Bubble column - Eulerian Eulerian

oj.bulmer · June 4, 2012, 13:56

Hello

I am a university student and working on my summer project which is about bubble column and I am using Ansys CFX 13.
Following are the details:

Mesh:
Axisymmetric, Hex elements

Model:
Eulerian Eulerian mono dispersed

Boundary conditions:
Inlet: Gas - Mass flow rate of gas 28 kg/s, volume fraction 0.1
Liquid - Normal speed 0, volume fraction 0.9

Outlet:
Degassing

Symmetry:
Two symmetric boubdaries and tip nipped at axis where I specified free slip boundary

Initialization: Gas - Velocity automatic, volume fraction automatic
Liquid - Velocity automatic, volume fraction automatic

Monitors: Effective density, Gas volume fraction, Pressure at some
internal points and Maximum velocity at outlet

As a result, I get convergence as shown in figure 1 (Monitors except velocity flat), but volume fraction everywhere is 1 in the contour plot as seen in CFD post!!

I changed the initialization of liquid to volume fraction 1. But then pressure monitor oscillates with least and max value being -25 bar and 0 bar and won't go flat.

Both the scenarios are unphysical and I dont seem to know what is going wrong.

Regards
OJ

ghorrocks · June 4, 2012, 19:57

Have you considered this FAQ: http://www.cfd-online.com/Wiki/Ansys...gence_criteria

oj.bulmer · June 5, 2012, 10:26

Thanks Glenn for quick reply.

Yes, I have tried all options for steady state simulation.
I found out approximate residence time and set value of physical timescale accordingly (I tried 3/4th, haldf, quarter of it). Since this was unsuccessful, I used local timescale factor as 5. Though the residuals are dropping and monitors seemed flat, still I get volume fraction of 1 everywhere in CFD post. Further, I found pressures at the bottom portion of the tank of the order of -300000 Pa. This can not be true given the tank is 15 m high and buoyancy is enabled, meaning there will be hydrostatic head.

Do you see I am missing something? Gas volume fraction of 1 everywhere doesnt make sense, as one of the tutorials in CFX has inlet air volume fraction more than my case, and yet the air volume fractions everywhere (except region just above sparger) are negligible.

I believe that some form of steady state (and physical) results should be possible for this simulation, before discarding SS approach entirely.

Regards
OJ

ghorrocks · June 5, 2012, 20:33

Rather than trying a smaller time step try a larger one. This is described on the FAQ.

June 4, 2012, 13:56	Bubble column - Eulerian Eulerian	#1
oj.bulmer Senior Member OJ Join Date: Apr 2012 Location: United Kindom Posts: 473 Rep Power: 20	Hello I am a university student and working on my summer project which is about bubble column and I am using Ansys CFX 13. Following are the details: Mesh: Axisymmetric, Hex elements Model: Eulerian Eulerian mono dispersed Boundary conditions: Inlet: Gas - Mass flow rate of gas 28 kg/s, volume fraction 0.1 Liquid - Normal speed 0, volume fraction 0.9 Outlet: Degassing Symmetry: Two symmetric boubdaries and tip nipped at axis where I specified free slip boundary Initialization: Gas - Velocity automatic, volume fraction automatic Liquid - Velocity automatic, volume fraction automatic Monitors: Effective density, Gas volume fraction, Pressure at some internal points and Maximum velocity at outlet As a result, I get convergence as shown in figure 1 (Monitors except velocity flat), but volume fraction everywhere is 1 in the contour plot as seen in CFD post!! I changed the initialization of liquid to volume fraction 1. But then pressure monitor oscillates with least and max value being -25 bar and 0 bar and won't go flat. Both the scenarios are unphysical and I dont seem to know what is going wrong. Regards OJ

Similar Threads
Thread	Thread Starter	Forum	Replies	Last Post
Bubble Column Modelling	dlaw	FLUENT	24	October 31, 2017 07:01
bubble column	ken	FLUENT	1	June 30, 2013 02:18
Modelling a Water-Air bubble column	GerhardHolzinger	OpenFOAM Running, Solving & CFD	1	February 28, 2012 05:12
bubble rising in a column	swamysrikanth	Main CFD Forum	2	September 27, 2010 09:59
Help required for CFD simulation of Trayed Bubble Column using Fluent	art705	Main CFD Forum	0	July 15, 2009 05:04

June 4, 2012, 19:57		#2
ghorrocks Super Moderator Glenn Horrocks Join Date: Mar 2009 Location: Sydney, Australia Posts: 17,870 Rep Power: 144	Have you considered this FAQ: http://www.cfd-online.com/Wiki/Ansys...gence_criteria

June 5, 2012, 10:26		#3
oj.bulmer Senior Member OJ Join Date: Apr 2012 Location: United Kindom Posts: 473 Rep Power: 20	Thanks Glenn for quick reply. Yes, I have tried all options for steady state simulation. I found out approximate residence time and set value of physical timescale accordingly (I tried 3/4th, haldf, quarter of it). Since this was unsuccessful, I used local timescale factor as 5. Though the residuals are dropping and monitors seemed flat, still I get volume fraction of 1 everywhere in CFD post. Further, I found pressures at the bottom portion of the tank of the order of -300000 Pa. This can not be true given the tank is 15 m high and buoyancy is enabled, meaning there will be hydrostatic head. Do you see I am missing something? Gas volume fraction of 1 everywhere doesnt make sense, as one of the tutorials in CFX has inlet air volume fraction more than my case, and yet the air volume fractions everywhere (except region just above sparger) are negligible. I believe that some form of steady state (and physical) results should be possible for this simulation, before discarding SS approach entirely. Regards OJ

June 5, 2012, 20:33		#4
ghorrocks Super Moderator Glenn Horrocks Join Date: Mar 2009 Location: Sydney, Australia Posts: 17,870 Rep Power: 144	Rather than trying a smaller time step try a larger one. This is described on the FAQ.