Performance Enhancement of Solar Collectors Using Partially Twisted-Lobe Absorber Tube and Nanoparticles

This paper presents a numerical investigation of the thermo-hydraulic performance of a partially twisted tube used in solar collector systems. The main objectives are to (i) quantify the effects of the twisted section length on heat transfer and pressure drop and (ii) assess the additional enhancement provided by a water/Al2O3 nanofluid under typical solar-collector operating conditions. Three-dimensional steady turbulent flow and heat transfer are simulated using the standard k−ε model with enhanced wall treatment, a non-uniform wall heat flux representative of concentrated solar irradiation, and a single-phase homogeneous nanofluid model based on Brinkman and Maxwell correlations for effective properties. The novelty of this work lies in identifying an optimal partial twist length, rather than a fully twisted tube, and in evaluating its performance with and without nanofluid for a realistic solar heat-flux distribution. The results show that the performance evaluation criterion (PEC) initially increases with twisted length and reaches a maximum when the twisted section is 700 mm, corresponding to 35% of the total tube length, which provides the best tradeoff between heat transfer enhancement and pressure drop penalty. Beyond this length, the frictional losses grow faster than the heat transfer gains, leading to a decrease in PEC. The addition of Al2O3 nanoparticles further augments the Nusselt number and improves the temperature uniformity of the working fluid; higher particle volume fractions yield larger temperature rises, indicating more effective conversion of solar heat flux into useful thermal energy for solar collector applications.