Whereas for vertical downward orientation in addition to Gomez et al. [22] correlation, Cai et al. [27] and Rouhani and Axelsson [24] are the other two recommended void fraction correlations.
The performance of the Rouhani and Axelsson [24] and Gomez et al. [22] against different fluid combinations for vertical upward and downward flow is shown in Figs. 17 and 18, respectively.
Both of these correlations are based on the concept of drift flux modeling and the Rouhani and Axelsson [24] models the distribution parameter as a variable with respect to the mixture quality while the Gomez et al. [22] treats the drift velocity as a variable with respect to the void fraction.
Gomez et al. [22] was initially proposed for vertical upward flow but in Bhagwat and Ghajar [34] we have shown its satisfactory results over the entire range of the void fraction. Rouhani
and Axelsson [24] is found to predict void fraction satisfactorily for downward flow as well, but does not perform well in comparison to the Gomez et al. [22] for the low range of the void fraction in general for 0 < a 6 0.25. As shown in Fig. 17, the correlation of Rouhani
and Axelsson [24] does not predict satisfactorily air–kerosene data in low range of the void fraction. This is probably due to the inaccurate void fraction data available in the literature. The accuracy of this data is not known since no uncertainty analysis was provided by Mukherjee