CFD Online Logo CFD Online URL
www.cfd-online.com
[Sponsors]
Home > Forums > Software User Forums > ANSYS > FLUENT

Turbulent boundary conditions for bubble column

Register Blogs Community New Posts Updated Threads Search

Reply
 
LinkBack Thread Tools Search this Thread Display Modes
Old   October 14, 2005, 01:49
Default Turbulent boundary conditions for bubble column
  #1
anjai
Guest
 
Posts: n/a
Dear friends, I am trying to simulate 2-D axisymmetric bubble column, operating liquid in Batch mode.I am using k-e (dispersed)model to treat the turbulence.I don't have any idea about the turbulent boundary conditions(liquid phase) .Can anyone help me on this issue? Looking forward to ur reply..... Thanks anji
  Reply With Quote

Old   October 14, 2005, 16:37
Default Re: Turbulent boundary conditions for bubble colum
  #2
pUl|
Guest
 
Posts: n/a
There is no basis for my answer. However, please try using low values. Not 1 (the default) for k and e. Use say for instance, 0.1 for k and 0.25 for e. I repeat, there is no basis for my answer. Only thing I remember is that I saw this in some of the bubble column tutorials.
  Reply With Quote

Old   October 15, 2005, 05:51
Default Re: Turbulent boundary conditions for bubble colum
  #3
anjai
Guest
 
Posts: n/a
I really appreciate your suggestion ..I will try simulations with low values of k and e. Actually I am using U(liquid)=0 boundary condition at inlet(batch liquid) and if i use this velocity to calculate k that will become zero..can i use gas velocity to calculate k at inlet? Do u have any idea about tubulent outflow conditions( by nature of problem i will get reverse flow of liquid ) Thanks, anji

  Reply With Quote

Old   October 15, 2005, 10:33
Default Re: Turbulent boundary conditions for bubble colum
  #4
pUl|
Guest
 
Posts: n/a
The inlet velocity of water is zero. You are correct. When you go the Solve -> Initialize -> Initialize panel and try to initialize values from inlet, usually Fluent estimates a value of 1 for k and 1 for e to avoid startup trouble. Unfortunately however, even these values do not often work out. Also it will be incorrect to use the gas velocity to estimate inlet values of k and e as the gas moves significantly faster than the liquid when the column is in operation and remember that the k and e that you specify are for the liquid. So try out 0.1 and 0.25 and see what you get. A more practical approach would be to look into experimental data (for example, the liquid velocity profiles) and try to get an estimate for the average liquid velocity and use those to roughly estimate k and e. Well, for the outlet, if you are using a pressure outlet, choose intensity and hydraulic diameter as the turbulence specification method and use a backflow volume fraction of 1 and the hydraulic diameter of corresponding outlet (which for a bubble column is simply the whole diameter of the column (Imp. Note: Just because you are using an axi-symmetry boundary condition, you should not assume that the hydraulic diameter is half the column diameter; always input the actual diameter of the column for the hydraulic diameter as this is just a cylinder).
  Reply With Quote

Old   October 15, 2005, 12:46
Default Re: Turbulent boundary conditions for bubble colum
  #5
anjai
Guest
 
Posts: n/a
Dear friend, I got the trend of results with <8% differ from literature data.The values mentioned for k and e are working properly for my case.. Thank you very much for ur help..can i get the information regarding tutorials(bubble column) u'he mentioned in the first mail.. thanks

  Reply With Quote

Old   October 15, 2005, 13:19
Default Re: Turbulent boundary conditions for bubble colum
  #6
pUl|
Guest
 
Posts: n/a
It is nice to know that you are able to predict reasonably good agreement with experimental data. I wish to know however, what parameters have you compared? For instance is it the:
a. Gas velocity profiles
b. Liquid velocity profiles
c. Gas holdup profiles
d. Average gas velocity
e. Average liquid velocity
f. Average gas holdup
Here are some of the tutorials. Note that some of them were prepared for Fluent 4; nevertheless you should be able to understand most of the input easily.
1. (Partially Aerated Bubble Column) http://www.fluentusers.com/fluent45/...tml/node97.htm
2. (Fully Aerated Bubble Column) http://www.fluentusers.com/fluent45/doc/doc_f.htm
3. (Hydrodynamics of Bubble Column Reactors) http://learningcfd.com/login/fluent/...ble-column.pdf
  Reply With Quote

Old   October 15, 2005, 13:20
Default Re: Turbulent boundary conditions for bubble colum
  #7
pUl|
Guest
 
Posts: n/a
Sorry, the second link is:

http://www.fluentusers.com/fluent45/...ml/node108.htm
  Reply With Quote

Old   October 16, 2005, 00:38
Default Re: Turbulent boundary conditions for bubble colum
  #8
anjai
Guest
 
Posts: n/a
I have compared the time averaged profiles of axial liquid velocity and gas holdup...Thanks for sending the links..Actually i am accessing fluent through the institue license so i don't have user name and password to view those tutorials..anyway i will try to get those from our representative...thank u very much anji
  Reply With Quote

Old   October 16, 2005, 02:35
Default Re: Turbulent boundary conditions for bubble colum
  #9
anjai
Guest
 
Posts: n/a
In addition to axial liquid velocity and gas holdup I also have to compare the timeaveraged kinetic energy/unit volume(dyne/cm2)..do u have any suggestions to do this? I have written a UDF using ( c_k(c,t)/c_VOLUME(c,t) )..but i have doubt about the units and the volume to be used in the denominator .I am thankful to your continuous help... anji
  Reply With Quote

Old   October 16, 2005, 02:44
Default Re: Turbulent boundary conditions for bubble colum
  #10
anjai
Guest
 
Posts: n/a
How can i calculate the turbulent intesity at the outlet?.. Thanks
  Reply With Quote

Old   October 16, 2005, 11:49
Default Re: Turbulent boundary conditions for bubble colum
  #11
pUl|
Guest
 
Posts: n/a
Use a custom field function:

Define turbulence_intensity as sqrt(2*k/3)/velocity_magnitude_water

where 'k' is the turbulent kinetic energy of water.
  Reply With Quote

Old   October 17, 2005, 06:38
Default Re: Turbulent boundary conditions for bubble colum
  #12
anjai
Guest
 
Posts: n/a
Hi, From my knoweldge custom field functions can be used only for intialization(patch) or for plotting the results.I don't know the procedure of using custom feild function as boundary condition.Can u please help with this problem.. Thanks
  Reply With Quote

Old   October 17, 2005, 07:34
Default Re: Turbulent boundary conditions for bubble colum
  #13
Raju
Guest
 
Posts: n/a
Hi, Turbulent Intensity is availble in the standard feild functions defined by fluent..so we no need to define it again i think... anji
  Reply With Quote

Reply


Posting Rules
You may not post new threads
You may not post replies
You may not post attachments
You may not edit your posts

BB code is On
Smilies are On
[IMG] code is On
HTML code is Off
Trackbacks are Off
Pingbacks are On
Refbacks are On


Similar Threads
Thread Thread Starter Forum Replies Last Post
mesh file for flow over a circular cylinder Ardalan Main CFD Forum 7 December 15, 2020 14:06
Turbulent Boundary Layer on a Flat Plate Hoshang Garda FLUENT 1 November 27, 2013 11:24
Water subcooled boiling Attesz CFX 7 January 5, 2013 04:32
Turbulent Boundary Conditions cfd seeker FLUENT 2 June 24, 2011 03:28
Help with boundary conditions Dan CFX 0 April 3, 2006 12:32


All times are GMT -4. The time now is 03:08.