evaporation on the surface of liquid nitrogen

dodo

i want to simulate a open tank in which liquid nitrogen is partially filled.when heat from outside is applied on the wall,natural convection occurs and evaporation takes place at the interface(surface).

1,how to use udf to simulate evaporation in this problem?i have got a udf code including three parts source,but in this problem,the evaporation only occurs at the interface and the interface changes always.

2.how to add the initiative interface position to fluent,if the liquid nitrogen is filled half

thx,dodo