Yet another variable in the drift flux model is the drift velocity and it should change with respect to the phase flow rates and hence the flow patterns. For bubbly and slug flow the drift velocity proposed by Haramathy [37] and Nicklin et al. [23] serves for the satisfactory performance of the void fraction correlations. But as the phase flow rates increase and the flow pattern transits to the annular flow the drift velocity should increase since the buoyancy force always aids the upward motion of the gas phase. In contrast to this, for vertical downward orientation as discussed earlier the drift velocity should be negative for the bubbly and slug flow regimes. With increasing phase flow rates the gas phase is forced to move in the direction of the mean flow and hence the drift velocity should reduce gradually to zero and attain some finite positive value