方腔内电场强化固液相变传热

Numerical investigation of electrohydrodynamic solid-liquid phase change in square enclosure

采用格子Boltzmann方法对方腔内介电相变材料的熔化过程进行数值模拟与分析,系统研究了电场力和浮升力耦合作用下固液相变传热过程的流体流动、电荷输运以及传热等基本特征,重点分析了电瑞利数T、斯蒂芬数Ste、离子迁移率M和普朗特数P r等多个无量纲参数对固液相变传热过程的影响.研究表明,与浮升力驱动下的固液相变情况相比,外加电场不仅会改变方腔内流体流动结构以及相界面的演化规律,而且还会提高介电相变材料的熔化效率,强化固液相变传热过程.特别地,上述现象会随着电瑞利数T的增大愈加明显.此外,引入了无量纲参数Φ,用以表征电场强化固液相变的实际效果.结果显示,随着斯蒂芬数Ste的增加,电场强化固液相变的实际效果会有所减弱,但对于较大的电瑞利数而言,改变斯蒂芬数Ste的实际大小并不会对电场强化固液相变的实际效果有太大影响.最后,发现电场强化固液相变的实际效果与离子迁移率M一般具有负相关的关系,即随着离子迁移率M的增加,电场强化固液相变的效果反而下降;而受电场力和浮升力的协同影响,普朗特数P r对电场强化固液相变效果的影响则依赖于离子迁移率M.

Melting of the dielectric phase change material inside a closed square enclosure is numerically investigated.The fully coupled equations including Navier-Stokes equations, Poisson’s equation, charge conservation equation and the energy equation are solved using the lattice Boltzmann method(LBM). Strong charge injection from a high temperature vertical electrode is considered and the basic characteristics of fluid flow, charge transport and heat transfer in solid-liquid phase change process under the coupling of Coulomb force and buoyancy force are systematically studied. Emphasis is put on analysing the influence of multiple non-dimensional parameters,including electric Rayleigh number T, Stefan number Ste, mobility number M, and Prandtl number P r on electrohydrodynamic(EHD) solid-liquid phase change. The numerical results show that comparing with the melting process driven by buoyancy force, the applied electric field will not only change the flow structure in liquid region and the evolution of the liquid-solid interface, but also increase the heat transfer efficiency of dielectric phase change material and thus enhance the solid-liquid phase change process. In particular, we find that this phenomenon becomes more pronounced when T is larger. Further, the dimensionless parameter Φ is introduced to characterize the effect of EHD enhanced solid-liquid phase change, and the results indicate that the effect of EHD enhancement solid-liquid phase change is weakened with the increase of Stefan number Ste,However the change of Ste does not make much difference in EHD enhancement solid-liquid phase change for a sufficiently high electric Rayleigh number T, and it is attributed to the fully developed convection cells at a very early stage of the melting process. Moreover, it is found that the effect of EHD enhancement solid-liquid phase change is negatively related to the mobility number M and that the effect of Prandtl number P r on the EHD enhancement solid-liquid phase change largely depends on the mobility number M, which is due to the simultaneous influence of electric field force and buoyancy force. In general, the electric field has a significant influence on the melting process of dielectric phase change material, especially at high T, P r and low Ste, M.And quantitatively, in all tested cases, a maximum melting time saves about 86.6% at T = 1000, Ra = 10000,M = 3, Pr = 20, and Ste = 0.1.