Ansys Employee

You can set the heat source to be in cells that are wet, or have some value of the liquid(solid) material. However, you can't then stop heat being convected/conducted into adjacent cells. Yes, the free surface may slow down heat transfer, but if you're adding it into a cell with some gas present you've already got hot gas. 

Divergence can be caused all sorts of things. Check the mesh, source terms etc and review the flow & energy fields as the divergence occurs. That'll hopefully highlight what's going on.