Recommendations for new Experiments able to Better Characterize Flashing Flows in Nozzles for Improving Critical Flow Modelling in System Codes

Abstract

The FONESYS network of system code developers made a benchmark of eight system codes against Two-Phase Critical Flow (TPCF) experiments, which updated the state of the art in TPCF modelling. 0-D and 1-D TPCF code models still have large prediction errors particularly for slightly sub-cooled inlet conditions. Three main reasons explain these errors: (1) 3D effects in complex geometry, (2) nucleation delay, which depends on non-well-controlled parameters such as purity of water or status of metallic surface, (3) interfacial transfers in rapidly changing non-established flashing flow with unknown flow regimes. It seems possible to progress at least on the third point using new measurements techniques. The SILENCE network of experimentalists in thermal-hydraulics conducted reflections on the type of measurements that should be used in future experiments on TPCF to provide new local information on flow regime, bubble size, film thickness and drop size. This paper first recalls the characteristics of TPCF and summarizes the conclusions of the benchmark of TPCF code models. Then, one presents the limitations of the current modelling, and the limitations of the existing TPCF database. The needs of new measurements are listed. Advanced measurement techniques are reviewed with an evaluation of their applicability to flashing flow in nozzles. Then, recommendations for any new experiment on TPCF are given.