## Fluids

#### How to define two different pressure condition in fluent 17.1

• pawar002
Subscriber

How to define two different pressure condition in fluent, in my geometry i want to define one region as a vacuum.

• Konstantine Kourbatski
Ansys Employee

1) have you tried patching different pressure values in different parts of the domain? 2) Fluent cannot take zero absolute pressure (vacuum), so you'll need to specify near-vacuum conditions, 3) if you only care about conductive and radiative heat transfer inside vacuum, then you can simply fix the velocities to zero inside the vacuum zone and apply near-zero thermal conductivity. In this case pressure value inside the vacuum zone won't matter as only the enrgy equation will be solved

• pawar002
Subscriber

Actually, I am simulating a 3D model of evacuated tube collector. so there is no outlet. so I will be going to try lower thermal conductivity as you have mentioned above. also because of the complex geometry, it is hard for me to define the coordinates of geometry to register cells( in adapt/mark region). is there any difference in patching cells and patching regions? will it affect on the final result?

moreover, in the same model, I am using radiation, VOF, and solidification and melting together, because of the solidification and melting I have to keep the finer mesh but in VOF model global courant no. is crossing over 250. is it fine if I use the implicit method instead of the explicit method to avoid global courant number error?

• DrAmine
Ansys Employee

It is fine to use either implicit or explicit but a Global Courant number is a symptom that the time step size you are using based on your BC and input is quite enthusiastic.

• pawar002
Subscriber

could you please guide me how to mark cells to define volume fraction in the 3D model, as geometry is very complex it is hard for me to find out the coordinates. is there any other way to mark cells to define volume fraction.?

• DrAmine
Ansys Employee

You need to create cell registers to mark the cells where you want to patch.  Under Solution you RMB click on "Cell Registers" then "New" and select Region. The other way would be to go back to pre-processing and create a dedicated cell zone which will be filled at the start stage with the desired phase.