Thus the original void fraction correlations evaluated here are assigned positive drift velocity values for vertical upward orientation and negative values for downward orientation. The correlations of Gomez et al. [22], Nicklin et al. [23], Rouhani and Axelsson [24], Bonnecaze et al. [25] and Sun et al. [26] were developed for vertical upward flow but could be used for the downward orientation by changing the sign of drift velocity from positive to negative. 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