From synthesis to optimization: thermal performance of MWCNT–ZnO hybrid nanofluids in brazed plate heat exchangers

Global energy demands and the inherent limitations of conventional heat transfer fluids have driven growing interest in nanofluid-based thermal systems. In this study, a novel hybrid nanofluid composed of multi-walled carbon nanotubes (MWCNTs) and zinc oxide (ZnO) nanoparticles in a 70:30 ratio was synthesized and characterized using FTIR, SEM, and EDX analyses to confirm functional groups, morphology, and composition. Evaluation of stability and thermal performance was conducted over a concentration range of 0.05–1.0 mass%. Among these, dispersing 0.1 mass% nanoparticles in a water–ethylene glycol (70:30) base fluid provided the optimum balance between stability, thermal enhancement, and hydraulic penalty. Thermal performance was experimentally assessed in a brazed plate heat exchanger (BPHE) under varying temperatures (35–55 °C) and cold-side flow rates (14–23 L.min −1 ). Response surface methodology (RSM) combined with central composite design (CCD) was applied to model and optimize the main thermal parameters. Compared to the base fluid, the hybrid nanofluid demonstrated enhancements of up to 17% in convection heat transfer coefficient (h) and 10.4% in heat transfer rate (Q). Furthermore, pressure drop and friction factor increased by 11 and 10%, respectively, indicating the hydraulic performance trade-off associated with thermal enhancement. The developed quadratic models demonstrated high predictive accuracy (R 2 > 0.96). These findings confirm the potential of MWCNT–ZnO hybrid nanofluids for improving thermal performance in compact heat exchanger applications.