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Electrothermal convection demonstrates the huge potential for the active enhancement of heat transfer, but an in-depth understanding of many related processes is still superficial. In this paper, electrothermal convection subjected to unipolar injection within rectangular cavities is investigated. A series of simulations under different Rayleigh number Ra and electric Rayleigh number T are carried out and the stability diagram of the flow motion in the Ra-T plane is obtained. The stability diagram can serve as a benchmark for the study of electrothermal convection. Besides, the impact of charge in the injection direction (with charge in injection from below or from above) and different aspect ratios are studied. It turns out interestingly that the injection directions do not modify the stability thresholds, the maximum velocity, and the heat transfer efficiency. Moreover, it is found that the geometric size has a significant influence on the flow pattern, and a more complex transition process occurs at a larger aspect ratio. [ABSTRACT FROM AUTHOR]

Electro-thermo-hydrodynamic (ETHD) flows induced by simultaneous Coulomb and buoyancy forces in a dielectric medium are studied. Previous results limited to two dimensions are extended to three dimensions. The fully coupled governing equations, including the Navier–Stokes equations, the electrohydrodynamic equations, and the energy equation, are solved using a unified lattice Boltzmann model. Various flow patterns, determined by the balance between the buoyancy-driven mechanism and the Coulomb-driven mechanism, can be observed for different combinations of the governing parameters (the electric Rayleigh number T and the Rayleigh number Ra). It is found that the electrical effect on enhancement of heat transfer becomes significant at a relatively low value of Ra. Besides, an approximately linear relationship is found between the Nusselt number Nu and T. Finally, ETHD flows for different directions of charge injection are investigated, and the results reveal that the heat transfer performance of the system is improved when the injection direction is the same as the direction of the temperature gradient. [ABSTRACT FROM AUTHOR]

The electro-thermo-convection of a perfectly insulating liquid lying between a square enclosure and a heated inner circular cylinder is numerically investigated using a unified lattice Boltzmann method (LBM). The flow motion is driven by the thermal buoyant force and the Coulomb force due to a direct current (DC) electric field exerting on free space charges injected from the inner cylinder. Four lattice Boltzmann equations are used to solve all governing equations including the Navier-Stokes equations, the energy conservation equation and a simplified set of Maxwell’s equations. It is found that the augmentation of heat transfer due to the electrical effect becomes significant when the electric driving parameter T above a critical threshold. For sufficiently high values of T , the flow is fully dominated by the Coulomb force, and the average Nusselt number becomes independent of Ra . The influence of the fluid properties and geometric parameter on heat transfer enhancement is also studied. [ABSTRACT FROM AUTHOR]

This paper numerically investigates the effect of unipolar charge injection on the onset of Rayleigh-Bénard convection (RBC) in a layer of insulating liquid contained between two parallel plates. A recent developed unified model on the basis of the mesoscopic lattice Boltzmann method (LBM) is employed to solve the entire macro control equations. Two different cases of charge injection from above and from below are considered. It is interesting to find that the charge injection advances the onset of convection regardless of the injection direction. Besides, the decrease of critical Rayleigh number, along with the increase of flow intensity and the enhancement of heat transfer resulting from charge injection remain the same for different injection directions. However, the results show that the charge injection directions have significant effect on temperature and charge density distribution, as well as the evolution of flow motion, especially for the case of larger electric Rayleigh number. In addition, the effects of Prandtl number, the mobility number and the injection number are also studied. [ABSTRACT FROM AUTHOR]

Highlights • Oscillatory flow of electro-thermo-convection in eccentric annulus is studied for the first time. • A unified lattice Boltzmann model is adopted to solve the whole set of governing equations. • Results are presented for different control parameters and radius ratio. • The flow undergoes steady state, periodic oscillation and chaotic state as electric Rayleigh number increase. • The formation of oscillating flows is strongly related to the value of radius ratio. Abstract A detailed numerical investigation for oscillatory electro-thermo-convection of dielectric liquids in horizontal eccentric annulus is carried out based on a unified lattice Boltzmann method (LBM). The liquid is under the simultaneous action of a strong unipolar charge injection and a thermal gradient. The self-oscillatory behaviors and heat transfer characteristics for different Rayleigh number Ra , electric Rayleigh number T , Prandtl number Pr , mobility parameter M and radii ratio are studied systematically. The results show that the values of Ra within the range from 103 to 104 have little effect on the periodic oscillatory convection, while the flow develops from steady to periodic oscillatory and finally evolves into chaotic as T increases, and the fundamental frequency and the fluctuation amplitude as well as the heat transfer rate increase with T. Moreover, it is found that the fluid properties have significant impact on the oscillatory convection, and the formation of oscillating flows is strongly related to the value of radius ratio. [ABSTRACT FROM AUTHOR]

