NUMERICAL STUDY OF NATURAL CONVECTION IN RECTANGULAR CAVITY FILLED WITH NANOFLUID UNDER MAGNETIC FIELD

Abstract

The natural convection mode of heat transfer has received a lot of attention due to its wide range of applications in several industries. Miniaturization of the devices in several sectors has compelled the researchers to adopt different approaches to improve efficiency, such as increasing the heat transfer area by mounting internal and external fins, increasing the temperature gradient, and improving the thermo-physical properties of the working fluid. The fluids, which have been commonly used, are water, ethylene glycol, engine oil, etc. Poor thermal conductivities of these fluids have forced researchers to look for alternatives. Initially, the micro-sized solid particles of material with high thermal conductivity were dispersed in the fluid to improve its thermal conductivity. Though the thermal conductivity of the fluid was increased, the problem of agglomeration and high viscosity was raised. The issue was raised due to the large size of the solid particles. In 1995, Choi developed a fluid by dispersing solid particles of size less than 100 nm into the base fluid and termed them nanofluids. It was found that the mixture was stable than the earlier one. Since then, the researchers have focused their attention on developing the theory related to this fluid. Though this fluid has better thermo-physical properties like good thermal conductivity and low viscosity, the stability of the fluid is still an issue. The stability of this fluid is inversely proportional to the particle concentration. Hence, it is essential to check whether nanofluid with extremely low particle concentration can significantly increase the heat transfer rate. First, the numerical simulations of natural convection in a square enclosure consisting of two mutually orthogonal heaters of the same size are under an external magnetic field and are filled with extremely low particle concentration of Fe3O4-water nanofluid are carried out for different Rayleigh numbers. The effect of particle concentration, magnetic field strength, Rayleigh number and positions of both the heaters have been investigated. The effect of particle concentration is found to be different for different Hartmann numbers. For Hartmann number equal to 25, there is an increase in mean Nusselt number by 13.88 % when the particle concentration increases from 0.01 % to 0.07%. In contrast, for Hartmann numbers of 50 and 75, mean Nusselt numbers are increased by 20.69 and 34.09%, respectively. The positions of the heaters have significant effects on the mean Nusselt number. When the horizontal heater is shifted from a position located in the upper half of the enclosure to the mid of the enclosure, the mean Nusselt number is increased by 10.71 %, whereas it is increased by 12.84% when it is shifted from center of the enclosure to a position in the lower half of the enclosure. In case of vertical heater, there is an increase in the mean Nusselt number by 11.33% when the position of vertical heater is shifted from a position in upper half to mid of the enclosure and by 12.04%, when it is shifted from mid to a position in the lower half of the enclosure. After this, a numerical investigation of the effect of spatially varying magnetic fields on free convection in an enclosure filled with Fe3O4 nanofluid is performed. A magnetic source is placed below the enclosure. A spatial varying magnetic field created by the magnetic source creates a gradient in the magnetization of the nanofluid; hence, along with magnetohydrodynamics, ferrohydrodynamics is also considered in this study. The effects of the relative positions of the heater and magnetic source have been investigated. At last, a numerical study of the Marangoni-Benard convection of Fe3O4 based nanofluid is conducted. The gradient of surface tension due to variation in temperature of the surface is found to govern the motion of the fluid. The flow inside the enclosure is examined under different parameters like Marangoni number, Biot number, Hartmann number, and inclination of the magnetic field. The effects of these parameters on the surface temperature of the fluid have been investigated and these parameters alter the fluid motion inside the domain.