Experimental investigation of ferrofluid-based parabolic trough solar collector employing twisted tape under variable flow conditions

This experimental study examines the thermo-hydraulic performance of a parabolic trough solar collector using water-based Fe₃O₄ nanofluids with twisted tape inserts under simulated solar flux. Fe₃O₄ nanofluids with volume fractions of ϕ = 0.01–0.06 were circulated through a copper absorber tube equipped with twisted tapes of twist ratios (H/D) = 2.8, 3.9, and 5.4 over a Reynolds number range of 3,000–25,000. Compared with plain water, nanofluids produced higher inlet–outlet temperature differences, increasing from about 7–8 K to approximately 34–37 K when combined with twisted tape inserts at lower Reynolds number. The Nusselt number increased markedly with Reynolds number and nanoparticle concentration, reaching approximately 2–7 times that of plain water for combined nanofluid–twisted tape configurations. The friction factor increased by about 2–3.0 times due to enhanced swirl flow and higher effective viscosity. Despite this hydraulic penalty, the Performance Evaluation Criterion remained greater than unity for all cases, with PEC values generally ranging from about 1.42 to 5.18, confirming an overall thermo-hydraulic advantage.