Abstract The natural convection heat transfer in a dielectric liquid confined in horizontal cylindrical annuli and enhanced by imposing a direct current electric field is numerically investigated using a unified lattice Boltzmann method. This augmentation of heat transfer is attributed to the flow motion induced by the Coulomb force due to the electric field exerting on free charges injected from the inner cylinder. Both concentric and eccentric configurations are considered. It is found that there exists a threshold of the electric driving parameter T , above which the heat transfer enhancement due to the electric effect becomes significant for concentric cases. For eccentric configuration, this change becomes relatively gentle. In addition, the influence of the eccentricity on heat transfer enhancement is also studied. [ABSTRACT FROM AUTHOR]

The externally imposed direct current (DC) electric field mightily impacts on the conjugate heat transfer existing somewhere between the solid and liquid phases, which is numerically investigated via Lattice Boltzmann method (LBM) in this paper. The model is a square cavity filled with dielectric liquid, in which has a centred heat-generating solid body, and the spatially inhomogeneous physical properties such as thermal conductivity, heat capacity, permittivity and mobility are considered distinctively. The lattice Boltzmann method is used to solve the entirely coupled equations and the developed code is first verified by three cases for the hydrostatic solution, conjugate heat transfer with and without heat source. The LBM results show the highly agree well with both the derived analytical solution and the results obtained by other papers. Then, enhancement effect of the flow and heat transfer when electrohydrodynamic (EHD) forces in existence are studied under the different governing parameters and the internal heat source intensity. It is found that the solid-liquid two phases conjugate heat transfer can be significantly enhanced when applying a DC electric field across the domain. [ABSTRACT FROM AUTHOR]

• Systematic numerical simulations of electro-thermo-convection in power-law fluids is studied. • Influence of different combinations of electric field and gravity direction is investigated. • Bifurcation diagram varies considerably with different power-law index. Electro-thermo-convection is a typical interdisciplinary problem, that has been widely studied in terms of its physical phenomena in Newtonian fluids. As an essential supplement to the study of electrohydrodynamics, electro-thermo-convection in power-law fluids within rectangular enclosures is investigated in this paper. Two geometrical configurations are considered, namely differentially heated vertical and horizontal wall configurations. The governing equations are discretized using the finite volume scheme, and the bifurcations, flow patterns, and heat transfer efficiency under different power-law indexes are studied. A comprehensive comparison between the Newtonian and non-Newtonian cases is presented. The results indicate that electro-thermo-convection in power-law fluids exhibits more complicated hydrodynamic behavior than in Newtonian fluids. The shear-thinning characteristic decreases the criterion of subcritical bifurcation and corresponds to a smaller hysteresis loop, whereas the shear-thickening characteristic has an opposite effect. The bifurcation diagrams exhibit various features under different power-law indexes, and some of which have not been observed in Newtonian fluids. Additionally, the effects of the non-Newtonian behavior on the temporal evolution of the flow are discussed, and the fluid motion is found to be sensitive to the power-law index. [ABSTRACT FROM AUTHOR]

Experiments on the electrohydrodynamic (EHD) flow characteristics of a dielectric liquid in a sharp needle-plate configuration under direct current (DC) and alternating current (AC) electric field are carried out. The current-voltage characteristics and the velocity field are simultaneously recorded for understanding the flow behavior and its mechanism. For the DC case, a critical voltage separating the conduction and injection regimes and the corresponding flow reverse is observed. Results indicate the polarity of voltage only influences the flow's intensity in the injection regime. For the AC case, two types of signals with the same 50% duty cycle are considered. For the pulsed voltage (−2.0kV to 0), the flow generally keeps the plume structure and shows weaker strength than the corresponding DC case. For the AC signal (-2.0kV–+2.0kV), the velocity amplitude first decreases and then tends to be constant with the increase of frequency. The weak flow motion with high frequency AC field is due to conduction. • EHD flow characteristics of liquid under DC and AC electric field are investigated. • The influence of DC polarity on charge generation mechanisms is studied. • The velocity field of liquid under applied different AC signals is reported. [ABSTRACT FROM AUTHOR]