In the Pre-Analysis step, we'll review the following:
Since we are neglecting the convection terms, the diffusion equation we want to use looks nearly identical to the heat equation. Therefore, we can solve the heat equation with the flow terms disabled and then simply reinterpret the results as if we had solved the diffusion equation.
The outer boundary will be set to “wall.” Then we will set a zero-flux boundary condition, which corresponds to an impermeable wall when discussing diffusion.
The horizontal boundary will be set to "axis," which will impose zero gradients perpendicular to the boundary and will tell Fluent that the full domain is rotated about this axis. Note that Fluent only allows axisymmetric rotation about the x-axis.
The vertical boundary will be set to “symmetry,” which will impose zero gradients perpendicular to the boundary. Note that Fluent allows symmetric boundary conditions along any axis.
Please see the handout below for more details: