In-depth flow investigation in subchannels of helical cruciform fuel using magnetic resonance velocimetry

Magnetic resonance velocimetry (MRV) was employed to measure the three-dimensional, three-component time-averaged velocity field in a 3 × 3 helical cruciform fuel (HCF) bundle at Re = 104. The HCF test-section was fabricated by stereolithography-based three-dimensional printing, and four consecutive measurement volumes were acquired using a 3-tesla (3 T) magnetic resonance imaging system with an isotropic spatial resolution of 0.30 mm. The volumetric flow rate reconstructed from the MRV-measured axial velocity agreed with the flowmeter reading within ±3.5%, providing a bulk-flow validation of the MRV dataset. Axial correlation analysis indicated that the bulk flow reached a pitch-wise periodic state after approximately one twist pitch, while local velocity profiles continued to develop over approximately two twist pitches. The flow split factor was used to quantify the relative axial-flow partitioning among subchannels and ranged from 0.83 to 1.22, with higher values in the interior subchannels and lower values in the corner subchannels. In the developed region, the secondary-flow intensity was approximately 3.66%–4.87% of the bulk axial velocity, with a minimum near half pitch due to partial cancellation between counter-rotating secondary-flow structures. In-plane streamlines of the time-averaged transverse velocity field revealed an off-axis inner-core vortex that exhibited a systematic displacement along the streamwise direction rather than remaining fixed at the bundle center. These observations provide quantitative experimental insight into the three-dimensional flow behavior, transverse momentum exchange, and subchannel-scale mixing in HCF bundles.