Natural Convection Heat Transfer Characteristics Of Nano Fluids In An Enclosed Square Cavity With Time-periodic Boundary Conditions

Transient natural convection with time-periodic boundary conditions exists in manyengineering applications such as cooling of electronic devices, bioengineering, electronicengineering, chemical engineering, environment and solar energy and so on. Meanwhile, as anew type of heat-transfer fluid，nanofluid has provided new challenges and opportunities forthermal science and gained increased interest． In view of practical importance of transientnatural convection and feasibility of nanofluid, the transient natural convection of nanofluidhas been studied numerically in a square enclosure with time-periodica boundary conditions.The flow and heat transfer characteristics of this kind of problems are analyzed. The mainresearch contents are follows:(1) In a square enclosure where the temperature of vertical sidewall is sinusoidallyoscillated with a constant average temperature, the opposing sidewall is cooled at a relativelylow temperature, the influence of pertinent parameters such as Rayleigh number, volumefraction of nanoparticles, dimensionless amplitude and time period on the transient naturalconvection heat transfer characteristics of nanofluid is studied. Buoyancy force and naturalconvection of nanofluid can be improved with increase of Rayleigh number while coefficientof heat conductivity increases with increase of volume fraction of nanoparticles. These twoparameters can enhance convective heat transfer of fluid. Geometric average Nusselt numberon the hot and cold walls presents sinusoidal variation with the same time period oftemperature of heat wall, but phase difference exists. Their size of amplitude is impacted byamplitude and time period of temperature of hot wall. The secondary vortex in the oppositedirection of mainstream periodically appears and disappears with temperature change on hotwall. Meanwhile heat from the hot wall is not completely removed from the enclosurethrough the cold wall, and there is a part of heat removed from the upper part of hot wall.Even in a given period of time, the whole hot wall has heat eduction and no heat in.(2) In an inclined square enclosure with positive or negative inclined angles where thesinusoidal oscillating wall temperature is on one side wall and relative low constanttemperature is on the opposing side wall, the effects of positive/negative inclined angles of theenclosure, Rayleigh number and volume fraction of nanoparticles are studied. The resultsdemonstrate that heat transfer rate of nanofluid is enhanced with increase of volume fractionof nanoparticles in any inclined angle. The greater the inclined angle is, the strongest naturalconvection is, but natural convection heat transfer has not to be strongest. The inclined angle has the effect on the natural convection heat transfer rate, and there is an optimum angle withthe best natural convection heat transfer. The bigger the Rayleigh number is, the smaller thisangle is. When the inclined angle is at about15°with Ra=10~6, the natural convection heattransfer rate of nanofluid obtains the best. Volume fraction of nanoparticles has no mucheffect on the best inclined angle.