Meaning of Porosity
 

Dear all,

This query is regarding liquid redistribution in a packed column, where as an intrinsic property, the porosity of the packed bed varies along the radial distance (ending up to almost unity at the walls). Can we define the porosity to vary like this in FLUENT? With reference to some previous suggestions, I tried to divide the bed radially into discrete zones, with different porosities, and then ran the simulation for a countercurrent Gas-Liquid flow. But whatever be the value of the porosities in the different zones, the velocity patterns, the gas phase fractions etc. do not seem to change at all. What I had presumed is that due to flow restrictions in a porous zone, as compared to an unpacked zone, the velocity profiles should have changed. Where am I wrong?

Sumi.

Re: Meaning of Porosity
 

Hi,

I also got confused about how Fluent treat porosities. I checked the manual and did not find any information of the way that Fluent uses porosity. Anybody has any idea on this? Thanks a lot.

Steven

Re: Meaning of Porosity
 

I assume you're referring to the porosity of a porous medium in FLUENT. The FLUENT porous media model only adds a sink term to the momentum equation, it doesn't constrict the flow physically. All calculations are performed using superficial velocities, not actual velocities. Other than the extra momentum sink, porous zones are identical to fluid zones.

Re: Meaning of Porosity
 

hi, sumi.

in past versions of FLUENT only superficial velocities could be computed in porous regions. functionality is now available in the latest FLUENT release, 6.1, to compute physical (interstitial) velocities in porous zones. this allows fluid acceleration due to constriction in the porous media to be considered. in FLUENT 6.1, porosity may also be specified as a spatially varying function.

eric.

Re: Meaning of Porosity
 

That is news. Can porosity be specified through a UDF ? Has that facility come yet ?

Murali

Re: Meaning of Porosity
 

hi, murali.

yes. spatially varying porosity has to be defined via UDF. the profile definition is:

DEFINE_PROFILE(por, t, nv)

eric.

Re: Meaning of Porosity
 

Sorry to be a tough ass. But does this mean that, we can "solve" for porosity using a UDS ? coz, there are models where we have solids depositing thus altering the porosity of the model in time and space. We would then have an eqn similar to:


de
-- = f(u,x,t)
dt



where u is the set of other dependent variables, x is the space vector and t time. Can this be solved by specifying the DEFINE_UDS_FLUX as zero and be solved along with other variables ?

Murali

Re: Meaning of Porosity
 

Hi eric. The version of Fluent which I am using is Fluent 6.0. Do you mean that I cannot do such calculations with the interstitial velocities in the pore constrictions in this version? That means that I have to wait for the Fluent 6.1 version? When has that been released?

Sumi

Re: Meaning of Porosity
 

hi, sumi.

you are correct. you cannot do such calculations with the interstitial velocities in the pore constrictions in FLUENT 6.0. you need to use FLUENT 6.1. FLUENT 6.1 was released in February 2003, and is available for FTP download from our User Services Center

www.fluentusers.com

eric.

Re: Meaning of Porosity
 

possibly, murali. i'll admit i'm not an expert with UDSs and UDFs, but i think this sort of approach would be worth investigating.

i would encourage you to contact your local technical support representative to discuss your particular needs in more detail.

eric.