Aydin, Duygu Yilmaz2026-06-192026-06-1920252076-3417https://doi.org/10.3390/app15147831https://hdl.handle.net/20.500.12899/5264This study presents an experimental evaluation of the thermal and exergetic performance of an air-to-air heat pipe heat exchanger using a cobalt oxide (Co3O4)-based non-Newtonian nanofluid, with the additional incorporation of carbon black (CB). Nanofluids were synthesized via a two-step method and tested under turbulent flow conditions across varying Reynolds numbers. The results demonstrated that increasing the Co3O4 nanoparticle concentration and adding CB substantially improved both the thermal and exergetic performance compared to deionized water. Specifically, maximum thermal efficiency improvements of 62.7% and 75.4% were recorded for nanofluids containing 1% and 2% Co3O4, respectively. The addition of CB further enhanced the thermal efficiency, achieving a maximum improvement of 79.2%. Furthermore, the maximum reduction in thermal resistance reached 61.4% with CB incorporation, while the 2% Co3O4 nanofluid achieved a maximum decrease of 50.2%. The use of nanofluids led to a significant reduction in exergy loss, with exergy-saving efficiencies reaching up to 33.6%. These findings highlight the considerable potential of Co3O4- and CB-based hybrid nanofluids in advancing waste heat recovery technologies and enhancing the thermodynamic performance of air-to-air heat pipe heat exchanger systems.eninfo:eu-repo/semantics/openAccessAir-To-Air Heat Pipe Heat ExchangerCo3O4 NanofluidCarbon BlackWaste Heat RecoveryArticle10.3390/app1514783115142-s2.0-105011852121Q1WOS:001550987400001Q20000-0003-0557-5279