Gomez et al. [22] achieved the maxi

Gomez et al. [22] achieved the maximum accuracy by predicting 93.7% of the data points in the ±20% error bands. It should be noted that the Gomez et al. [22] and Nicklin et al. [23] correlations were developed for vertical upward flow but could be successfully employed to predict the void fraction in the downward orientation by changing the sign on the drift velocity term from positive to negative. In comparison to the vertical upward flow the RMS error associated with the downward orientation is observed to be significantly less and typically less than ±15%.