EHD-enhanced pool boiling heat transfer: Experimental methodologies and parametric studies

Among active heat transfer enhancement methods, Electrohydrodynamic (EHD) technique has shown promising results for substantial improvement of single-phase and phase change heat transfer in heat exchangers. This technique couples the flow field with a high voltage, low current electric field in a dielectric fluid medium. The result is an increase in heat transfer coefficient often in the range of one order of magnitude higher than that offered by conventional techniques.;Two main contributions of this work were: (1) quantifying the EHD applicability in an industrially sized heat exchanger and (2) a systematic study of electric heating method versus water heating in a pool boiling setup. It is of extreme interest to academia and industry to know the limitations and conditions for use of these two techniques for experiments such as pool boiling. The present study began by a thorough analysis of the electric and fluid fields interactions and the governing equations for the electric body force. The experimental part included experimenting on smooth and enhanced surfaces. Both fluid and resistive (electric) heating were tested. The electrode tested included straight wire and wire mesh geometries. Each heat transfer process was studied parametrically in terms of operating pressure and high voltage levels. In most cases enhancements were obtained with negligible power consumptions when compared to the heat exchanger capacity.;Boiling heat transfer process over 19 fpi tube with R123 and R134a as the boiling fluids was studied at different pressures. At lower pressures and therefore lower corresponding saturation temperatures, the dielectric constant of the fluid is higher and the electric body force is more effective. The results of water heating and resistive heating were compared for different tube surfaces. It is the pseudo-exponential water temperature profile along the tube length in a water heating set up that causes the difference between the two methods. In addition, the trend of heat transfer coefficient for different tube surfaces is important to determine the degree of difference between the two heating methods.;Visualization studies were performed for Corona 36igc and Turbo-BII tubes to better understand the EHD phenomena in boiling heat transfer processes. It was concluded that the electro-convection and the increased evaporation rate due to increase in the interface surface between bubbles and liquid film were the major contributions to the enhancement of boiling heat transfer for the Corona 36igc tube. However, the electric force had an adverse effect for the case of Turbo-BII tube, as the bubbles were trapped in the cavities, causing up to 50% reduction in the heat transfer coefficient.