Hi John, I am working on a very similar problem to the one you are describing. I am using Fluent instead of CFX and I have a column of water on top of a heated wall attempting to model evaporation and the interface movement. (See my previous posts) I observe either the interface remaining fixed similar to what you have or a flash boiling but no natural evaporation from the interface. I believe I need to restrict the mass transfer to just the cells at the interface and have a UDF to track the volume of fluid across the domain. Would be happy to discuss approaches if you are interested.