Applocation of new closure models for bubble coalescence and breakup to steam-water vertical pipe flow

The new closure models for bubble coalescence and breakup proposed in our previous work (Liao and Lucas, 2011) are tested in the case of condensing steam-water pipe flows. Steady-state CFD calculations are performed with the commercial CFD solver CFX12.1. The predicted evolution of cross-section averaged bubble size distribution and gas volume fraction along the pipe is compared with the measured one provided by the TOPFLOW facility (Lucas et al., 2010). It is shown that for cases with small initial bubble size and low gas volume fraction, bubble coalescence and breakup is nearly negligible and the change of bubble size is mainly due to condensation. Nevertheless, with the increase of initial bubble size or gas volume fraction bubble coalescence and breakup become important phenomena. The performance of the new closure models and the standard ones available in CFX is investigated for two examples of such a case. It is shown that both sets of the models give an overestimation of breakup rate, especially the standard ones. Furthermore, the results are found to be dependent sensitively on the inlet liquid temperatures, which might exceed the accuracy of the measurements. In addition, inter-phase heat transfer models have a significant impact on the results.