Figure 6 shows the variation in Nusselt number with different coolant flow rate of various nanofluids (CNT-water and Al2O3-water) at 0.15% volume concentration. While, the variation in Nusselt number with different coolant flow of various nanofluids (CNT-water and Al2O3-water) at 1% volume concentration is shown in Fig. 7. It is observed that the Nusselt number increases with the increase in the coolant flow rate, thereby increases the heat transfer (Figs. 3 and 4). This may be due to the fact that the enhanced thermal conductivity of nanofluids increases the heat transfer performance. CNT-water nanofluid was found to exhibit enormous heat transfer performance compared to Al2O3-water nanofluid for any value of coolant flow rate and nanoparticle concentration. Compared to water, CNT-water nanofluid (1% by volume) exhibited 45.87%, 66.64%, 76.55%, and 90.76% increase in the Nusselt number at flow rates 2 l/min, 3 l/min, 4 l/min, and 5 l/min, respectively. On the contrary, Al2O3-water nanofluid (1% by volume) exhibited 24.66%, 39.17%, 44.18%, and 52.03% increase in the Nusselt number at coolant flow rates 2 l/min, 3 l/min, 4 l/min, and 5 l/min, respectively, compared to the results with water as a coolant. The increase in the Nusselt number of CNT-water nanofluid (0.15% by volume) compared to water was found to be 15.30%, 22.09%, 32.18%, and 39.95% for the flow rates of 2 l/min, 3 l/min, 4 l/min, and 5 l/min, respectively. While, Al2O3-water nanofluid (0.15% by volume) exhibited 10.21%, 13.63%, 20.83%, and 23.07% enhancement in Nusselt number at coolant flow rate 2 l/min, 3 l/min, 4 l/min, and 5 l/min, respectively, compared to the pure water. Compared to water, the maximum enhancement in Nusselt number for CNT-water and Al2O3-water nanofluid are found to be 90.76% and 52.03%, respectively. The experimental result shows that CNT-water nanofluid gives enormous enhancement in heat transfer compared to the Al2O3-water nanofluid. CNT nanoparticles of 20–30 nm diameter, 3–8 μm length and Al2O3 nanoparticles with 100 nm length are used in the present experimental investigation. CNTs have higher thermal conductivity, higher aspect ratio, lower specific gravity, large SSA and lower thermal resistance compared to Al2O3-water nanofluid. The higher thermal conductivity and larger SSA of CNT nanoparticles compared to Al2O3 nanoparticles play an important role for the better heat transfer enhancement of CNT-water nanofluid [15,18]. Therefore, greater enhancement is shown by CNT-water compared to Al2O3-water nanofluid.