Electroconvection of a poorly conducting fluid in a steady electric field

The emergence of electroconvection and nonlinear flows of a nonisothermal poorly conducting fluid in a steady electric field of a horizontal capacitor is investigated. The main mechanism of charge formation in the fluids under investigation is electric conduction due to the temperature dependence of the charge mobility. The Galerkin method is used for obtaining a system of nonlinear differential equations for the amplitudes of spatial modes; on the basis of this system, steady-state, periodic, and random nonlinear regimes of the fluid flow are investigated. The scenarios of transition to chaotic flows (via intermittence and a subharmonic cascade) are discovered. The interrelation between the electroconductive and thermogravitational mechanisms of convection is analyzed, and hysteretic transitions between monotonic and oscillatory regimes are studied. Qualitative comparison with the experiment is carried out.     

Electroconvection in the presence of autonomous unipolar injection and residual conductivity

The problem of the finite-amplitude electroconvection of a nonuniformly heated fluid filling a horizontal capacitor in the presence of the autonomous unipolar injection of a charge from an anode has been solved. In addition to the injected charge in a carrier fluid, the presence of proper ions (appearing owing to dissociation) ensuring the low residual conductivity of the fluid is assumed. The effect of the ratio of proper and injected ions, as well as the heating degree, on the thresholds of monotonic and oscillating electroconvection has been analyzed. The bifurcation diagrams of the solutions have been plotted for the cases of the heating of a layer from below and above. The electroconvection modes of the nonuniformly heated liquid that appear because of forward and backward bifurcations have been revealed.     

Investigation of electroconvective flow regimes in liquid dielectrics possessing unipolar injection conductivity

Isothermal electroconvection in a dielectric liquid arising in a plane-parallel electrode system due to unipolar injection of charges from the cathode is considered. Stationary flow regimes formed after the crisis of the equilibrium layer stability loss are numerically modeled. Variations of initial conditions yield two flow regimes providing comparatively higher volume charge densities in the first case near the cathode and in the second case near the anode. __________ Translated from Izvestiya Vysshikh Uchebnykh Zavedenii, Fizika, No. 6, pp. 3–7, June, 2008.     

Low-mode model of electroconvection of an ideal dielectric

A three-mode model of electroconvection of an ideal dielectric in the ac electric field of a horizontal capacitor is developed. With the help of this model, hysteretic transitions between different nonlinear regimes of electroconvection of the perfect liquid dielectric are found and investigated. The cases of zero-gravity and of heating from above in the gravitational field are considered. Frequency dependencies of the electroconvection oscillatory regimes are studied. Diagrams of the regimes are constructed. The following scenarios of transition to chaos are explored: through a subharmonic cascade, through quasi-periodicity, and from equilibrium. Behavior of the electroconvecton oscillations in the high-frequency limit is also studied.     

Numerical simulations of electro-thermo-convection and heat transfer in 2D cavity

In this article the electro-thermo-convective phenomena in a dielectric liquid enclosed in a 2D cavity and subjected to the simultaneous action of an electric field and a thermal gradient is studied. We solved directly the full set of coupled equations of Electro-Hydro-Dynamic (EHD) and energy equation using a finite volume method. In order to characterize the influence of the electric field on heat transfer the liquid is first heated (from a lateral wall) till the thermal steady state is obtained and then the electric potential and injection of electric charge is applied. Two cases of injection are considered: from the lower electrode and from a lateral wall (left or right). The flow pattern and Nusselt number strongly depend on the non-dimensional characteristic parameters: electrical parameter, Rayleigh number, Prandtl number and mobility parameter M. The convective motion passing from a purely thermal convection to a purely electrical convection and the number of electro-thermo-convective rolls patterns are investigated.As a consequence of the analysis of the combined effect of electric and thermal fields on the flow structure and on Nusselt number, we have also evaluated the heat transfer enhancement due to electroconvection. It is shown that the injection of electric charge increases the heat transfer and Nusselt number is independent of Rayleigh number for high enough values of T.     

Electric convection of low-conductivity liquid in a horizontal capacitor in the presence of unipolar charge injection

Electric convection of low-conductivity liquid in a horizontal plane capacitor is analyzed with allowance for unipolar charge injection. Dynamics of charge transfer through stationary isothermal liquid in the presence of modulated electric field is studied. Effect of modulation amplitude and frequency on the spatiotemporal distribution of charge density and electric potential is considered. Nonlinear electric convection of nonisothermal low-conductivity liquid is studied in the presence of static electric field in a 2D system. Hysteresis transitions between two different (with respect to intensity) regimes of electric convection are analyzed.     

