Numerical investigation on mixed convection in narrow rectangular channel under inclination condition

Narrow rectangular channels have been widely adopted in small modular reactors (SMRs), which show particular promise for maintaining stable driving forces under marine operating conditions. The flow and heat transfer characteristics of these channels are significantly affected by oceanic motions, particularly under low Reynolds number flow regimes. This study employs computational fluid dynamics (CFD) to model a 3 mm (gap) × 50 mm (width) × 1000 mm (length) channel for investigating mixed convection phenomena under various inclined conditions. Following rigorous mesh independence verification, the numerical framework is validated against both analytical solutions and experimental data from comparable rectangular channel configurations. The research mainly focuses on the coupled effects of thermodynamic instability and lateral forces on mixed convection dynamics in inclined narrow channels. The flow structure and temperature distribution on the cross-section are obtained under longitudinal inclination and transverse inclination. A comprehensive evaluation of mixed convection reveals that the onset of significant buoyancy effects can be characterized by a dimensionless criterion combining Richardson number, inclination angle, and normalized channel length. The critical value of ${\text{Ri}}^{i}z/D_h$ is determined as 0.45 under typical inclination conditions, beyond which buoyancy-driven secondary flows enhance thermal transport capacity compared to pure forced convection baselines.