Study On Electro-thermo-convection Characteristics Of Dielectric Liquids Based On The Injection-conduction Coupling Mechanism

Electro-convection(EC)and electro-thermo-convection(ETC)in dielectric liquids study the interaction between the electric field,flow field,temperature field,and free charges,presents abundant characteristics of flow field structure and transition,which owns important research value.In recent years,electrohydrodynamic(EHD)pump and electric enhanced heat transfer technology has broad application prospects in flexible robots,space thermal management,chip heat dissipation,and other fields due to its simple structure,no moving mechanical parts,convenient control,rapid response,and good environmental adaptability.However,at the present stage,most of the existing researches focus on a single charge,and there is a lack of researches on the flow heat transfer characteristics,flow state transition,instability and bifurcation characteristics of electric(heat)convection under multiple charge generation mechanisms.The EC and ETC problems involve the complex nonlinear coupling of multiple physical fields.Numerical simulation provides an effective means to study the nonlinear characteristics of EC and ETC systems.In this dissertation,we first introduce two main charge generation mechanisms in dielectric liquids: the injection mechanism and the conduction mechanism.We also describe the transport mechanism of free charge under the action of an electric field.On this basis,the macroscopic governing equations of EC,ETC under various mechanisms are given.Furthermore,the complete dimensionless governing equation and related dimensionless governing parameters of EC,ETC are derived by dimensional analysis.A solution algorithm was developed based on the finite volume method,and the transition and instability characteristics of EC and ETC with typical electrode structures were explored by combining nonlinear analysi s methods.The specific research work is summarized as follows:The numerical simulations of the unipolar injection induced EC between concentric and eccentric cylinders are carried out.It is found that for concentric cylinders,mobility parameters significantly affect the critical point and transition sequence of the instability in EC.Three transition sequences from steady state to chaos for EC were observed under different mobility parameters.A power exponential relationship between the Nusselt number Ne and the Rayleigh number T is identified for the chaotic EC system.For the eccentric cyli nders,the eccentricity and radius ratio have significant effects on the flow characteristics and the transition sequence to chaos in the EC system,meanwhile,more abundant steady-state convection structures are observed before the flow instability.The s imulation of the cell structure,evolution process,and instability characteristics of the EC flow between three-dimensional concentric cylindrical electrodes shows that the system is influenced by the roll structure and the hexagonal structure at the init ial stage of the evolution of the EC flow,and then evolved into a polygonal cell structure.When the computational domain is large,the polygonal cell structure is not stabl e.When the computational domain is limited to 1/4 circumferential length,the steady triangular cell structure is observed.The symmetry and periodicity can be clearly observed in the structure of three-dimensional EC flows.For dielectric liquids with residual conductivity in practice,the EC between parallel plate electrodes and concentric cylindrical electrodes was numerically simulated based on the charge conduction-injection mechanism.It is found that injection strength and residual conductivity significantly affect the flow structure,flow strength,flow transition,and other flow characteristics of the EC system.A further study demonstrates that the residual conductivity of dielectric liquid has two effects on the EC system: inhibition and stabilization.For parallel plate electrodes,the flow field is dominated by a single main vortex and accompanied by several angular vortices,the EC flow always oscillates for weakly injected convection.For concentric cylinders,the difference between linear and nonlinear critical criteria decreases with the increase of residual conductivity.The residual conductivity significantly affects the evolution sequence from steady state to chaos and the recovery sequence from chaos to steady state.For unsteady convection,the initial flow field significantly affects the fully developed flow state and critical point.Based on the charge conduction-injection mechanism and coupled the temperature field with the EC system,the numerical simulations of the ETC system in a rectangular cavity and concentric cylinders are carried out.For a rectangular cavity,it is found that the direction of charge injection significantly affects the characteristics of flow structure and flow transition of ETC.Compared with that of upper injection,the transition takes place earlier,and the heat transfer enhancement after the transition is almost the same for the same electric Rayleigh number.T he temperature field and injection strength also significantly affect the characteristics of transition and heat transfer of the ETC system.Flow transition under strong injection strength takes place at a lower T value while the heat transfer enhancement is greater.For the ETC between concentric cylinders,the flow gradually evolves from one pair of vortices to 8 pairs of vortices with abundant transitions when the temperature field is kept constant and increases the value of T,at the same time,the dominant force also changes from buoyancy to coulomb force.The Rayleigh number Ra and residual conductivity significantly affect the transition of steady-state convective structures.When the residual conductivity is small,the intermediate flow states of the transition process are abundant.With the increase of the residual conductivity,the intermediate flow state decreases,and the maximum velocity and heat transfer enhancement of the system decreases.With the increase of electric Rayleigh number T,the ETC system becomes unstable,and periodic,quasi-periodic,and even chaotic convective states take place.The simulation results show that Ra has little effect on the periodicity of the ETC system,but significantly affects the transition sequences of ETC flows from steady state to chaos.