In this study the flow characteristics of a down-flowing gas-liquid column incorporating a submerged entry porous nozzle system are considered. A model based on one-dimensional drift flux analysis and critical Weber number for stable bubble size is presented. The model is used to predict the bubble size and gas void fraction as a function of the gas and liquid flow velocities within the bubbly flow regime, and can also be used to predict the gas and liquid flow conditions at which a transition from bubbly to churn-turbulent flow occurs. Reasonable agreement was found between the model predictions and experimental measurements carried out in the column using an optical fibre probe. The implications of these results for operation of submerged entry nozzle systems in the continuous casting of steel are also discussed.