Thermo-hydrodynamic analysis of an absorber tube containing water–metal oxide nanofluid in a parabolic trough solar collector under non-uniform heat flux conditions adapted to the Birjand climate

Abstract

higher outlet fluid temperatures in parabolic trough solar collectors (PTSCs) are essential for enhancing their thermal efficiency and applicability in high‑temperature solar thermal systems. In this study, a numerical investigation is conducted to analyze the thermo‑hydrodynamic behavior of a water–copper oxide (CuO) nanofluid flowing through the absorber tube of a PTSC under non‑uniform heat flux conditions representative of the summer climate of Birjand. The spatially varying solar heat flux on the absorber tube wall is realistically modeled using a User‑Defined Function (UDF), accounting for the non‑uniform distribution of concentrated solar radiation. The nanofluid enters the absorber tube at a constant mass flow rate of 0.0112 kg/s, with inlet conditions selected based on local climatic data.The results indicate that the non‑uniform heat flux leads to localized enhancement of wall temperature and heat transfer rates along the absorber tube. Although the addition of CuO nanoparticles increases the effective viscosity of the working fluid and results in a higher pressure drop, it simultaneously improves the convective heat transfer coefficient and overall thermal performance, particularly at higher flow velocities. The findings demonstrate that an appropriate balance between heat transfer enhancement and hydraulic penalties can be achieved through the use of water–CuO nanofluids. Overall, the study confirms the strong potential of parabolic trough solar collectors operating under favorable climatic conditions such as Birjand for efficient solar energy concentration and high‑temperature thermal applications, including hot water and steam generation.