Experimental investigation of the effect of nanoparticle size on the Thermophysical properties and heat transfer performance of sol–gel synthesized MgO/water Nanofluids for heat exchanger applications

This study experimentally investigates the effect of nanoparticle size and its associated surface properties on the heat transfer performance and thermophysical properties of water-based magnesium oxide (MgO) nanofluids synthesized via the sol–gel method. High-purity MgO nanoparticles with average sizes of 45 nm and 90 nm were dispersed in distilled water at volumetric concentrations of 0.25%, 0.33%, and 0.50%, utilizing carboxymethyl cellulose (CMC) as a stabilizer. Solid-state differential scanning calorimetry (DSC) revealed that the 45 nm particles exhibit higher specific heat capacity ( c p,np ) than the 90 nm particles due to increased surface-to-volume ratios. Colloidal stability analysis via zeta potential measurements indicated that lower concentrations (0.25 vol%) provide excellent stability (up to 50.82 mV), whereas increasing the concentration to 0.50 vol% prompts aggregation risks, particularly for the 90 nm sample. Thermal conductivity measurements, together with thermal performance tests conducted using a custom-designed finned-tube heat exchanger, revealed that the apparent heat transfer coefficient ( U app ) reached its maximum value at 0.25 vol%, corresponding to the best-performing concentration among those tested, yielding a 6.67% ± 2.63% enhancement in U app for the 45 nm nanofluid. At this threshold, the 45 nm nanofluid exhibits a statistically distinguishable heat transfer enhancement over the 90 nm counterpart due to intensified Brownian motion. Beyond 0.25 vol%, a severe degradation in thermal performance was observed for both particle sizes, driven by Van der Waals-mediated agglomeration and subsequent thickening of the momentum boundary layer. These findings provide crucial insights into the structure-property relationships of metal oxide nanofluids for the design of energy-efficient thermal management systems.