Wide wall liquid film thickness distribution in narrow rectangular geometry for annular flow

Annular flow in narrow rectangular geometries is critical for high-efficiency heat transfer systems, yet existing models—primarily developed for circular pipes—fail to capture the two-dimensional film thickness distribution inherent to asymmetric channels. This study addresses this gap by proposing a novel methodology that adapts conventional one-dimensional film thickness models to rectangular geometries through geometric and hydrodynamic considerations. Three characteristic lengths (hydraulic diameter, center-to-wall distance, and narrow gap) and two velocity profiles (1/7th power law turbulence and pressure-drop-derived phase separation) are systematically integrated into six methodologies to predict wide-wall film thickness. Validated against experimental data from a 200 mm × 10 mm rectangular channel, the results demonstrate that Method 6—combining hydraulic diameter and pressure-drop-derived superficial liquid velocity—achieves better accuracy by accounting for gas-core dominance at the channel center and confinement effects. While the framework reliably predicts wide-wall film thinning, challenges persist near the narrow wall due to flow regime transitions. This work advances the theoretical foundation for annular flow modeling in non-circular geometries, offering critical insights for enhancing thermal–hydraulic safety in nuclear reactors, compact heat exchangers, and high-power electronics cooling systems.