Numerical Study Of Natural Convection Heat Transfer Characteristics Of Nanofluids In Enclosures

As a new type of high efficient heat transfer fluid,nanofluid has broad application prospect in many engineering application fields such as utilization of solar energy,cooling of electronic devices,nuclear power system,refrigeration and air conditioning,and others.In recent twenty years,studies on nanofluid have gained more and more attention in the world.Many scholars have carried out many theoretical and experimental studies on the heat transfer characteristics of nanofluid and they have gained fruitful results.In this paper,the natural convection heat transfer characteristics of nanofluid in two-dimensional and three-dimensional enclosures is investigated numerically.The research contents and conclusions are as follows:?1?Natural convection of water-based nanofluid between two-square concentric duct annuli is studied numerically.The outside wall of the annulus enclosure is maintained at a constant high temperature,and the inside wall is cooled at a constant low temperature.Four types of nanoparticles,Ag,Cu,Cu O and Ti O2,are taken into consideration.The effects of Rayleigh number and volume fraction of nanoparticles on the natural convection heat transfer characteristics of nanofluid are analyzed.The results show that conduction dominates the heat transfer process at low Rayleigh number,and the velocity and temperature gradients of nanofluid within the enclosure are changed slightly.At high Rayleigh number,convection dominates the heat transfer process.For all Rayleigh numbers,all of the average Nusselt numbers for water-based nanofluid containing different nanoparticles increase with the increase of nanoparticles volume fraction.Meanwhile,the average Nusselt numbers for water-based nanofluid containing metal nanoparticles are larger than those for water-based nanofluid containing metal oxide nanoparticles at the fixed values of Rayleigh number and solid volume fraction.?2?Natural convection of Ag-water nanofluid between two-square eccentric duct annuli is studied numerically.The outside wall of the annulus enclosure is maintained at a constant high temperature,and the inside wall is cooled at a constant low temperature.The effects of Rayleigh number,aspect ratio of the annulus AR,eccentricity W and nanoparticles volume fraction on heat and fluid flows are investigated.The results indicate that with increasing aspect ratio AR from 0.25 to0.75,the fluid flow intensity becomes weaker and the average Nusselt number for the hot wall increases.As eccentricity W increases,the fluid flow intensity becomes stronger and the average Nusselt number for the hot wall increases.Furthermore,theaverage Nusselt number for the hot wall increases with increasing nanoparticles volume fraction at any given Rayleigh number.?3?Three-dimensional natural convection heat transfer of Al₂O₃-water nanofluid in a cubic enclosure heated from below is investigated numerically considering nanoparticles size.The bottom wall of the enclosure is maintained at a constant hot temperature,while the top wall,left and right sidewalls have a constant cold temperature.Two other walls of the enclosure are adiabatic.The effects of Rayleigh number,nanoparticles volume fraction and nanoparticles size on the natural convection heat transfer of Al₂O₃-nanofluid are discussed.The results show that with the increase of Rayleigh number and nanoparticles volume fraction,the average heat transfer rate increases.Moreover,it is also found that nanoparticles size has a significant effect on the natural convection heat transfer.As nanoparticles diameter decreases,the average heat transfer rate increases at the fixed value of nanoparticles volume fraction.