Similarly
the correlations of Cai et al. [27], Hasan [28] and Goda et al.
[29] were developed for vertical downward orientation, yet can be
used to predict the void fraction in upward orientation with a sheer
flip of the drift velocity sign from negative to positive. The correlation
of Woldesemayat and Ghajar [30] could be used for both upward
and downward flow since it incorporates the inclination
term in its physical structure and automatically flips the sign of drift
velocity with change in the orientation.