Development of frictional pressure drop models for single- and two-phase flows in a helically coiled tube

Helically coiled tubes (HCTs) are widely employed in small modular reactors (SMRs), due to their compact geometry and superior heat transfer performance. Accurate prediction of frictional pressure drop for single-phase and two-phase flows in HCTs is crucial for the design and safety analysis of SMRs. In this study, the characteristics of frictional pressure drop in HCTs were investigated using an experimental database comprising 1,489 single-phase and 876 two-phase data points. A systematic physics-based model development framework was applied, including key parameters identification and statistically supported modeling to ensure the significance and independence of all selected parameters. Through this, dominant factors influencing frictional pressure drop in HCTs were determined. It was found that, in addition to the well-known flow variables, centrifugal force induced by main axial flow has a significant impact on pressure drop. Based on the finding, new empirical correlations were developed by explicitly incorporating a dimensionless centrifugal force number. The final formulation is designed to preserve asymptotic consistency with straight tube when the curvature effect vanishes. Assessment results indicate that the proposed correlations reduce the root mean square errors by 7.0%, 9.1%, and 15.0%, respectively, compared to the best-performing existing models for single-phase laminar, single-phase turbulent, and two-phase flows.