Viscosity and thermal conductivity of ZnO–water-based nanofluids stabilized by grafted SMA-g-MPEG comb-shaped copolymer for heat transfer applications

In this work, the effect of poly(styrene-co-maleic anhydride)-grafted methoxy (polyethylene glycol) (SMA-g-MPEG) copolymer as dispersants on the viscosity and thermal conductivity of zinc oxide (ZnO)–water-based nanofluids was studied. Various molecular weight MPEGs were grafted to SMA copolymer by esterification reaction, and ZnO nanoparticles (NPs) were prepared by microwave irradiation method. Prepared ZnO NPs and copolymers were characterized by UV, FESEM, TEM, XRD, FTIR, NMR and TGA techniques. The grafted copolymer was used to enhance the dispersibility of ZnO–water-based nanofluids. The effect of SMA-g-MPEG comb-shaped copolymer on the viscosity and thermal conductivity of the suspensions was investigated at different concentrations (0.1, 0.3 and 0.5 wt%) and solid volume fractions of ZnO NPs (φ = 0.5–3.0%). The suspension with SMA-g-MPEG 2000 dispersant showed improved stabilization at higher particle concentration. The variation of viscosity with shear rate showed the nanofluids behaved as a non-Newtonian fluid at the lower shear rate and Newtonian behaviour with the increase in shear rate. However, thermal conductivities of the ZnO–water-based nanofluids increased with increasing of the particle volume concentration and decreased with increase in the chain length of the grafted molecules. The average chain length of the grafted molecule exhibited enhanced thermal conductivity as compared with that of the base fluids. Finally, experimental values of the thermal conductivity and viscosity were compared with the estimations done by several simple theoretical models.