Jet Engine Doubt: How Does Temp & Pressure Affect Efficiency?

[perfectz]

A Jet Engine doubt...

Hi folks...
My aerospace propulsion professor taught me that, in the combustion chamber of a jet engine, during combustion, the pressure instead of increasing, decreases.
I have 2 questions.
1) Is it true?
2) If yes, how?

How and in what way does temperature, and pressure affect the efficiency of a turbine in the jet engine?

I am counting on you little Einsteins out there...

 

[russ_watters]

Let me answer the question with another question: what direction does a fluid flow: from low pressure to high pressure or from high pressure to low pressure?

As far as efficiency goes, it is a function of pressure: http://www.adl.gatech.edu/classes/propulsion/prop5.html

 

[perfectz]

a fluid flows from high pressure to low pressure...
so what? please tell me...

 

[perfectz]

and u say efficiency of the turbine plainly depends on the pressure alone?

 

[Langbein]

Remember pressure is two or tree things, static and dynamic pressure and the sum of dynamic and static pressure.

I would believe that the static pressure decreases slightly, while the dynamic pressure and the temperature increases, so that the total amount of energy in the exhaust gasses increases even though there might be a slightly drop in static pressure. (If I remember it right.)

The increase in temperature and dynamic pressure, and with other word the increase of energy content is due to the burning fuel.

 

[Langbein]

By the way .. I allmost wrong, I forgot one thing.

Most jet engines use to have an difuser in the area between the comressor outlet and the front end of the combustion chamber. In this area the static pressure goes up and the dynamic pressure goes down.

Then it is a rather stable area trough the combustion chamber. In the last part of the comustion chamber, the combustion gasses increases velocity, and static presure and temperature goes down, before stream of combustion gasses enters the turbine blades.

I found this link that tells something about how static pressure and velocity (dynamic pressure) changes trough the jet engine. The picture is not very precise as it does not mention "the spesial things" that happened when the air stream enters and leaves the comustion chamber area. In this way the drawing is a bit "oversimplified".

http://science.radioelectronics.biz/aeronautics390/jetEngine.htm

 

[russ_watters]

a fluid flows from high pressure to low pressure...

Ok, so for air to flow from the compressor to the combustion chamber to the diffuser, which must be at the highest and which must be at the lowest pressure?

 

[russ_watters]

Remember pressure is two or tree things, static and dynamic pressure and the sum of dynamic and static pressure.

For the purpose of the OP, we need only discuss the total pressure. By Bernoulli's principle, the total pressure along a streamline is constant. When losses are taken into account, the total pressure will drop as the fluid travels through the duct.

 

[russ_watters]

and u say efficiency of the turbine plainly depends on the pressure alone?

Did you read what the link said and see how the equations work?

 

[FredGarvin]

In a well designed burner, the heat addition step, in practice, results in a pretty constant pressure exiting the burner and going into the HP turbine. The pressure does drop for a couple of reasons:

1) It is an unrestrained expansion.
2) There are flow losses within the burner
3) There is a decent dilution amount of cooling done to the flow to maintain a cool enough gas temperature to not have too high of an TIT.

 

[Langbein]

For the purpose of the OP, we need only discuss the total pressure. By Bernoulli's principle, the total pressure along a streamline is constant. When losses are taken into account, the total pressure will drop as the fluid travels through the duct.

No, I don't think it will work like that.

The gas stream trough a combustion chamber does not follow the Bernoulli's equitation at all. This prinsipple will only be valid when the airstream has one fixed amount of energy. In the case of the combustion chamber you add on a lot of burning fuel so that the total pressure will certainly not be constant or decrese.

It is this increase in total pressure that makes the jet engine to work as a jet engine. If no build up in total pressure, there would in the end (in the exhaust nozzle) be no jet trust.

To make a jet trust the air that passes trough the jet engine will need to have added the dynamic and the total pressure. A jet engine that does not accerlerate the air masses, that will increase the total pressure, will be a shut down jet engine.

