A comprehensive review of experimental investigations of forced convective heat transfer characteristics for various nanofluids

Nanofluids are suspension of nanoparticles (less than 100 nm) in the conventional base fluids. The dispersed solid metallic or non-metallic nanoparticles change the thermal properties like thermal conductivity, viscosity, specific heat, and density of the base fluid. Past studies focused on measuring the thermal properties of nanofluids. These suspended nanoparticles effectively improve the transport properties and heat transfer characteristics of the base fluids. Recently, heat transfer augmentation using suspensions of nanometre-sized solid particles in base liquids have been investigated by various research groups across the world. This paper reviews the state-of-the-art nanofluid studies in the area of forced convection heat transfer enhancement. The results for the heat transfer characteristics in internal flow with constant heat flux and constant wall temperature boundary conditions reported by various researchers have been compiled and reviewed. Further, in heat exchangers, the real boundary conditions are different from the constant heat flux and constant wall temperature boundary conditions. Over a span of 2 decades, the literature in this field is widespread; hence, this review would be useful for researchers to have a precise screening of a wide range of investigations in this area. Introduction Energy concerns have come up as the most important problem for the world's scientists and engineers. Thermal loads are increasing day by day and have wide variety of use in electronics, transportation, power plants, food industry, air conditioning, refrigeration, etc. The conventional heat transfer fluids, such as water, oil, ethylene glycol, propylene glycol are mostly used in industries. These fluids contain poor thermal properties. In order to increase heat transfer rates, the use of extended-surface thermal control technologies such as fins and micro channels, vibration of heated surface, injection or suction of fluid and applying electrical or magnetic fields has reached to the bottleneck. Therefore, new technologies with the potential to improve the thermo-physical properties of the conventional cooling fluids have been an area of great potential for researchers. The solids have better thermal properties than fluids. Ahuja (1975) and Liu et al. (2999) carried experiments to enhance the thermo* Correspondence: [email protected] Department of Mechanical Engineering, Guru Jambheshwar University of Science and Technology, Hisar, Haryana 125001, India Full list of author information is available at the end of the article © 2014 Gupta et al.; licensee Springer. This is a Attribution License (http://creativecommons.or in any medium, provided the original work is p physical properties of fluids by adding micrometreand millimetre-sized solid particles in the base liquids. However, real-world applications of these fluids are fewer due to the reasons, i.e. large-sized particles tend to quickly settle out of suspension and thereby, in passing through micro channels, cause clogging and a considerable rise in the pressure drop. Furthermore, the abrasive actions of these particles cause erosion of components and pipelines. To overcome these problems, nanosized particles dispersed in the base fluid known as nanofluids, were firstly introduced by Choi (1995) at the Argonne National Laboratory. These novel fluids indicated improved heat transfer properties such as higher thermal conductivity, long-standing stability and uniformity along with the negligible obstruction in flow channels due to very small sizes and large specific areas of the nanoparticles. The nanoparticles used to prepare the nanofluids are basically metals (e.g. Cu, Ni, Al), oxides (e.g. Al2O3, TiO2, CuO, SiO2, Fe2O3, Fe3O4, BaTiO3) and some other compounds (e.g. CNT, TNT, AlN, SiC, CaCO3, graphene) with a size of 1 to 100 nm. The great quantum of research on heat transfer enhancement shows the appreciable growth and the n Open Access article distributed under the terms of the Creative Commons g/licenses/by/4.0), which permits unrestricted use, distribution, and reproduction roperly credited. Gupta et al. International Journal of Mechanical and Materials Engineering 2014, 9:11 Page 2 of 21 http://www.springer.com/40712/content/9/1/11 necessity of heat transfer enhancement technology in the field of nanofluids. This paper presents the comprehensive review of various experimental investigations in convective heat transfer with the use of nanofluids in laminar and turbulent flow regimes under constant wall temperature and constant heat flux boundary conditions. Further, a detailed review on the use of nanofluids in different types of heat exchangers has been presented. It is vital for reliable applications in engineering thermal systems. Preparation of nanofluids This section presents different methods used by researchers for the synthesis of nanoparticles and preparation of nanofluids. For making nanoparticles, the current processes for the synthesis include inert-gas condensation process, chemical precipitation, mechanical milling, chemical vapour deposition, micro-emulsions, spray pyrolysis and thermal spraying. The nanoparticles are mostly used in powdered form for making nanofluids. In experimental studies, the preparation of nanofluids is the next most essential step. The nanofluids are not simply formed by mixing of solid particles in base liquids. Some special requirements are necessary including uniform, stable and durable suspension, minimal accumulation of particles, no chemical alteration of the fluid, etc. There are mainly two techniques used to produce nanofluids: the single-step and the two-step methods.