Gas Dispersion in Open Pond

park.1148 · December 18, 2013, 16:27

Hello.

My name is Stephen, and I am a CFD newbie. This is my first post - I hope I can contribute as much as I get help from all of you. If I am missing any information in my descriptions, please let me know and I will be happy to provide them to clarify anything.

I am trying to model the gas dispersion from a bubbling sparger located at the bottom of an open pond. My geometry is set up so that I have air bubbles entering the system through 147 holes in a sump (inlet), and exiting the system via the pond surface (outlet). I followed Chapter 20 of the Tutorial Guide (Using the VOF Model) to model the 2-phase flow.

My primary phase is water, which the pond is full of. My secondary phase is air, coming in through the 147 holes with a fixed velocity. I used the VOF mixing model with surface tension (72 dyn/cm) as my only interaction. My gas phase proportion was set to 1 at the inlet. I also added the energy model and standard k-epsilon (SKE) turbulence model. Aside from my inlet gas velocity, the other boundary conditions is that my pond surface is a pressure outlet and all other fluid boundaries are no-slip walls.

Here are the problems I am experiencing.

1. When I initially tried to run the calculation, I get an error message stating "Global Courant number is greater than 250.00 The velocity field is probably diverging. Please check the solution and reduce the time-step if necessary. Error Object: #f"

Even after reducing the time step, I saw the same message. The solver recommended that I use the realized k-epsilon (RKE) instead of SKE, and when I did so, I did not see the Courant number error, but the solution still did not converge. The Courant number remained around 30.

How would I go about helping my system to converge better (faster)?

2. My time step size is set at 0.01 s, and it is taking about 1 minute per time step calculation (iteration). Any ways in speeding up the process? My mesh is composed of 280,400 elements, so that may be an issue...

Thank you for keeping up with my rambling!

Stephen P.

Zaktatir · December 19, 2013, 12:43

I guess the volume fraction of your gas phase is bigger than 10% i will suggest to use mixture model with assuming local equilibrium over short spatial length scale between the phases. There you can use an algebraic formulation for the slip velocities between the phases. This may be appropriate for your problem if you have particle relaxation time smaller than 0,1 s and no counter-current flow.

Concerning the problems that you are facing that is normal to use smaller time steps in VOF where we are applying tracking or capturing methods to resolve the interface.

December 18, 2013, 16:27	Gas Dispersion in Open Pond	#1
park.1148 New Member Join Date: Dec 2011 Posts: 4 Rep Power: 14	Hello. My name is Stephen, and I am a CFD newbie. This is my first post - I hope I can contribute as much as I get help from all of you. If I am missing any information in my descriptions, please let me know and I will be happy to provide them to clarify anything. I am trying to model the gas dispersion from a bubbling sparger located at the bottom of an open pond. My geometry is set up so that I have air bubbles entering the system through 147 holes in a sump (inlet), and exiting the system via the pond surface (outlet). I followed Chapter 20 of the Tutorial Guide (Using the VOF Model) to model the 2-phase flow. My primary phase is water, which the pond is full of. My secondary phase is air, coming in through the 147 holes with a fixed velocity. I used the VOF mixing model with surface tension (72 dyn/cm) as my only interaction. My gas phase proportion was set to 1 at the inlet. I also added the energy model and standard k-epsilon (SKE) turbulence model. Aside from my inlet gas velocity, the other boundary conditions is that my pond surface is a pressure outlet and all other fluid boundaries are no-slip walls. Here are the problems I am experiencing. 1. When I initially tried to run the calculation, I get an error message stating "Global Courant number is greater than 250.00 The velocity field is probably diverging. Please check the solution and reduce the time-step if necessary. Error Object: #f" Even after reducing the time step, I saw the same message. The solver recommended that I use the realized k-epsilon (RKE) instead of SKE, and when I did so, I did not see the Courant number error, but the solution still did not converge. The Courant number remained around 30. How would I go about helping my system to converge better (faster)? 2. My time step size is set at 0.01 s, and it is taking about 1 minute per time step calculation (iteration). Any ways in speeding up the process? My mesh is composed of 280,400 elements, so that may be an issue... Thank you for keeping up with my rambling! Stephen P.

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December 19, 2013, 12:43		#2
Zaktatir Senior Member Join Date: Jan 2010 Location: Germany Posts: 268 Rep Power: 17	I guess the volume fraction of your gas phase is bigger than 10% i will suggest to use mixture model with assuming local equilibrium over short spatial length scale between the phases. There you can use an algebraic formulation for the slip velocities between the phases. This may be appropriate for your problem if you have particle relaxation time smaller than 0,1 s and no counter-current flow. Concerning the problems that you are facing that is normal to use smaller time steps in VOF where we are applying tracking or capturing methods to resolve the interface.