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.