Numerical Investigation of Heat Transfer and Pressure Drop in Partially Twisted Five-Lubed Tube With Al2O3 Nanofluid

This study examines the heat transfer and pressure drop characteristics of water/aluminum oxide (Al₂O₃) nanofluid flow in a semi-twisted tube within solar collector systems. The investigation focuses on the influence of varying nanofluid concentrations and twisted tube lengths on thermal performance and friction factor. Numerical simulations employing the k − ɛ turbulence model incorporate solar radiation effects by modeling heat flux as a discrete, angular-dependent function. The upper half of the surface is subjected to direct solar radiation, implemented via a C-programed user-defined function (UDF) in Ansys Fluent to resolve wall flux dynamics. Additionally, the performance evaluation criterion (PEC) was employed to quantitatively assess system efficiency across different configurations. Notably, the highest PEC value was achieved at a twisted length of 700 mm, corresponding to 35% of the total tube length. The study demonstrates that increasing the number of twisted tube number of swirls (N) from 3 to 7 leads to a significant 17% improvement in the Nusselt number (Nu), outweighing the accompanying 12% increase in friction factor. The comparative analysis highlights that integrating a 4% nanoparticle concentration with a 7-twist tube configuration leads to a substantial boost in thermal performance, resulting in an approximate 80% increase in the Nu. This optimized setup offers strong potential for enhancing the efficiency of solar collectors.