Systematic numerical analysis of thermal and hydraulic performance of nanofluids in double-tube heat exchangers

Heat exchangers are central to thermal management in industrial, energy, and HVAC systems, motivating continued interest in advanced working fluids that enhance thermal performance. This study presents a computational investigation of the thermo-hydraulic performance of three nanofluids (MWCNT, SiO₂, and ZnO) flowing through a double-tube heat exchanger under identical geometrical and boundary conditions. An experimentally validated CFD model is used to isolate the effects of flow rate, nanoparticle concentration, and operating temperature on heat-transfer enhancement and pressure drop. Relative to water, nanofluids enhance heat transfer rates by approximately 6–40%, with a pressure drop of 8–106%. Increasing the volumetric flow rate to 600 l/h and the nanoparticle concentration to 0.25% yields the most significant thermal enhancement among all nanofluids. However, thermo-hydraulic evaluation shows that maximum heat-transfer enhancement alone does not guarantee optimal performance. When thermal and hydraulic effects are combined using the performance evaluation criterion (PEC), SiO₂ at 0.25% concentration yields a thermo-hydraulic improvement of approximately 11% relative to water, whereas ZnO, despite enhanced heat-transfer performance, incurs substantially higher-pressure losses. At high operating temperatures, heat transfer is further intensified, with ZnO exhibiting the highest sensitivity to thermal loading. The results establish a controlled reference for nanofluid comparison and provide practical guidance for selecting working fluids for compact, high-efficiency heat exchangers in HVAC, energy recovery, and industrial thermal management applications.