Validity of performance factors used in recent studies on heat transfer enhancement by surface modification or insert devices

•The performance factors have been used for evaluating heat transfer enhancement.•The validity of the performance factors used in previous studies are examined.•The performance factors used in previous studies are not valid.•Alternative performance factors for flows under an identical pumping power are proposed. Based on literature survey, the performance factors were used in 62 articles which have been published in 2021 in International Journal of Heat and Mass Transfer, International communications in Heat and Mass Transfer, International Journal of Thermal Sciences and Applied Thermal Engineering, to evaluate the heat transfer enhancement by geometry modification or insertion of tapes, wires or turbulence promoters under identical pumping power. However, the validity of the performance factors was not examined yet. The heat transfer rate in a periodic fully developed region of a circular duct with periodic annular baffle plates was numerically obtained. Comparing the heat transfer rate with that of a circular smooth duct, the validity of the performance factors used in previous studies was examined. It was found that the performance factors used in the previous studies are not valid for the evaluation of heat transfer enhancement. 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.