Chapter 7 Natural Convection in Nanofluids 7.1 Natural Convection in Water As discussed in preceding chapters, the suspension of nanoparticles is one of the best suitable alternatives to enhance the profitable thermophysical properties of working fluids. But before focusing on the effect of nanoparticles dispersion on the natural convection heat transfer, a brief discussion on the natural convection in water is essential for the comparison and validation purpose. There has been abundant research work, comprising theoretical and experimental, conducted on the study of natural convection in water or gases, and different kinds of outcomes have been presented for innumerable geometrical-shaped test sections. The Rayleigh–Bénard convection is the classical model used by the researchers to study the natural convection in an enclosure. In the Rayleigh–Bénard convection, the fluid is enclosed between two plates and heated from below and cooled from above, as shown in Fig. 7.1, and the model is presented by Barna et al. [1] to show the convection rolls in two spatial dimensions: a horizontal and a vertical. The effect of non-uniform temperature heating of side wall on natural convection in a parallelepiped enclosure filled with water was explored by Filis et al. [2]. Two different cells having a half height of enclosure were used to create a non-uniform heating, in which one top cell filled with hot water and other bottom cell with cold water. A forced direct thermal contact due to flow structure for the aspect ratio (ratio of the enclosure depth to the enclosure width) of 0.749 was observed and resulted in the weak dependency of the Nusselt number (Nu) on the Rayleigh number (Ra). The transient natural convection in an inclined rectangular enclosure which is heated and cooled by two opposed walls was studied through experiments by Upton and Watt (1997). The temperature and velocity field during the evolution of the flow to steady state for different inclination angles of 45°, 90° and 135° were examined. For inclination angle 45°, a rapid growth in Nu was found at the center of the enclosure as compared to the wall for aspect ratio of 1.0. Turan et al. [3] have numerically investigated the effect of constant heat flux conditions on natural convection in a rectangular enclosure with heated side walls © The Author(s), under exclusive license to Springer Nature Singapore Pte Ltd. 2021 A. Kumar and S. Subudhi, Thermal Characteristics and Convection in Nanofluids, Lecture Notes in Mechanical Engineering, https://doi.org/10.1007/978-981-33-4248-4_7 151