Learning Forum

[SR786]

Hi,

I want to implement a population balance model in fluent via solving for species transport and user defined scalars (where scalar quantity will be moments). My framework is homogeneous VOF with two phases, gas and liquid. Both species and UDS will be solved for liquid phase only. From my understanding of the manual, Fluent solves the conservative form of the transport equations as defined in https://www.afs.enea.it/project/neptunius/docs/fluent/html/th/node14.htm and https://www.afs.enea.it/project/neptunius/docs/fluent/html/th/node345.htm. For species transport I will be solving the following

My source term will therefore be:

Which has units consistent with the LHS of the equation.

For the UDS, the transport equation is 

  

Where source term will be 

With units consistent with LHS

My question is that when it comes to implementing this in UDF file do I need to multiply by alpha (volume fraction) or is this automatically handled in Fluent.

i.e.

For Species:

S = alpha*S_c

For UDS:

S = alpha*rho_liquid*S_mj

 

 

[Mark O]

Hi, for a phase specific species the source term is automatically multiplied by the volume fraction. But for a phase specific UDS the source term is not multiplied by the volume fraction. If you want it to be multiplied by the volume fraction you will have to do that yourself for the source. So if you want your sources to be proportional to the volume fraction then for the species source you do not multiply by alpha but for the UDS source you do.

 

[SR786]

Hi Mark,

Thank you for the reply. So for species it is automatically handled by Fluent solver but for UDS I will need to multiply myself.

Also as the conservation equations all have density multiplication factor, do I need to multiply by density for UDS soruce term as well for dimensional consistency?

[Mark O]

Yes, the simple rule is to just look at the transient term on the left and match it to the source term on the right. For species, alpha is already matched up. For UDS is not. For the density is just depends on how you calculated the source in the first place. If you calculated it as a rate of change of variable then multiply by the density, but if you calculated it as a mass rate per unit volume then it already has the density built-in.

[SR786]

Hi,

Thank you very much for the clarifications!