Experimental investigation of the effect of pH on the stability and thermal conductivity of metal oxide nanofluids

The pH level of nanofluids plays an important role in stability and thermal conductivity. But limited studies have been done in this field. In this research, the effect of pH on the stability and thermal conductivity of ZnO-EG nanofluid at concentrations of 0.05 and 0.75% volumetric fraction and MgO-W at concentrations of 0.05 and 0.5% volumetric fraction were investigated. Experimental measurements of the thermal conductivity were performed by KD2 Pro at a constant temperature of 25 °C. The results showed that the pH strongly affected the stability of nanofluids, so that at the pH of the isoelectric point (IEP), complete aggregation and sedimentation were observed. The thermal conductivity of nanofluids have the lowest value at the pH of the isoelectric point, but as the pH moves away from the isoelectric point, the thermal conductivity has increased. The highest enhancement in the thermal conductivity of ZnO-EG nanofluid was 63%, which was obtained at a volume fraction of 0.75% and pH = 12. However, the highest enhancement in the thermal conductivity of MgO-W nanofluid was 49%, which obtained at a volume fraction of 0.5% and pH = 12. Finally, using the experimental results and with the help of curve fitting, equations with good quality were presented to predict the effective thermal conductivity of metal oxide nanofluids.