Numerical Study On Oscillatory Instability And Transition To Chaos In Electro-thermo-convection Of Dielectric Liquids

Electro-thermo-convection in dielectric liquids describes the complex multiphysics coupling problem with unqiue flow phenomenon and heat and mass transfer process caused by the interaction between flow field,thermal field,electric field and free charges,which covers multiple subjects including fluid mechanics,electrochemistry,heat transfer,electrodynamics,etc.This work has rich physical connotation and high academic value,and has rather wide application prospects in lots of important fields including aerospace,pharmaceutical,food processing,and electronic industry.Owning to the complexity of the mathematical model as well as the abundant flow pattern transition,instability and bifurcation in the electrothermo-convection,the present related research work is still rather scare,and most of which focuses on the steady-state problems in simple geometric structures.This greatly hinders the develpoment of electrohydrodynamics in industrial applications.In view of this situation,this thesis amis to conduct an deeper research on the hydrodynamic behavior and heat transfer characteristics of unsteady electro-thermoconvection in different geometric models.First,the main generation and transport mechanisms of free charges in dielectric liquids under the action of a strong direct current electric field are discussed in detail,and then the macroscopic governing equations of each physical field in electro-thermo-convection system are defined,including the Maxwell equations under the quasi-electrostatic limit,the simplified energy conservation equation,the continuity equation,and the momentum conservation equation considering thermoelectric effects.The corresponding dimensionless governing equations are derived through dimensional analysis.Furthermore,the lattice Boltzmann method is adopted to establish the corresponding mesoscopic numerical model.Subsequently,the oscillation instability and the chaotic transition mechanism of the electro-electro-convection under different charge generation mechanisms are analyzed by using the above numerical model together with some nonlinear dynamic analysis methods.The main works of this thesis can be outlined as follows:Taking the electro-thermo-convection in horizontal concentric and eccentric cylindrical annuli as the object,the flow patterns and heat transfer enhancement under different driving parameters and geometric structures are compared and analyzed.For the concentric case,when the electric field strength exceeds a certain threshold,the radial convection induced by charge injection greatly enhances the heat transfer of the system.For the eccentric case,because the symmetry of the force is broken,the flow and heat transfer behaviors become more complex.Furthermore,the self-sustained oscillation of electro-thermo-convection in eccentric annulus is analyzed systematically.The typical characteristics of the oscillatory behavior are given and analyzed.It is shown that the competition between Coulomb force and thermal buoyancy force is the main reason for the oscillatory instability.The flow patterns in a wide range of driving parameters are obtained,and the influence of the radius ratio on the oscillatory behavior and heat transfer efficiency is further studied.Taking the electro-thermo-convection in a closed square cavity as the research object,and by carefully adjusting the values of the driving parameters,a series of nonlinear dynamic phenomena and heat transfer characteristics within the system that appear in the process of transition from the steady-state flow to the periodic oscillation flow,and finally to the chaotic flow are observed and analyzed.The internal mechanism of oscillatory instability is then discussed by visual analysis of the flow field structure and charge density distribution.By using nonlinear dynamic analysis techniques,three different routes to chaos are identified for the first time,namely,the quasi-periodic sequence involving four incommensurable frequencies,the intermittency sequence,and the alternating periodic-chaotic sequence.Moreover,the influence of shear force on three typical single frequency periodic oscillation modes of electro-thermo-convection in a closed square cavity is studied,complex flow evolutions caused by the interaction between shear force,thermal buoyancy force and electric field force are observed and discussed.It is found that the shear force has a certain stabilizing effect on the system.Based on the classical Rayleigh-Bénard convection between two flat plates and considering different injection direction,it is found for the first that strong unipolar charge injection induced by a uniform electric field can inhibit the secondary instability of thermal convection.Further increasing the electric field strength will cause the system to lose stability again and enter into a high-frequency oscillation mode,the frequency of which is two orders of magnitude higher than the original ones.In addition,the results show that the flow motion and heat transfer intensity are closely related to the charge injection direction.Then a lattice Boltzmann model for electro-thermo-convection considering the ion dissociation and recombination mechanism is established.The pure electro-convection is simulated,and the effects of non-dimensional ion dissociation parameter as well as the electric Rayleigh number on flow structure and distribution of positive and negative charge density are studied.Finally,by considering the thermal effect,the oscillatory instability of electro-thermo-convection under the ion dissociation and recombination mechanism is analyzed,and the effect of flow motion on heat transfer is discussed.