Investigations of density wave oscillation in helical tubes with different configurations sharing the same inlet and outlet headers

Helical steam generators (HSGs) are crucial in the design of small modular reactors, offering excellent heat transfer performance and compact design characteristics. These features make HSGs viable for both light water and non-light water reactors. However, they pose significant design and operational challenges, particularly with respect to two-phase flow instabilities such as density wave oscillation (DWO). DWO arises from complex thermal-hydraulic interactions within the helical tubes, and failure to clearly define stability boundaries can adversely impact the safety and performance of steam generators and reactor operations. This study provides an in-depth experimental and computational analysis of DWO in HSGs to deepen the understanding of thermal-hydraulic characteristics and DWO mechanisms. The analysis focuses on DWO characteristics observed in two configurations: two identical helical tubes and three helical tubes with differing geometries. An experimental facility utilizing R245fa as the working fluid was designed and constructed to provide detailed insights into two-phase pressure drop and DWO characteristics. The data were further used for the validation and improvement of the MARS-KS code, contributing to the establishment of stability boundaries that can represent complex HSGs. This study aims to serve as foundational data for the design and safety assessment of HSGs in the future.