Numerical Simulation On Flow Pattern Multiplicity And Evolution Of Rayleigh-bénard-marangoni Convection Of Cold Water In Cylindrical Containers

The Rayleigh-Bénard-Marangoni(R-B-M)convection driven by the coupling of thermocapillary force and buoyancy is a natural phenomenon widely existing in industrial production and daily life.In this relatively simple physical system,the rich and diverse flow structures and complex flow pattern evolution have been the research hot spot of many scholars.At present,the research on R-B-M convection mainly focuses on the conventional fluids whose density varies linearly with temperature,but less on R-B-M convection of the fluids with the extreme density.Therefore,this thesis presents a series of three-dimensional numerical simulations on R-B-M convection of the cold water with the extreme density in the cylindrical containers.The critical conditions and flow patterns of the convection appearance are determined.The first bifurcation flow structure and flow pattern evolution of R-B-M convection of the cold water with extreme density in the cylindrical containers under different Biot numbers,density inversion parameters and the aspect ratio are obtained.The influences of various factors on the heat transfer characteristics are analyzed.The main conclusions are as follows(1)The critical Rayleigh number of the appearance of R-B-M convection of the cold water is related to the density inversion parameter,the aspect ratio and Biot number.With the increases of Biot number and the aspect ratio,and the decrease of density inversion parameter,the critical Rayleigh number decreases.The critical flow pattern is multi-cell flow structure.With the increase of the aspect ratio,the number of flow cells increases.(2)With the increase of Rayleigh number,R-B-M convection of the cold water will evolve.When the aspect ratio is small,the thermal conduction state of cold water transits to multi-cellular first,then polygonal flow cells appear,and finally turns into concentric rings with the increase of Rayleigh number.When the aspect ratio is larger,the primary bifurcation flow is a multi-cell flow structure.With the increase of Rayleigh number,the number of flow cells decreases,the flow is enhanced and the flow cells fuse with each other.There exists a lag phenomenon of flow pattern transition of R-B-M convection of the cold water.Therefore,several flow patterns can be observed at the same Rayleigh number.(3)With the increase of Biot number,the convective heat transfer between the liquid layer and the external environment is enhanced,the surface temperature distribution becomes more uniform,and the thermocapillary force is weakened.Furthermore,the axial temperature gradient increases and the buoyancy force is enhanced.The coupling driving force of buoyancy and thermocapillary force first increases and then weahens.Therefore,there is a maximum value of the coupling driving force.At this time,the flow in the liquid pool is the strongest and the heat transfer performance is the best.(4)The average Nusselt number on hot wall is related to Rayleigh number,density inversion parameter and the aspect ratio.With the increase of Rayleigh number,the average Nusselt number increases monotonously.With the decrease of density inversion parameter and the increase of the aspect ratio,the average Nusselt number is improved.