I have to simulate a emptying tank, with a bi-phasic fluid inside (while emptying the pressure falls down and when reach the saturation pressure the fluid starts to evaporate). As boundary condition i have only a fixed mass outflow.

I tried with Star-cd but i haven't got any results, so i'm now using fluent. Do you think it is better?

u can work with fluent on this one but the evaporation has to be implemented by a UDF I feel... look at phase chnege tutorials to understand how to do this.