I think I have it approx right above

 

[Langbein]

In a well designed burner, the heat addition step, in practice, results in a pretty constant pressure exiting the burner and going into the HP turbine. The pressure does drop for a couple of reasons:

1) It is an unrestrained expansion.
2) There are flow losses within the burner
3) There is a decent dilution amount of cooling done to the flow to maintain a cool enough gas temperature to not have too high of an TIT.

Remember that the combustion chamber consist of "two halves":

1. The area outside the internal cobustion liner, that will work allmost as you mention. (Exept for that you have not mentioned the diffusor zone between the compressor area and the combustion chamber, that will increase the static pressure quite a bit.)

2. Then there is the area (volume) inside the internal combustion liner. Here the static pressure will be allmost a constant or drop slightly, while the dynamic pressure and speed is increasing due to the added amount of energy from the fuel. (And this increase in energy is what makes the engine, in the end, capable of producing trust and work.)

 

[russ_watters]

Have a look at the P-V diagram for a jet engine, Langbein: http://en.wikipedia.org/wiki/Brayton_cycle

Pressure rises only in the compression stage. Combustion happens at constant pressure (in an idealized engine).

All thermodynamic cycles are pretty much variations on the same themes. In this case, it is just like how in a steam cycle, water goes into the boiler and is heated at constant pressure.

 

[Langbein]

The p-v diagram of a jet engine menetioned in wikipedia would not be that one you would use if you got an F-16 engine up in the test bed to make a test run. The p-v diagram of wikipedia leaves out a few details, I think.

One detail that is left out is that the front diffusor of the combustion chamber (the area/voulme between the end og the axial compressor and the front of the combustion chamber inner liner) will work as and should be considered to be a part of the compressor. (So that the compressor actually will consist of one part that is build up with a moving rotor and one part that is buildt up by a static diffusor that continues into the combustion chamber up to the front of the combustion chamber inner liner.

When you relate to the brayton cycle, these two parts, the dynamic compressor and the static diffusor will be "the compressor".

(Step 1-2 of the brayton cycle extend into the combustion chamber.)

The combustion at the approx constant static pressure will take place inside the combustion chamber internel liner.

(Step 2-3 take place inside the combustion chamber internal liner.)

As one can se from the bryton cycle at step 2-3 the static pressure is approx constant (innside the inner liner) while the dynamic pressure (the speed) is increasing. As total pressure is the sum of static pressure and dynamic pressure, the total pressure gores up.

One other thing that is left out from the simplified p-v diagram in wikipedia, is that most real jet engines has a nozzle at the rear og the combustion chamber inner liner and in front of the first stage turbine rotor. This sets up the speed, it reduces the static pressure and increases the dynamic pressure. This will also cool down the air stream so that the first stage turbine is not burned.

A lot of these smaller details is not possible to read from such a overview that is mentioned in wikipedia.

 

[FredGarvin]

Remember that the combustion chamber consist of "two halves":

1. The area outside the internal cobustion liner, that will work allmost as you mention. (Exept for that you have not mentioned the diffusor zone between the compressor area and the combustion chamber, that will increase the static pressure quite a bit.)

2. Then there is the area (volume) inside the internal combustion liner. Here the static pressure will be allmost a constant or drop slightly, while the dynamic pressure and speed is increasing due to the added amount of energy from the fuel. (And this increase in energy is what makes the engine, in the end, capable of producing trust and work.)

Who mentioned anything about the diffuser?

The total pressure across a well designed burner is what remains almost constant. If you look at the pressure ratio across a burner, it should be very close to 1.

Attached is a plot of expected parameters as varying with engine station.

 

[FredGarvin]

The p-v diagram of a jet engine menetioned in wikipedia would not be that one you would use if you got an F-16 engine up in the test bed to make a test run. The p-v diagram of wikipedia leaves out a few details, I think.

You are absolutely wrong. That is the exact cycle to use as a reference (not that I have ever had a Brayton cycle next to me when we have done an engine test, but who's asking?).

 

[Langbein]

Well, I think I'm right. What kind of engine(s) do you test on ?

What was the test conditions ?

How do you check what happens inside the combustion chamber ?

Did you dissassembly the engine to take a look at the different details ?

