Numerical Study On Marangoni-thermocapillary Convection In Annular Pool Under Axial Magnetic Field

The fluid would be driven by the surface tension,which is caused by the uneven distribution of temperature on the free surface.The flow is named as thermocapillary convection or Marangoni convection.It widely exists in various natural phenomena and the actual production process,and the temperature gradient in different directions would lead to different flow patterns.In the present study,a majority of researchers paid their attention to the flow that is caused by the horizontal temperature gradient or vertical temperature gradient,the researches on the Marangoni-thermocapillary convection driven by bidirectional temperature gradients are few.The Marangoni-thermocapillary convection driven by the bidirectional temperature gradient is more complex,and it is more suitable for industrial practice.In the process of crystal growth by Czochralski method,Marangoni-thermocapillary convection would lead to uneven growth of crystals,which is not conducive to the production of high quality crystals.The axial magnetic field has been widely used in the inhibition of thermocapillary convection and Marangoni convection.Therefore,this paper will focus on the influence of axial magnetic field on Marangoni-thermocapillary convection,and discuss many influencing factors such as magnetic field intensity,the horizontal temperature gradient,heat flux on the bottom,depth of the pool and the buoyancy convection caused by gravity.The main works and results are as follows:Firstly,we investigated the Marangoni-thermocapillary convection in the annular pool under the combination of horizontal temperature gradient and vertical temperature gradient by three-dimensional numerical simulation,and divided the flow into stead flow and unsteady flow.Also,it has been concluded that there are two types of unsteady flow: one is dominated by the heat flux on the bottom,while the other one is dominated by the horizontal temperature gradients.Secondly,a series of simulations is carried out in a shallow annular pool with the depth of 3mm under microgravity.The influences of axial magnetic field on steady flow and unsteady flow are investigated,and the inhibition effect of axial magnetic field has been proved.Also,the different inhibition process in different unsteady flow types could be different according to the research.Thirdly,a series of simulations is carried out in a shallow annular pool with the depth of 6mm under microgravity.The influences of pool depth on Marangoni-thermocapillary convection are investigated.Also,the comparison between Marangonithermocapillary convection under microgravity and buoyant-Marangonithermocapillary convection under normal gravity has been carried out,the result showed that the buoyancy convection that is caused by gravity would enhance the instability of the flow.Finally,the effect of axial magnetic field on Marangoni-thermocapillary convection and buoyant-Marangoni-thermocapillary convection has been studied,respectively.The velocity,temperature distribution and temperature fluctuation have been analyzed.And the effect of buoyancy convection could be ignored when the axial magnetic field is strong enough to suppress the unsteady flow to basic flow.