diff from sphere - zero flow

jdb

I am new to Fluent and so may be asking obvious questions. I'm trying to model a sphere ("blob") of liquid contaminant immersed in a large volume of water. We want to begin with zero water velocity, and model simple steady state diffusion. I want the species to diffuse *out* of the surrounding "control volume" of water. I do not want the diffusing species to "fill up" that volume as if it had impermeable walls.

My problem is that the solution converges immediately - without iteration. Why is this?

Setup: Fluent 6.2.16 2DDP, axisymmetric, cutting through the blob. (picture a setting sun, half cut off by the bottom of your monitor!) Solver: Multiphase diffusion B/C: all boundaries pressure outlet

At first we tried this with three sides of the "control volume" box assigned as velocity inlets (zero velocity), with the fourth side a pressure outlet. But that kept diffusing species from leaving the control volume on 3 sides...

Thank you for your patience and advice.