Numerical Study on Flow Characteristics of Spiral Channel with Coiled Wire

This paper aims to investigate the flow characteristics of the primary side fluid in a casing once-through steam generator (COTSG) under vertical conditions, providing theoretical support for its design in nuclear power plants. The study employs the three-dimensional CFD software STAR-CCM+, utilizing the Reynolds stress transport turbulent model for numerical simulations, and presents a method for determining the fully developed section of the heat transfer channel based on dimensionless velocity overlap analysis (entrance length L∗ = 80 De). Through analysis, the frictional resistance characteristic curve of the spiral channel with coiled wire is divided into three regions: laminar region, transition region, and turbulent region. Over a Reynolds number range of 1000–30,000 and heat transfer powers of 1–30 kW, an expression between the frictional resistance coefficient and the Reynolds number for the spiral channel with coiled wire is established, achieving a prediction error within ±10% through a kinematic viscosity correction factor (ct) accounting for heat transfer effects. This paper conducts a detailed study of the fully developed fluid in the spiral channel with coiled wire, revealing significant axial variations in the frictional resistance coefficient and identifying distinct velocity distribution patterns in different flow regimes (maximum velocity in central sub-channels for laminar/transition regions vs. boundary sub-channels for turbulent regions). The critical Reynolds number for laminar-to-turbulent transition increases with higher heat transfer powers, demonstrating the stabilizing effect of enhanced cooling on flow regimes. These findings provide quantitative criteria for optimizing heat exchanger design under vertical operating conditions with varying thermal loads.