Numerical Study Of Natural Convection Of Nanofluid In Enclosures Filled With Porous Medium

Natural convection in porous media is often encountered in many engineering applications fields such as utilization of solar energy,oil exploitation,storage of nuclear waste materials,cooling of electronic devices,and others.The working fluids in porous media are conventional heat transfer fluids with low thermal conductivity and this limits the enhancement of natural convection heat transfer.An innovative and new technique to enhance heat transfer is dispersing nanoparticles into the base fluid.In recent years,some research works on natural convection in porous media filled with nanofluids have been performed by some researchers.However,studies on this topic are relatively scarce.In view of this,natural convection in porous media filled with nanofluids is investigated numerically using the local thermal equilibrium(LTE)model or local thermal no-equilibrium(LTNE)model.The contents and conclusions of the present study are as follows:(1)Natural convection in isosceles triangular porous enclosures filled with Cu-water nanofluid are investigated numerically using LTE model.The solid phase of the porous medium is glass balls.The middle part of the bottom wall of the enclosure is maintained at a constant high temperature while the rest are adiabatic,and the inclined sidewalls of the enclosure are cooled at a constant low temperature.Darcy's law is used to describe the fluid flow in porous media.The effects of Rayleigh number Ra,nanoparticles volume fraction ?,porosity of the porous medium ?,the diameter of glass balls dp,and dimensionless length A of partially heated bottom wall on natural convection of the isosceles triangular porous enclosure with different top angles.The results show that the average heat transfer rate decreases with increasing ? and A when the top angles are ?/2 and 2?/3.Meanwhile,the average Nusselt number increases with the increase of Ra,? and dp when the top angles are ?/2 and 2?/3.However,the average Nusselt number decreases with the increase of ?,? and A when the top angle is ?/3.(2)Three dimensional simulations of natural convection in a right angled prismatic porous enclosure filled with nanofluid is studied using LTE model.The bottom wall of the enclosure is kept at a constant high temperature,the inclined wall is cooled at a constant low temperature,and the other walls are adiabatic.The solid matrix of the porous medium is glass balls.Cu-water nanofluid and Cu O-water nanofluid are considered.The effects of porosity of the porous medium ?,volume fraction of nanoparticles ?,Rayleigh number and diameter of glass balls on natural convection are inspected.The results show that the average Nusselt number of the bottom wall decreases linearly with the increase of ?.At low Rayleigh numbers(Ra=10,50,and 100),the average heat transfer rate increases with increasing ?.However,at high Rayleigh number(Ra=500),the average heat transfer rate decreases with increasing ?.Furthermore,the average Nusselt number decreases slightly with the increase of the particle diameter of glass balls.(3)Natural convection around a heated cylinder in a square enclosure filled with porous medium saturated with Cu-water nanofluid is investigated numerically using LTNE and LTE models.The solid matrix of the porous medium is aluminum foam.The heated cylinder placed concentrically inside the enclosure is maintained at a constant high temperature while four walls of the enclosure are assumed to be at a constant low temperature.The influence of dimensionless volumetric heat transfer coefficient Nhs,porosity of the porous medium ?,nanoparticles volume fraction ? and Rayleigh number Ra on the flow and heat transfer characteristics are analyzed.The results show that the average Nusselt number Nunf for nanofluid decreases with increasing Nhs and ?,and it increases with the increase of Ra.Meanwhile,the average Nusselt number Nus for solid phase increases with increasing values of Nhs,? and Ra.It is also found that using nanofluid can not enhance the heat transfer rate.At low Rayleigh number,the variation of Nhs has little influence on Nunf and LTE model can be used instead of LTNE model.However,at high Rayleigh number,the variation of Nhs has great effect on Nunf and LTNE model should be used for accurate simulation.