Thermal Rectification Enhancement of Coalescence–Jumping Phase Transition Thermal Diodes using Cu–Al₂O₃ Hybrid Nanofluids

Herein, a novel coalescence–jumping phase transition thermal diode using hybrid nanofluids, an emerging thermal fluid with proven supreme thermal properties, is first proposed and investigated. The thermal rectification enhancement mechanism is investigated by the evaporation performances of a working fluid-filled superhydrophilic porous tank. Furthermore, a developed mathematical model is well-matched with the experimental results in terms of the heat transfer and thermal rectification performance of the thermal diodes using hybrid nanofluids. It is worth noting that increasing the volume fraction of the hybrid nanofluids does not necessarily improve the thermal rectification performance of the thermal diodes due to the aggregative nanoparticle deposition on the evaporation surface. In addition, there is a need to balance the thermal conductivity (higher copper proportion) and stability (higher alumina proportion) of the hybrid nanofluids used in the thermal diodes for long-term operation. The experimental results show that the thermal rectification performance of the thermal diode using Cu-Al₂O₃ hybrid nanofluid can be increased to 637.4, showing a nearly 300% improvement compared with the thermal diode using water. Overall, the remarkable results in this study promote the applications and studies of thermal diodes using hybrid nanofluids in various engineering systems for thermal management.