The onset of convection of a poorly conducting fluid in a modulated thermal field

The stability of a poorly conducting fluid in a constant electric field of a horizontal capacitor is investigated under a variable temperature gradient. It is assumed that free charge in the fluid is generated only due to the nonhomogeneous conductivity of the fluid. The Floquet theory is used to determine the convection thresholds. The instability boundaries and the characteristics of critical perturbations are determined. In addition to the synchronous and subharmonic responses to an external action, the instability can be attributed to quasiperiodic perturbations. The low-frequency limit of modulation is considered by an asymptotic method. The critical electric Rayleigh number is represented as a function of inverse frequency and heating level.     

Visualization of transient natural convection heat transfer from a vertical rectangular fin

Experiments on thermal visualization of transient natural convection from short vertical rectangular fins were conducted using the technique of laser holographic interferometry. A sequence of infinite-fringe interferograms recorded for the heating regime of an aluminum fin demonstrates the effect of fin base heating on local convection coefficients and reveals alternating and oscillatory buoyancydriven flows similar to those over the top surface of heated horizontal plates. The effect of fin base heating results in greater uniformity of the local heat transfer coefficient along the fin. It also significantly reduces the steady-state heat transfer coefficients of short vertical fins compared to their transient values. Hence, the use of steady-state solutions for the design of short vertical fins operating in transient conditions may not introduce as much error as was previously thought.     

Onset of parametric electroconvection in a nematic liquid crystal

The behavior of a nematic liquid crystal (NLC) in an external ac electric field is studied in terms of the weak electrolyte model. Since electroconvection of certain types of NLCs can occur in a constant field in an oscillatory mode, a quasi-periodic response of a nematic to the external action is possible in an ac field, as well as a sharp reduction of the electroconvective stability thresholds at resonance frequencies. Stability maps for NLCs are plotted on the plane of amplitude-frequency parameters of the external field. Phase portraits are constructed for subharmonic and synchronous perturbations. The optical response of a NLC in the ac electric field is studied; the time dependences of the relative intensity of light, which passed through the nematic liquid crystal cell, are obtained.     

The measurement of subcritical electroconvection

Under idealized conditions, electroconvection in a nematic liquid crystal sets in at a well-defined threshold of the driving ac voltage. Fluctuating convection rolls of a small amplitude below that threshold have been observed recently. The measurement of the amplitude of these fluctuations is described in detail in this paper. It is based on averaging the light intensity signals using the structure function and a quantitative analysis of the light deflection.     

Dynamics of electroconvective wave flows in a modulated electric field

Wave regimes of finite-amplitude electroconvection in a poorly conducting fluid heated from above in a modulated field of a horizontal capacitor are analyzed. Low conductivity of the fluid is ensured by the charges injected from the anode and intrinsic (dissociated) ions. A bifurcation diagram of solutions is constructed. Apart from the regimes of modulated standing and traveling waves, stable waves changing their direction of motion are detected, as well as waves with a chaotically varying phase velocity.     

Dynamics of electroconvective nematic liquid crystal structures in a nonharmonic electric field

The emergence of electroconvection in a nematic liquid crystal under the action of a nonharmonic electric field is investigated. Analysis is carried out using a 2D model. We propose new forms of the varying electric field acting on the system, for which subharmonic oscillations exist: (a) electric field of a trapezoidal form and (b) external field varying in accordance with the law of “joined cosines.” The behavior of synchronous excitations in the insulating and conducting regimes, as well as subharmonic oscillations, is analyzed. The parametric instability domains are found, and the critical frequencies of transition between different response regimes are determined. The stability maps of the nematic liquid crystal are constructed on the frequency-voltage amplitude plane.     

Experimental study of coherent transition radiation from relativistic electron bunches entering a dihedral angle

The angular distributions of the intensity of transition radiation from a bunch of relativistic electrons entering a dihedral angle between two conducting planes have been measured in a millimeter wavelength range. A microtron with a particle energy of 7.4 MeV is used as a source of electrons. The effect of the particle injection direction and the magnitude of the dihedral angle on the angular distribution of the radiation intensity has been analyzed. The measurements show that the character of the distribution of radiation from a charge entering the dihedral angle significantly differs from that for a charge escaping the angle. A comparatively small change in the magnitude of the dihedral angle can lead to qualitative changes in the angular distribution of radiation from a charge entering the dihedral angle.     

