Rob
Ansys Employee

OK. That makes sense: I'm sure someone else had a similar case on here recently.   


Think about the UDF logic, and how the VOF model works.  If you have a cell that's tagged for phase change you will have a cell with some vapour & some liquid.  Depending on the flow you could get a "neck" of liquid, so you have a few nearly adjacent cells with some liquid & gas. If this happens you can very quickly get a smudged region.