Numerical Investigation On Spontaneous Droplet/bubble Migration Under Thermal Radiation

The dyanmics of droplets and bubbles are the fundamental problems in phase change heat transfer in power cycles,and the spontaneous migration of droplets/bubbles are causing more and more interest from the researchers.In the gas-liquid flows,heat and mass transfer occurs across the gas-liquid interface.The temperature variations along the interface will lead uneven distribution of the surface tension,thus the flow around droplet/bubble is induced,leading to spontaneous Marangoni migration of droplet/bubble The study in this area has great significance in enhancing the efficiency in phase change heat transfer.In the meanwhile,it also has great significance in actively controlling the droplets/bubbles dynamics under microgravity.In my study,the sponentaneous migration of droplet/bubbles are investigated through numerical simulation,the migration is caused by the thermo-capillary effect due to uneven distribution of temperature along the droplets/bubbles.Firstly,the literature review is carried out on the spontaneous migration of droplets/bubbles,the theoretical analysis,numerical simulation and experimental studies are summarized.Secondly,the two-dimensional transient axisymmetrical numerical model is developed based on the level set method,and it can used to simulate the spontaneous migration of droplets/bubbles under thermo-capillary effect.The numerical results under low Reynolds number and low Marangoni number are compared with the theoretical solution to verify the accuracy of the numerical model.Through the numerical simulation,the droplets/bubble migration under high Reynolds number and high Marangoni number is studied.In the migration process,the ratio of dynamic viscosity ratio,thermal conductivity ratio,density ratio and heat capacity ratio of different Reynolds number is studied.The numerical results show that under the low Reynolds number,the migration velocity is mainly affected by the ratios of dynamic viscosity and thermal conductivity.Under high Reynolds number,the density ratio and the ratio of specific heat capacity of inside and outside phase will also affect the migration velocity.In addition,it is found under the arbitrary Reynolds number,when the dynamic viscosity or thermal conductivity of continuous phase equal to the corresponding values of the droplets/bubbles diffusion phase,the migration velocity will reach the minimum value.With the increasing difference of the physical values of the continuous phase and the diffusion,the migration rate increases gradually.At the same time,the Nusselt number is defined in the migration process,and found that the Nusselt number increases with the increase of Reynolds number.Under arbitrary Reynolds number,in the case of the migration of bubbles,when the ratio of dynamic viscosity or thermal conductivity of continuous phase or the ratio of heat capacity to the corresponding the physical values of the bubbles reaches the maximum,Nusselt number reaches the minimum.This study might be of great significance for manipulating the droplet/bubble through thermocapillary force with incoming thermal radiation.