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

Abstract

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.