Experimental investigation of flow regime transitions and frictional pressure drop in a 9x9 helical cruciform fuel bundle

• Frictional pressure losses were measured in a 9 × 9 HCF rod bundle over 200 ≤ Re ≤ 22,000. • Flow regime boundaries were identified at Re bL ≈ 999.6 and Re bT ≈ 8274.2. • Empirical correlations for friction factors were developed. This study experimentally investigates the frictional pressure loss and flow regime behavior of a 9 × 9 Helical Cruciform Fuel (HCF) rod bundle, a novel design proposed for Small Modular Reactors (SMRs). The unique cruciform cross-section, featuring four twisted petals, eliminates the need for conventional spacer grids, offering higher fuel packing fraction and enhanced coolant mixing. To assess these advantages, a high-precision differential pressure measurement system was employed over a Reynolds number range of 200–22,000, covering the laminar, transition, and turbulent flow regimes. The experimentally determined friction factors showed statistically similar trends between the “one pitch” and “bundle-averaged” axial segments, confirming fully developed flow in both regions. Empirical correlations for friction factor and differential pressure per unit length were then developed for each flow regime and validated by comparison with previous HCF and wire-wrapped fuel bundle studies. Results identified flow regime boundaries at approximately Re ≈ 1000 for laminar-to-transition and Re ≈ 8274 for transition-to-turbulent, highlighting distinctly different hydraulic behavior in the three regimes. The findings significantly broaden the limited experimental database on HCF rod bundles, providing new insights into regime-dependent pressure drop characteristics. By refining existing correlations and offering high-fidelity benchmark data, this work advances the development of more efficient and accurate reactor core designs that leverage HCF technology for enhanced thermal performance.