Payam Sharifi

Computational Studies on the Behavior of an Interface Separating Two Fluids Under Uniform Electric Field

Direct Numerical Simulations (DNS) are carried out to study the dynamics of a horizontal interface separating two fluids, having different electrical properties, under the influence of a uniform electric field. A front tracking/finite difference scheme is used, in conjunction with Taylor’s leaky dielectric model, to solve the governing electrohydrodynamics equations in both fluids at finite Reynolds numbers. The methodology and the code is validated by comparing the results with those of the analytical studies developed at the linear stability limit and it is shown that a very good agreement exists between the two. The results of this study show interesting interface behavior depending on the parameters of the problem. In all the cases considered, the interface becomes unstable beyond a critical voltage and starts to oscillate as it moves toward its steady-state shape which is a vertical column pointing from the liquid of higher electric conductivity to the one with a lower conductivity. The shape of the column, however, will vary depending on the individual governing parameters.Copyright

Computational Studies of EHD-Enhanced Condensation Heat Transfer on a Downward-Facing Horizontal Plate

This study aims to investigate the effect of uniform electric fields on the enhancement of condensation heat transfer from a downward facing horizontal plate by direct numerical simulations. The governing equations of fluid flow and electric field are solved using a front tracking/finite difference technique in the framework of Taylor’s leaky dielectric model. The electric force comprises of the dielectrophoretic and the Coulomb forces. Both forces act on the phase boundary and their relative magnitude and directions affect the condensation rate. For the results shown here, the condensate drops are more elongated compared to the those in zero-electric field. It is shown that the electric field enhances the condensation rate in two ways: by increasing the number of the drops that are generated per unit surface due to destablizing the interface and by increasing the frequency of drop generation and pinch off. The mechanism of elongation of the condensate drops are explained by detailed examination of the distribution of the electric field at the phase boundary.© 2009 ASME

The Effect of Film Thickness on EHD-Driven Instability of Interface Separating Two Liquids

Most of the studies conducted so far on EHD-driven instability of superimposed fluids have been concerned with liquid layers of modest depths. In many applications, however, the liquid layers can be very thin. Since the dynamics in thin films is generally governed by lubrication equations rather than full Navier-Stokes equations, it is expected that the interface dynamics will be quite different from that of the liquids with modest depths. The objective of this study is to explore the effect of initial liquid thickness on the dynamics of the phase boundary. To do this end, we perform Direct Numerical Simulations (DNS) using a front tracking/finite difference scheme, in conjunction with Taylor’s leaky dielectric model. For the physical parameters used here, it is shown that for sufficiently thick liquid layers, the interface instability leads to formation of liquid columns that merge together to form a big column. However, for thin layers, the interactions between the columns are weaker and lead to a short and a longer column that are connected by a thin liquid film.Copyright

Dynamics of Film Boiling in AC Electric Fields

Direct numerical simulations are performed to explore the effect of AC electric fields on film boiling of liquids at horizontal flat surfaces. The simulation results show that the application of the electric field leads to a substantial increase in the Nusselt number and that the Nusselt number correlates well with the instantaneous strength of the electric field. The effect of the dielectric properties of the liquid and vapor phase on the interface stability and free charge build up is discussed and it is shown that the direction and strength of the electric stresses at the interface depends on these parameters in a subtle way.

Dynamics of the Interface Separating Two Superimposed Fluids in AC Electric Fields

Direct Numerical Simulations (DNS) are carried out to study dynamics of a horizontal interface separating two fluids, having different electrical properties, under the influence of an AC electric field. A front tracking/finite difference method is used in conjunction with Taylors leaky dielectric model to solve the governing electrohydrodynamic equations in both fluids at finite Reynolds numbers. The behavior of the interface in an AC field at a high and a low frequency is compared with that in the corresponding DC electric field. In all the cases considered, the interface forms a vertical conic column which originates at the fluid with a higher electric conductivity and extends to the one with a lower electric conductivity. In the DC electric field, the interface height (represented by its midpoint) reaches a steady state which is determined by the balance between the electric force (that pulls the interface upward) in one hand, and the surface tension and the column weight (that try to bring the interface back to its original position) on the other hand. In the low frequency AC field, the height oscillates between a minimum that is nearly zero and a maximum that is substantially higher than the steady-state DC height. In the high frequency, however, the minimum and maximum heights are close to the steady state DC one.

Direct Numerical Simulation of EHD-Enhanced Film Boiling

In applications involving boiling in micro-devices or under microgravity conditions it is extremely desirable to enhance the heat transfer rate to increase the efficiency of these systems. Here, a possible mechanism is to increase the convective effects by application of an electric field on the bulk of the fluid. While the enhancement of heat and mass transfer by electric field has been known for decades, a fundamental understanding of the problem is still lacking, primarily due to the difficulties in conduct of experimental studies. Direct Numerical Simulations (DNS) opens up enormous possibilities for detailed understanding of EHD-enhanced film boiling. Such simulations can make it possible to capture the dynamics of the boiling flows. Here, we present a front tracking/finite difference algorithm, in conjunction with a leaky-dielectric electrohydrodynamic (EHD) model, to study EHD-enhanced film boiling on horizontal surfaces. According to this study, the bubble shape and its frequency of release are highly dependent on the dielectric properties of fluid, and electric field strength. Our results show an improvement of about 50% in the Nu number over that of the regular boiling in the range of parameters that are explored here.© 2008 ASME