Flow pattern map and multi-scale entropy analysis in 3 × 3 rod bundle channel

Highlights: • Flow patterns of steam–water two-phase flow in a 3 × 3 rod bundle were visualized. • The flow pattern map obtained for the rod bundle was compared with some existing flow pattern maps. • Multi-scale entropy was used to characterize the dynamic characteristics of the different flow patterns. • Rate of multi-scale entropy can effectively identify flow patterns in a rod bundle channel. - Abstract: The characteristics of the heat transfer and two-phase flow resistance in a rod bundle channel are closely related to flow patterns. In the present study, two-phase flow patterns for vapor–water flows in a 3 by 3 rod bundle channel were obtained at atmospheric pressure and relatively low mass flow condition. Under the current experimental conditions, slug flow found in circular tubes was not observed. Comparisons with some existing flow pattern maps and transition criteria were likewise conducted. Results show that the transition boundary of circular tube deviated from the transition boundary of the boiling two-phase flow of the rod bundle channel. Using the differential pressure signal of the vapor–liquid two-phase flow, multi-scale entropy algorithm is employed to reveal the dynamic characteristics of the different flow patterns in the rod bundle. The multi-scale entropy rate could effectively classify the flow patterns in the rod bundle channel. Results suggest that multi-scale entropy can be an effective method to reveal the dynamic details of macro and local vantages