Flow visualization study at the interface of alternating pitch tube bundles in a model helical coil steam generator using particle image velocimetry

•Simplified novel steam generator model to study flow behavior is proposed.•Refractive index matching PIV is conducted and results displayed.•Analysis shows discrepancy in area specific flow behavior compared to other designs.•Analysis suggests local behavior a major influence on transverse and streamline flow. A simplified Helical Coil Steam Generator (HCSG) model was constructed to study the flow on the shell side of rod bundles that coil against one another, using a 12-degree single rod bundle interface. The test section maintained a transverse pitch-to-diameter ratio of 2.98 while lateral pitch-to-diameter ratio constantly changed. To resolve optical distortions, experiments used para-cymene to match the refractive index of the test section. Three equally spaced regions of interest along the length of the model each have unique lateral rod pitch ratios. Experiments focused to capture the flow fields at each section between the adjacent coils at Reynolds number of 9,000. Particle image velocimetry is used for velocity measurements. The flow velocity in both the stream-wise (y-axis) and transverse (x-axis) directions for each plane, vorticity and streamline plots, used Reynolds Stresses for u′u′, v′v′ and u′v′ for different geometric coordinates are presented. The wake and center streamline regions between the rod bundles demonstrate the complex flow structures and interactions between local adjacent rods that the changing geometry of the helical coil steam generator produces throughout its configuration. Previous studies on straight tube heat exchanger models identify the formation of vortices behind rods that form dependent on an in-line or staggered tube bundle reliant on the pitch ratios. Averaged flow visualization data for the tested model HCSG suggest that flow characteristics follow their relative location to bundle formations instead of pitch-ratio.