Greskovich and Cooper [32] gave the maximum accuracy of 45.2% in the restricted ±10% error bands while the Rouhani and Axelsson [24] correlation gave the maximum accuracy of 71.9% in the less restrictive ±20% error bands. The RMS error associated with all the correlations was greater than 50%. It should be noted that Hasan [28] correlation though developed for downward orientation worked satisfactorily for upward orientation except for the void fraction range of 0.5 < a 6 0.75. For vertical downward flow as seen in Table 4, the void fraction correlations gave a better accuracy in the lower range with the Hasan [28], Gomez et al. [22], and Cai et al. [27] being the outstanding correlations. 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%.