Numerical Study Of Unsteady Natural Convection Heat Transfer Of Nanofluids In A Two-Dimensional Isosceles Triangular Cavity With Time-Oscillating Heat Flux

At present,most studies on natural convection heat transfer in the cavity have focused on steady natural convection.However,the unsteady natural convection heat transfer has a wide application background in the fields,such as electronics,biology,chemical engineering and solar energy utilization.Moreover,as a new material with excellent heat transfer performance,nanofluids gradually replaces the traditional heat transfer media.Therefore,it is necessary to study the unsteady natural convection heat transfer of nanofluids.In this paper,the unsteady natural convection heat transfer of nanofluids?Cu-water,Al₂O₃-water and TiO₂-water?in a two-dimensional isosceles triangular cavity with oscillating heat flux has been studied numerically.The main contents and conclusions of this paper are as follows:?1?Under the condition that the heat flux of the bottom wall oscillating with cosine law,the unsteady natural convection of nanofluids in a two-dimensional isosceles triangular cavity is studied numerically.The effects of Rayleigh number,volume fraction of nanoparticles ?,types of nanoparticles?Cu,Al₂O₃ and TiO₂?and oscillation period of heat flux?_p on natural convection heat transfer are examined.The results show that the increase of the values Ra and ? can enhance the buoyancy force in cavity and the heat transfer effective coefficient of the fluid respectively.Moreover,it reduces the temperature range of heat source in one period by adding nanoparticles to the base liquid.Besides,the heat transfer rate is affected by the dimensionless oscillation period of heat flux.During 0.01??_p?10,the dimensionless oscillation period of heat flux has a significant effect on the average Nusselt number and dimensionless temperature of heat source.However,during?_p?10,the influence of oscillation period of heat flux on heat transfer can be neglected.In addition,compared with Al₂O₃ and TiO₂ nanoparticles,Cu nanoparticles have higher thermal conductivity so that the efficiency of natural convection of Cu-water nanofluid is higher.?2?The influence of geometrical conditions of the cavity,such as enclosure aspect ratio,heat source location as well as heat source length,on the unsteady natural convection heat transfer of Cu-water nanofluid is studied numerically.The results show that the enclosure aspect ratio has different effects on the heat transfer rate for different Ra numbers.Moreover,as the local heat source is located at the center of the bottom wall,the heat transfer rate decreases while the temperature of the heat source increases.And the effect of the location of heat source on heat transfer is obvious especially at low Ra number.Besides,with the increase of the length of the heat source,more heat is generated by the heat source.After which,the buoyancy force in the cavity is improved,the temperature of the heat source increased and the flow velocity of the fluid enhanced.In addition,the heat source end near the cold wall is the place where heat transfer rate is highest.And the highest temperature of the heat source will appear in the center of the heat source.?3?The unsteady natural convection heat transfer of Cu-water nanofluid in a two-dimensional inclined isosceles triangular cavity is studied numerically.It is found that the effect of inclination angle on the heat transfer rate is different for different Ra numbers.At low Ra number,the effect of inclination angle on heat transfer rate is not obvious.But with the increase of Ra number,the effect is obvious,and the temperature of heat source decreases.When the length of heat source is short,the flow of Cu-water nanofluid in the cavity as well as heat transfer rate is enhanced with the inclination angle increasing.As the length of the heat source is long and the Ra number is high,an adverse value for inclination angle exists during 0°<?<30°,which results in the minimum heat transfer rate.At the same time,with the increase of the inclination angle of the cavity,the heat exchange between the fluid and the left end of the heat source becomes intense gradually,and more heat will flow out of the cavity from the left cold wall.For all Ra numbers and heat source lengths,the effect of the inclination angle on heat transfer rate is not obvious when the inclination angle is greater than 60°.