【作者】 张波；

【导师】 程永攀； 蔡正敏；

【作者基本信息】 华北电力大学(北京) ， 工程硕士（专业学位）， 2018， 硕士

【摘要】 液滴/气泡的运动规律是动力循环相变传热的基础问题,而液滴/气泡的自发迁移运动正在引起研究学者越来越多的兴趣。在气液两相流中,两相界面处发生着传热传质过程,常常伴随着温度不均并产生界面张力分布不均,使得相界面附近的流体介质发生流动进而自发产生迁移现象,这样的运动过程被称作是Marangoni迁移运动[1]。这方面的研究对于提高相变传热的传热效率具有重要的意义,同时对于微重力下主动控制液滴/气泡的运动规律,也有重要的意义。本文主要是通过数值模拟的方法,研究热辐射条件下,由于界面温度分布不均产生的热毛细力驱动液滴/气泡迁移现象。首先概括了液滴/气泡的Marangoni自发迁移现象研究的发展过程,阐述了现有的理论分析,数值模拟以及实验方面的主要成果;然后采用Level Set Method捕捉两相界面,建立二维瞬态轴对称模型模拟了液滴/气泡的迁移过程,将低Ma数、低Re数下的数值模拟计算的结果与理论解进行比较,验证了模型的准确性,而后将数值模拟进一步推广到高Re数和高Ma数下的液滴/气泡迁移过程来展开研究。主要研究迁移工程中不同Re数下动力粘度比,导热系数比,密度比,比热容之比对液滴/气泡迁移速度的影响。研究结果发现低Re数下,迁移速度主要受动力粘度比和导热系数比的影响,而在高Re数下,内外相密度之比以及比热容之比也会对迁移速度产生影响。除此之外,研究还发现在任意Re数下,当连续相的动力粘度或者导热系数与扩散相对应的液滴/气泡参数值相等时,迁移速度达到最小值。随着连续相与扩散项的物性参数差异的逐渐增大,迁移速度也会相应变大。同时,我们还定义了Marangoni迁移过程中的努塞尔数,得出了努塞尔数会随着Re数的增大而增大的变化规律。任意Re数下,对于气泡的迁移,当连续相的动力粘度、导热系数或者比热容与气泡对应物性参数之比达到最大值时,努塞尔数值最小。这一研究对于在热辐射情况下,通过热毛细力操纵液滴或气泡运动有着重要的指导意义。更多还原

【Abstract】 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.更多还原