Visualization experimental investigation on flow boiling and critical heat flux characteristics of helical fuel

Helical fuel represents an innovative nuclear fuel design characterized by an extended heat transfer surface and reduced fuel temperatures compared to conventional cylindrical rods, significantly enhancing thermal–hydraulic performance. Critical Heat Flux (CHF), a key parameter in light water reactor thermal design, dictates the operational safety margin by marking the threshold of the boiling crisis. However, the flow boiling and CHF characteristics of helical fuel have not been fully elucidated. This study employs high-speed visualization to systematically investigate flow boiling dynamics and CHF characteristics in a single helical fuel rod under atmospheric pressure. During the experiments, visualization measurements are performed to reveal the boiling crisis triggering mechanism. Various flow patterns, including bubbly flow, bubbly-cap flow, slug flow, and annular flow, are identified in steam-water two-phase systems. Our observations indicate that bubble nucleation and aggregation initiate preferentially in the elbow region (the zone of peak heat flux), with subsequent vapor transport along helical blade-induced swirling paths. At elevated steam qualities, localized vapor accumulation and temperature escalation at the elbow heating wall jointly trigger CHF onset. This work examines the effects of thermal–hydraulic and geometric parameters on CHF. The increasing R₂/R₁ ratio (where R₁ is the outer radius of the petal and R₂ is the inner radius of the fuel rod) improves CHF due to a more uniform circumferential heat flux. Conversely, a reduction in CHF is observed when the helical pitch decreases from 600 mm to 300 mm, as the enhanced lateral flow promotes vapor phase accumulation. These findings showed that, regardless of the tested geometry, CHF values were about 14.1 % higher than those predicted by the lookup table. The results highlight the importance of optimizing helical fuel geometry to further enhance CHF performance and provide valuable insights for developing advanced safety analysis methods.