- Much work has been done in the area of two-phase upflow, while the flow of gas and liquid in the downward direction has not received much attention. This paper presents a model for estimating void fraction in downward flow for vertical and inclined systems for the two dominant flow regimes, bubbly flow and slug flow. We use the drift-flux approach to determine slip between the phases and the transition between the flow regimes. The flowing mixture densities calculated from the model are useful for pressure gradient estimation. Extensive data are used to validate the model. The data show that for both bubbly and slug flow the effect of buoyancy, expressed by the terminal bubble-rise velocity, has the same magnitude as that for the case of upflow. In addition, the flow distribution parameter in bubbly flow appears to have the same value of 1.2 as in upflow. For slug flow, however, the flow parameter is represented by a somewhat lower value (1.12) than for upflow.