Numerical Study Of Natural Convection Heat Transfer Characteristics Of Al₂O₃-Water Nanofluids In Enclosures

As a new type of heat transfer fluid, nanofluid has potential application prospects. Nanofluid can be used to improve heat transfer efficiency of vehicle cooling system and manned spacecraft, and develope a liquid thermal control system with small size, light weight and compact structure. Nanofluid can be also used in many enginnering applications such as nuclear system, chemical process, refrigeration and air conditioning, biological medicine and so on. In view of potential application value of nanofluid in flow and heat transfer field, it is very important to study the flow and heat transfer characteristics of nanofluid. In the present study, natural convection of Al₂O₃-water nanofluid in two-dimensional and three-dimensional enclosures is investigated numerically. The research contents and results are as follows:（1） Three-dimensional numerical simulations of natural convection heat transfer of Al₂O₃-water nanofluid in a differentially-heated, cubic enclosure is performed using four different viscosity models of nanofluid. The effects of nanoparticles volume fraction and Rayleigh number on natural convection of Al₂O₃-water nanofluid are analyzed with four different viscosity models. The results show that the average Nusselt number increases with increasing nanoparticle volume fraction with defferent viscosity models at Ra=103. When Ra is larger, the average Nusselt number increases with increasing nanoparticles volume fraction for nanofluid viscosity model Ⅰ, and the average Nusselt number decreases for nanofluid viscosity models Ⅱ, Ⅲ and Ⅳ. Meanwhile, the heat transfer rate increases with increasing Rayleigh number.（2） Three-dimensional numerical simulations of natural convection heat transfer of Al₂O₃-water nanofluid in a differentially-heated, cubic enclosure is performed considering nanoparticles shape. The effects of nanoparticles shape, nanoparticles volume fraction and Rayleigh number on natural convection heat transfer of Al₂O₃-water nanofluid are analyzed. The results show that the thermal conductivity of nanofluid increases with the addition of Al₂O₃ rod-shaped nanoparticles, and the increasing degree of viscosity of nanofluid is higher than that of thermal conductivity. This indicates that the heat transfer rate of nanofluid decreases with the increase of flow resistance. When Al₂O₃ spherical nanoparticles are added in water, the strength of flow and heat transfer increases with increasing nanoparticles volume fraction. The average Nusselt number increases with the increase of nanoparticles volume fraction at Ra=103 for Al₂O₃-water nanofluid with different nanoparticles shape. When Ra is larger, the average Nusselt number decreases with the increaing volume fraction of Al₂O₃ rod-shaped nanoparticles, and the average Nusselt number increases with the increase of spherical nanoparticles volume fraction. Meanwhile, the heat transfer rate increases with increasing Rayleigh number for nanofluid with different nanoparticles shape.（3） Two-dimensional numerical simulations of natural convection heat transfer of Al₂O₃-water nanofluid in a cavity is performed. A heat source is embedded on the bottom wall of the cavity, the left and top walls are kept at a constant low temperature, and the other walls are adiabatic. The effects of nanoparticles volume fraction, position and size of heat source on natural convection of nanofluid Al₂O₃-water are analyzed. The results show that the strength of nanofluid flow decreases as nanoparticles volu me fraction increases and this causes the deterioration in the heat transfer. When heat source is located on the left bottom wall with the dimensionless distance between the left wall and heat source D=0.2, the average Nusselt number of nanofluid is larger. The average Nusselt number of nanofluid decreases with moving heat source to right. When D=0.8, the average Nusselt number is relatively lower. Meanwhile, the heat transfer rate of nanofluid decreases with the increase of heat source size.