Detailed review of hybrid and carbon-based nanofluids for double-tube heat exchangers: Recent advances and performance evaluation

Nanofluids have high thermal conductivity, which is why they have become the focus of researchers, developers, and designers who want to use them as working fluids to maximize the thermal performance of thermal applications, including heat exchangers. Nanofluids made of metal particles and their oxides are widely used in thermal applications. However, in recent years, hybrid and carbon nanostructured nanofluids have gained widespread use due to their unique thermophysical properties. In this regard, this paper presents a comprehensive review of studies that have used hybrid nanofluids as working fluids in double-tube heat exchangers, either individually or in comparison, such as CuO-Al₂O₃ and Al₂O₃-TiO₂ nanofluids. Additionally, it examines studies that utilized carbon nanostructure nanofluids for the same purpose, including multi-walled carbon nanotube nanofluids. The review found that hybrid nanofluids and carbon nanostructured nanofluids improved the overall heat transfer coefficient, Nusselt number, and effectiveness, with a relative increase in friction coefficient, compared to the base fluids. These nanofluids also exhibit thermal superiority over other nanofluids, such as metal oxide nanofluids, and are therefore considered pioneers in this field.