Validation of models for bubbly flows and cap flows using one-group and two-group average bubble number density

Gas-liquid flows with wide range of bubble sizes are commonly encountered in many nuclear gas-liquid flow systems. In tracking the changes of gas volume fraction and bubble size distribution under complex flow conditions, numerical studies have been performed to validate predictions of the onegroup and two-group approaches against experimental measurements for upward gas-liquid flows in vertical pipes. These experiments have been strategically chosen because of particular flow conditions yielding specific trends of bubble size evolution, which provided the necessary means of carrying out thorough assessments of bubble coalescence and break-up kernels. Predictions of one-group approach for bubbly flows were in good agreement with experimental data. Predictions of two-group approach for bubbly and cap flows were in reasonable agreement with experimental data; additional insights into the appropriate bubble interaction mechanisms are still required for cap flows. Nevertheless, the encouraging results demonstrated the capability of both approaches in capturing the dynamical changes of bubbles size due to bubble interactions and the transition from “wall peak” to “core peak” gas volume fraction profiles caused by the presence of small and large bubbles.