[Mnasso]

Hello, I have a problem modelling natural convection, I have a 3D complex solid (which has internal heat generation) which is inside a vertical tube, the tube has its inlet and outlet ends connected to an ambient and air circulates inside the tube cooling the solid.
We understand the setting and how natural convection it is modelled, the problems is with the fluid boundary conditions.

We need to estimate which is the air flow rate which establishes.

Regarding the boundary conditions:
1- the couple pressure inlet / flow outlet (velocity or mass flow rate) or flow inlet /pressure outlet are discarded as with them I will impose the flow.
2- outflow boundary conditions (that do not impose neither flow nor pressure) are also discarded as they are only compatible with mass/velocity inlets.
3- pressure inlet and pressure outlet are also discarded as this boundaries request that I impose the pressure in each of them, and if I impose the difference of pressure I indirectly impose the flowrate

Modelling the complete ambient so as the flow establishes as it wants it is also discarded as the part of interest of the model is quite big and we will be limited in computational resources.

So how can I set the boundary conditions in a model which has an clear inlet and outlet so i do not impose, directly nor indirectly, the flow rate???