###### Abstract
Among carbon-based materials, CB has attracted considerable attention in thermal system applications due to its strong light absorption capacity, high surface area, cost-effectiveness, and favorable dispersion characteristics. CB contributes to increased heat transfer efficiency, particularly in systems involving turbulent flow, heat exchangers, or energy-harvesting technologies. [PERSON] et al. reported a 44% enhancement in the heat transfer coefficient when employing  
Figure 1: Application areas of air-to-air HPHEs. carbon-water nanofluids in double-pipe heat exchangers, accompanied by a 42.2% improvement in thermal-hydraulic performance at a Reynolds number of 10,200 [28]. Similarly, [PERSON] et al. reported a 40.7% enhancement in convective heat transfer using CB-water nanofluids in helical-coil heat exchangers [29]. [PERSON] et al. used CB nanofluid as a coolant for photovoltaic modules. They reported that the highest overall efficiency was achieved at a concentration of 0.21 wt [30]. [PERSON] et al. investigated the use of CB nanoparticles in thermal oil for solar thermal applications. Their findings demonstrated that CB-enhanced nanofluids maintain high thermal and optical stability under elevated temperatures, making them promising candidates for direct-absorption solar collectors [31]. Despite their promising thermal properties, nanofluids often encounter stability challenges caused by nanoparticle aggregation and sedimentation. To mitigate these issues, stabilizing agents, such as carboxymethyl cellulose (CMC), are employed. CMC is an anionic polysaccharide, produced from natural cellulose by carboxymethylylation, and is readily soluble in water. It is widely utilized as a stabilizing, thickening, and dispersing agent in food, pharmaceutical, and industrial applications. In nanofluid formulations, CMC plays a crucial role as a surfactant and rheological modifier, improving the dispersion stability of nanoparticles by preventing agglomeration and sedimentation. Its polymeric structure provides steric hindrance and enhances colloidal stability over extended periods. Owing to its biodegradability, non-toxicity, and environmental compatibility, CMC is considered a green additive for nanofluid systems. CMC solutions also exhibit shear-thinning (pseudo-plastic) rheological behavior [32; 33; 34; 35]. [PERSON] and [PERSON] experimentally investigated nanofluids prepared with 0.4 wt% CMC and three different metal oxide nanoparticles (CuO, Al\({}_{2}\)O\({}_{3}\), and TiO\({}_{2}\)) and reported shear-thinning (non-Newtonian) behavior based on power-law fitting, along with enhanced colloidal stability demonstrated by extended sedimentation times up to 30 days [36]. Within this context, evaluating the thermal performance of CMC-stabilized Co\({}_{3}\)O\({}_{4}\) nanofluids in heat transfer systems is highly relevant for advancing energy recovery technologies and enhancing system-level efficiency. Numerous experimental studies have explored the thermal performance of HPHEs using various nanofluids, aiming to enhance heat transfer efficiency and reduce thermal resistance under different operating conditions. These studies have employed a wide range of nanoparticle types, base fluids, and stabilizing agents to improve the thermophysical properties of working fluids and system effectiveness. Table 1 presents a comparative summary of recent experimental research on nanofluid-enhanced HPHE systems, highlighting their working fluids, nanoparticle concentrations, and thermal performance outcomes. The main objective of this study is to enhance the thermal and energetic performance of air-to-air HPHEs by utilizing Co\({}_{3}\)O\({}_{4}\)-based non-Newtonian nanofluids stabilized with CMC while specifically investigating the additional improvements provided by CB incorporation.