In flow boiling, the fluid enters the heated tube with certain values of vapor quality x and velocity, and due to the heating process, the vapor quality value increases, therefore, the fluid velocity varies along the tube as a result of varying the fluid phase (consequently, properties).

What is preferred;

(1) calculating Re, friction factor ..... etc, based on inlet properties?

or

(2) calculating Re, friction factor ..... etc, at mean properties and mean vapor quality assuming linear variation of x?