## Fluids

#### Reaction in multiphase domain. Should the phase volume fraction be consider in the source term?

• michele rocchi
Subscriber

Dear Support,

I would like to clarify a doubt that I have regarding reaction in a multiphase domain.

Considering a domain where a disperse phase is present, and a reaction ongoing only in the continuous phase (a variable composition mixture)

Given the rate of consumption or production of a species per unit volume, when inserting the expression in the fluid source of the subdomain used to model the reaction, should the value be multiplied by the local volume fraction of the phase in which the reaction occur?

In other word given the value "r" [kg/m^3/s]  and the element volume in the domain "v" [m^3] and the volume fraction of the continuous phase "a" [vol/vol] is CFX calculating the consumption or the production of a specie as "dmi/dt" [kg/s] as dmi/dt=r*v or dmi/dt=a*r*v?

BR

Mauro

• Karthik R

Hello Mauro,

Since this is a CFX question, I am moving this to the 'Fluid Dynamics' thread so it gets better traction.

Thank you.

Best Regards,

Karthik

• raul.raghav
Subscriber
Since the reaction occurs only on the continuous phase, you can define the continuity source term using the Fluid Source tab inside the subdomain. If you created the subdomain based on the entire domain, select the “Total Fluid Mass Source” in the continuity source term definition and assign “r” as its value. The volume “v” of the subdomain will be calculated internally. In other words, your “dmi/dt” would be “r * v”.

Read the quoted text below carefully to make a sound decision. Quoting text from CFX User Guide:

“Fluid-specific sources are applied directly to fluid as specified and are not multiplied by volume fraction; that is, the sources are assumed to be specified per unit volume of geometry, not per unit volume of the phase. If you have a source expression that is per unit volume of the phase, you must either multiply the source by volume fraction or else use Bulk Sources, which are discussed below.

Fluid-specific sources are not available for equations in homogeneous multiphase flow that span all fluids; Bulk Sources are the only appropriate option for these situations.”

• michele rocchi
Subscriber

Dear Rahul

thank you for the clear answer

KR

Mauro