فهرست نشریات علمی دسترسی آزاد ایران

This study presents the first investigation of TiO₂–MgO hybrid nanofluids in water for Ground Heat Exchangers (GHEs) with advanced divergent and convergent helical geometries, including variable-pitch configurations. Addressing a gap in prior work that largely relied on simplified single-phase assumptions, a comparative analysis between single-phase and Euler–Euler multiphase Computational Fluid Dynamics (CFD) models is performed, capturing interphase slip, particle interactions, and volume fraction (φ) -dependent effects. Simulations are carried out for Reynolds numbers between 1000 and 5000, complemented by an exergy analysis to evaluate thermo–hydraulic and thermodynamic performance. Using a hybrid TiO₂–MgO (50:50)–water nanofluid with 2% volume fraction, the divergent variable-pitch configuration increases thermal–hydraulic performance by 4.3% over the convergent case at the same Reynolds number. Adding 2% hybrid nanoparticles in the divergent heat exchanger increases the Nusselt number by up to 5.2% and total heat transfer by up to 6.3% compared to pure water in a convergent exchanger. Multiphase modeling at Re = 5000 reveals that for volume fractions up to 1%, thermal conductivity predictions are within −5.16% (single-phase) and +14.14% (multiphase) of experimental values, with Nusselt number deviations of −6.30% to −1.02% (single-phase) and −3.19% to +4.60% (multiphase). Exergy efficiency improvements align with the observed heat transfer gains, demonstrating that nanoparticle enhancement is achieved at moderate pumping penalties. Trade-offs were quantified via a pumping index (Δp·Vin) and a thermal-capacity index (Nu·Aw): the divergent-pitch coil raised Nu by 24% relative to a uniform-pitch helix. These findings validate multiphase CFD as a more accurate predictive tool at higher volume fraction (φ) and confirm the engineering relevance of TiO₂–MgO nanofluids for scalable, high-efficiency geothermal heat exchanger applications.