FUNDAMENTAL STUDIES ON THE EFFECT OF ELECTRIC FIELD ON INTERFACE INSTABILITY, FILM BOILING, AND FILM CONDENSATION

AN ABSTRACT OF THE DISSERTATION OF PAYAM SHARIFI, for the Doctor of Philosophy degree in ENGINEERING SCIENCE, presented on April 2011, at Southern Illinois University Carbondale. TITLE: FUNDAMENTAL STUDIES ON THE EFFECT OF ELECTRIC FIELD ON INTERFACE INSTABILITY, FILM BOILING, AND FILM CONDENSATION MAJOR PROFESSOR: Dr. A. Esmaeeli This research focuses on investigation of uniform electric field on three inter-related interfacial phenomena including interface under electric field, film boiling under applied electric field, and film condensation under applied electric field. The idea of applying electric field to enhancement boiling and condensation heat transfer has been considered one of the active enhancement methods. However, understanding the details of interaction of electric field and phase change demands a strong tool to go beyond the limitations of experimental and theoretical approaches. We perform Direct Numerical Simulations (DNS) using front-tracking/finite-difference techniques to fully resolve the electric, flow, and heat transfer fields in continuum scales. In terms of electric field-induced interface instability problem, we studied the dynamics of interface under AC/DC uniform electric fields for a wide range of fluid physical properties and investigated the individual effect of their corresponding nondimensional numbers. We observed that application of DC electric field destabilizes the interface in such a way that it goes over several cycles of oscillations and then settles to its steady-state form and remains quiescent. However, for AC electric field, the interface oscillations follows the frequency of applied electric potential source. For the film boiling under applied electric field, we studied the effect of individual governing nondimensional numbers on the behavior of film boiling under DC/AC electric fields. Electric field makes the interface more unstable by elongating the bubbles, decreasing the most dangerous wavelength, and expediting the formation of bubbles. The impact of these effects on heat transfer can be observed from the evolution of Nu number in the course of film boiling. We realized that for the same conditions AC field alters the transient spatially averaged Nu number in a way that it follows the oscillations of applied electric potential source. However, the heat transfer enhancement does not get affected by applying either AC or DC electric fields. We extended our research to multimode film boiling to observe the interaction of bubbles growing next to each other. Also, we carried out a study on the effect of electric field on downward-facing film condensation over a horizontal flat plate. This problem is similar to film boiling over a horizontal flat plate which we already studied although the phase change occurs in opposite direction. Like the effect of electric field on film boiling, electric field made the interface of condensate more unstable by decreasing its most dangerous wave length. However, in this case, the enhancement becomes more effective due to cooperation of gravitational and electrical forces. Our studies show that phase change heat transfer coefficient can be enhanced in the presence of electric field by more that 70%. Condensation of vapors over the bank of horizontal tubes has always been the host of many engineering applications in power plants, chemical and petrochemical plants, etc. To take the first step toward the study of enhancement effect of electric field on complex geometries, we also carried out a study on the condensation over tube banks in the absence of electric field. This study mainly concentrates on the effect of tube distance on heat transfer coefficient in a vertically in-lined tube bank. Our study reveals that heat transfer coefficient can be highly dependent on tube diameter and spacing such that choosing an appropriate spacing can lead to a more than 50% enhancement