Thermo-Hydraulic Performance of Ni-Fe3O4 Hybrid Nanofluids in a Shell and Helical Tube Heat Exchanger: Two-Phase Flow Modeling

The thermo-hydraulic performance of Ni-Fe3O4/water hybrid nanofluid (HNF) in a shell and helical tube heat exchanger (SHTHE) was investigated numerically and experimentally. A Eulerian mixture model was employed to simulate the two-phase flow behavior of the HNF and analyze the impacts of nanoparticle volume fraction and mixing ratio on the heat-transfer and flow properties under turbulent flow. Numerical simulations were performed at nanoparticle volume fractions (φ) of 0.1%, 0.5%, and 1% and Ni:Fe3O4 mixing ratios (MRs) of 1:4, 1:1, and 4:1. Experimental measurements at a nanoparticle volume fraction of 0.1% for different MRs were used to validate the numerical predictions. The numerical results showed that the nanoparticle volume fraction and mixing ratio significantly affected the thermal performance, whereas their influence on the hydraulic behavior remained relatively limited. At the highest Dean number, MR (4:1) and φ = 1% yielded the highest thermal performance, reaching a 56% enhancement in the Nusselt number, whereas the friction factor increased by only 6%. In addition, MR (4:1) exhibited the highest average thermal performance index of 1.4. Furthermore, good agreement was achieved between the numerical predictions and experimental measurements, confirming the validity of the proposed numerical model. These findings demonstrate the applicability of the validated Eulerian mixture model for evaluating the thermo-hydraulic performance of Ni-Fe3O4/water HNF in SHTHEs, particularly for achieving heat-transfer enhancement with limited hydraulic penalties.