Assessment of CFD predictive capacity for flash boiling

Flash boiling of water inside a converging-diverging nozzle and a vertical pipe under pressure release transients is investigated. Some representative assumptions and simplifications encountered in the literature are assessed. It is shown that a fully non-equilibrium model with two velocity fields and two temperature fields should be applied. Non-drag forces are necessary to predict the lateral distribution of bubbles, which is usually non-uniform. For large bubbles (dg>1mm), bubble translation has a significant contribution to the interphase heat transfer, and thus heat conduction would give a significant under-prediction. The most important point in the assessment of these interphase exchange closures is to have a reliable prediction of bubble size. On this topic there is still much to do, since a realistic modelling of bubble number density transport is still missing. Great efforts need to be invested in the development of generally applicable closure models for nucleation, bubble growth, coalescence and breakup, etc.