Electroconvection between coaxial cylinders of arbitrary ratio subjected to strong unipolar injection

This study deals with the electroconvection phenomenon that takes place in a dielectric liquid layer placed between two annular electrodes and subjected to the action of an electric field.The full set of governing equations describing the combined Electro-Hydro-Dynamic (EHD) flow is directly solved with the finite volume method.The development of electroconvective motion is investigated in details in the case of strong injection when the emitter electrode is the inner cylinder.We first examine the stability of such flow. As in the plane–plane configuration we have highlighted the existence of a linear and non-linear critical electric Rayleigh numbers giving rise to a hysteresis loop. The flow structure under the effect of the electric field is analyzed and the distribution of electric charge density is presented. In particular we investigate the effect of various parameters involved in this configuration, such as the radius ratio, injection strength and electric Rayleigh number.A multicellular convective pattern is particularly observed and it is shown that the number of cells is increased when the annular gap is narrower. Finally a significant increase of the rotational speed of the vortex with the electric Rayleigh number is recorded.     

Negative space charge and electric field intensity in polymeric insulators at low temperatures

The effect of negative space charge accumulation due to injection of electrons from cathode microprotrusions on the steady-state and transient electric field distributions in polymer dielectrics is discussed. An isolated microprotrusion is modeled by a spherical capacitor in which an electrode of smaller radius is the cathode. The calculations include the fact that the distribution of negative space charge depends on the rate of capture and liberation of electrons by traps, while the activation energy of this process depends on the electric field intensity. An exponential energy distribution is proposed for the traps. It is shown that significant electrical overvoltages can only appear near the cathode microtips immediately after switching on the voltage. In the course of 10⁻⁶–10⁻⁵ s, the coefficient of electrical overvoltage drops to a few units and approaches its steady-state value. The region of significant electrical overvoltage is localized, and is the same order as the dimensions of the microtip. Fiz. Tverd. Tela (St. Petersburg) 40, 1167–1172 (June 1998)     

星载行波管高效率阴极热子组件优化设计

为减小X波段星载脉冲行波管的热子功率,对阴极热子组件结构进行了优化设计,利用ANSYS有限元软件对该阴极热子组件结构进行了热分析,得到结构的稳态温度场分布、阴极温度瞬态解以及加热功率与阴极温度的关系,并与实测结果进行对比,对比结果表明,阴极温度模拟与实验结果误差在1.3%以内,说明了所用模型和方法的可行性。在此基础上通过研究阴极支持筒不同开槽宽度、壁厚及材料下热子加热功率-阴极温度关系,对阴极支持筒进行了优化。模拟结果显示,优化后的阴极热子组件结构加热功率由8.2W降到6.7 W。     

On couple-stress fluid heated from below in porous medium in hydromagnetics

A layer of electrically conducting couple-stress fluid heated from below in porous medium in presence of magnetic field is considered. For stationary convection, the couplestress and magnetic field postpone the onset of convection whereas the medium permeability hastens the onset of convection. The magnetic field introduces oscillatory modes in the system which were non-existent in its absence. A sufficient condition for the non-existence of overstability is obtained.     

Acceleration of a multicomponent plasma in the cathode region of a vacuum arc

A general solution to the problem of the steady-state spherical expansion of a current-carrying multicomponent plasma into a vacuum is derived. It is shown that, in vacuum arc discharges, the main force accelerating the cathode material, which becomes a plasma at distances of 1 to 300 μm from the cathode surface, is the electron pressure gradient force maintained by Joule heating. It is established that ions of different charges move with the same hydrodynamic velocity, which is uniquely determined by the mass and mean charge of the ions and the maximum electron temperature in the cathode region.     

Oscillations of an electron stream with a virtual cathode

The results of an investigation of the oscillations of a virtual cathode and the hysteresis of the state for a one-dimensional electron stream in a flat drift space are given. It is shown that in the case of a given mean space charge density and a given injection current different oscillatory regimes can exist in a stream with a virtual cathode.Translated from Izvestiya Vysshikh Uchebnykh Zavedenii, Fizika, No. 10, pp. 74–77, October, 1987.In conclusion, the authors thank L. V. Shelomentsev for carrying out the numerical calculations for the nonrelativistic stream on the computer.     

Electric convection in a nematic liquid crystal in a varying electric field

The emergence of electric convection in a nematic liquid crystal under the action of a varying electric field is analyzed in the 1D approximation. A new mode of external field variation with subharmonic oscillations is proposed. The behavior of several classes of synchronous perturbations and subharmonic oscillations is analyzed; the evolution of the fields of the director and of the volume charge is studied. Parametric instability regions are specified and the critical frequencies of transitions between response modes are determined. Stability maps for a nematic liquid crystal are plotted on the frequency-amplitude plane.