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.

Reflux condensation occurs in a vertical tube when there is an upward core flow of vapour (or gas-vapour mixture) and a downward flow of the liquid film. The understanding of this condensation configuration is crucial in the design of reflux condensers and in loss-of-coolant safety analyses in nuclear power plant steam generators. A range of modelling approaches exists for co-current film condensation from gas-vapour mixtures in parallel-plate channels and tubes. These methods are based on marching from the inlet down the tube and do not apply to the reflux condensation. In this research, however, a two-dimensional two-phase model was developed that solves the steady, full elliptic governing equations in both the film and the gas-vapour core flow on a non-orthogonal mesh that dynamically adapts to the phase interface. Gas-vapour shear and heat and mass transfer at the interface were accounted for fundamentally. This modelling is a big step ahead of current capabilities by removing the limitations of previous reflux condensation models which inherently cannot account for the detailed local balances of shear, mass, and heat transfer at the phase interface. The model was developed and applied for co-current and counter-current flows in vertical parallel-plate channels, followed by vertical tubes. In each stage, the model results were compared against the available experimental and numerical data for validation purposes. A wide range of boundary conditions and geometries have been studied to examine the details of co-current and counter-current condensation phenomena. Velocity, temperature, pressure, and gas mass fraction profiles along with the axial variation of various parameters such as local Nusselt number, film thickness, interface and centre-line temperature and gas mass fraction are presented in parametric studies. / February 2017

In this study laminar forced film condensation of vapor flowing in cross-flow direction through the annular space between two concentric cylinders was investigated numerically. To achieve this, governing equations of the vapor and the condensate flow in cross-flow direction between two concentric cylinders were developed. After obtaining the equations in integral forms by using the finite difference technique the vapor boundary layer thicknesses on the inner and outer cylinders and the condensate layer thickness was obtained as a function of the angular position on the cylinders. It was assumed that the condensation took place on the outer surface of the inner cylinder only and the outer cylinder was assumed to be insulated. The computer program developed is capable to calculate the condensate film thickness, vapor boundary layer thickness, the heat flux and the heat transfer coefficient and the interface velocity between the condensate and the vapor layer as a function of the angular position on the cylinders. Effects of changing the free stream velocity flowing in the channel, the radius of the inner cylinder, the temperature difference between the saturated vapor and the wall and the annular space between the concentric cylinders were investigated numerically by using the computer program and the results were presented graphically. Results showed that by increasing the free stream velocity of the vapor in the core, the film thickness decreased and by increasing the radius of the inner cylinder, the temperature difference between the saturated vapor and the wall and the annular space, the film thickness increased.

The objective of this study is to determine the mean heat transfer coefficient and heat transfer rate and to analyse the effect of inclination angles,the effect of subcooling temperatures and the effect of vapour velocity for laminar filmwise condensation of water vapour on a vertical tier of spheres experimentaly and analyticaly. For this purpose, the experimental aparatus were designed and manufactured. In the free condensation experimental study &Oslash / 50mm and &Oslash / 60 mm O.D. spheres were used to analyse the diameter effect . In the experimental studies of free and forced condensation &Oslash / 60mm O.D. spheres on which vapour flows at 2,75 bars were used to analyse the effect of vapour velocity. For the experimental study of the annular condensation in the concentric spheres the effect of vapour velocity was studied by forcing the vapour to flow in the area between two concentric spheres. In the free condensation experiments it is observed that at smaller diameters the heat flux and mean heat transfer coefficients for sphere is higher. In the free and forced condensation experiments increasing the velocity of vapour increases the mean heat transfer coefficient. At the experiments with annular condensation between the concentric spheres high mean heat transfer coefficient values have been obtained compared to the free and forced condansation over the surface of spheres experimental studies.

This work analytically solves the governing equations of the laminar film condensation from pure vapors in vertical tubes to find the condensate film thickness. The solution is then extended to turbulent flow conditions for steam. All other relevant quantities are derived from the film thickness solution. For laminar film condensation of quiescent vapors, an exact explicit solution and an approximate closed-form solution were found by using a new definition of the dimensionless film thickness, the Lambert W-function, and a logarithmic function approximation. For laminar mixed-convection film condensation with interfacial shear stress, an approximate closed-form solution was found by using a new definition of the pressure gradient. For turbulent film condensation of steam, correlations of the turbulent vapor and liquid viscosities were formed by asymptotically comparing this approximate laminar closed-form solution to a turbulent flow numerical solution. The present solutions compared very well to published numerical and experimental data.

