Alberto T. Pérez

Two-dimensional numerical analysis of electroconvection in a dielectric liquid subjected to strong unipolar injection

Two-dimensional numerical simulations are carried out to examine the problem of transient electroconvection stability of dielectric liquids subjected to unipolar injection. The entire set of electrohydrodynamics equations associated with the electroconvective phenomena that occur in a layer of a dielectric liquid between two parallel electrodes subjected to a potential difference are solved numerically. We first validate the numerical simulation by comparing our linear stability electroconvection criteria with those obtained by other authors with a stability approach. In this paper, we restrict the study to the strong injection case, which corresponds to values of the non-dimensional injection parameter C greater than or equal to 10. The numerical solution of the electroconvective problem is then presented for rigid lateral boundary conditions. A detailed analysis of the scenario that occurs for different characteristic values of the stability parameter T is provided. The flow structure and its behaviour highlight the existence of different regimes, from laminar to chaotic. The development of charged plumes has been observed in particular. We compute the electrical Nusselt number for different values of the stability parameter and ion mobility. The electrical Nusselt number saturates with increasing T, a fact that it is in agreement with available experimental data. Finally, a spectral analysis is conducted for different aspect ratios of the computational domain. The spectral analysis gives an insight into the physical origin of the velocity and current oscillations.

Electroconvection and its effect on heat transfer

The use of electric fields in order to increase heat transfer in fluids dates back some time. One part of the previous work deals with the effects of the dielectric forces resulting from permittivity gradients which are themselves induced by the imposed temperature gradients. The rest of the work is concerned with the Coulomb force which the electric field (E) exerts on the free charge (or space charge) which results from the thermally induced conductivity gradient within the liquid.

The electrohydrodynamic plume between a line source of ions and a flat plate-theory and experiment

The electrohydrodynamic (EHD) flow between a line source of ions and a flat plate due to an electric field acting on an injected space charge is examined both theoretically and experimentally. A number of simplifying assumptions (e.g., laminar flow and negligible diffusion of charge) are made such that the governing equations are tractable. The mathematical model used is taken from heat transfer analysis and the experimental results are very close to theoretical ones. With such encouraging agreement it may be possible to present a more formal analogy between electroconvective and natural convection flows, leading to a more precise definition of the role of each parameter involved, as well as indicating methods of analysis for EHD problems. >

Thermal and electrohydrodynamic plumes. A comparative study

In this paper we deal with self‐similar thermal and electrohydrodynamic (EHD) plumes. The former arises from hot lines or points, whereas the latter arises when sharp metallic contours submerged in nonconducting liquids support high electrostatic potential, resulting in charge injection. Although the motive force is buoyancy in one case and Coulomb force in the other, it is shown that the solution for EHD plumes is the same as for thermal plumes in the limit of large Prandtl numbers. We present the analysis of axisymmetric plumes for large values of Prandtl number, and this analysis is subsequently applied to EHD plumes. The validity of the approximations for EHD plumes is discussed in the light of experimental data.

Heat transfer enhancement by electroconvection resulting from an injected space charge between parallel plates

Abstract An examination of heat transfer between two parallel plates due to electroconvection is presented. Unipolar ion injection can be brought about at either the upper or lower electrode with heating of the liquid layer from above or below (stable or unstable stratification). The four possible configurations are treated separately. For the transitory regime, an analysis is carried out from which it is possible to calculate the thermal Nusselt number by considering an analogy between the experiment and an electric circuit of a resistance and capacitor in parallel. The values thus approximated, showing increases of up to an order of magnitude in the Nusselt number show very good agreement with the steady-state data. It is thus demonstrated that the electrical effects dominate totally over buoyancy effects for all cases considered. A short analysis of the steady state yields the relevant parameters governing the heat transfer : Id 3 for the fully turbulent flow and IVd 3 in non-turbulent conditions.

An automated apparatus for measuring the tensile strength and compressibility of fine cohesive powders

This paper describes an apparatus based on a novel use of a powder bed, whereby the relationship between consolidation stress, tensile strength, and free volume of fine powder is measured. The powder to be tested is first initialized to a reproducible state. The initialized powder is next consolidated either beyond its own weight or below its own weight by means of a controlled flow of gas. An ultrasonic device measures the height of the bed, thus providing an average value of the powder free volume. Next the consolidated bed of powder is subjected to a slowly increasing gas flow, so directed as to put the powder under tension. The overpressure causing the powder to break provides a measure of the tensile strength of the powder, which in turn is a function of the consolidation and free volume. The relationship between consolidation stress, tensile strength, and free volume is related to powder flowability.

Rose-Window Instability in Low Conducting Liquids

Abstract Rose-window instability appears when corona discharge from a needle is applied over a thin layer of low conducting liquid. When the tip is at high voltage a corona current passes through the liquid and surface deformations appears due to the destabilization of the liquid surface. This instability gives place to a pattern, referred to as rose-window, whose cells are six to ten times greater than the layer thickness.

Dynamics of electrohydrodynamic laminar plumes: Scaling analysis and integral model

In this paper electrohydrodynamic plumes are examined in the region far from the injecting electrode and the collector plate, for both two-dimensional and axisymmetric geometries. The relative importance of the conduction mechanisms (convection, drift and diffusion of electric charge) is analyzed. Diffusion turns out to be negligible compared to convection and drift for the experimental conditions. But the transverse drift (Coulomb repulsion) is of the same order of magnitude than convection. We find a set of three differential equations giving the evolution of the velocity at the center of the plume and the widths of the plume and the charged core inside.

Dynamics and linear stability of charged jets in dielectric liquids

The physical system to be considered is a blade-plane configuration in a dielectric liquid. For high electric fields, injection from the blade takes place with ions of the same polarity. The Coulomb force acting upon the injected charges originates an electrohydrodynamic (EHD) flow, referred in what follows as the charged jet. A laminar solution of this EHD jet is obtained using similarity analysis. If transport of charge is dominated by convection, i.e., neglecting molecular diffusion and ion drift, and the electric field is assumed constant, the problem is mathematically equivalent to the bidimensional thermal plume in the limit of large Prandtl numbers. The authors examine the stability of this EHD jet using linear theory and parallel-flow approximations. Neutral stability curves are computed numerically in terms of a nondimensional parameter which is the electrical analogous to the Grashof number. Finally, some experimental observations are presented, followed by a short discussion. The role played by the viscosity correlates reasonable well with the theoretical analysis. >

Instability in a non-ohmic/ohmic fluid interface under a perpendicular electric field and unipolar injection

We set the equations for the linear electrohydrodynamic instability of an interface between two fluids, subjected to a perpendicular field and a unipolar charge injection. One of the fluids is modeled as being in non-ohmic regime (insulating), whereas the other is ohmic. A new interfacial instability mechanism is described, which may account for the Rose-window instability. The equations are analytically solved in the limit of long wavelength and neglecting the fluid motion. We show that this limit applies well to the case of an air–ohmic liquid interface. The applicability to a liquid–liquid interface is also analyzed.