Análise experimental e numérica do escoamento em coletores solares de tubo a vácuo sob diferentes inclinações

Abstract

This thesis presents an integrated experimental and numerical investigation of the thermal and fluid dynamic behavior of water in evacuated tube solar collectors operating under natural circulation (thermosiphon regime). The study focuses on the influence of the tube set inclination on the internal circulation of the thermal fluid and the stability of the flow. The experimental methodology involved test bench trials under real operating conditions, with continuous monitoring of temperature and mass flow rate for inclinations of 20°, 30°, and 45°. In parallel, numerical simulations were conducted using CFD (Computational Fluid Dynamics), employing validated numerical models to analyze the local velocity and temperature fields inside the tubes. The results indicate that inclinations higher than 30° enhance natural convection but also induce transient flow instabilities, characterized by abrupt oscillations in the mass flow rate. The coupled analysis confirmed the consistency between experimental and numerical patterns, revealing internal mechanisms associated with the breakdown of thermal stratification and the formation of recirculation. The daily global efficiency of the system was calculated for each inclination configuration, resulting in the following efficiency values: 46.4% for 20°, 69.5% for 30°, and 54.4% for 45°. These results highlight that the 30° inclination provided the highest overall thermal efficiency, due to its more stable behavior throughout the daily cycle. Although the 45° inclination promoted the highest values of flow rate and temperature increase, the system’s behavior did not remain stable at this elevated level throughout the day, which resulted in a lower efficiency compared to the 30° inclination. The conclusions highlight the importance of inclination adjustment to optimize the thermal performance of the collectors and demonstrate that the combination of numerical and experimental methods is effective in understanding the phenomena involved.