This work analytically solves the governing equations of the laminar film condensation from pure vapors in vertical tubes to find the condensate film thickness. The solution is then extended to turbulent flow conditions for steam. All other relevant quantities are derived from the film thickness solution. For laminar film condensation of quiescent vapors, an exact explicit solution and an approximate closed-form solution were found by using a new definition of the dimensionless film thickness, the Lambert W-function, and a logarithmic function approximation. For laminar mixed-convection film condensation with interfacial shear stress, an approximate closed-form solution was found by using a new definition of the pressure gradient. For turbulent film condensation of steam, correlations of the turbulent vapor and liquid viscosities were formed by asymptotically comparing this approximate laminar closed-form solution to a turbulent flow numerical solution. The present solutions compared very well to published numerical and experimental data.

The objective of this study is to analyze theoretically the laminar film condensation of water vapor flowing on a sphere. For this purpose, the problem was handled by including all of the two-phase boundary layer parameters such as gravity, effect of vapor shear, inertia, energy convection and pressure gradient. For this full two-phase boundary layer system, the boundary layer equations, boundary conditions and the interfacial conditions were first analyzed, and then discretized. A computer program in Mathcad, solving these discretized equations, was written to obtain the velocity and temperature profiles within the condensate, the velocity profiles within the vapor, the condensate film thickness and the local Nusselt number. The effects of pressure gradient, gravity, vapor oncoming velocity and sphere radius on these parameters were examined. By alternating the formulation of the problem, the results for the flow over a horizontal cylinder were obtained. These results were then compared with those for the sphere. Finally, the results for the system with Mercury vapor flowing on a sphere were obtained. All of these results were represented as diagrams and tables, and were discussed at the end of the study.

EFFECTS OF OFF-CENTER ANGLE ON THE HEAT TRANSFER COEFFICIENT ON VERTiCAL TIER OF MULTIPLE SPHERICAL SURFACES Kaya, Ebubekir M.S., Department of Mechanical Engineering Supervisor: Assoc. Prof. Dr. Cemil Yamali December 2004, 112 pages The purpose of this study is to investigate the laminar film condensation phenomenon of steam on a vertical tier of multiple spherical surfaces by using both analytical and experimental methods. The analytical heat transfer results were obtained by following the Nusselt type of analysis and represented graphically. In addition, in order to observe the real behavior of the film condensation, an experimental setup was manufactured and experiments were done. In analytical section / mass flow rate, (mean) velocity, film thickness, local heat flux and local heat transfer coefficient values were obtained and plotted as depending on angular position. Moreover, mean heat flux and mean heat transfer coefficient variations were presented with respect to diameter of the sphere and sub-cooling. On the other hand, for the experimental section, heat flux and mean heat transfer coefficient values were obtained and expressed as depending on sub-cooling. To see the effects of off-center angle, setup was inclined for different angles and experiments were repeated for each inclination angle. At the end of the study, mean heat transfer coefficients belong to analytical and experimental studies were compared to each other as well as to the literature.

A complete two-phase numerical model of film condensation from a mixture of a vapour and a non-condensing gas that is based on the two-dimensional elliptic governing equations with variable physical properties is presented. The model predicts the full viscous flow and heat and mass transfer for the mixture around the tube and in the entire liquid film from the top of the tube to the falling film below the tube. A finite volume method is used with a segregated solution approach and a dynamically moving computational grid that tracks the phase interface sharply. Fundamental balances of mass, energy, and force are enforced accurately at the phase interface. The model was developed in steps and validated against various experimental and theoretical works in the literature for different two-phase flows. The validation tests included stratified flow of liquid and gas in a horizontal channel, falling liquid film over a vertical wall, and condensation of steam from a steam-air mixture in a vertical channel. The model was used to simulate laminar film condensation from a downward flowing steam-air mixture over an isothermal horizontal tube. The validity of this new model is demonstrated by comparisons with previous theoretical and experimental studies. New results are presented on the effects of free-stream-to-tube temperature difference, upstream Reynolds number, free-stream gas mass fraction, and free-stream pressure on the condensate film development, the local and average heat transfer coefficients, and the total condensate mass flow rate. It was found that the temperature difference had the greatest effect on the condensation rate and film thickness. The presence of non-condensing gas in the vapour has a strong negative impact on the condensation process. For the pure steam case, moderate changes in the upstream Reynolds number showed slight increases in condensate mass flow rate with increased Reynolds number. For the mixture case, however, moderate increase in upstream Reynolds number increases significantly the condensate mass flow rate and film thickness. This trend becomes more noticeable as the free-steam gas mass fraction increases. Changing the free-stream pressure demonstrated that property variation had a relatively smaller effect than temperature difference and gas mass fraction changes. / February 2017