Water flow due to natural convection in vertical pipe
 

Hello dear colleagues,

I have simple problem, but I don't know how to solve it. I have a pipe, which side walls are heated. I would like to have top and bottom walls opened - it means 0 m/s velocity in the beginning of simulation. Due to the heat, water density should decrese near pipe walls and water starts to flow upward. Meanwhile, feedwater/new water from the bottom wall starts to flow inside the pipe due to natural convection. So, I set top and bottom wall as pressure-outlet, but water just fall down from top wall through the pipe and bottom wall. See Fig.:
https://scontent.fvno3-1.fna.fbcdn.n...91&oe=5B59FFB3

How to set conditions for top and bottom walls, that water through the bottom wall start to flow due to natural convections, which occurs inside a pipe?

Thank You.
Regards.