Abstract: Liquid metal reactors, widely used in advanced nuclear systems, utilize liquid metals with low Prandtl numbers as coolants. The flow of liquid metal within the fuel assembly often exists in a mixed convection regime. In such systems, natural convection caused by buoyancy plays an important role in influencing the heat transfer characteristics of the liquid metal. In this research, a second-order differential heat flux model (DHFM) was developed based on OpenFOAM, and a buoyancy term was introduced to study the mixed convection characteristics of sodium-potassium (NaK) alloy in a pipe with different inclination angles and different wall heat flux. The distribution of temperature, velocity, turbulent kinetic energy k, temperature fluctuation k_θ were investigated. The temperature distribution and transverse flow in the x and y directions were showed, exhibiting asymmetry with changes in the inclination angle. It is shown that forced convection is the main factor affecting heat transfer of liquid metal, and natural convection caused by buoyancy is negligible and axial inertia force plays a leading role in convective heat transfer when Re is large. When Re is small, natural convection plays a dominant role, and the inclination angle and wall heat flux affect heat transfer of liquid metals, which manifests itself as distortion of the temperature and transverse flow distributions, and inhomogeneity. With the decrease of Re, the k gradually decreases and the k_θ gradually increases. When Re is small, Nu_local increases with the increase of inclination angle, and the increase is about 1.3%-3.5%. With the increase of Re, the influence of natural convection is weakened, and the Nu_local difference caused by the inclination angle is not significant. When the liquid metal flow is dominated by natural convection, the increase of wall heat flux helps to enhance the influence of natural convection on heat transfer, which is manifested as the increase of Nu_local, the increase range is about 1.05%-11.55%. This research not only provides data on the mixed convection characteristics of liquid metals but also enriches research in the field of mixed convection in liquid metals. In particular, the analysis of convective heat transfer under different flow conditions, pipe inclination angle, and wall heat flux is of great significance for a deeper understanding of the heat transfer mechanisms of liquid metal coolants in actual reactors. These data offer a theoretical basis for further optimizing the design of liquid metal reactors, especially for the optimization of convective heat transfer under mixed convection conditions.