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 prom August 31, 2013 13:29

pressure development

Dear friends,

I was trying to simulate non-reacting flow characteristics in a simple can type gas turbine combustion chamber. The pressure contour showed that the pressure increased from inlet face to exit face. My question is what is the reason to increase in pressure. As the velocity increases, the pressure should decreases no? Please correct me if I am wrong.

Prom

 FMDenaro August 31, 2013 16:06

Quote:
 Originally Posted by prom (Post 449072) Dear friends, I was trying to simulate non-reacting flow characteristics in a simple can type gas turbine combustion chamber. The pressure contour showed that the pressure increased from inlet face to exit face. My question is what is the reason to increase in pressure. As the velocity increases, the pressure should decreases no? Please correct me if I am wrong. Thanks in advance. Prom
You made an assumption based on the Bernouilli relation but that applies for isoentropic, incompressible flows. Are you solving viscous flows?

 prom September 1, 2013 03:25

It is compressible model, the geometry is simple can type combustor solved for steady state, isothermal process. Air is used as fluid.

 FMDenaro September 1, 2013 17:58

Quote:
 Originally Posted by prom (Post 449142) It is compressible model, the geometry is simple can type combustor solved for steady state, isothermal process. Air is used as fluid.
ok, could you post the figures (iso-contour) of pressure, density and velocity?

 prom September 2, 2013 00:56

 michujo September 2, 2013 03:05

Hi. Is that the static pressure? I think it makes sense since the flow is decelerated along the combustor. Flow is injected from a nozzle at very high speed and small section area. As the gas flows along a wider section it is decelerated (to conserve total mass flow rate) and therefore its static pressure is increased.

Did you try plotting the total pressure (static + dynamic). I guess you'll then obtain a decreasing profile along the combustor.

Cheers,
Michujo.

 FMDenaro September 2, 2013 03:51

Quote:
1) what pressure you are plotting? I see Pascal in the scale but I also see negative values, therefore what is plotted is not p= rho*R*T
2) your model is compressible, therefore without density and velocity the pressure plot alone does not make sense
3) are you using also some turbulence modelling? because for compressible model it enters into the pressure

 prom September 2, 2013 10:23

yes I used k-epsilon model

 prom September 2, 2013 10:43

Yes it is static pressure plot... I am pretty convinced by Mr. Michujo's explanation. thank you sir. And I am new to CFX, I don't know how to plot the total pressure. BUt I'll give it a try...

 prom September 4, 2013 01:15

Dear Filippo Maria Denaro,
Is it wrong if negative pressure comes. please explain it. kindly elaborate the concept of it.
thank you...

 FMDenaro September 4, 2013 03:50

If you plot the thermodinamic pressure rho*R*T it must be always positive, if you plot the dynamic pressure rho*V^2 it must be positive, too.
Therefore, or you are plotting a scaled pressure or your results are wrong.
In any case your plots say nothing if are not completed by other variables

 prom September 4, 2013 09:56

Dear Filippo,

In CFX post I used pressure which we get by clicking on pressure in contour ploting. I dont have knowledge whether it is thermodynamic or dynamic pressure. But if I plot total pressure, then also I get negative pressure. I have very little knowledge in this. Plz tell me where it has gone wrong. The total pressure is in this link: https://docs.google.com/file/d/0B9tq...it?usp=sharing
Thank you

 FMDenaro September 4, 2013 10:07

Quote:
 Originally Posted by prom (Post 449845) Dear Filippo, In CFX post I used pressure which we get by clicking on pressure in contour ploting. I dont have knowledge whether it is thermodynamic or dynamic pressure. But if I plot total pressure, then also I get negative pressure. I have very little knowledge in this. Plz tell me where it has gone wrong. The total pressure is in this link: https://docs.google.com/file/d/0B9tq...it?usp=sharing Thank you
Sorry to say that using a CFD code is not as same as using a Play Station... You must be aware of the physics of the problem, having knowledge of fluid dynamics, numerical analysis and eventually have a look to the manual of CFX

 prom September 4, 2013 10:19

I am very sorry for my stupid questions sir. I am a beginner. Thanks for all your replies.

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