The physical structure of the Rouhani and Axelsson [24] and Gomez et al. [22] correlations as shown in Table 5 is revisited below.
It is clear that both of these correlations perform well over the entire range of the void fraction since they partially meet the requirements of an ideal void fraction correlation for vertical flow.
As reported by Rouhani and Axelsson [24] the distribution parameter takes the following form of Co ¼ 1 þ 0:2ð1 xÞðgDq21
=G2Þ0:25 for a < 0.25, while for a > 0.25, Co = 1 + 0.2(1 x). Thus Rouhani and Axelsson [24]
model the distribution parameter as a variable which is observed to reduce from 1.2 for a < 0.25 or alternatively the bubbly flow and approach close to unity as the flow pattern turns from bubbly to annular flow. The Gomez et al. [22] model the drift velocity as a variable by defining UGM ¼ 1:53 gr qlqg q2l 0:25 ð1 aÞ0:5 sin h. Here the (1 a)0.5 render the drift velocity negligible with increasing void fraction in downward two phase flow and thus satisfying the
requirement of variable drift velocity for ideal void fraction.