can any one help me in defining boundary conditions

my problem is natural convection over heat rod bundles in the axial direction so i do not have have any initial velocity and the fluid which is flowing has to enter the domin with a known temperature because of the buyoncy effect and has to leave the domin. like the it is a through flow not an encloser flow.

for that i have taken a representative element for which one wall whose temperature i know and other side walls of the domin are symmetry but in the specification of top and bottom wall of the domine i have specified as pressure outlet (both).

the back flow temperature of the both the pressure outlet i gave the known temperature but i have still convergance difficulty

i have used standard k-e model for the specification of turbulance

can any one help in this regard??? please

Did you activate gravity in the operating conditions?

Which material is flowing? air?

Which density 'method' did you specify? With 'constant' density you will never get buoyancy.

Why do you specify top and bottom boundaries both as outlets? From where should the flow come when there are only outlets?

the flowing material is liquid sodium and i have used boussnique's approximation for the buoyancy. my big problem is i don't know which is outlet and which inlet i mean if the rod temperature is less than the fluid temperature top one will become inlet and bottom will become outlet and if it is heating case the reverse will happen that is why i have specified as both the openings as pressure outlets as i know the temperature with which the fluid is entering, and it is doing the flow is reversing from heating case to cooling case.