Validity of Performance Factors Used in Recent Studies on Heat Transfer Enhancement of Nanofluids

Many previous studies used the performance factors for the evaluation of heat transfer enhancement of nanofluids under the identical pumping power. The validity of the performance factors was not examined yet. The validity of the performance factors used in previous studies examined considered only flows in a circular tube based on the empirical correlations and experimental data. It was found that the performance factors used in the previous studies are not valid for the evaluation of heat transfer enhancement of nanofluids. Furthermore, this paper shows that the direct comparison of heat transfer rates without the assumptions of equal surface area and the equal temperature difference is suitable for the evaluation of heat transfer enhancement of single-phase fluids.