you should set the operating pressure near zero(in operating condition)

Thanks for reply Montana.

Actually I am patching liquid in some portion then it's automatically taking rest of the region as other defined phase. Instead I need to have vaccum in that region.

Any solution to this?

you can mark region which you need in adapt window of fluent(without press on adapt,only mark the region). after that you can patch separately for each region.

hello, I would like to ask the question in another way. in very low pressure the viscosity of fluid is low so that naiver stokes equation doesn't work well. So how should we simulate flow in vacuum systems?

Mojtaba Mokhtari I have one question here.

Do you have any idea how to create a vacuum in OpenFoam? Do I just need to put the zero pressure or something else?

Suppose I have one rectangular region (the blue region in the attached figure), and inside it is a heater (green region in the attached figure). I want to create the blue region a vacuum, so that I can simulate the radiative heat transfer in a vacuum.

I shall be very thankful if you can help me in this.

Thank you