## Fluids

#### Defining constant density for mixture

• Chris1opher
Subscriber

Dear all,

I am modeling a mixture in a closed reactor so that the volume can not change. As mass conservation applies, the density can not change.

It is not possible to choose [constant] as the density property of my mixture, when I define the physical properties of the mixture. As a workaround, I could use a mixing law and use the same density for each pure component. I wonder if there is a reason that I cannot choose [constant] as the density property for my mixture.

Kind regards.
Christopher

• Rob
Ansys Employee

If you're in Fluent check the density for each/all of the materials - are any variable? I'm pretty sure you can set a constant for a mixture density.

• Chris1opher
Subscriber

I just checked fluent and attached a screenshot.
No [constant] is available when i want to define the density.
It is possible to choose constant for other parameters like viscosity

• Rob
Ansys Employee

And what are the densities of the various mixture materials?

• Chris1opher
Subscriber

I want my mixture to have always the same density no matter of the composition.

The density is 1042 kg/m^3

When I'm specifying the viscosity of the mixture as constant I'm no longer able to edit the viscosity of the pure materials. I was expecting that the same thing works for the density.

• vinitraj3
Subscriber

For that you need to define the density of each component of mixture as 1042 kg/m^3. Then in the mixture panel you can specify the density as volume weighted mixing law. It will always return 1042 as the density of mixture

Regards

Rajeev

• Chris1opher
Subscriber

Do you know why there is no possibility to set the density of the mixsture as constant like it is possible for other proberties?

• Rob
Ansys Employee

There's probably some maths that needs a mixture value. Normally we'd use ideal gas for a sealed system where heat was expected otherwise you'll miss any buoyancy effects.