We need more information to answer your question. What do you mean by "dryness fraction"? Are you referring to moist air and relative humidity or steam or some mixture of substances? What vapor? Are you evaporating some liquid or are you referring to moist air or some drying process? What kind of heat pipe? What do you expect this heat pipe to do?

As Dudley says, the question is not clear; an unclear question has an unclear answer.