of the combustion chamber and the turbines themselves. ITT will rise due to the reduced mass flow rate of air going to the combustion chamber as you climb.

Less mass of air going into combustion chamber, but same mass required for combustion itself = less left over for cooling. I'm not saying this is the only cause (I have a mere pilot's understanding of turbines) but it is the only cause I was taught at the Royal Naval Flying Training School. It is important to helicopter pilots because you can become power limited in the hover due to turbine temp before you reach the service ceiliing - a factor in so called 'hot and high' operations.

Last edited by oggers; 19th Nov 2011 at 11:29.
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Old 20th Nov 2011, 18:54
  #45 (permalink)  
 
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...if you didn't have a compressor it would be a ram jet. Anyone mentioned stoichiometric yet ? As someone said, need to have the pressure gradient to avoid a bonfire - imagine starting a ram jet at zero speed.
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Old 20th Nov 2011, 20:14
  #46 (permalink)  
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Thermodynamics

Gentlemen, as a mere physical chemist and at the risk of causing more confusion, may I suggest we try to stick to what is to me the fundamental process here?

The more we compress the air the more oxygen molecules we supply per unit time to the combustion chambers so we can burn more mostly hydrocarbon fuel per unit time and release more energy to do useful work per unit time.

That's the simple central point here.

Let's dig a little deeper.

The Second Law of Thermodynamics can be expressed as

ΔG = ΔH -TΔS

The combustion products are hotter than the fuel or the air reactants so the ΔS term becomes more positive as their entropy is now larger, the negative sign above then makes the TΔS term negative.

T is large and positive again making the TΔS term negative

The combustion process (yes yes ideally stoichiometrically , we'll have that discussion if you wish but not here for the moment please) is an exothermic reaction that mostly releases energy to the surroundings in the form of heat so ΔH has a negative sign on this side of the pond.

So ΔG (the free energy) is large and negative and we have lots of free energy available to do useful work - such as provide thrust.

Finally, to go back to the original question of "why compress?", we can now say "so as to release more energy per unit time from the fuel"

CW
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Old 20th Nov 2011, 21:52
  #47 (permalink)  
 
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oggers:
ITT will rise due to the reduced mass flow rate of air going to the combustion chamber as you climb.
Well, this may or may not be true for a given engine type, depending on the characteristics of the control system (whether governing to a constant core speed, constant corrected speed, constant torque, constant CDP, etc etc.)

If an electronic control, it's all done with ones and zeroes; if hydromechanical, then it's bellows and cams and flyweights.
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Old 21st Nov 2011, 02:33
  #48 (permalink)  
 
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Blimey - PPRuNe at it's best. A simple question asked by someone seeking enlightenment and they get a thesis and a sh!t fight!

I would like to answer the original queries by by answering the second question first. In simple terms, the power output of a reciprocating engine can be expressed as bore x stroke x rpm x compression ratio. Based on this, if you force more of a charge into the engine by using a turbo or superchager, the compression ratio will increase and the power output will go up.

As for the first part, obviously you understand the OTTO cycle an the principle of induction, compression, conmbustion and exhaust, or suck, squeeze bang and blow. The turbojet engine does the same. The suck is the flow into the engine, the squeeze comes from the compressor, the bang occurs in the combustion chamber and the blow is the thurst. If you look at it like this, if you didn't have the compressor, the engine wouldn't work and as someone else said, all you would have is a bonfire!
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Old 21st Nov 2011, 09:54
  #49 (permalink)  
 
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Barit:

Well, this may or may not be true for a given engine type [that ITT rises in the climb due to reduced mass flow rate of air], depending on the characteristics of the control system (whether governing to a constant core speed, constant corrected speed, constant torque, constant CDP, etc etc.)
No doubt you're quite right. However, I was responding to aerobat's specific scenario regarding his specific type in which he observes an ITT rise.

If you follow the thread back you will find this comment by aerobat which gives context:

i think slippery pete is pretty right with the higher compression- lower exhaust temperatures at pistons . you can see this effect also in tubine engines where maintaining the same power output / fuel flow in a climb will result in exhaust temperature RISING due to thinner air and worsening compression of the compressor stages
My point is that if you observe such a rise in those circumstances there is a good alternative explanation; namely less cooling air available.

Last edited by oggers; 21st Nov 2011 at 10:58.
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Old 21st Nov 2011, 11:08
  #50 (permalink)  
 
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Pedant head on.

Internal combustion engine (Otto) cycle - suck, Squeeze, bang, fart.

Gas turbine (I refuse to call it Brayton as Mr Whittle formulated it years before) cycle - Suck, squeeze, burn, blow.

Just sayin'.




Anyway, to answer the original question...

If the turbine engine didn't have a compressor, then the turbine is just a spinning disc getting in the way. It has to drive something.

