Air-water flow in a vertical pipe with sudden changes of superficial water velocity

For further model development and the validation of CFD codes for two-phase flow applications experiments were carried out with a sudden change of the superficial velocity of water in a vertical pipe of 51.2 mm diameter. Measurements of the cross sectional gas volume fraction distributions were taken by two wire-mesh sensors (24x24 points, 2500 Hz) mounted in a short distance (16 mm) behind each other. This sensor assembly was placed 3030 mm downstream of the gas injection. The change of the superficial water velocity was jump-like reduced and in a second series of tests jump-like increased. The tests enable the observation of the restructuring process of bubbly flow between two steady state conditions. The experimental results for tests showing a monodisperse bubble size distribution were compared to CFD calculations using the code CFX-5. Applying the two fluid approach, the momentum interaction between the liquid and gas phase was considered. For the experimental conditions of dispersed bubbly flows without or with neglectable bubble coalescence & breakup the main flow features observed in the experiments could be reproduced qualitatively and quantitatively by the numerical simulation. Further research will be undertaken for the investigation of flow regime transition from gaseous phase volume fraction wall peak to core peak dominated flows. Further investigations will also include compressibility effects for the disperse bubbly phase.