High Resolution Measurements of Shell-Side Velocity Fields in Helical Coil Steam Generators Using Particle Image Velocimetry

The helical coil steam generator is a type of tube and shell heat exchanger that is proposed for many small modular and Gen IV reactor designs. It is favored due to the higher heat transfer it provides compared to traditional U-tube steam generators, compact design and thermal stress flexibility. The arrangement of tubes, where the secondary two-phase fluid flows, determines the shell side, pressurized single phase flow. This study focuses on the fluid structures that form in the shell side flow when adjacent helical tube bundles coil in opposite directions. Two helical coil steam generator designs were created and studied using a high-resolution flow visualization technique, refractive index matching particle image velocimetry (PIV). Experiments focused on multiple planes across each test section for different lateral pitch tube arrangements at Re ~ 8,500. Two-dimensional image sequences captured with a high speed camera were processed to determine average velocity flow fields for x- and y-direction velocity components. Two main regions of flow develop between tube bundle arrangements, center streamline flow and recirculation regions below the tubes. Previous studies have shown these recirculation regions are responsible for the increase in heat transfer this geometry offers. Nevertheless, vortices that form in these regions have the potential to grow and enter the streamline, a phenomenon called vortex shedding. The frequency of this shedding behavior is currently correlated to lateral and transverse pitches between bundles. A comparative study showed that while the center streamline development remained consistent between helical coil steam generator models, the recirculation regions varied in strength and relative size. Areas with a larger lateral pitch, expected to have large recirculation development between the rods, did not have the highest velocity magnitude. These average velocity fields suggest that relative location along the coiling bundle also contributes to the vortex shedding behavior of the recirculation regions below the tubes. Future work aims to study the transient behavior of the flow to have an in-depth analysis of the relationship between lateral pitch, transverse pitch, radial location around the helical coil tube bundle and recirculation region development