Experimental Study On Flow Structures And Heat Transfer In Turbulent Rayleigh-benard Convection With Viscoelastic Fluid

Rayleigh-Bénard convection(RBC)is a classical model for turbulent thermal convection,and it is also considered as one of the simplest models to study turbulence.RBC system widely exists in people's daily life,industrial production and nature.It has very important theoretical value and research significance.When a small amount of polymer or surfactant is added to water,the flow resistance can be greatly reduced.And this is the drag reduction effect of viscoelastic fluid.Recently,the influence of polymers on heat transfer in RBC system has attracted much attention.However,the research results conclude both heat transfer enhancement and heat transfer reduction with the addition of polymer.It can be seen that the influence of polymer on heat transfer in RBC system is complex,while it is still unkown whether there exists the influence of surfactant solution(another kind of viscoelastic fluids)on heat transfer in RBC system.Therefore,the experiment of RBC system with viscoelastic fluids is carried out in this paper.The heat transfer and flow structure in RBC system with different kind of viscoelastic fluids and with aspect ration are deeply studied.The mechanism of heat transfer change is expounded.Firstly,the experimental device of RBC system was established.The physical parameters of viscoelastic fluids were measured.The results show that the shear viscosity of viscoelastic fluids is obviously higher than that of water,while the other physical parameters are not obviously different from that of water.The shear viscosity of polymer polyethylene oxide(PEO)aqueous solution does not change with shear rate.The surfactant cetyltrimethyl ammonium chloride(CTAC/Na Sal)aqueous solution is a typical shear thinning fluid.The experimental methods for studying the flow characteristics in RBC system are established: particle image velocimetry(PIV)and shadow method.Secondly,the experiments of RBC system with polymer aqueous solution were carried out.Nuseelt number(Nu),Reynold number(Re)and large scale circulation scale in RBC system were calculated.And the flow velocity field of RBC system was measured by PIV.The results shows that the heat transfer of RBC system is weakened by polymer,while the flow state of RBC system has no change with the addition of polymer,but the large scale circulation area is reduced.By measuring the thickness of the viscous boundary layer in RBC system,it is found that the polymer increases the thickness of the boundary layer,which is the reason for the weakening of heat transfer in RBC system.The temperature distribution law of RBC system with polymer aqueous solution is similar to that of water case,and the temperature distribution satisfies the logarithmic distribution law.Thirdly,flow and heat transfer characteristics of RBC system with surfactant aqueous solution were studied experimentally.It was found that surfactant greatly reduced the heat transfer in RBC system.The flow characteristics of RBC system were studied by shadowing method and PIV technology.It is found that the shear dilution of surfactant aqueous solution inhibits the formation of large scale circulation in RBC system,resulting in the weakening of heat transfer.Because of the difference of flow pattern,the temperature distribution of RBC system with surfactant aqueous solution is different from that of water.The addition of surfactant also increases the thickness of viscous boundary layer in RBC system,which weakens heat transfer in RBC system.Finally,the experiments of viscoelastic fluid RBC system with aspect ratio of 2 were carried out.It is found that the addition of polymer enhances the heat transfer of RBC system,while surfactant still weakens the heat transfer.By measuring the flow velocity field,temperature distribution and the thickness of viscous boundary layer in RBC system,it is found that the polymer aqueous solution makes the plume flow more regular,which enhances the heat transfer in the bulk region of RBC system.And it will also increase the thickness of the boundary layer,which weaken the heat transfer in the boundary layer region of RBC system.The heat transfer proportion in the bulk region of RBC system with aspect ratio of 2 is larger,so the heat transfer enhancement appears under the combined action.The large scale circulation in RBC system with surfactant aqueous solution is still not formed,and the heat transfer of RBC system is weakened.At the same time,the flow pattern of RBC system with aspect ratio of 2 is similar to that of RBC system with aspect ratio of 1,so it has the same temperature distribution.The scaling laws of Nu?Ra and Re?Ra for RBC system with viscoelastic fluid are given in combination with Nu,Re and Rayleigh number(Ra)under all experimental cases.