Enhancing energy efficiency in heat pipe heat exchangers using silver nanofluids: Performance and optimization

• Silver nanofluids improved HPHE thermal conductivity by 20% and heat transfer by 15%. • Zeta potential above 30 mV confirmed excellent nanofluid stability and dispersion. • Hot fluid mass flow rate was the most influential factor in heat transfer optimization. • Thermal resistance decreased by 10% using silver nanofluids compared to DI water. • Uncertainty-aware desirability optimization (Derringer–Suich) reported with a 95% optimum ellipse Heat pipe heat exchangers were used for low-grade heat recovery; however, internal and interfacial resistances limited their performance. Dispersing nanoparticles in water improved conductivity and interfacial transport, provided stability, and maintained viscosity control. Many studies treated single responses and lacked uncertainty-aware, multi-response tuning, leaving little guidance on energy efficiency setpoints within pumping-power limits. The objective was to quantify changes in the overall heat-transfer coefficient and thermal resistance using a 1 vol% silver nanofluid, and to identify energy efficiency operating conditions that maximise heat transfer and minimise thermal resistance. A copper HPHE with a mesh wick was tested using a central composite design spanning hot- and cold-mass flow rates, thermal loads, and hot-inlet temperatures. The nanofluid was prepared by ultrasonication and characterised; its responses were then analysed using ANOVA and response surface methodology, incorporating desirability optimisation and uncertainty propagation. The hot-side mass flow rate dominated all responses: raising it from 0.1 to 0.6 kg·min⁻¹ increased OHTC from ∼150 to ∼313 W·m⁻²·K⁻¹ and decreased thermal resistance from ∼0.09 to 0.043 K·W⁻¹; evaporator and condenser temperature differences rose by about 80–120%. The optimal setting was 0.6/0.6 kg·min⁻¹ (hot/cold) with a 90 W load and a 60°C hot inlet, yielding a desirability of 0.746. These results indicated that internal convective transport was the principal bottleneck and that cold-side flow played a secondary stabilising role; the nanofluid's ∼20% conductivity gain amplified improvements at high hot-side velocity, enabling energy-efficient HPHE operation.