*8.1. Challenges in Carbon-Based Nanofluids*

Carbon allotropes in the nanoscale have shown to be promising in enhancing the thermal performance of liquids when homogeneously dispersed, and their products are commercially available through a wide range of companies. Nevertheless, these powders are very expansive from an economic perspective compared to other sorts of nanomaterials, which makes their utilization quite questionable in the sense of their feasibility towards the targeted application [484]. Therefore, one of the challenges that need to be focused on is how to fabricate large quantities of these nanopowders at minimal production cost. In the current situation and before even introducing such type of nanofluids to the industry, researchers need to initially evaluate the gained performance enhancement and economic benefits of carbon-based nanofluids for each selected application before hands, and hence more work is needed in this area. Some scholars have proposed combining carbon-based nanomaterials with other cheaper types of nanoparticles (e.g., Cu, Al, and Fe) to form hybrid nanofluids containing carbon allotropes, and thus reducing the suspension cost [485–488]. However, the feasibility of such an approach remains limited, and the consideration of such types of hybrid nanofluids remains in the exploration stages. As it is well-known by now that the favorable thermophysical properties of nanofluids have made such a category of working fluids beneficial when used for enhancing the system performance of many thermal applications. Yet, the stability of the dispersed particles remains a major drawback and thus limiting the widespread of these suspensions. This is because, in an unstable state, the particles tend to cluster into larger forms of agglomerate, and therefore the benefits of the high surface area of the nanoparticles losses its optimum effectiveness on the exposed host (i.e., base fluids). For such a reason, it is essential that any proposed nanofluid to the industry maintains its long-term stability. This is where the preparation phase of the product plays a critical role. In order to overcome this difficulty, scientists have suggested using physical approaches (e.g., sonication) and/or chemical methods, such as surfactants and surface functionalization. Although this can help solve the aggregation problem, the changes caused to the surface of the dispersed particles remain another uncertainty that needs to be understood. In addition, further exploration on combining the two stabilization methods (i.e., physical and chemical routes) need to be conducted. Moreover, a joint international standard database on the thermophysical properties and physical stability of different types of nanofluids, their dispersed nanomaterial(s) concentration, and fabrication approach is strongly needed [198]. This is because even after more than 25 years from the first discovery of nanofluids, scientist are still reporting different thermophysical properties and physical stability for similar synthesized suspensions. In terms of properties prediction, it was shown previously that both effective thermal conductivity and effective viscosity lacks universal correlations and can only be determined through experimental means. However, artificial neural networking that is based on data mining has started to show good accuracy in predicting these properties, but further research is still required in this area [489–492].