I suppose in theory you could have a turbo-prop with no compressor and rely on a very efficient ram effect for compression but as no-one has done it yet I suspect it won't work, for all the very clever and frankly complex answers given on the last 3 pages.
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Old 21st Nov 2011, 12:24
  #51 (permalink)  
 
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If the turbine engine didn't have a compressor, then the turbine is just a spinning disc getting in the way. It has to drive something
actually it won't spin unless there is a pressure drop across it. The turbine needs a forced movement of air through it.
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Old 21st Nov 2011, 20:03
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Quote:
If the turbine engine didn't have a compressor, then the turbine is just a spinning disc getting in the way. It has to drive something

actually it won't spin unless there is a pressure drop across it. The turbine needs a forced movement of air through it.
Erm, I don't quite understand your point.

EG. A ram jet with a turbine stuck up it's arris. Pointless but you get my drift?
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Old 21st Nov 2011, 20:36
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EG. A ram jet with a turbine stuck up it's arris. Pointless but you get my drift
yea, but how do you get it up to speed to ram it? Difficult to start one on the ground.
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Old 21st Nov 2011, 20:59
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Yes, I know but that wasn't the point I was trying to make.
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Old 21st Nov 2011, 21:47
  #55 (permalink)  
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A ram jet has sufficient compression without the need for a fan assembly.

Go sub routine velocity dependence!

Why do we compress - to release more energy to do useful work per unit time within the engine (be it piston or gas turbine).

CW
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Old 21st Nov 2011, 23:29
  #56 (permalink)  
 
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Why do we compress - to release more energy to do useful work per unit time within the engine (be it piston or gas turbine).
It's so that less heat is added to the air over the entire cycle.

Burning a certain amount of fuel produces a certain amount of energy, let's call it X. You can't change the amount of energy released by burning a fuel by burning it faster or slower or under more pressure. Burn it as fast or slow or under as much pressure as you like, the amount of energy stored in that fuel is constant.

The energy produced by X is divided into waste heat, useful energy and sound by an engine. The sound is so negligible against the power output of the engine, it can be ignored.

So energy from the fuel = useful work + waste heat

The ONLY way to get more energy out of the same amount of fuel is to make less waste heat. Higher compression results in less waste heat transferred to the air by the end of the cycle.
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Old 22nd Nov 2011, 03:20
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Any aircraft engine supplies a propulsive force by capturing a mass of air and then accelerating that air. It may be a propeller doing the job, or it may be a gas turbine (we call it a jet engine). In any case, a pressure increase is needed to force (accelerate) the air mass aft. Thus, the need for a compressor. The prop is a compressor; the fan in a turbofan is a compressor; and so is the compressor (doh!) in a straight jet or a turboprop.
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Old 22nd Nov 2011, 10:48
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slippery....

The ONLY way to get more energy out of the same amount of fuel is to make less waste heat. Higher compression results in less waste heat transferred to the air by the end of the cycle.
If you confine yourself to framing everything in terms of thermal efficiency you will never be able to explain how or why an engine works.

The topic of this thread is simply "why do turbine engines require a compressor section". That is the question that was being answered by CW. His answer emphasised the requirement for power.

Nobody wants an aircraft that can taxi out on a thimble of fuel before failing miserably to accelerate to flying speed.

Last edited by oggers; 22nd Nov 2011 at 14:17.
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Old 22nd Nov 2011, 11:17
  #59 (permalink)  
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Power v heat waste losses

Slippery Pete, sir,

Saying that "the main reason for thermodynamic efficiency increase in high compression is reduced heat waste to the fluid" is, in my view, wrong. It is the "main reason" aspect of the statement that I quarrel with.

The primary reason for compressing the air is the enhanced power per unit time the engine can then produce; heat transference losses are also reduced it is true for the reasons you give but I submit that that it is a secondary (albeit welcome) effect of the compression.

With compressed air you have more oxygen molecules in the combustion chamber and you can burn more fuel per unit time. You release more energy to do useful work per unit time. The time dependence is critical.

The main reason is power but you are right in that, under high compression, the loss of heat this way is a smaller % of the process i.e. it helps but it's not the key driver.

If we really want to get obscure the parallel with Le Chateliers Principle when justifying running big pressures on systems that are making fewer moles of gas in production processes is a good one - you "notice" the drop in entropy less than at low pressures.

Yes yes there's a rate of reaction factor at elevated pressures too but let's not go there either!

I suspect we shall have to agree to differ short of doing a lot of sums in public.

CW
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Old 22nd Nov 2011, 12:51
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Methinks you are all missing the point. Suppose I build an aircraft that is electrically powered (motor driving a prop), or hydraulically powered, or compressed air, or human powered. In all cases the prop is a compressor - a device which captures a mass of air and accelerates it aft.

Now if we put a simple turbojet in the plane, it still has to - ahem - capture a mass of air and accelerate it aft. But we have to somehow drive the compressor, and so we have a burner and turbine to recover some of the energy in the airflow to create a continuous Brayton cycle (named after American engineer George Brayton (18301892)).
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