various geometric definitions are used for specifying the void fraction local ,chordal ,cross-sectional and volumetric,which are represented schematically .the local void fraction refers to that at a point or very small volume when measured experimentally and thus void fraction local as 0,when liquid is present and void fraction local as 1 when vapor is present. Typically ,the local time-averaged void fraction is cited , or measured using a miniature probe, which represents the fraction of time vapor ,was present at that location in the two-phase flow.