Numerical investigation of synergistic interactions between fill ratio and inclination angle in nanofluid heat pipes

Nanofluid-based heat pipe solar collectors represent an efficient conversion device for solar energy. The fill ratio and inclination angle synergistically influence the thermal performance of heat pipes by altering the size and path of the vapor-liquid flow region. However, the quantitative relationship governing this synergistic interaction remains unclear. This study presents a novel Computational Fluid Dynamics (CFD) numerical model tailored for nanofluid heat pipes, accurately simulating evaporation and condensation processes. The model is validated against experimental data, exhibiting a maximum deviation of 6.99 % in wall temperature. By conducting an in-depth analysis of the wall temperature, wall liquid film velocity, and thermal resistance, this study quantitatively elucidates the synergistic mechanism of the fill ratio and inclination angle on the heat pipe's thermal performance. The results demonstrate that the fill ratio and inclination angle primarily influence the thermal resistance of the heat pipe by altering the vapor-liquid flow velocity, which in turn affects the convective heat transfer intensity. The results indicate that the fill ratio exerts a more pronounced impact on the thermal resistance of the heat pipe than the inclination angle, although this effect gradually diminishes as both parameters increase. The predictive modeling has identified the optimum fill ratio and inclination angle as 73.4 % and 61.9°, respectively, resulting in a thermal resistance of R = 0.6390 $\pm 0.163$ K/W. This represents a 47.4 % $\pm 13.4\%$ reduction compared to the thermal resistance observed at a 10 % fill ratio and a 10° inclination angle.