Numerical Study On Unsteady Natural Convection Of Cu-Water Nanofluids In Cavity With Transient Boundary Conditions Of Local Heat Source Temperature

Unsteady natural convection with time-varying temperature of a localised heat source at the wall of an enclosure exists in many engineering applications such as electronic engineering,chemical engineering,solar energy and so on.Due to the low thermal conductivity of conventional heat transfer fluids such as water and oils,the enhancement of this type of natural convection has been limited.Many researchers have found that natural convection can be enhanced using nanofluids.In view of practical importance of this type of natural convection and potential application of nanofluids in heat transfer enhancement,unsteady natural convection of Cu-water nanofluids in enclosure with time-varying temperature of a localised heat source has been studied numerically.The contents and conclusions of this present research are as follows:(1)Localised heat source temperature at the bottom wall of an enclosure is sinusoidally oscillated with time and transient natural convection of Cu-water nanofluids in this system is studied numerically.Two types of the enclosure model C1 and C2 with different location of cold and adiabatic walls are considered.The effects of Rayleigh number,dimensionless length of localised heat source B and dimensionless amplitude of time-varying temperature a on natural convection of Cu-water nanofluids in two types of the enclosure are presented.The numerical results show that the flow and heat transfer characteristics of Cu-water nanofluids in C1-type and C2-type enclosure are related to the values of Ra,B and a.The time-averaged heat transfer rate of the localised heat source for two types of enclosure increases with the increase in one parameter and the other two parameters having the fixed values.Moreover,the heat transfer rate of C1-type enclosure is higher than that of C2-type enclosure.(2)Localised heat source temperature at the bottom wall of an enclosure is sinusoidally oscillated with time and transient natural convection of Cu-water nanofluids in C1-type enclosure is studied numerically.The influence of volume fraction of nanoparticles ?,dimensionless location of localised heat source D and aspect ratio of the enclosure AR on natural convection of Cu-water nanofluids is studied.The numerical results indicate that adding nanoparticles into pure water improves the flow intensity and heat transfer rate in enclosure.As D increases,the circulation strength of nanofluids increases and the heat transfer rate changes slightly.With the increase of AR,the circulation strength of nanofluids increases.Furthermore,the variation trend of heat transfer rate with AR is related to the value of Rayleigh number.(3)By considering Brownian motion,localised heat source temperature at the bottom wall of an enclosure is sinusoidally oscillated with time and transient natural convection of Cu-water nanofluids in C1-type enclosure with the inclined angle is investigated numerically.The effects of volume fraction of nanoparticles ?,Rayleigh number and inclination angle of the cavity on unsteady natural convection heat transfer are analyzed.The results show that when Brownian motion is considered,the circulation strength of nanofluids is related to Rayleigh number.For a high Rayleigh number,the circulation strength of nanofluids is higher when Brownian motion is considered.However,at a lower Rayleigh number,the circulation strength of nanofluids changes slightly when Brownian motion is considered.The heat transfer rate considering Brownian motion is larger than that without considering Brownian motion and it increases as Ra increases.The influence of inclination angle of the enclosure on the flow and heat transfer characteristics of nanofluids is related to the value of Rayleigh number.For a lower Rayleigh number,the influence of inclination angle of the enclosure on the flow and heat transfer characteristics of nanofluids is not obvious.However,at a higher Rayleigh number,increasing the inclination angle can not enhance the circulation strength and heat transfer rate.