Why does burner pressure drop when internal energy increases in a gas turbine?

[sid_galt]

Why does the pressure in the burner of a gas turbine engine drop when you are increasing the internal energy of the air?

 

[Clausius2]

Why does the pressure in the burner of a gas turbine engine drop when you are increasing the internal energy of the air?

The pressure doesn't drop at all.

 

[FredGarvin]

Where did you get this information?

 

[sid_galt]

From the NASA site.

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

In the first paragraph they say

The burning occurs at a higher pressure than free stream because of the action of the compressor. The pressure in the burner remains nearly constant during burning, decreasing by only 1 to 2 per cent.

So how is the burner designed such that the pressure doesn't increase at all even while heat is added to the air through combustion?

 

[FredGarvin]

Now I understand your question.

The first thing is that, in the burner, static pressure does rise due to combustion at the point where it is happening. The amount will vary from engine to engine. One thing I think you may be missing when looking at the outlet pressure is that not all air is combustion air in the burner. The majority of the air entering the burner is used as cooling and dillution air. Maybe 25% of the total air flow is used in combustion.

Secondly, the turbine nozzle at the exit of the burner is usually going to be designed such that the flow is choked. This helps to maintain a rather constant flow into the turbine that doesn't get effected by the downstream conditions. That also means there is an appreciable acceleration of the airflow into the turbine. If you'll notice that the equation stated for BPR is in terms of total pressures. The exit of the burner into the nozzle is basically going to trade that static pressure increase and convert it into a dynamic increase.

Edit: I did forget to mention, that there is a head loss due to the highly turbulent nature of the flow in the combustor. Mixing flow is a big culprit of the losses in this sense. The more mixing involved, the better the temperature distribution (i.e. no hot spots) the higher the rpessure loss. Yet another engineering tradeoff.

If you REALLY want to get into the nuts and bolts, I can pull out a couple of books and start throwing equations at you.

 

[sid_galt]

Thank you for the reply.

I have another question.
Does combustion always occur near the fuel injecters or does sometime the fuel get diverted to other areas of the burner due to the static pressure of combustion? If it does, then are there additional spark plugs present in the burner to ensure full combustion of fuel?
If the fuel always burn mostly at or near the same points, this means the static pressure is constantly getting converted to dynamic pressure. How does this happen so fast? Is it because of the velocity of the aircraft?

 

[FredGarvin]

In a can type of burner, the actual combustion will happen in around the first 30% of the burner. The rest of the length is used to mix the secondary flows with the combustion products to get a more even temperature distribution entering the turbine. As far as other injector locations, as far as I know, they are usually in one location. I know of engines that have start nozzles and then running nozzles, but they are in the same axial location.

As far as how this happens so fast, the combustion is a constant process. The whole engine is for that matter. You have a big expansion happening in a constant area. So, ideally, I would think that the expansion would be as fast as the combustion process itself.

 

[sid_galt]

So when the expansion happens, it happens in all directions, right? So doesn't it resist the flow of the compressed air inside the burner? Doesn't the energy from combustion go waste there?

 

[FredGarvin]

The flow into the burner is going through a diffuser which brings up the pressure and slows down the velocity from the last stage of compression. The expansion is supposed to happen mostly in the downstream direction, towards the turbine nozzle. There is an appreciable momentum in the incoming airstream. I think of it in the basic terms of the path of least resistance. What direction is the gas most likely to expand? In the direction of the incoming, highly compressed side or towards the nozzle side?

I would think that what you are thinking does indeed happen to a point and is part of the losses involved with the burner.

 

[sid_galt]

Thanks for the info.
Then are the fuel injectors in gas turbine engines oriented in the direction of airflow or are they normal to it?

If the orientation is normal to the airflow, do you think there will be any minimal advantage gained were they oriented in the direction of the airflow? That way, the outer layer of the fuel would burn off and expand compressing the inner layers of the fuel which would then exert pressure on the fuel injectors increasing the thrust, not a lot of increase maybe 0.01 %. When the outer layer of fuel is at a sufficient distance, the inner layer will be exposed to airflow and will burn off and so on. Do you think this will minimally increase the thrust?