Evaluation methods of thermo-hydraulic performance in nanofluids: A comprehensive review

Widely employed for enhancing heat transfer in various devices, nanofluids are still seriously challenged by contradictory conclusions on their effectiveness under different evaluation methods. Because of the lack of literature explicitly addressing this issue, misjudgment of nanofluids’ effectiveness will continue to occur, unless the reasons behind flawed evaluation methods are elucidated. In response, this study provides a comprehensive review of the evaluation methods for assessing the thermo-hydraulic performance of nanofluids when applied to single-phase forced convective heat transfer. By comparing various evaluation methods used in the literature in assessing nanofluids’ performance, we have identified unreasonable evaluation methods that use inappropriate constraints or evaluation indexes. The use of Reynolds number as a constraint can distort assessment results, due to improper consideration of fluid velocities’ impact on heat transfer and flow resistance. Meanwhile, many evaluation indexes struggle to accurately weigh heat transfer enhancement against the simultaneous increase in flow resistance caused by nanofluids. Moreover, most assessments mainly focus on specific heat devices, while neglecting the comprehensive impact of nanofluids on entire heat exchange systems. To address these issues, we proposed a novel evaluation method to assess nanofluids’ effectiveness in systems from a practical engineering perspective, and investigated critical factors that influence the assessment of their effectiveness in reducing heat exchange surface area, enhancing heat transfer, and decreasing operational energy consumption. Based on these findings, key factors that warrant thorough exploration for future studies are also highlighted.