the pH of each nanofluid sample has been justified at different values by adding the adequate quantity of HCl and NaOH solutions to the suspensions. The pH is measured using a pocket-sized pH meter (AZ Co. 8686). The photographs, which are shown in Fig. 3, demonstrate the stability of CuO/water and Fe2O3/ water nanofluids at the concentration of 0.65 vol.% and at different pH. These photos were taken a week after the preparation. As can be seen in Fig. 3a and b, the stability condition at pH ¼ 10.1 and pH ¼ 11.1 is the best for CuO and Fe2O3 nanoparticles, respectively. Therefore, all the experimental data reported in this study have been gathered at above mentioned pH