###### Abstract  
This study presents an experimental evaluation of the thermal and exergetic performance of an air-to-air heat pipe heat exchanger using a cobalt oxide (Co\({}_{3}\)O\({}_{4}\))-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 Co\({}_{3}\)O\({}_{4}\) 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% Co\({}_{3}\)O\({}_{4}\), 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% Co\({}_{3}\)O\({}_{4}\) 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 Co\({}_{3}\)O\({}_{4}\)- 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. air-to-air heat pipe heat exchanger; Co\({}_{3}\)O\({}_{4}\) nanofluid; carbon black; waste heat recovery 2025 28 June  
Heat pipes, the core components of HPHE systems, rely on phase-change principles to transport heat efficiently over considerable distances with minimal temperature loss. A typical heat pipe comprises three primary sections: an evaporator, where the operating fluid takes in thermal energy and undergoes evaporation; an adiabatic section, through which vapor moves without significant heat loss; and a condenser, where the vapor emits its latent energy and transitions into a liquid phase [10]. The condensed working fluid flows back to the evaporator either through capillary forces or under the influence of gravity [11; 12]. The thermal performance of heat pipes is strongly influenced by the thermophysical properties of their working fluids, thus making fluid selection crucial for enhancing overall heat exchanger efficiency [13; 14; 15; 16]. Recent advancements in nanotechnology have enabled the development of nanofluids, which are fluids containing uniformly dispersed nanoparticles that exhibit superior thermal properties compared to conventional working fluids. Various studies have demonstrated that incorporating nanoparticles into working fluids improves thermal conductivity, increases convective heat transfer coefficients, and reduces viscosity, thereby enhancing overall heat transfer performance [17; 18; 19]. By reducing the thermal resistance of the heat pipe and increasing the evaporation-condensation cycle's effectiveness, nanofluids offer a promising approach for optimizing the performance of HPHEs. Among the different types of nanoparticles used in heat transfer applications, Co\({}_{3}\)O\({}_{4}\) nanoparticles stand out due to their excellent thermal stability, high surface area, and superior heat transfer properties [20; 21]. Co\({}_{3}\)O\({}_{4}\) has been observed to enhance heat transfer rates by increasing the effective thermal conductivity of the fluid [22]. By integrating Co\({}_{3}\)O\({}_{4}\)-based nanofluids into heat pipe systems, it becomes possible to extend the operating temperature range of heat exchangers and improve their overall effectiveness in capturing low-grade waste heat. Furthermore, carbon-based materials have recently attracted considerable attention for their ability to enhance the thermal properties of fluids. In particular, nanofluids consisting of Co\({}_{3}\)O\({}_{4}\) nanoparticles combined with carbon-based additives have been investigated due to their potential to further enhance heat transfer performance [23; 24; 25; 26; 27].