## Fluids

Topics relate to Fluent, CFX, Turbogrid and more

#### Unit source and mass transfer

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• prebenjs
Subscriber

What is stated in the manuals is that both source and mass transfer is given in kg/(m^3 *s). Taking a closer look on the mass-transfer, let's say that I want to specify the rate in m/s, or kg/s ? How do you calculate this and what density should you take into account ? This question is also valid for the source, however I assume one just uses the density of the phase produced / removed here ?

• KR
Hello Fluent is a finite volume code. So when you are specifying source terms, you need to account for the local cell volume so that when you integrate your source term over the cell volume, you obtain the desired value in either W or kg/s. This is the reason why you see these units in the manuals.
Karthik
• DrAmine
Ansys Employee
Density of incoming phase also called upwind Density yo get speed after you first calculate the mass transfer flux.
But safer to just multiply with cell volume to have kg/s as integrated mass transfer rate.
• prebenjs
Subscriber
So when you say density of incoming phase, you mean density of the phase that is after the mass transfer?
Let's see if I understand this correctly, If I want to specify the mass transfer in a cell register in kg/s I just take the mass flow rate I want out of the cells and divide this by the amount of cells in the register, then divide this value with the volume of the cells (doing this for every cell. Which in mathematical terms would be:
kg/m^3*s= (Wanted flow rate / amount of cells which flow rate should happen) / volume of the given cell
And then the mass transfer rate for the whole cell register combined would be my wanted flow rate ? Wouldn't this also depend on the density of the phase before and after the mass transfer ?

Furthermore, If i want to set a sink term where a set amount of mass is removed in the cell_register defined, would this amount to only be "wanted flow rate (kg/s) * volume of cell".
I'm a little bit confused here. And also clarifications if it's possible to do this with a wanted m/s is happily received.
• DrAmine
Ansys Employee
Kg/s is easy we multiply with cell volume. Speed is bit complicated as you need to have interfacial area and then the upwind density.
Water liquid evaporation: upwind density is the liquid density interfacial area will be gradient of vof if using resolving mesh or any other formulation if Eulerian disperse.
• prebenjs
Subscriber
Okay, but lets say I have 300 cells and I want to have a combined kg/s, then i need to do ([kg/s]/300) * C_V(c,t) ? Let's say this is implemented in a mass sink term, do you need to apply a correction to the momentum sink as well ? (As i've understood you should always implement momentum sink when implementing mass sinks?)
And how is this accounted for in the mass transfer rate, will you have the wanted kg/s by just multipling with the volume of the cell if the densities in the phases differ ?
Regarding velocity, is it just to take ?:
wanted mass flow rate * C_R(c,thread_to_phase_before_transform)*C_VOF_G(c, mixture_thread)

• prebenjs
Subscriber
Any suggestions?
• DrAmine
Ansys Employee