Second-law analysis of TiO2-SiO2 binary nanofluids in shell-and-tube heat exchangers: Entropy generation and exergy efficiency

Shell-and-tube heat exchangers (STHE _X ) are widely used in industry; however, first-law analysis alone cannot capture the energy degradation caused by irreversibility. This study experimentally evaluated the second-law performance of a parallel-flow, adiabatically insulated STHEX operated with TiO ₂ –SiO ₂ binary nanofluids dispersed in a monoethylene glycol–water mixture (MEG-40). The influences of the Reynolds number (500 ≤ Re ≤ 2222) and nanoparticle loading (0–3.0 vol.%) on the entropy generation, exergy destruction, and exergy efficiency were examined. Characterization confirmed a stable hybrid dispersion and nearly Newtonian behavior. Entropy generation and exergy destruction increased with Re and were dominated by tube-side processes. Entropy generation exhibited a nonmonotonic dependence on concentration, with intermediate loading (φ ≈ 1.75%) minimizing irreversibility, whereas a greater loading intensified viscous dissipation. The dimensionless exergy loss followed quadratic ε–Re correlations (R² = 99.5–99.6), consistently identifying φ ≈ 1.75% as the minimum irreversibility condition. However, the exergy efficiency was highest at φ = 3.0%. The results highlight that optimal nanofluid operation depends on the selected second-law metric, which requires a balance between heat-transfer enhancement and friction-induced irreversibility.