The next two ranges of the void fra

The next two ranges of the void fraction, 0.25 < a 6 0.5 and 0.5 < a 6 0.75 represented the slug, froth and churn in vertical upflow while slug and froth flow regimes in vertical downflow. From Tables 3 and 4, it is observed that the Nicklin et al. [23] and Rouhani and Axelsson [24] predict the void fraction outstandingly in both up and down orientations. For vertical downward flow Cai et al. [27],Gomez et al. [22] and Yamazaki and Yamaguchi [17] were among the top five performers as shown in Table 4. For 0.25 < a 6 0.5 the RMS error associated with the upflow was almost twice in comparison to the downward flow while it was comparable in the void fraction range of 0.5 < a 6 0.75. For downward orientation Woldesemayat and Ghajar [30] emerged to be the best correlation in this range of 0.5 < a 6 0.75 by predicting 80.2% and 97.6% of the points within ±10% and ±20% error bands, respectively. The quantitative performance of Woldesemayat and Ghajar [30] in comparison to other top performing correlations is reported in Table 4. For both
orientations superior performance of the Nicklin et al. [23] correlation was anticipated since it was devised for the slug regime, i.e. approximately for the void fraction range of 0.25 < a 6 0.75.