Recent theoretical studies show that the convective thermal performance of nanofluids in cooling applications depends crucially on the effective thermophysical properties and, if the performance comparisons are made under different flow constraints, contradictory conclusions can be drawn regarding the effectiveness of the same nanofluid. In this work, an experimental study was reported on the laminar forced convection of Al2O3-water and Al2O3-Polyalphaolefin (PAO) nanofluids through a circular minichannel. Based on the experimental data, the thermal effectivenesses of the nanofluids were evaluated critically using various figures of merit (FOM) under three typical flow constraints, i.e., constant flow rate, constant Reynolds number and constant pumping power. Although the nanofluids enhance convective heat transfer, the results show their overall thermal effectiveness is adversely affected by the combined effects of higher viscosity and lower specific heat. In particular, when the cooling application is constrained by the constant pumping power condition, there is virtually no difference in the thermal performances between the base fluid and the nanofluids.

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