pressure development

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 August 31, 2013, 13:29 pressure development #1 New Member   Join Date: Jul 2013 Posts: 14 Rep Power: 4 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

August 31, 2013, 16:06
#2
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Filippo Maria Denaro
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Quote:
 Originally Posted by prom 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?

 September 1, 2013, 03:25 #3 New Member   Join Date: Jul 2013 Posts: 14 Rep Power: 4 It is compressible model, the geometry is simple can type combustor solved for steady state, isothermal process. Air is used as fluid.

September 1, 2013, 17:58
#4
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Filippo Maria Denaro
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Quote:
 Originally Posted by prom 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?

 September 2, 2013, 00:56 #5 New Member   Join Date: Jul 2013 Posts: 14 Rep Power: 4 https://docs.google.com/file/d/0B9tq...it?usp=sharing follow this link to see pressure contour

 September 2, 2013, 03:05 #6 Senior Member   Join Date: Dec 2011 Location: Madrid, Spain Posts: 133 Rep Power: 6 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. Last edited by michujo; September 3, 2013 at 03:06.

September 2, 2013, 03:51
#7
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Filippo Maria Denaro
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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

 September 2, 2013, 10:23 #8 New Member   Join Date: Jul 2013 Posts: 14 Rep Power: 4 yes I used k-epsilon model

 September 2, 2013, 10:43 #9 New Member   Join Date: Jul 2013 Posts: 14 Rep Power: 4 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...

 September 4, 2013, 01:15 #10 New Member   Join Date: Jul 2013 Posts: 14 Rep Power: 4 Dear Filippo Maria Denaro, Is it wrong if negative pressure comes. please explain it. kindly elaborate the concept of it. thank you...

 September 4, 2013, 03:50 #11 Senior Member   Filippo Maria Denaro Join Date: Jul 2010 Posts: 1,793 Rep Power: 24 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

 September 4, 2013, 09:56 #12 New Member   Join Date: Jul 2013 Posts: 14 Rep Power: 4 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

September 4, 2013, 10:07
#13
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Filippo Maria Denaro
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Posts: 1,793
Rep Power: 24
Quote:
 Originally Posted by prom 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

 September 4, 2013, 10:19 #14 New Member   Join Date: Jul 2013 Posts: 14 Rep Power: 4 I am very sorry for my stupid questions sir. I am a beginner. Thanks for all your replies.

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