Application of new closure models for bubble coalescence and breakup to steam–water vertical pipe flow

•New development in closure models for bubble coalescence and breakup.•Application of new coalescence and breakup model for condensing steam bubbles.•Considerable improvement brought by the new model.•General applicability of the new model for air–water and steam–water flows.•Uncertainty in inlet liquid sub-cooling affects the results significantly.

New closure models for bubble coalescence and breakup proposed in Liao et al. (2011), Nucl. Eng. Des. 241, 1024, are assessed for the case of condensing steam–water pipe flows. Steady-state CFD calculations are performed employing the commercial CFD solver ANSYS CFX. Predicted evolution of cross-section averaged bubble size and gas volume fraction distribution along the pipe is compared with the measurements provided by the TOPFLOW facility (Lucas et al., 2010. CFD4NRS-3, Int. Workshop on Experimental Validation and Application of CFD and CMFD Codes to Nuclear Reactor Safety Issues, Paper 13.1, 14.16.09, Washington D.C., USA.). It is shown that for cases with small initial bubble size and low gas volume fraction, bubble coalescence and breakup can be taken to be nearly negligible and the change of bubble size is primarily due to condensation. Nevertheless, with the increase of initial bubble size or gas volume fraction, bubble coalescence and breakup become more prevalent. Performance of new and standard closure models of bubble coalescence and breakup is investigated It is shown that both models overestimated the breakup rate; in particular, the standard model. The numerical results are also found to be dependent on the inlet liquid temperatures and inter-phase heat transfer models have a significant impact on the results.