(Drs. Abdel-Khalik and  Sheldon Jeter, co-advisors)

"EHD Enhancement of Nucleate Pool Boiling"

Abstract

 Electrohydrodynamic (EHD) enhancement of single phase natural convection and nucleate boiling heat transfer allows for significant increases in heat transfer coefficients. By applying an electric field to a boiling dielectric fluid, both the vapor bubble dynamics and the liquid motion are modified. Three major components of the electric force contribute to this behavior: 1) Electrophoresis, 2) Dielectrophoresis, and 3) Electroconvection. Electrophoresis is concerned with the conduction of electric charge in an electric field. Dielectrophoresis is the translational motion of neutral matter caused by polarization effects in a non-uniform electric field. Electroconvection is the mixing of the thermal boundary layer caused by the temperature dependence of the dielectric constant. An experimental investigation of the effect of EHD on single phase natural convection and on bubble dynamics in saturated nucleate pool boiling of HCFC-123 was conducted to elucidate the nature of the electric forces, and quantify the extent of heat transfer enhancement.

Single phase natural convection experiments from a horizontal heated platinum wire in a pool of liquid R-123 with an applied uniform electric field were conducted. As a result, a new correlation for predicting EHD enhancement of natural convection from a heated horizontal cylinder based on the dimensionless electric Rayleigh number was developed. Saturated nucleate boiling experiments in HCFC-123 with an applied uniform electric field were also conducted. The bubble dynamics was quantified by measuring the average number density of active nucleation sites, the average bubble departure frequency per nucleation site, and the frequency distribution of bubble departure diameters. These measurements were conducted through high-speed video imaging of the platinum wire. At a given heat flux, with the application of an electric field, ebullition was suppressed because of the enhancement of natural convection which lowered the surface temperature. As a result, there were fewer active nucleation sites with an applied electric field. In addition, the applied electric field caused a reduction in the average departure diameter of the vapor bubbles. The results of this investigation will provide the means to validate the applicability and accuracy of various mechanistic nucleate boiling models in the presence of an electric field.