This paper is concerned with a numerical model for the steady-state flow characteristics of the gas-liquid two-phase mixture flowing upward in a vertical pipeline with an abrupt enlargement in cross sectional area. The system of governing equations used is based upon the multifluid model and the transitions of gas flow pattern are taken into consideration in the system of governing equations. For the case of an abrupt enlargement in diameter in a coaxial pipeline, the procedure of the numerical calculation to obtain the flow characteristics in the pipeline section after a sudden change in diameter has been proposed in the present investigation. At the same time, the experiments have also been performed using two kinds of lifting pipes in order to confirm the validity of the present numerical model. As a result of the comparison of the calculated results with the experimental data, we have confirmed the applicability of the system of governing equations as well as the validity of the procedure of the numerical calculation proposed here to the prediction of the flow characteristics of the gas-liquid two-phase mixture in a vertical pipe with a sudden change in diameter.
