Enhanced heat transfer of laser-fabricated copper nanofluid at ultra-low concentration driven by the nanoparticle surface area

Abstract

As solar thermal energy systems are an important pillow toward green energy production, the enhancement of their thermophysical properties using nanofluids is a highly relevant topic. However, when nanofluids are designed by the addition of nanoparticles (NPs), their colloidal stability is frequently impaired during high- temperature processing, a phenomenon related to particle size, morphology, and concentration. In this work, we synthesized nanofluids composed of ligand-free colloidal CuNPs dispersed in ethylene glycol by continuous- f low, picosecond-pulsed laser ablation in liquids synthesis, yielding monomodal-CuNPs with mean diameters of 2.5 and 4.8 nm. The nanofluids' thermal conductivity (k nf nf ) was measured using a guarded-hot-plate method in the temperature range from 298 to 318 K. We observed a nanoparticle surface area-dependent enhancement of the k up to 30 % at ultra-low volume concentration of 20 ppm. This corresponds to 30 times higher concentration-normalized k nf in comparison to the state-of-the-art, while the resulting nanofluids retain their rheological properties. The findings are matched with Yu-Choi's theoretical model calculations, indicating that heat transfer at the nanoparticle-solvent interface is driven by an interfacial layer of solvent molecules. These f indings highlight the suitability of laser-fabricated ligand-free CuNPs as additives for heat transfer fluids, maximizing performance in mid-temperature heat transfer applications like solar thermal collectors.