As shown in Table 3, for vertical upward flow
among the top performing correlations the Armand [33] correlation
is based on the a–b relationship while rest of the correlations
are based on the concept of the drift flux model. In case of vertical
downward flow Rouhani and Axelsson [24] and Woldesemayat and
Ghajar [30] were among the top performing correlations predicting
more than 90% of the data points within the restricted ±10% error
bands and the RMS error of 6.8% and 7.4%, respectively. The comparative
performance of these two correlations is shown in Table 4