Up to date review on the synthesis and thermophysical properties of hybrid nanofluids

Research studies about nanofluids are on the rise owing to the mounting interest and demand for nanofluids as heat transfer fluids in a wide variety of applications in the recent years. This can be noticed from the number of papers published and in particular by journal special issues. To establish the field further, the purpose of this paper is threefold. First, it offers a literature review on hybrid or composite nanofluids, which are prepared either by dispersing different nanoparticles as individual constituents or by dispersing nanocomposite particles in the base fluid, taking 160 papers published from before 1995 to 2017 into account. Second, it offers a contemporary investigation on preparation and thermophysical properties of hybrid nanofluids. Lastly, this review outlines the applications and challenges associated with hybrid nanofluid; which should stimulate further research. This review also discusses several factors affecting the thermophysical properties; including types of nanoparticles, solid volume fraction, different base fluid, stability, temperature, particle size, shape, pH, sonication, and surfactants. There are many contradictory results found in the literature on the influence of effective parameters on thermophysical properties. It has been observed that the thermophysical properties are affected by the mentioned parameters. This review also reveals that proper characterization of hybrid nanofluids results in more efficient heat transfer fluids compared to single nanoparticle based nanofluid. However, more intense research is needed towards the selection of proper hybrid nanoparticles, their preparation, characterization and long-term stability to exploit their full potential as well as to overcome the barriers in applying these new fluids in industrial scale applications. Both practitioners in companies and academics might find this review useful, as it outlines significant lines of research in the field.