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-   -   Density and pressure correlation (http://www.cfd-online.com/Forums/fluent/95721-density-pressure-correlation.html)

shk12345 December 30, 2011 03:00

Density and pressure correlation
 
Hi friends

i am trying to simulate a nozzle in which i am using a pressure inlet for air and nitrogen. The inlet pressure is 7 bars.The outlet pressure is 0 atm guage.
I am using a multiphase as water as the secondary phase.
I am using pressure based solver and transient simulation

1) The mass flow rate is to high as compared to experimental.
2) The velocity is to high as compared to calculated.
3) The density is not varying according to the change in pressure .

Kindly shed some light in enlightening my knowledge in the above described field. And tell me what is to be done so that i can get appropriate mass flow rate and velocity. And how to change the density as accordance with the change in pressure .

goami December 31, 2011 14:23

I think shift from pressure based to density solver make some differenc......

duri January 2, 2012 06:16

Select ideal gas for density model. Check whether exit pressure is matching with experiment. If not you solution could be correct. You may need to adjust your boundary condition to get you flow rate, or use target mass flow rate in pressure outlet boundary.

shk12345 January 2, 2012 11:37

Quote:

Originally Posted by goami (Post 337475)
I think shift from pressure based to density solver make some differenc......

thanks for replyingh but i am using multiphase model as i have tried that but
density based solver doesn't work in the case of multiphase solvers.

shk12345 January 2, 2012 11:41

Quote:

Originally Posted by duri (Post 337537)
Select ideal gas for density model. Check whether exit pressure is matching with experiment. If not you solution could be correct. You may need to adjust your boundary condition to get you flow rate, or use target mass flow rate in pressure outlet boundary.

Thanks for replying
As i am selecting the ideal gas t he solution is diverging after few itterations.
I know the internal pressure but exit pressure is atmospheric.
I tried all the boundary conditions velocity inlet, mass flow inlet and also pressure inlet.
I also have tried the target mass flow at the pressure outlet but that option is there only in single phase and not in multiphase.

I need more new suggesion friends i hope that someone can give me sugeestion and my problem may be solved.

thanks

ComputerGuy January 2, 2012 21:36

Are you calculating pressure and temperature-dependent properties? How are the velocities compared to the acoustic velocity of the fluid? How large a conduit (diameter and length) are you taking a 7 bar drop across?

Make sure you aren't exceeding choked-flow conditions (http://en.wikipedia.org/wiki/Choked_flow) due to improperly-calculated fluid properties.

Give us a hint at the type of system you're simulating. the mix of air and nitrogen (remember, air is ~79% nitrogen), and geometries involved.

ComputerGuy

shk12345 January 3, 2012 10:14

Quote:

Originally Posted by ComputerGuy (Post 337606)
Are you calculating pressure and temperature-dependent properties? How are the velocities compared to the acoustic velocity of the fluid? How large a conduit (diameter and length) are you taking a 7 bar drop across?

Make sure you aren't exceeding choked-flow conditions (http://en.wikipedia.org/wiki/Choked_flow) due to improperly-calculated fluid properties.

Give us a hint at the type of system you're simulating. the mix of air and nitrogen (remember, air is ~79% nitrogen), and geometries involved.

ComputerGuy

My problem is with pressure and velocity first then comes density and temperature correlation.
The velocities as calculated by me are not in acoustic range they are around 25-30 m/s.
the length of the tube is 1.2 m approx. But there is a outlet nozzle which is giving a pressure drop of 3-4 atm. The other pressure drop is due to the converging section in between the nozzle. As that pressure is then making the water to splash into the droplet after the nozzle portion.
My system is 1 air inlet, 2 nitrogen inlets and 4 water inlets.
i am giving the pressure of 5.78 atm at air and nitrogen inlet and a velocity boundary condition at the water inlet.

Kindly comment


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