# Density and pressure correlation

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 December 30, 2011, 03:00 Density and pressure correlation #1 Senior Member   shoeb khan Join Date: Nov 2011 Posts: 179 Rep Power: 5 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 .

 December 31, 2011, 14:23 #2 New Member   goami Join Date: Dec 2011 Posts: 6 Rep Power: 5 I think shift from pressure based to density solver make some differenc......

 January 2, 2012, 06:16 #3 Senior Member   duri Join Date: May 2010 Posts: 130 Rep Power: 7 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.

January 2, 2012, 11:37
#4
Senior Member

shoeb khan
Join Date: Nov 2011
Posts: 179
Rep Power: 5
Quote:
 Originally Posted by goami 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.

January 2, 2012, 11:41
#5
Senior Member

shoeb khan
Join Date: Nov 2011
Posts: 179
Rep Power: 5
Quote:
 Originally Posted by duri 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.
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

 January 2, 2012, 21:36 #6 Senior Member   Real Name :) Join Date: Jan 2010 Location: United States Posts: 180 Rep Power: 7 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

January 3, 2012, 10:14
#7
Senior Member

shoeb khan
Join Date: Nov 2011
Posts: 179
Rep Power: 5
Quote:
 Originally Posted by ComputerGuy 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.

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