Did you then reassemble it, testrun it to send it out do do the flying with pilot and or pax after ?

Which engine was it ?

 

[FredGarvin]

If you take a look at my occupation, you will see what I do for a living. I test full up engines as well as engine components for a living.

My company makes a few different engines for both military and civilian applications.

 

[Langbein]

OK, I did, do the same before, but not any more. (Does some other technology stuff now.)

Used to do F-100/F16 O/H and test drive in teststand and installed on aircraft before. (Worked as a F-16 test engineer) Later on civilian with helicopter and aircraft turbine engines.

But back to the interesting stuff .. :-)

Will you really say:

1. That if you open a jet engine, you will not find a diffusor behind the axial compressor and in front of the combustion chamber inner liner ? And this diffusor will not increase the static pressure into the inner liner ?

2. In the rear part of the inner liner and in that section that goes over to the turbine area, the air/exaust channel is not formed like a nozzle that increases the exhaust gas velocity before entering the turbine area ??!

I think actually all jet engines I have looked into have been designed like this.

Which one (engine type/model) is not designed like this, som practical examples ?

 

[Langbein]

By the way, I will try to se if I can get up the picture you sendt before.

https://www.physicsforums.com/attachment.php?attachmentid=10253&d=1181648089

If you see the area after the compressor, you can see it is formed as a nozzele. The pressure curve is not accurate because it should show a pressure build up in this area.

On the other side the increase in speed does near the end of the combustion chamber does show. The drawing is allmiost correct on that one.

I think this drawing is a good one but it is still a bit to simplified to show all of the prosess. The main thing that is missing is the pressure build up after the last compressor stage.

By the way, I do agree tha general course books on jet engine theory does explain the pressure diagram for a jet engine like the drawing on your link. But I also think that if you go a bit deeper down on each jet engine model, this courve will look a little bit different.

.. Just my point of view :-)

 

[FredGarvin]

Will you really say:

1. That if you open a jet engine, you will not find a diffusor behind the axial compressor and in front of the combustion chamber inner liner ? And this diffusor will not increase the static pressure into the inner liner ?

No. I did not say that at all. What I said was the diffuser is technically part of the compressor group and not part of the burner. Of course that is what a diffuser does. Why would you think I said anything to the contrary?

2. In the rear part of the inner liner and in that section that goes over to the turbine area, the air/exaust channel is not formed like a nozzle that increases the exhaust gas velocity before entering the turbine area ??!

There is a turbine nozzle that does exactly that. I never even mentioned the turbine nozzle. What I did say was:

The total pressure across a well designed burner is what remains almost constant. If you look at the pressure ratio across a burner, it should be very close to 1.

Now, where you get me saying that the diffuser and the nozzle do not exist is beyond me. You are throwing in a bunch of "facts" that have absolutely no bearing on the OP's question which has to do with the pressure drop across a burner.

 

[Langbein]

Well, I think he got a long and well explained answer.

In the end I think we agree .. allmost :-)

 

[perfectz]

Ok folks...
I went through your posts, and sadly I understood nothing.
Can some one summarize all the posts and come to a crisp and short and simplest conclusion/an answer to my question in the first post, so that my stupid brain gets it.....
Pleaseeeeeeeeeeeeeeeeeeeeeeeeee....

 

[russ_watters]

Sure, one more time:

Fluids move from areas of high pressure to low pressure unless there is a physical device (like a compressor) to push it backwards. Therefore, the pressure in the combustion chamber will stay roughly the same or drop slightly in the direction of flow (depending on the efficiency and other effects Fred mentioned).

 

[Langbein]

[post deleted by Russ]

I'm sorry to have to do this because I know you are putting a real effort into it, but you are completely wrong here and you're just going to confuse the OP. Fred is an industry professional and a resident guru in this particular subject, but you don't need to be to understand this particular question. The answer to this question is learned in first semester thermodynamics. Post deleted.

-Russ

 

[FredGarvin]

Lookie at what I found. Please notice the section on burner pressure ratio.

http://www.grc.nasa.gov/WWW/BGH/burnth.html