Proposals for Improving Interphase Drag Modelling for the Bubbly and Slug Regimes in RELAP5.

摘要

The proposal is put forward that the effective interphase drag coefficient for the bubbly and slug regimes in RELAP5 should be calculated using best-estimate void fraction correlations. It is argued that this will lead to improvements in the code's modelling of interphase drag and evidence is given to corroborate this. The need for such improvements has been prompted by the poor performance of the current models in simulating rod bundle experiments. There is also concern that the models do not account for profile slip effects, which could be important in a variety of geometries, and that the slug flow equations may not be appropriate for large diameter vertical pipes. To support the proposal, a set of void fraction correlations is identified which is believed to cover the full range of geometries and flow conditions encountered in PWR safety analysis including the analysis of small-scale experimental facilities. This set is selected from a detailed appraisal of the most appropriate correlations found in the literature which takes account of comparisons with experimental data and physical considerations. This Report forms part of the UK's commitment to the ICAP Code Improvement Plan. The recommendations will now be implemented in a development version of RELAP5/MOD3 and a preliminary assessment made. The interphase drag models used in the annular-mist regime will also be examined and, if necessary, appropriate improvements will be proposed. (Atomindex citation 20:049792)